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SAMPLING AND INTERPRETATION OF DRILL CUTTINGS FROM GEOTHERMAL WELLS

J e f f r e y B. Hulen and Bruce S. Sibbett

.. from: Society o f Professional Well Log Analysts, Geothermal Log

I n t e r p r e t a t i o n Handbook, 2nd e d i t i o n ( i n press)

)

Also presented a t : Introduct ion t o Geothermal Log I n t e r p r e t a t i o n , Geothermal Resources Council Tech. Training Course No. 7, Apr i l 22-23, Reno, Nev. (1981).

e I

c I;

ABSTRACT

D r i l l c u t t i n g s from geothermal and mineral explorat ion boreholes, by

con t ras t with those from most petroleum wells, commonly are der ived h i g h l y

f ractured and faul ted, hydrothermally a l t e r e d igneous and metamorphic rock

sequences, and are l i k e l y t o be severely contaminated. Character izat ion of a

subsurface resource from c u t t i n g s thus requires no t on l y espec ia l l y ca re fu l

sample co l l ec t i on , preparation, storage and examination, bu t a l s o a thorough

knowledge of d r i l l i n g technology, l o c a l geology and the f u l l range o f

p o t e n t i a l borehole contaminants.

Accurate i d e n t i f i c a t i o n o f l i t h o l o g y from c u t t i n g s i s c r i t i c a l f o r

recogni t ion and c o r r e l a t i o n o f rock types l i k e l y t o s e l e c t i v e l y host t he

desired commodity, However, many o f the rocks encountered i n geothermal and

mineral exp lo ra t i on boreholes (such as gneisses and g r a n i t i c rocks) can

resemble one another c lose ly as cu t t i ngs even though d i s s i m i l a r i n outcrop o r

core,

can be determined by comparison wi th simulated c u t t i n g s f o representat ive

surface rocks, and w i t h various geophysical and other wel l logs. Many other

I n such cases, t h e actual rock type(s) i n a cu t t i ngs sample genera l ly

c lues i n cut t ings, such as d iagnost ic metamorphic mineralogy, o r sedimentary

rounding and sort ing, may help i d e n t i f y subsurface l i t h o l o g i e s .

Faul ts and f ractures commonly are t h e dominant physical con t ro l s on . r '

geothermal and minral resources. Faul ts occasional ly can be recognized

I d i r e c t l y i n cu t t i ngs by t h e presence of s l ickensid ing, gouge, o r o ther crushed

mater ia l , More commonly, however, t h e "gouge" observed n cu t t i ngs a c t u a l l y i s L

pseudo-gouge created beneath a b i t dur ing d r i l l i n g . Since most f a u l t s and a l l

f r ac tu res produce no d i r e c t evidence apparent i n cut t ings, they are best

recognized i ndi r e c t l y , e i t h e r by commonly associated hydrothermal a1 te ra t i on ,

1

o r by responses on appropr ia te geophysical we l l logs.

Hydrothermal a l t e r a t i o n , use fu l f o r l o c a t i n g and de f i n ing a geothermal o r

mineral resource, i s f a r more d i f f i c u l t t o recognize and i n t e r p r e t i n cu t t i ngs

than i n core o r outcrop. A l t e r a t i o n tex tu res and paragenetic re la t i onsh ips

can be obscured o r o b l i t e r a t e d as cu t t i ngs are produced.

a l t e r a t i o n (and rock-formi ng) minerals can be disaggregated dur ing d r i l l i n g

and l o s t from cu t t i ngs dur ing sampling o r washing. R e l i c t and contemporary

a l t e r a t i o n can be ind is t ingu ishab le , and a wide va r ie t y of borehole

Less r e s i s t a n t

contaminants can c lose ly resemble na tura l a l t e r a t i o n produts encountered

dur ing d r i l l i n g . These contaminants a lso can produce confusing geochemical

signatures.

2

INTRODUCTION

D r i l l cut t ings, because o f t h e i r low cost and ease o f recovery r e l a t i v e

t o core, are t h e standard borehole rock samples.

source o f t h e d i r e c t downhole geologic in format ion essent ia l f o r a successful

subsurface i nves t i ga t i on . Cuttings, however, a re fragmentary, mixed and

frequent ly contaminated samples of formerly more o r l ess coherent rock, and

They a re commonly t h e only

the re fo re are more d i f f i c u l t than core t o i n t e r p r e t r e l i a b l y .

reconst ruct ion from c u t t i n g s o f t h e rock penetrated by a borehole requires, i n

add i t i on t o thorough knowledge o f l o c a l geology, a f u l l understanding o f t he

means by which these samples a re produced and transported i n d r i l l i n g f l u i d ,

col lected, prepared, s tored and exam1 ned.

i n t h i s paper, then appl ied t o i n t e r p r e t a t i o n o f l i t h o l o g y , contact

Imaginary

These processes a re each discussed

. re la t ionships, f rac tu r i ng and f a u l t i n g , geochemistry and hydrothermal

a l t e r a t i o n .

Logging o f r e l a t i v e l y undisturbed sediments and sedimentary rocks i s

thoroughly covered by previous pub l i ca t i ons (e.g. Swanson, 1981; Low, 1977;

Maher, 1964; H i l l s , 1949).

za t ion o f c u t t i n g s from fractured, a l tered, dominantly igneous and metamorphic

t e r r a i n such as commonly penetrated i n geothermal and mineral exp lo ra t i on

Therefore, t h i s paper w i l l emphasize character i -

,

boreholes. Many o f t h e concepts and techniques discussed a l so should be

useful f o r i n t e r p r e t i n g c u t t i n g s from petroleum we l l s i n s t r u c t u r a l l y complex

sedimentary sequences 1 i ke those o f t h e Rocky Mountai n Overthrust B e l t . PRODUCTION OF CUTTINGS

Thorough knowledge o f how cu t t i ngs are produced and transported t o a

sampling p o i n t w i l l ease t h e i r i n t e r p r e t a t i o n by a l lowing a c l e a r d i s t i n c t i o n

3

between o r i g i n a l rock features and those induced by d r i l l i n g .

aspects o f t h e d r i l l i n g process which d i r e c t l y a f f e c t c u t t i n g s w i l l be

discussed i n t h i s paper. A more comprehensive survey o f d r i l l i n g techniques

Only those

and equipment i s provided by Moore (1974).

Cutt ings are produced beneath a b i t by a combination of impact, which

crushes and f rac tu res a rock, and drag, a gouging and f r a c t u r i n g act ion. The

r e l a t i v e importance o f drag and impact i n generating c u t t i n g s depends l a r g e l y

on b i t type.

Drag and claw b i t s , designed f o r s o f t e r rocks, penetrate by gouging and

f rac tu r ing wi thout impact (Fig. 1). Rotat ion combined w i th drawdown (weight-

on-b i t ) produces crushed zones i n f r o n t o f t he t e e t h o f these b i t s

(Nishimatsu, 1972). Chips are formed between f rac tu res propagated from these

crushed zones t o t h e rock surface o r t o pre-ex is t ing f rac tu res (Appl and

Row1 ey , 1963 ; Varnado, 1980).

FIG. 1 -- Conceptual i l l u s t r a t i o n o f cu t t i ngs product ion by a drag b i t .

Tricone b i t s produce cu t t i ngs by impact with subordinate drag. Those

designed f o r moderately hard rocks have o f f s e t cones, t h e axes o f which do not

cross the b i t s ' center o f r o t a t i o n (Moore, 1974). Because o f t h i s o f f s e t , t h e

m i l l e d t e e t h o r tungsten carbide i n s e r t s on t h e cones cannot t rack properly,

and a s l i g h t gouging a c t i o n resu l t s . Hard rock t r i c o n e b i t s have l i t t l e o r no

cone o f f s e t and correspond1 ngly reduced gouging action. A conceptual i r e d

penetrat ion sequence fo r a t r i c o n e b i t , based i n p a r t on rock penetrat ion

experiments by Sikarsk ie and Cheatham (1973) and Somerton (1959), i s

i l l u s t r a t e d i n Figure 2. The b i t t o o t h (or button) s t r i k e s the rock

e s s e n t i a l l y v e r t i c a l ly , formi ng a compacted crushed zone d i r e c t l y beneath the

4

A B Figure 1, Conceptual i l l u s t r a t i o n o f cu t t i ngs product ion by a drag b i t . A,

r o t a t i o n combined w i th drawdown causes the b i t t oo th t o gouge t h e rock, forming a crushed zone from which f rac tu res are propagated forward t o the rock surface o r t o p re-ex is t ing f ractures, B, f u r t h e r movement o f the too th dislodges the crushed zone and a d r i l l ch ip wh i l e forming a new crushed zone and f ractures. The d is lodged crushed zone MY s u r v i v i e t ranspor t up the borehole t o appear i n c u t t i n g s as pseudo-gouge. Modi f ied from Nishimatsu (1972), Appl and Rowley (1963) and Varnado (1980).

A B Figure 2. Conceptual i l l u s t r a t i o n of cu t t i ngs product ion by a t r i c o n e b i t .

A, b i t t oo th ( o r i n s e r t ) (1) s t r i k e s the rock under heavy drawdown, forming a crushed zone f r o m which f rac tu res are propagated t o the surface or t o p re -ex i s t i ng f ractures. b i t cone r o l l s forward, t oo th (2 ) s t r i k e s the rock. This second impact, together w i t h s l i g h t l a t e r a l t o o t h movement and d r i l l i ng f l u id c i r c u l a t i o n , dislodges the chips and crushed m a t e r i a l (pseudo-gouge) formed by the previous impact. experimental work by S ikarsk ie and Cheatham (1973).

8 , as the

Based on

t o o t h po int . Fractures are propagated downward and outward from t h i s crushed

zone t o i n t e r s e c t p re-ex is t ing na tura l f rac tu res o r those induced by previous

impacts. Chips formed between these f rac tu res are dis lodged by subsequent

impacts together w i t h s l i g h t l a t e r a l t oo th movement and t h e j e t t i n g ac t i on of

d r i l l i n g f l u i d .

f r i a b l e pseudo-gouge ( t o be subsequently discussed i n d e t a i l ) , which i n

cu t t i ngs commonly resembles n a t u r a l l y occurr ing f a u l t gouge.

The crushed zone may surv ive t ranspor t up t h e borehole as

FIG. 2 -- Conceptual i l l u s t r a t i o n o f cu t t i ngs product ion by a t r i c o n e b i t .

Diamond d r i l l b i t s , o f u n i t const ruct ion and studded w i t h small diamonds,

penetrate exc lus ive ly by r o t a r y g r ind ing and produce extremely f i n e

cut t ings. Cable too ls , by contrast , d r i l l by repeated impact o f a heavy

chisel-shaped b i t w i t h a s i n g l e tooth. Although such a b i t could produce very . ,

la rge chips, cable t o o l cu t t i ngs are general ly ra the r small, s ince @ey are

no t cont inuously removed from t h e hole and there fore are extens ive ly recut.

Cut t ings product ion i s a f fec ted not on ly by b i t type, bu t a lso by such

o ther var iab les of t h e d r i l l i n g process as r o t a r y d r i l l i n g speed, drawdown o r

weight-on-bit, and densi ty and c i r c u l a t i o n r a t e o f d r i l l i n g f l u id .

wa te r -d r i l l ed cu t t i ngs t r a v e l up a borehole annulus a t a moderate r a t e

(average about 100 f t /m in -- Low,'1977) w i t h l i t t l e o r no m i l l i n g and

mixing.

mixture, however, may be s i g n i f i c a n t l y mixed and abraded.

e f f e c t i v e l y , a i r o r air-based d r i l l i n g f l u i d s must be c i r c u l a t e d a t 3000 t o

7000 f t / m i n (Hobbs, 1979; Wolke, 1980), ra tes which temporar i ly may be boosted

even h igher by penetrat ion i n t o a high-pressure gas o r steam zone.

Mud- or

Those d r i l l e d and t ranspor ted i n a i r o r an air-water-foaming agent

To remove cu t t i ngs

Such h igh

f l u i d v e l o c i t i e s cause considerable turbulence and resu l tan t abrasion o f

cu t t i ngs by impact w i t h the hole w a l l , the d r i l l s t r i n g and other chips.

t Cutt ings fran deeper a i r - d r i l l e d holes, which requ i re t h e highest f l u i d

c i r c u l a t i o n rates, canmonly a re rounded and p i t t e d through s e l e c t i v e erosion

o f s o f t e r minerals such as micas and clays. I n shallower a i r - d r i l l e d holes o r

L i n those where a foaming agent i s added t o he lp t ranspor t cut t ings, sample 1

abrasion and se lec t i ve mineral a t t r i t i o n may be minor.

Chip s ize, shape and t e x t u r e a re s t rong ly in f luenced by t h e phys ica l I' L charac te r i s t i cs o f t h e rocks penetrated.

which are so f t , heterogeneous, coarse grained and poor ly consolidated, and

h igh l y f ractured. By contrast , hard and unf ractured rocks, o r those which are

Large cu t t i ngs are favored by rocks

i c f i n e grained and r e a d i l y disaggregated, tend t o y i e l d small1 cut t ings.

monomineralic i s o t r o p i c rocks, b i t type and d r i l l i n g method w i l l be t h e

dominant fac to rs i n ch ip formation, but rock cha rac te r i s t i cs remain important

cont ro ls .

I n

L r , Cutt ings f r u n a b r i t t l e i s o t r o p i c rock such as quar tz i te , f o r u. example, tend t o be t h i n conchoidal fragments (Somerton, 1959) whereas i n a

s o f t rock such as c laystone shavings may be produced. 1 I SAMPLE COLLECTION

f" E f f e c t i v e logging and subsurface i n t e r p r e t a t i o n requ i re care fu l

c o l l e c t i o n and handl ing of cu t t i ngs samples. Poor samples can cause a v a r i e t y u o f i n t e r p r e t i v e er rors .

l o s t from mud-dr i l led cu t t i ngs if openings i n shaker screens (used t o separate

~ L * cu t t i ngs from mud) a re t o o l a rge These can be minimized Or e l iminated

Fine grained ore minerals, as one example, can be

1

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ii i f sampli ng procedures are desi g

w i t h d r i l l i n g methods and w i t h the geology o f t he s i t e t o be d r i l l e d .

Cutt ings samples should be as representat ive as poss ib le o f an e n t i r e

d r i l l e d i n t e r v a l . Since t h e geology of geothermal systems and m i nera l i zed

6

t e r r a i n can be very complex and can change abrupt ly, c u t t i n g s should be

co l l ec ted a t r e l a t i v e l y shor t i n t e r v a l s t o minimize o r avoid homogenization o f

m u l t i p l e l i t h o l o g i e s o r a l t e r a t i o n types i n a s i n g l e sample. Ten f e e t

probably should be t h e maximum i n t e r v a l f o r r o u t i n e sampling.

continuously, when p r a c t i c a l , are more representat ive than "grab" o r spot

samples, which can miss e n t i r e l y a geologic u n i t smaller than t h e sample

i n t e r v a l .

Samples taken

Mud- o r water-dr i l l e d cut t ings, especia l ly those from deep, large-bore

wells, canmonly a re separated from d r i l l i n g f l u i d on sloped, v i b r a t i n g and/or

r o t a t i n g shaker screens ("shale shakers"), t h e many designs o f which a re

beyond t h e range of t h i s d i scussi on . C u t t i ngs, which through screen v i b r a t i o n

may be p a r t i a l l y c l a s s i f i e d according t o ch ip s i t e and density, a re co l l ec ted

e i t h e r d i r e c t l y from t h e shaker o r from i t s discharge. Screen openings should

be as small as p r a c t i c a l t o r e t a i n f i n e r grained mater ia l , t h e l oss of which

can b ias sample i n t e r p r e t a t i o n and analysis. Openings o f 0.77 mm t o 1.54 mm

are t y p i c a l , b u t f i n e r screens commonly are used f o r f i n e grained sediments

such as penetrated i n Imperial Valley, Ca l i f o rn ia , geothermal we l l s (Westin,

1980). Mater ia l which passes through shaker screens can be removed from t he

d r i l l i n g f l u i d by desanders, d e s i l t e r s and decanting centr i fuges, and

considered along w i t h corresponding coarser c u t t i n g s i f necessary.

Shall ow mud- o r w a t e r - d r i l l ed boreholes are general l y completed by smal 1

r i g s wi thout shaker screens.

d r i l l i n g f l u i d e i t h e r from a mud d i t c h between t h e open hole and a mud p i t ,

or, i f surface casing i s i n s t a l l e d , i n a bucket a t the end o f a short

discharge pipe. The bucket can be emptied and cleaned a t t he end of each

i n t e r v a l t o ensure representat ive sampling.

Cutt ings from these we l l s are c o l l e c t e d w i t h

I

A l t e rna t i ve l y , samples from the

7

b

pipe can be caught i n a s t ra ine r , although t h i s method w i l l r e s u l t i n loss o f

f i n e r grained mater ia l w i th d r i l l i n g f l u i d through openings i n t h e s t r a i n e r

screen. Yet another method, successful l y used on several geothermal boreholes

i n Nevada, invo lves channeling f l u i d and cu t t i ngs over a stacked se t o f sample

s p l i t t e r s (H. D. P i lk ington, AMAX Exploration, Inc., pers. comm., 1980).

A i r - d r i l l e d boreholes do not requ i re shaker screens. On shallower holes

o f t h i s type, samples a re co l l ec ted from t h e mound o f cu t t i ngs which forms

around t h e o r i f i c e . On deeper holes, such as t y p i c a l l y d r i l l e d i n The Geysers

geothermal area, Ca l i f o rn ia , re tu rns ( a i r and cu t t i ngs ) are conducted away

from t h e we l l head through a b loo ie l i ne , where water may be added. The

moistened mix tu re then enters a muf f le r , o r cyclone separator. The gaseous

f r a c t i o n o f t h e returns, along w i t h some rock powder, i s exhausted from t h e

top o f t he muf f le r ; remaining coarser cu t t i ngs and water f low out t h e

bottom.

c u l v e r t l i n e d d i tch, from which samples are co l l ec ted a t regular i n te rva l s .

The d i t c h i s cleaned a f t e r each sample t o minimize contamination (M.

Twi tchel l , Aminoil, Inc., pers. comm., 1980).

A t The Geysers, t h i s coarser f r a c t i o n genera l ly i s trapped i n a h a l f -

Sample Lag

As cu t t i ngs are co l lected, a sample general ly i s l a b e l l e d w i t h cur ren t

d r i l l i n g depth.

t r a n s i t t o the c o l l e c t i o n point , t h i s recorded depth i s greater than the

actual depth o f t h e sample.

However, because d r i l l i n g continues wh i le cu t t i ngs are i n

The d i f fe rence between the two depths i s t h e

sample lag, discussed i n d e t a i l i n Low (1977) and Hobbs (1979), who a lso

provide methods f o r i t s ca lcu la t ion .

boreholes, i n which cu t t i ngs t ranspor ted i n a h igh-ve loc i ty a i r stream reach

the surface almost immediately a f t e r they are produced. Lag a lso i s minimal

Lag i s n e g l i g i b l e f o r a i r - d r i l l e d

li i n mud- o r wa te r -d r i l l ed boreholes penetrat ing t h e hard igneous and

metamorphic rocks t y p i c a l of geothermal systems and minera l ized t e r r a i n , s ince c d r i l l i n g ra tes i n these rocks r a r e l y exceed 3-10 f t (1-3 m) per hour.

I' Fie1 d Preparation, Packagi ng and Storage . Borehole cu t t i ngs genera l ly w i l l be s tud ied no t on ly on s i t e wh i le

d r i l l i n g i s i n progress, b u t l a t e r , when de ta i l ed examination and analys is may u be necessary f o r f u r t h e r charac ter iza t ion o r l o c a t i o n o f a resource. Thus,

cu t t i ngs samples should be f ie ld-prepared ._ and packaged so t h a t physical and

chemical changes dur ing storage are minimized o r el iminated.

F i e l d preparat ion general l y i nvol ves only washing the sampl es t o remove

b forei-gn mater ia l , although t h i s process commonly i s deferred u n t i l cu t t i ngs

are a c t u a l l y needed f o r study, Among t h e advantages o f f i e l d washing are

reduct ion o f subsequent labora tory preparat ion t ime and acce le ra t ion o f sample

dry ing through removal o f d r i l l i n g f l u i d and 'o ther contaminants.

f l u i d may conta in a c t i v e chemicals such as caus t ic soda which can reac t w i t h

cu t t i ngs i f they are s tored wet before washing.

washing inc lude lack o f t ime and proper equipment and u n a v a i l a b i l i t y o f de-

ion ized water i f required. Hasty washing, genera l ly through i n s u f f i c i e n t l y

f i n e screens, may r e s u l t i n l o s s of t he f i n e r grained component o f cu t t i ngs

D r i l l i n g

Disadvantages o f f i e l d c

c w i t h d r i l l i n g f l u i d . Washing w i t h de-ionized water may be requi red i f

c chemical analys i s o f c u t t i ngs i s a n t i c i pated.

Sample containers should be la rge enough t o conta in 500 g o r more of

cut t ings, s u f f i c i e n t l y durable t o wi thstand t ranspor ta t i on and storage, and

I completely i d e n t i f i e d (borehole, depth i n t e r v a l etc.) w i t h permanent

waterproof i n k on securely attached labels. Type o f package requi red w i l l

depend on the nature o f s tud ies planned f o r t h e cut t ings. Those t o be

9

F

analyzed f o r v o l a t i l e elements such as arsenic and mercury, f o r example,

should be packaged i n a i r - t i g h t g lass o r heavy p l a s t i c . Although containers

made of these ma te r ia l s a r e bu lky and may cause handling problems (such as

glass breakage), they e l im ina te element l oss and cross-contamination o f

samples.

which a re r e l a t i v e l y inexpensive and easy t o handle.

these bags a l s o f a c i l i t a t e sample drying.

Generally, however, c u t t i n g s w i l l be placed i n c l o t h o r paper bags,

Since they are porous,

Most samples should be s tored dry, s ince moisture may a l t e r t h e

appearance and composition o f cut t ings. D r i l l s tee l , f o r example, r e a d i l y

rus ts when moist and may s t a i n and obscure c u t t i n g s o r may resemble Indigenous

1 imonite (Hulen, 1978). Wet storage a lso promotes hydrat ion and resu l tan t

sof ten ing o f formation clays, which then are l o s t from samples when

subsequently washed.

e a s i l y redissolved and removed by washing.

Dr ied d r i l l i n g mud may weakly b ind cut t ings, but i t i s

SAMPLE PREPARATION

Careful preparat ion o f d r i l l cu t t i ngs f o r examination o r analys is he lps

The ensure they w i l l y i e l d maximum informat ion about a subsurface resource.

wide range o f a n a l y t i c a l techniques avai lable, and t h e special sample

preparat ion each requires, exceed t h e breadth o f t h i s discussion. This

sect ion w i l l be conf ined t o sample cleaning, a pre l iminary process requi red

f o r near ly a l l methods o f cu t t i ngs study, and t o preparat ion o f samples f o r

b inocular microscopic and petrographic examination.

Mud-dr i l led c u t t i n g s should be washed t o remove d r i l l i n g mud and other

fo re ign debris, such as l o s t c i r c u l a t i o n mater ia l , i n such a way t h a t l o s s o f

natura l const i tuents o f t h e d r i l l e d rock i s minimized. An e f f e c t i v e washing L

method i nvol ves pouring water

and careful l y decanti ng . The

mater ia l has f l o a t e d frm t h e

d r i l l i n g mud. This method i s

over a sample i n a shallow pan, gen t l y s t i r r i n g ,

process i s repeated u n t i l most l o s t c i r c u l a t i o n

cu t t i ngs and t h e decanted water i s c l e a r o f

preferable t o washing c u t t i n g s i n a screen,

through which much of t h e f i n e f r a c t i o n o f a sample can be l o s t along w i t h

d r i l l i n g f l u i d .

c lay and s i l t from such mater ia ls as a l luv ium o r s o f t a r g i l l i z e d rock i f

Even t h e decanting method, however, can remove some na tu ra l

samples a re over-washed.

De-ionized water may be useful f o r washing c u t t i n g s i n preparat ion f o r

chemical studies. I n many cases, however, t h i s special technique may be

superfluous. Mud-dr i l led cut t ings, f o r example, a1 ready may be contaminated

with a wide v a r i e t y o f i ons der ived from d r i l l i n g f l u i d s and add i t i ves and

adsorbed on natura l rock c lays and micas.

A f t e r washed c u t t i n g s have been dr ied, e i t h e r n a t u r a l l y o r i n an oven a t

low heat, remaining l a r g e r o r heavier f o re ign debr is such as cottonseed h u l l s

and various metals can be removed by hand-picking.

magnet ical ly removed, b u t t h i s method w i l l a l so remove some natura l magnetite

as wel l as any mineral wf th which i t i s intergrown.

D r i l l s tee l can be

A i r - d r i l l e d c u t t i n g s are devoid of t h e introduced clays, l o s t c i r c u l a t i o n

mater ia ls and other add i t i ves used f o r mud d r i l l i n g and therefore requ i re

washing only t o rmove f i n e rock dust, which may obscure l a r g e r cu t t i ngs

dur ing b inocular microscopic examination. Metals, however, are very common

contaminants i n these c u t t i n g s (Wolke, 1980). Steel and i ron, t h e most J

prevalent, can be removed magnet ical ly ( i n e v i t a b l y along w i t h some natura l

magnetite) i f not thoroughly rusted.

s p i t e o f meticulous e f f o r t , t h e cu t t i ngs may remain s l i g h t l y contaminated.

Other metals can be hand-picked, but i n

11

L

Such res idual contamination should be taken i n t o account i f t h e cu t t i ngs are

f u r t h e r analyzed by chemical o r o ther methods.

Binocular microscopic study requi res l i t t l e preparat ion o f d r i l l c u t t i n g s

a f t e r washing, The samples can be examined loose i n small dishes, but pouring

cu t t i ngs ou t o f and back i n t o bags i s t ime consuming and messy. This problem

can be avoided by examining samples i n shallow paper cups, which then can be

capped f o r storage.

Cutt ings can be examined most. e f f i c i e n t l y when glued on chipboards (Fig.

3) o r canvas s t r i p s .

i n t e r v a l s c l e a r l y marked, a1 low rap id examination and comparison o f l a rge

numbers of samples whi le e l im ina t i ng unneccessary sample handling. Chips are

firmly secured f o r easy scratch tes t ing .

tex tu re , which cou ld be overlooked wi thout m u l t i p l e sample comparison, a re

detected e a s i l y on chipboards.

r o l l e d up f o r easy storage. The small amount o f each sample ( t y p i c a l l y about

These devices, a t an appropr ia te scale and w i th depth

Subtle va r ia t i ons i n c o l o r and

Canvas s t r i p s , as an added advantage, can be

1.5 g) used t o prepare these boards o r s t r i ps , however, may not always be

adequately representat ive.

FIG. 3 -- D r i l l cu t t i ngs mounted on chipboards.

For petrographic study, cu t t i ngs are embedded i n epoxy which then i s

s l i c e d t o make a grain-mount th in-sect ion. These sect ions must be prepared

M i th care, s ince d i f f e r e n t i a l s e t t l i n g ra tes o f chips w i th d i f f e r e n t s izes

and/or s p e c i f i c g r a v i t i e s I n t h e viscous epoxy can r e s u l t i n a non-

representat ive s l i ce . Even w i t h care fu l preparat ion, th in-sect ions can be

l ess representat ive than chipboard i n te rva l s , s ince they incorporate a f a r

smal l e r po r t i on of an e n t i r e cu t t i ngs sample.

12

1 SAMPLE B I A S CAUSED BY DRILLING AND SAMPLE PREPARATION

The d i f f e r e n t mineral cons t i tuents o f a rock may o f f e r va r iab le ~

res is tance t o crushing dur ing d r i l l i n g . Softer, more b r i t t l e o r otherwise

weaker minerals; f o r example, a re l i k e l y t o be s e l e c t i v e l y pu lver ized or:

disaggregated (Somerton, 1959). Thus reduced i n g ra in size, they may be l o s t

from a cu t t i ngs sample, e i t h e r w i th d r i l l i n g f l u i d a t t he c o l l e c t i o n po in t o r

l a t e r when t h e sample i s washed. Harder, more r e s i s t a n t cons t i tuents thereby

may become enr iched i n cu t t i ngs r e l a t i v e t o t h e rock from which they were

d r i l l e d .

a i r - d r i l l e d we l ls i n The Geysers geothermal area are r i c h e r i n quar tz than the

corresponding rock i n the borehole (M. Twi tchel l , Aminoil, Inc., pers. comm.,

I: u

For example, graywacke cut t ings. from t h e deeper por t ions o f many

1980).

Cut t ings samples enr iched i n c e r t a i n minerals and depleted i n others

compared t o t h e i r source rocks can cause a v a r i e t y o f i n t e r p r e t i v e er rors .

Soft, a r g i l l i z e d and s e r i c i t i z e d i n t e r v a l s i n a borehole, f o r example, can be

overlooked o r underemphasized i f c lays and s e r i c i t e are e l iminated from

cut t ings.

d imin ish po ten t i a l geochemical anomalies, s ince many d iagnost ic t race elements

Removal o f c lays and l imon i tes from a sample may s l g n i f i c a n t l y

are known t o be concentrated i n these minerals (Levinson, 1974, 1980).

Samples enr iched i n quartz, one o f t h e best na tura l heat conductors

(Kappelmeyer and Haenel , 1974) w i 11 y i e l d erroneously h igh thermal

F

conduc t i v i t y values and correspondingly low thermal gradients.

Select ive mineral dep le t ion i n cu t t ings a l s o a f f e c t s poor ly consol idated Li

c l a s t i c sedimentary rocks. These rocks tend t o break around, ra the r than

through grains, l i b e r a t i n g f i n e r grained mat r ix const i tuents and cement t o be

removed from samples along w i th d r i l l i n g f l u i d . I n t h i s way, f i n e grained,

13

w uranium-bearing organic carbon i s commonly washed out o f cu t t i ngs fran r o l l -

f r o n t sandstone uranium deposi ts such as those o f t h e Crooks Gap and Gas H i l l s

d i s t r i c t s o f Wyoming. S i m i l a r l y , in te rgranu lar l i m o n i t e o r hematite, which

are commonly coprec ip i ta ted w i t h (or adsorb) valuable ore metals, can be

f lushed from cu t t i ngs and can s t a i n d r i l l i n g f l u i d s b r i g h t orange- o r reddish

brown.

u \ CONTAMINATION OF CUTTINGS

D r i l l cu t t ings , p a r t i c u l a r l y those from boreholes i n h i g h l y f rac tu red

t e r r a i n , f requent ly are contaminated w i t h a v a r i e t y o f indigenous and

introduced debris. Recognition and descr ip t ion of these contaminants i s

c r u c i a l f o r an accurate subsurface inves t iga t ion . Fragments from i n t e r v a l s .

h igher i n t h e borehole, f o r example, i f incorporated i n t o subsequent samples,

w i 11 m i srepresent t h e actual rock penetrated.

cu t t i ngs w i l l be i n e r r o r r e l a t i v e t o t h e parent rock. This sec t ion b r i e f l y

discusses common contaminants, methods by which they can be i d e n t i f i e d , and

Analyses o f contaminated a .

t h e i r i n f l uence on phys ica l and chemical measurements o f cu t t i ngs samples, u Indigenous contaminants i n cu t t i ngs samples are those der ived from

prev ious ly d r i l l e d i n t e r v a l s i n t h e borehole. e Generally, they are re fe r red t o

as caved o r sloughed fragments, although ra the r than always caving i n t o t h e

borehole (as i s common dur ing " t r i p s " f o r b i t changes -- Low, 1977) they are ," \ i probably o f t e n eroded o r plucked from i t s wa l ls by upward-flowing d r i l l i n g

I f l u i d . They a l so may be recyc led cu t t i ngs -- e i t h e r those no t removed from

I d r i l l i n g f l u i d a t t h e c o l l e c t i o n p o i n t and then rec i rcu la ted , o r those c a r r i e d

i n t o and subsequently f lushed from c a v i t i e s i n t h e borehole w a l l .

lu

Caved o r sloughed fragments i n cu t t ings samples t r a d i t i o n a l l y are

14

i d e n t i f i e d by t h e i r l a rge s ize r e l a t i v e t o coex is t ing actual d r i l l ch ips (e.g.

Low, 1977). Thus, samples commonly a re passed through coarse screens and t h e

1 arger fragments discarded p r i o r t o examination and in te rp re ta t i on . We

be l ieve t h i s technique genera l ly should no t be appl ied t o geothermal we l ls o r

o ther boreholes i n f rac tu red te r ra in . Cutt ings from f r a c t u r e zones i n these

wel ls can be anomalously l a rge as wel l as representat ive o f t he i n t e r v a l

d r i l l e d . I n samples from we l l Utah State 14-2 a t Roosevelt Hot Springs KGRA,

f o r example, w i t h i n a f ractured, hot water en t r y zone between 487.7 and 548.6

m (1630 and 1800 ft) (Glenn and Hulen, 1979), average ch ip s i r e i s th ree t o

f i v e times as l a r g e as i n surrounding unf ractured rock (Fig. 4). These l a r g e r

cu t t ings probably r e f l e c t t h e ease w i t h which f rac tu red rock i s dislodged by

t h e d r i l l b i t .

FIG. 4 -- Anomalously l a rge cu t t i ngs from a f rac tu red probable hot water en t ry zone i n Thermal Power Company geothermal we l l Utah State 14-2, Roosevelt Hot Springs KGRA, Utah.

>

Caved fragments may be recognized more re1 i ably, though no t u n f a i l i ngly , by t h e i r i ncongru i t y i n cu t t i ngs samples. As a c l e a r hypothet ica l example, a

d iscrete, rounded and f ros ted quar tz pebble i n a sample cons is t ing most ly of

angular gabbro chips has c e r t a i n l y caved i n t o o r otherwise contaminated t h e

sample, probably f r a n an over ly ing a l l u v i a l horizon. Angular g r a n i t i c ch ips

i n the gabbro sample, however, may be contaminants o r may represent i n s i t u

dikes o r xenol i ths. Thorough knowledge o f surface geology i s essent ia l i n

determining which o f these p o s s i b i l i t i e s i s most l i ke ly . ’

Introduced o r fo re ign contaminants i n d r i l l cu t t i ngs inc lude:

d r i l l i n g muds and mud addi t ives; lost -c i rcu la tSon mater ia ls ; metal from

cement;

d r i l l s t r i n g s , b i t s , casing and d r i l l a b l e components o f cementing assemblies;

thread compounds and other greases; pa in t and rubber; and mater ia l sloughed

15

' \

!

Figure 4.

I;

i

Figure 3. Drill cut t ings mounted on ch

I

pboards.

Anomalously l a r g e cutt ings f r o m a fractured probable hot water ent ry zone (above 1800 f t ) i n Thermal Power Company geothermal wel l Utah S t a t e 14-2, Roosevelt Hot Springs KGRA, Utah.

L 1

from the sides o f d i tches and s e t t l i n g o r sampling p i t s . Most o f these

substances are r e a d i l y recognized as contaminants, but several may s t rong ly

resemble na tura l ly -occurr ing mater ia ls l i k e l y t o be encountered i n t h e

borehole. A l i s t of t h e more common contaminants (many of which a re f u r t h e r

discussed i n subsequent sect ions on l i t h o l o g y and hydrothermal a l t e r a t i o n ) i s

provided i n Table 1.

Table 1. Common introduced contaml nants i n d r i l l cu t t ings .

Borehole cement i s an espec ia l l y troublesome contaminant i n cu t t ings ,

s ince i t can resemble a v a r i e t y of rock types and a l t e r a t i o n products.

t y p i c a l l y a sof t , l i g h t gray t o buff, f i n e l y speckled substance (Fig. 5) which

may appear spongy o r ves icu lar , and which ef fervesces v igorous ly i n d i l u t e

hydrochlor ic acid. Depending on t h e add i t i ves i t contains (Table l ) , cement

e a s i l y can be confused i n cu t t i ngs w i t h chalk, marl, calcareous claystone o r

s i l t s t o n e , calcareous t u f f , t r a v e r t i n e , o r any bleached, c a l c i t i c ,

hydrothermal l y a l t e r e d rock, p a r t i c u l arly an aphan i t i c volcanic.

incorporates cu t t i ngs o r rock debr is from the borehole w a l l i t can a l s o

It i s

If cement

resemble brecc ia w i t h a mat r ix o f rock f l o u r and ca l c i t e . Cement i s most

l i k e l y t o contaminate cu t t i ngs a t and immediately below t h e bottoms o f casing

s t r ings , bu t may slough i n t o any sample co l l ec ted from lower i n t h e borehole.

FIG. 5 -- Per l i te-bear ing borehole cement cu t t i ngs from Getty O i l Company geothermal we l l Utah State 52-21, Roosevelt Hot Springs KGRA, Utah.

Physical and chemical analyses o f cu t t ings , f requent ly h e l p f u l i n

1 ocat ing and c h a r a c t e r i r i ng a subsurface resource, can be ser ious ly

compromised by sample coniarni nat . For instance, thermal conduc t i v i t y

measurements, used t o det a m i n e h i a t f low i n geothermal exp lo ra t ion (Sass e t

-1

I

Tab1 e 1, Common i n t roduced cont ami nant s i n d r i 1 1 cu t t i ngs .

MINERALS

D r i l l i n g mud c lays :

-Bent on i t e (mon tmor i -

- A t tapul g i t e o r

D r i l l i n g mud weight ing

l l o n i t e f i l l i t e , mixed- l a y e r c l a y and k a o l i n i t e )

s e p i o l i t e

Agents

- B a r i t e -Hematite -Ca lc i t e -Gz l l ena - I 1 men i t e

Cement

.Cement addi t,i ves

- S i l i c a f l o u r -Per1 i t e -Pozto l a n -Diatomaceous e a r t h -Bent on i t e -Hematite -Bar it e -Mica -Quartz sand

..

-G Y DS um

~~ ~~

ORGANIC AND SYNTHETIC MATERIALS

D r i l l i n g mud th inners and dispersants

- L i g n i t e and l i gnosu l fona te

D r i l l i nq mud 1 ubr icants

- S i l i c a o r glass spheres

L o s t - c i r c u l a t i o n ma te r ia l s

-Nut Shel ls -Wood f i b e r -Cane f i b e r -Seed h u l l s .- Pa pe r - L i g n i t e (coarse) -Cell ophane -Processed formica and o the r p l a s t i c s

-M i scel 1 aneous 1 oca1 l y a v a i l a b l e ma te r ia l such as a l f a l f a cubes

\

Cement Add i t i ves

- g i l s o n i t e and coal

Rubber and P l a s t i c

-Jackets on cement plugs -.

METALS {SEE TABLE 2)

Metal shavings from t h e d r i l l i n c l svstem

-Steel, brass, etc.

D r i l l a b l e metal from downhol e cementi na assembl i es

-Lead, i r o n and a 1 umi num

Col o r i ng agents

Metals i n thread . compounds and o the r greases

i I, I I

i I,

e 5. P e r l i t e - b geothermal we1 1 U Utah. A, cement urn gray wi th white speckles) wi th

t h i n sect ion ( transmitted l i g h t ) unted i n epoxy; m a t r i x i s cement;

white, angular fragments are quartz; l a r g e r subrounded 1 ight gray inclusions are p a r t i a l l y d e v i t r i f d p e r l i t e ; dark inclusions are an un ident i f ied opaque miperal.

-21, Roosevelt Hot Springs KGRA, u u

other conductive metal.

samples w i l l a l so be h igh r e l a t i v e t o t h e corresponding u n d r i l l e d rock.

Perhaps most f requent ly m i s l eadi ng , though , are geochemical analyses of

Density measurements on these metal--contami nated

cont ami nated cu t t i ngs . Inf luence o f Contami nat fon on t h e Geochemistry o f D r i 11 Cutt ings

Multi-element s o l i d s geochemistry has long been successfu l ly app l ied by

t h e mining i ndus t r y t o t h e search f o r and charac ter iza t ion o f mineral

deposits, These deposi ts commonly are surrounded by geochemical ha1 oes, t h e

nature and i n t e n s i t y o f which, as detected i n s o i l , rock, core and cut t ings,

can be used t o p r e d i c t d i r e c t i o n s t o and distances from ore-grade

m ine ra l i za t i on (Levinson; 1974,

geochmi cal l y zoned. Ewers and

1980). Geothermal systems a l so are

Keays (1977), Bamford e t a l . (1980) and

Chri stensen ( 1980a, 1980b) have demonstrated t h a t so l i ds geochemistry can be

usefu l i n est imat ing the s i ze and conf igura t ion o f a geothermal systkm and i n

l o c a t i n g o r p red ic t i ng appropch t o thermal f l u i d e n t r i e s w i t h i n these

systems.

f low paths are commonly s i g n a l l e d by anomalous concentrat ions o f mercury,

arsenic and l i t h i u m i n d r i l l cu t t i ngs and i n s p e c i f i c g r a v i t y concentrates

prepared from .these cu t t i ngs (Bamford e t a1 .

A t Roosevelt Hot Springs KGRA in \ l l tah, f o r example, thermal f l u i d

1980).

The geochemistry o f a cu t t i ngs sample, however, may not accurate ly

r e f l e c t t h e geochemistry of t h e rock from which i t i s obtained. Clays, i r o n

oxidesp s u l f i d e s and o ther minerals i n which d iagnost ic t r a c e elements

commonly are concentrated (Levi nson , 1974, 1980), f o r exampl e, may be enr iched

o r depleted i n cu t t i ngs r e l a t i v e t o t h e i r source rocks by d r i l l i n g , sampling

and sample preparation. A p o t e n t i a l l y usefu l geochemical i n d i c a t o r may be

d i l u t e d beneath i t s de tec t ion l i m i t by contamination o f a cu t t i ngs sample.

17

Most important ly, though, m e t a l l i c contaminants i n c u t t i n g s can produce

confusing o r f a l s e geochemical signatures. The more common o f these

contaminants, t h e i r sources and chemistry are presented i n Table 2.

Table 2. Chemistry o f common m e t a l l i c and metal-bearing contaminants i n d r i l l c u t t i n g s .

~~ ~~

~

F igure 6 exempl i f ies spurious cu t t i ngs geochemistry due t o m e t a l l i c

contamination. Lead values i n c u t t i n g s are p l o t t e d on t h e f i g u r e opposite

cementing l oca t i ons i n t h e upper p a r t o f a we l l i n t h e Roosevelt Hot springs

KGRA, Utah. Strong lead anomalies i n the wel l c l e a r l y co inc ide with cementing

s i t e s and are, i n fact , due t o incorporat ion i n cu t t i ngs o f d r i l l a b l e lead

components o f downhole cementing assemblies (Bamford e t a1 . FIG. 6 -- Lead geochemistry o f cu t t i ngs compared w i t h cementing depths i n t h e upper p o r t i o n o f Thermal Power Company geothermal wel l Utah State 14-2,

1980).

Roosevelt Hot Springs KGRA, Utah. ..

EXAMINATION AND DESCRIPTION OF CUTTINGS

The methods used t o examine c u t t i n g s and t h e ' d e t a i l w i t h which they a re

logged w i l l depend l a r g e l y on t h e unique requirements o f each d r i l l i n g

project .

o f fundamental features, some o r a l l o f which a re c e r t a i n t o he lp character ize

o r l o c a t e t h e resource under i nves t i ga t i on .

no t be l i m i t e d t o :

o r i g i n a l mineralogy, a l t e r a t i o n mineralogy and i n t e n s i t y , presence o f gouge o r

Even cursory c u t t i n g s logs, however, should record a minimum number

These features include, but may

color(s) , rock type(s), g r a i n s ize(s), rock fab r i c (s ) ,

o ther evidence o f f au l t i ng , presence o f d r i l l -produced pseudo-gouge, s i ze and

shape o f d r i l l chips, and types and amounts o f contaminants.

Binocular microscopic examination, a t 5X t o 50X, i s adequate f o r r o u t i n e

18

Table 2. Chemistry of canmn metallic and metal-bearing contaminants in

cuttings . I

Contaminant I Source I Elements Present I

Drill pipe, dr i l l collars, bits, subs, reamers, stabilizers, casi ng , mi scel laneous surface components

Steel (and rust) I; (1) Fe, Mo, Cr, N i , V , S

Tungsten carbide

u Thread compounds and other greases ' Monel metal

u Cast a l u m i n u m , lead and , i ron; iron filings

..

u Paint

c .

Bits, stabi 1 izers, W reamers, hol e openers

Tool j o in t s , etc. (2 ) Pb, Zn, Mo, Cu, Li, A1

Non-magnetic dril l (3) C u , N i col 1 a rs

Drillable metal ( 4 ) Al, Pb; Fe (A1 and components of downhol e cementing assembl i es

Fe may be alloyed with '

or contaminated by minor amounts of other metals)

Painted components of (3 ) (Exampl es) the dril l i ng system Blue: Cu, Co, Sn

Red: .Cd, Fe White: Pb, Zn, T i

.

CEMENT

3

200

600

data from Barn ford et all, 7980 Figure 6. Lead geochemistry o f cuttings compared with cementing

the upper portion o f Thermal Power Company geothermal State 52-21, Roosevelt Hot Springs KGRA, Utah.

depths i n well Utah

1 t cu t t i ngs logging. Wetting the cu t t i ngs may enhance the v i s i b i l i t y o f

otherwise obscure features such as f i n e l y disseminated su l f ides . As

prev ious ly discussed, most cu t t i ngs can be logged r o u t i n e l y us ing

chipboards. For i n t e r v a l s o f special i n t e r e s t o r complexity,. however, a

f a i r l y l a rge sample p o r t i o n ( a t l e a s t log) should be examined.

port ions, such as those used t o prepare chipboards o r grain-mount t h i n

sections, may not be adequately representat ive o f t h e rocks penetrated,

p a r t i c u l a r l y i f t h e rocks a re heterogeneous, coarse-grained, o r hydrothermal ly

1 1

Smaller L I

I' a1 tered.

Simulated d r i l l cu t t i ngs o f representat ive surface rocks, both a l t e r e d

and unaltered, g rea t l y f a c i l i t a t e i d e n t i f i c a t i o n and desc r ip t i on o f downhole

rock types and a l te ra t i on .

screening se lected hand samples, should be consul ted w i th caution, however,

s ince not a l l rocks encountered i n a borehole w i l l have surface equivalents.

1 t These simulated cut t ings, prepared by crushing and

I L Petrographic examination may be necessary t o conf i rm or expand

i l pre l iminary descr ip t ions o f cut t ings. However, i t should complement, ra the r M

than supplant, ca re fu l b inocu la r microscopic study. For example, a f i n e

I grained, l i gh t - co lo red bu t otherwise nondescript ch ip which could be e i t h e r i

cement o r a calcareous t u f f o r mudstone, general ly can be p o s i t i v e l y

i d e n t i f i e d only i n grain-mount t h i n section.

such as those which are s t rong ly a r g i l l i z e d and i ron-stained, remain obscure

even pet rographica l ly . Thin sect ions are most useful f o r i d e n t i f y i n g

unaltered, homogeneous rocks w i th g ra in s i ze l ess than d r i l l ch ip s ize.

g ra in s i ze exceeds ch ip size, t h e c h a r a c t e r i s t i c i n t e r g r a i n re la t i onsh ips o f a

rock are ob l i te ra ted .

Cer ta in o ther rocks, though, I

IJI i L

; I I f I

I

Cut t ings l o g formats and s t y l e s should be t a i l o r e d t o the p a r t i c u l a r

19 i

needs o f each subsurface invest igat ion.

sedimentary sequences a re provided by H i l l s (1949), Low (1977), Maher (1964)

Useful examples o f l o g formats f o r

and Hobbs (1979). Log formats fo r geothermal and .mineral exp lo ra t i on

boreholes w i l l be s i m i l a r t o these examples but a l so should provide columns

f o r g raph ica l l y recording a l t e r a t i o n and minera l izat ion, which are very useful I

i n resource exp lo ra t i on and character izat ion. Regardless o f format and

d e t a i l , logs should i nc lude both descr ip t ions of cu t t i ngs as w e l l as some

i n t e r p r e t a t i o n o f those descr ipt ions, s ince an examiner can best i n t e r p r e t how

the rocks penetrated i n a borehole r e l a t e t o the o v e r a l l geology o f a p r o j e c t

area . INTERPRETATIOh OF LITHOLOGY FROM DRILL CUTTINGS

Correct i d e n t i f i c a t i o n o f subsurface l i t h o l o g y i s important n o t on l y f o r

s t r a t i g r a p h i c and s t r u c t u r a l corre la t ion, b u t a lso because c e r t a i n rocks may

s e l e c t i v e l y host t he resource under i nves t i ga t i on . Careful rock type

determi nat fon a l s o a1 lows more accurate geophysical we1 1 l o g i n te rp re ta t i on .

The use of geophysical l ogs i n petroleum wel ls t o character ize sedimentary 'i

sequences i s well-documented ( f o r example, Maher, 1964).

usefu l

Such logs a re a l so

moreover, f o r deciphering t h e complex igneous and metamorphic geology

canmonly encountered i n geothermal and mineral exp lo ra t i on boreholes (Keys,

1979; Sanyal e t al., 1980). Potassic igneous rocks such as g ran i te and

r h y o l i t e , f o r example, produce conspicuous p o s i t i v e gamma-ray anomalies (Keys,

1979). Rocks r i c h i n hydrous minerals such as b i o t i t e and c h l o r i t e a r e

character ized by t h e i r spur iously high responses on neutron po ros i t y logs

(Nelson and Glenn, 1975).

confused w i t h coQductive thermal water en t r y zones i n geothermal we l l s

(S ibbet t and Glenn, 1981).

Graphi t ic rocks which cause r e s i s t i v i t y lows can be

20

i Rocks which are recognized r e a d i l y i n outcrop o r core f requent ly can be I

d i f f i c u l t t o i d e n t i f y and d i f f e r e n t i a t e as d r i l l cut t ings. Larger-scale

d iagnost ic features, i nc lud ing boulders, bedding and segregation banding,

phenocrysts, f low-breccia, xenol i ths, and g ra in boundaries i n coarser grained

rocks, can be o b l i t e r a t e d by t h e d r i l l i n g process. D iss im i la r rock types thus

can c lose ly resemble one another i n a cu t t i ngs sample.

Springs KGRA, f o r example, massive Te r t i a ry g r a n i t i c rocks are bel ieved t o be

f a r super ior as rese rvo i r hosts t o the f o l i a t e d Precambrian gneisses they

i n t rude (Sibbet t and Nielson, 1980), y e t t h e two rock types can be mistaken

f o r one another i n d r i l l cu t t i ngs (Fig. 7). As prev ious ly discussed, rock

i d e n t i f i c a t i o n can be complicated f u r t h e r by contamination and by

composit1ona.l discrepancies between cu t t i ngs and t h e i r source rocks.

FIG. 7 -- Hand samples and simulated d r i l l cu t t i ngs o f Precambrian gneiss and Te r t i a ry quar tz monzonite from Roosevelt Hot Springs KGRA, Utah.

i :I I ' u 1

11 i ; I I

; L

I

A t t h e Roosevelt Hot

I

i IL i

!

Local l i t h o l o g i e s t y p i c a l l y vary widely i n character and composition from !

general descr ip t ions provided by reg ional repor ts and maps, textbooks o r

manuals.

i n t h e type of geologic environment being d r i l l e d , are essent ia l f o r

determinat ion o f rock types fran cut t ings , espec ia l l y i n complex igneous and

Thus, d i r e c t knowledge o f 1 oca1 geology, as we1 1 as f i e l d experience 1

4

I t ;

'1

1

I U metamorphic t e r r a i n .

The remainder o f t h i s sec t ion w i l l discuss i n d e t a i l recogn i t ion and

i n t e r p r e t a t i o n o f t h e major l i t h o l o g i c groups as d r i l l cu t t i ngs -- a l luv ium

and other unconsolidated deposits, sedimentary rocks ( c l a s t i c and chemical ),

vol cani c and hypabyssal rocks, and p lu ton i c and metamorphi c rocks. The

discussions on sediments and sedimentary rocks w i l l emphasize those occurr ing

i n t h e s t r u c t u r a l l y complex and commonly a l t e r e d sequences l i k e l y t o be

F " '

i b I i

1 I

li; 1

i '

21

c c

A

c Li 2 ,

Figure 7.

C

Hand samples and simulated drill cuttings o f Precambrian gneiss and Tertiary quartz monzonite from Roosevelt Hot Springs KGRA, Utah. gneiss. C, simulated cuttings o f quartz monzonite.

A, hand samples (gneiss at left). B , simulated cuttings o f

encountered i n geothermal and m i neral expl o ra t i on boreholes.

from d r i l l c u t t i n g s o f t h e unaltered, r e l a t i v e l y undisturbed sediments and

sedimentary rocks which host petroleum reservo i rs i s thoroughly discussed i n

I n t e r p r e t a t i o n

H i l l s (1949), Maher (1964) and low (1977).

provide guidel ines f o r logging, respect ively, cu t t i ngs from p lacer deposits

Wells (1969) and Hobbs (1979)

and coal sequences.

Al luvium and Other Unconsolidated Deposits

Many geothermal and m i nera l expl o r a t i o n boreholes are c o l l ared i n

unconsolidated s u r f i c i a l deposits. A good l i t h o l o g i c l o g may be t h e on ly

source o f data f o r these deposits, since t h e upper pa r t s o f boreholes commonly

are cased t o depths o f several hundred f e e t before any geophysical logs a re

run . Because unconsolidated mater ia ls are disaggregated almost t o t a l l y when

d r i l l e d , many o f t h e i r deposi t ional features a re obl i terated. Much o f t he

matr ix mater ia l , f o r example, i nc lud ing clay, s i l t and f i n e sand, may be l o s t

w i t h d r i l l i n g f l u i d a t t h e sampling p o i n t o r dur ing washing. Matr ix g r a i n

s i ze and degree o f s o r t i n g l a r g e l y determine a l l u v i a l permeabi l i ty , so the

l oss o f matr ix from a sample precludes accurate assessment of t he e f f i c i e n c y

o f an a l l u v i a l hor izon as an aqui fer .

contains most a l t e r a t i o n and other secondary minerals, i t s loss a lso can

Since t h e matr ix mater ia l general ly

e l im ina te valuable c lues t o ’ t h e nature o f and distance from a resource.

I n s p i t e o f these d i f f i c u l t i e s , considerable in format ion can be gained

from carefu l logging o f a l l u v i a l cut t ings.

rounding and sor t ing, l i t h o l o g i e s and minerals present, g ra in s i z e

d i s t r i b u t i o n , cementation and a l t e r a t i o n w i 11 a s s i s t w i t h s t r a t i g r a p h i c

c o r r e l a t i o n between holes and may provide con t ro l on s t ructures and a l l u v i a l

Such features as degree of

22

aqui f e r s . Post-deposit ional a l t e r a t i o n i n al luvium can be good evidence fo r

contemporary o r recent geothermal a c t i v i t y .

deposi t ional a l t e r a t i o n by t h e re la t i onsh ip o f a l t e r a t i o n t o g r a i n surfaces.

Nhereas pre-deposit ional a l t e r a t i o n i s confined t o i n d i v i d u a l grains, post-

It can be d is t inguished from pre-

e

deposi t ional a l t e r a t i o n may a f f e c t g ra in surfaces and the matr ix between

grains. Euhedral c r y s t a l s and s ta ins o r c rus ts on g ra in surfaces a re

especia l ly i n d i c a t i v e o f late-stage a l te ra t i on .

bedrock a l t e r a t i o n alsoI i s use fu l i n deciding age re la t i onsh ips o f a l t e r a t i o n

minerals i n alluvium.

Thorough know1 edge o f surface

The depth and nature o f t h e contact ( f a u l t e d o r deposi t ional ) between

a l luv ium and bedrock i n a borehole are extremely important f o r t h e

i n t e r p r e t a t i o n o f var ious geophysical surveys as we1 1 as subsurface

s t ructure. Generally t h i s contact i s e a s i l y determined unless t h e bedrock i s

immediately o v e r l a i n by composi t ional ly i d e n t i c a l gravel , conglomerate or, i n

the case of weathered c r y s t a l l i n e rock, by a t h i c k grus. Al luvium genera l ly

contains variably-colored, m u l t i p l e l i t h o l o g i e s , whereas bedrock tends t o be

homogeneous and uni formly co lo re Angular chips are t y p i c a l o f bedrock, but

a l s o may be produced from l a r g e r pebbles, cobbles o r boulders i n gravel o r

conglomerate. Chips from these sediments, however, may r e t a i n a few rounded

surfaces which b e l i e t h e i r or ig ins. Gravels general ly are ye l l ow ish t o

brownish colored from surface weathering dur ing erosion, t ranspor t and

deposition.

s i m i l a r l y , but w i l l be i n v a r i a b l y angular.

Chips from paleo-weathered zones on bedrock may be colored

D r i l l i n g r a t e and o the r borehole logs may help i d e n t i f y a bedrock-

a1 1 uv i um contact. For exampl e, d r i l l i ng r a t e t y p i c a l l y decreases dramat

23

i c a l l y

c

i n bedrock, w i t h a corresponding increase i n t h e densi ty response. D r i l l i n g

rate, however, i s a f fec ted by o ther fac to rs than formation hardness, and

hardness contrast , i n fact , may be greater a t t he base of a weathered zone

than a t t h e base o f alluvium. A gradat ional contact between grus and i t s

parent c r y s t a l l i n e rock sometimes can be i n te rp re ted from t h e d r i l l i n g r a t e

and t h e degree o f weathering observed i n cut t ings.

A number o f features may help d i s t i n g u i s h a bedrock-alluvium contact as

f a u l t e d o r deposi t ional . A deposi t ional contact, f o r example, may e x h i b i t a

pa leosoi l , rego l i t h , o r weathered zone a t t h e top o f bedrock.

contact, by contrast , genera l ly lacks a weathered zone, although i t may be

A f a u l t

character ized by b recc ia t i on and a l t e r a t i o n o f one o r both formations.

f a u l t e d contact may cause a sharp de f l ec t i on o f a d r i l l ho le which w i l l be

apparent on a d i r e c t i o n a l survey. Surface t races o f known f a u l t s can be

pro jected downward t o t h e borehole, and t h e angles o f t he pro jec t ions compared

w i t h actual measured d ips on t h e f a u l t s t o determine t h e p r o b a b i l i t y t h a t

these s t ruc tu res i n t e r s e c t t h e borehole t o form t h e bedrock-a1 luvium

contact. S imi la r downward pro jec t ions o f bedrock erosional slopes should make

A

c l e a r i f t h e borehole bedrock-alluvium contact i s more l i k e l y t o be

deposi t ional . Sedimentary Rocks

Common sedimentary rocks -- sandstones, s i 1 tstones, shales and carbonates *

-- are very wel l documented (see, f o r example, t he references i n t h e

i n t roduc t i on t o t h i s sect ion), s ince they host o r are associated w i t h

petroleum reservoirs. T e r r e s t r i a l vo l can ic las t i c rocks, however, p a r t i c u l a r l y

as d r i l l cu t t ings , are l e s s thoroughly underst od.

commonly encountered i n geothermal and mineral exp lo ra t ion boreholes, they

Since these rocks are

24

t

L

u

w i l l be emphasized i n t h e discussion which fol lows.

Tuffaceous o r vo lcanoclast ic sediments possibly a re t h e most d i f f i c u l t

c l a s t i c s t o log, s ince they tend to 'occu r i n complex sequences which may

inc lude water la in, a i r - f a l l and non-welded ash-flow t u f f s , lahars, lava f lows

and dikes. Tuffaceous sediments t y p i c a l l y are very heterogeneous, poor ly

sorted and al tered.

def ined t o be evident, although bedding may be i nd i ca ted by r a p i d changes i n

c o l o r and t e x t u r e from sample t o sample.

Bedding i n these immature sediments general ly i s t o o ill-

Volcanoclastic o r tuf faceous sediments commonly are so a r g i l l i z e d t h a t

most o r i g i n a l t ex tu res a re destroyed. The a r g i l l i z e d sediments thus may

resemble s i m i l a r l y a1 tered glassy volcanic o r subvolcanic i n t r u s i v e rocks o r

t u f f s .

grains.

The sediments, however, commonly conta in r e l e c t rounded d e t r i t a l

Free euhedral c r y s t a l s i n t h e a r g i l l i z e d ma t r i x are more t y p i c a l of

a l t e r e d volcanic o r subvolcanic i n t r u s i v e rocks,'whereas broken c r y s t a l s

general ly i n d i c a t e a p y r o c l a s t i c o r i g in .

Descr ipt ions o f tuffaceous o r volcanoclast ic sedimentary rocks should

include, i n a d d i t i o n t o g ra in size, sort ing, rounding, l l thology, mineralogy

and a l t e r a t i o n , t he type o f cementing mater ia l and po ten t i a l porosi ty.

Although formation c o l o r i n t h e subsurface i s o f t e n d i f f e r e n t from t h a t o f the

outcrop, i t should be noted f o r he lp i n c o r r e l a t i o n between boreholes.

degree o f l i t h i f i c a t i o n o r metamorphism a l so i s important.

shale has poor rese rvo i r p o t e n t i a l but a s l a t e -- i t s metamorphic equivalent

-- may develop f r a c t u r e permeabi l i ty .

The

For example, a

Si l iceous and calcareous s i n t e r , d i s t i n c t i v e hot spr ing deposits i n

outcrop, can as cu t t i ngs resemble a wide va r ie t y o f other rock types and

i

I:

i L I I

c w

I,

a l t e r a t i o n products. S i l i ceous s i n t e r i s of p a r t i c u l a r i n t e r e s t i n geothermal

explorat ion, s ince i t ind ica tes past o r present rese rvo i r temperatures of a t

l e a s t 180OC (White e t al., 1971).

f o r a bedded s i l i c e o u s sediment o r hydrothermal s i l i c i f i c a t i o n . Some

s i l i c e o u s s i n t e r can be i d e n t i f i e d by a tendency t o be porous and t o

I n cu t t ings , however, i t can be mistaken

incorporate frothy-appearing p e t r i f i e d p lan t debr is o r casts, as a t Bal tazor

Hot Springs KGRA, Nevada (Hulen, 1979). Spring-deposited calcareous s i n t e r

( t r a v e r t i n e ) can be near ly impossible t o d i s t i ngu ish i n cu t t i ngs from cave

t rave r t i ne , f resh water limestone, a lga l deposits, borehole cement,

hydrothermal ve in c a l c i t e , o r p r a c t i c a l l y any f i n e grained calcareous

sedimentary rock.

o r i g i n of a suspected t r a v e r t i n e sample.

Volcanic, Subvolcanic and Hypabyssal Rocks

Petrographic study may be'necessary t o help c l a r i f y t h e

Volcanic, subvolcanic and hypabyssal rocks inc lude a wide va r ie t y o f rock

types, f a b r i c s and modes o f formation. Pyroc las t ic rocks are p a r t i c u l a r l y

d i f f i c u l t t o work w i t h i n cu t t i ngs because most large-scale tex tu res such as

so r t i ng and bedding w i l l be destroyed by d r i l l i n g . F a m i l i a r i t y w i th t h e

unique tex tu res and mineral compositions o f these rocks w i l l a i d t he logger.

Uncompacted pumice, f o r example genera l ly w i l l be destroyed dur ing d r i l l i n g

bu t fiamme (compacted pumice), shards and e u t a x i t i c t ex tu re may be v i s i b l e

w i th a b inocular microscope. The fragmental t e x t u r e o f a py roc las t i c rock

(which may requ i re petrographic conf i rmat ion) may g ive chips a mot t led

appearance even when a l tered. Phenocrysts genera l ly a re present both i n t u f f s

and flows, bu t i f broken are much more t y p i c a l o f a py roc las t i c o r i g in .

Cinders w i l l conta in unbroken phenocyrsts, but also rounded vesicles. As

prev ious ly discussed, non-welded t u f f s may be hard t o d i s t i n g u i s h from

tuf faceous sediments, but c l a s t rounding and so r t i ng as wel l as a h igh content

26

o f rounded mul t i -1 i t h o l o g i c c l a s t s suggests water deposition. Rounding alone,

however, i s no t d e f i n i t i v e evidence o f sedimentary o r i g in , s ince s o f t t u f f

ch ips can be h igh l y rounded dur ing t ranspor t up t h e d r i l l hole.

Lava f lows and subvolcanic o r hypabyssal i n t r u s i v e s may be impossible t o

d i f f e r e n t i a t e as cut t ings, bu t several features, i f present, may be helpfu l

f o r t h e i r i d e n t i f i c a t i o n . Flows, p a r t i c u l a r l y near t h e i r tops and bottoms,

are more l i k e l y t o conta in rounded ves ic les o r amygdules and c h i l l e d and

ox id ized f low breccia. Subvolcanic o r hypabyssal rocks, on t h e other hand,

may have c h i l l e d borders bu t general ly lack margin brecc ia and, u n l i k e flows,

may have f a u l t contacts.

body i n quest ion would be evidence f o r an i n t r u s i v e or ig in . M inera l i za t ion

Baking o f t h e country rock on both contacts o f t h e

and a l t e r a t i o n i n the country rock adjacent t o t h e contact a l so would suggest

t he body t o be an in t rus ive .

Netamorphic and P lu ton ic Rocks

The metamorphic rocks, as a group, are perhaps t h e most d i f f i c u l t t o

recognize i n d r i l l cut t ings. Lower-grade metamorphic rocks as cu t t i ngs may be

i nd i s t i ngu ishab le from t h e i r igneous and sedimentary parent l i t h o l o g i e s .

Chips o f high-grade gneiss and migmatite, on t h e other hand, may c lose ly

resemble p lu ton i c rocks i n t e x t u r e and mineralogy.

p l u t o n i c rocks from coex is t ing gneisses can be c r u c i a l t o a geothermal

evaluation.

rocks and are less l i k e l y t o develop and sus ta in f r a c t u r e permeabi l i ty f o r

D is t ingu ish ing downhole

Gneisses, i f fo l i a ted , are less competent than massive p lu ton i c

- t h e m a l f l u i d flow.

Charac ter is t i c mineral assemblages i n d r i l l chips are t h e best i nd i ca to rs

o f metamorphism, espec ia l l y t h e h igher grades.

rock cons is t ing mostly o r e n t i r e l y o f quartz and b i o t i t e , o r conta in ing

For example, cu t t i ngs of a

27 . .

I,

L I

L 1

L

rl L L

b

c

diagnost ic minerals such as c o r d i e r i t e , s i l l i m a n i t e and kyanite, are almost

c e r t a i n l y of h i gh-grade met amorphi c o r i g i n . such as c h l o r i t e , a l b i t e Bnd epidote, are much less useful as genet ic

Lower-grade met am o r ph i c m i ne ra l s ,

indicators , s ince they a re a l s o common i n hydrothermally a l t e r e d t e r r a i n (Rose

and Burt, 1979).

Cer ta in rock textures apparent i n d r i l l cu t t i ngs s t rong ly suggest a

metamorphic o r i g i n , whereas others are ambiguous, p a r t i c u l a r l y i n t h i n -

section. C r y s t a l l o b l a s t i c textures, r e s u l t i n g from c r y s t a l growth i n a s o l i d

medium, provide good evidence f o r metamorphism.

character ized by " t r i p l e po in t " g r a i n boundaries and/or by a mosaic of more o r

Such tex tu res are

l ess polygonal grains w i t h boundaries bear ing no d e f i n i t e r e l a t i o n s h i p t o

c r y s t a l lographic axes o r cleavage d i rect ions.

metamorphic rock only if p a r t i c u l a r l y wel l developed (as i n p h y l l i t e s and mica

schists) , since i t may a l s o form i n response t o flowage i n volcanics and a t

F o l i a t i o n i s diagnost ic of a

t he margins o f i n t rus i ves . Cataclasis, e longat ion o f grains and g ra in

aggregates, mortar texture, kink-banding and strain-shadowing, can be produced

not on ly by dynamic metamorphism, but a l so by renewed movement o f p a r t i a l l y

c r y s t a l l i z e d magma.

produced by t h e d r i l l i n g process.

Cataclasis and s t r a i n shadowing of minerals a lso may be

The c r y s t a l l o b l a s t i c Series, which ranks minerals by tendency t o form

euhedral c r y s t a l s i n a so l i d ,g rowth medium, may help i d e n t i f y a rock ch ip as

igneous o r metamorphic.

faces I n metamorphic rocks, f o r example, but are q u i t e common i n many p l u t o n i c

rocks.

Quartz and fe ldspar r a r e l y develop euhedral c r y s t a l

. A l t e rna t i ng mafic and f e l s i c segregations i n gneisses and sch is t s may be

homogenized i n a c u t t i n g s sample. I n the absence o f d iagnost ic minerals,

l d 28

these layered metamorphic rocks can be d is t inguished from mine ra log i ca l l y

s i m i l a r but massive i n t r u s i v e rocks by t h e i r geophysical wel l l o g responses.

Whereas massive rocks produce f a i r l y uniform geophysical signatures, 1 ayered

rocks a re character ized by f l u c t u a t i n g and h i g h l y va r iab le responses.

RECOGNITION OF FAULTS AhD FRACTURES FROM DRILL CUTTINGS

Recognition o f f a u l t s and f rac tu res i s one o f t he most important aspects

o f subsurface exp lo ra t i on and development. Faul ts and f ractures commonly a re

the domi nant physical con t ro l s on t h e commodities under invest igat ion.

Faults, f o r example, host, t runca te and displace ore bodies, provide t raps f o r

petroleum accumulation and, i n geothermal systems, serve as major channels f o r

thermal f l u i d flow. Permeabi l i ty i n most geothermal systems, i n fac t , i s

provided by f a u l t s and f rac tu res developed i n otherwise t i g h t and impermeable

host rocks (Sanyal e t al., 1980).

Faul ts and f rac tu res penetrated i n d r i l l holes t r a d i t i o n a l l y are

i d e n t i f i e d i n d i r e c t l y . They a r e commonly " l o s t c i r c u l a t i o n " zones which

remove f l u i d s from a d r i l l i n g system.

system, causing a wel l "kick".

recognized by t h e i r responses on appropri a te we1 1 1 ogs , 1 nc lud i ng c a l i per

(borehole enlargement), acoust ic v e l o c i t y (which decreases i n d isrupted rock)

and r e s i s t i v i t y (which decreases i n f a u l t s o r f rac tu res if saturated wi th

conduct 1 ve f l u i d ) .

They a lso may con t r i bu te f l u i d t o the

In most cases, however, these s t ructures a r e

Faults, p a r t i c u l a r l y those developed i n competent rocks, occasional ly can

be recognized d i r e c t l y from d r i l l c u t t i n g s by the presence of s l i ckens id ing

and chips o f gouge, breccia, c a t a c l a s f t e and mylonite.

however, provide such physical evidence o f t h e i r existence.

Not a l l f a u l t s ,

Gouge and

29

F ' u

i ; . c

L

u t' ! b

b

microbreccia genera l ly are s o f t and f r i a b l e , e a s i l y disaggregated by the

d r i l l 1 ng process and subsequently removed from cu t t i ngs dur ing sample

c o l l e c t i o n o r washing. Fau l ts d i s rup t i ng incompetent rocks such as mica

sch is ts may even lack a crushed zone, and there fore may completely escape

de tec t ion i n cu t t i ngs samples.

O r i l l i n g produces a r t i f i c i a l gouge, ca tac las i te and s l ickensides which

can be confused w i th t h e i r na tura l counterparts.

block o f massive, unf ractured quartz d i o r i t e dur ing laboratory a i r d r i l l i n g

Cutt ings produced from a

experiments conducted i n 1980 by Terra Tek, S a l t Lake City, Utah, conta in 1.5-

2 volume percent pseudo-gouge1 (Fig. 8; see a lso Figs. 1 and 2).

mater ia l , i d e n t i c a l t o t h a t observed i n cu t t i ngs from many boreholes, forms

conspicuously l ight -co lored, crudely d i sco id o r t abu la r chips which, although

f r i a b l e , a r e commonly slickensided.

This

FIG. 8 -- Dr i l l -produced pseudo-gouge chips hand-picked from laboratory a i r - d r i l l e d quar tz d i o r i t e cut t ings.

Several promi nent cha rac te r i s t i cs o f pseudo-gouge d i s t i n g u i s h it, though

no t i nvar i ably, from fau l t-produced gouge and breccia.

f r i a b l e , whereas na tura l mater ia l may be wel l - indurated and very r e s i s t a n t t o

Pseudo-gouge i s a1 ways

crumbling.

(o r cemented), although such a l t e r a t i o n can be simulated by crushing and

s t reak ing of rock-forming o r a l t e r a t i o n minerals i n the rock penetrated.

Slickensides on pseudo-gouge chips, probably produced mainly beneath tungsten

carbide "button" i n s e r t s i n t r i c o n e b i t s , commonly a re concave (Fig. 8), very

smooth though no t polished, and t h i n l y smeared w i t h maf ic minerals o r streaked

Only na tura l gouge and ca tac las i te can be hydrothermally a l t e r e d

l/ Actua l l y pseudo-cataclasite, but here in re fe r red t o as pseudo-gouge fo r convenience.

30

c

i Figure 8. Drill-produced pseudo-gouge chips handpicked from 1 aboratory air-

dril led quartz diorite cuttings. surfaces. Dark 'material i s crushed and smeared biotite. Largest chip a t upper left i s 5 mm i n length.

Note characteristic concave

/

t

c L

I L

w i t h dark b i t metal.

more l i k e l y t o be f l a t o r convex, polished, and furrowed o r s t r i a ted .

Natural sl ickensides, although ra re i n cut t ings, are

Fractures, along which no movement has occurred, general ly cannot be

d i r e c t l y detected i n c u t t i ngs by v i sua1 examination.

provided pathways f o r thermal f l u i d flow, however, as i n geothermal systems

and mineral deposits, t h e i r wall rocks may be hydrothermally al tered. Thus

Where f rac tu res have

a l t e r a t i o n o f cu t t i ngs may i n d i r e c t l y s ignal subsurface f rac tu r ing . Such

a l t e r a t i o n , however, commonly i s accompanied by reduct ion or e l im ina t i on o f

o r i g i n a l permeabil i t y through formation o f hydrothermal v e i n l e t s by open-space

f i l l i n g o f former f ractures.

Anomalously l a rge cu t t i ngs a l so may I n d i c a t e f ractur ing. I n w e l l Utah

State 14-2 a t Roosevelt Hot Springs KGRA, f o r example, w i t h i n a geophysical ly

confirmed (Glenn and Hulen, 1979), f rac tu red hot water en t r y zone between

487.7 and 548.6 m (1600 and 1800 ft), average ch ip s i z e (3-5 mm) i s t h ree t o

f i v e t imes as l a r g e as i n surrounding unfractured rock (Fig. 4).

c u t t i n g s may r e f l e c t t he r e l a t i v e ease w i t h which f rac tu red rock i s dislodged

by the d r i l l b i t . I n some cases however, l a r g e r cu t t i ngs may i n d i c a t e only a

change o f b i t s ; a l l o ther var iables equal, a new b i t w i l l produce l a r g e r

c u t t i n g s than a d u l l one.

These l a r g e r

INTERPRETATION OF HYDROTHERMAL ALTERATION I N CUTTINGS \

Most mineral deposits and geothermal systems a re accompanied by

hydrothermal a l t e r a t i o n o f t h e i r host rocks.

deposits commonly i s strong , pervasive and arranged i n we1 1 -devel oped zones o r

halos which i n exp lo ra t i on boreholes can be used t o determine vectors and

A l t e r a t i o n around mineral

approximate distances t o mineral i zat ion (Levinson, 1974, 1980; Rose and Burt,

t 31

i

I L i 1979). . I n many geothermal systems, by contrast , a l t e r a t i o n i s genera l ly

weaker and more e r r a t i c a l l y d is t r ibu ted . Nonetheless, i t can be used t o he lp I

def ine the three-dimensional geometry o f these systems and t o l oca te and

p red ic t approach t o thermal f l u i d en t r y zones (Elders e t al., 1979; Browne,

1978; Sum1 and Takashima, 1976; Browne and E l l i s , 1970; Steiner, 1968). I

Recognition and i n t e r p r e t a t i o n o f a l t e r a t i o n i n cu t t ings i s somewhat more pi

l L d i f f i c u l t than i n core o r outcrop.

l a r g e r scale d iagnost ic features o f t h e a l t e r a t i o n and may r e s u l t i n

e l im ina t i on o f c e r t a i n a l t e r a t i o n minerals from t h e cu t t i ngs w i t h d r i l l i n g

f l u i d dur ing sample c o l l e c t i o n o r washing.

The d r i l l i n g process commonly destroys 1L 1

A l t e r a t i o n tex tu res and paragenetic re la t i onsh ips may be obscured o r l i I t :

b I

o b l i t e r a t e d i n cut t ings. A l l but the smal lest ve in le t s and t h e i r cross-

c u t t i n g re la t i onsh ips are destroyed dur ing cu t t i ngs production. I I n t e r p r e t a t i o n becomes more d i f f i c u l t as d r i l l ch ip s i z e decreases.

cu t t i ngs prepared from representat ive hydrothermally a1 te red surface rocks as

wel l as knowledge o f surface a l t e r a t i o n paragenesis are very usefu l f o r

subsurface paragenetic i n t e r p r e t a t i o n from actual cut t ings.

Simulated

Softer, more b r i t t l e , more r e a d i l y cleaved and f iner-gra ined a l t e r a t i o n c

(and rock-forming) minerals are r e a d i l y disaggregated by the d r i l l i n g process

and there fore may be depleted n cu t t i ngs r e l a t i v e t o corresponding u n d r i l l e d

rock. Hydrothermal c lays enco ntered i n mud-dr i l led we l ls commonly w i l l be

completely e l iminated from cu t t i ngs (along w i t h b e n t o n i t i c c lays present i n

t h e mud) dur ing sample c o l l e c t i o n o r washing.

r '

L I '

R e l i c t or paleohydrothermal a l t e r a t i o n , p a r t i c u l a r l y i n cu t t ings , can be b i nd is t ingu ishab le from a l t e r a t i o n produced by a present ly ac t i ve geothermal

32 *

c L

u

system.

t y p i c a l l y occupy zones o f s t r u c t u r a l weakness which are read1 l y ref ractured,

prov id ing channels f o r renewed thermal f l u i d flow. A t Roosevelt -Hot Springs

KGRA, Utah, f o r example, r e f r a c t u r i n g o f dense, s i l i c i f i e d , c h l o r i t i t e d and

s e r i c i t l z e d c a t a c l a s i t e zones i s bel ieved t o be responsible f o r much o f t h e

geothermal rese rvo i r ' s present permeabi l i ty (Nielson e t a1 . , 1978; Sibbet t and

Even r e l i c t a l t e r a t i o n zones are o f i n te res t , however, as they

N i e l son, 1980).

A l t e r a t i o n mineralogy r e l a t i v e t o depth o f occurrence i n a geothermal

system czn provide clues t o t h e age of a l t e r a t i o n r e l a t i v e t o present

geothermal a c t i v i t y . Minerals which t y p i c a l l y form a t elevated temperatures

and/or pressures and occur a t shallow l e v e l s i n a c t i v e systems, such as

epidote (Zen and Thompson, 1974) a t Roosevelt Hot Springs KGRA are probably

pa 1 eo hydrot he rmal . Various contaminants i n c u t t i n g s may st rongly resemble natura l a l t e r a t i o n

D r i l l i n g muds and mud add i t i ves are t h e minerals encountered dur ing d r i l l i n g .

contaminants most commonly m i staken f o r indigenous a l t e r a t i o n products, but

cements and m e t a l l i c shavings can a l so cause i n t e r p r e t i v e er rors .

The bas ic ingredients o f d r i l l i n g muds are c lays which may a l s o occur as

hydrothermal a l t e r a t i o n minerals i n and around ore deposits and geothermal

systems. Bentonite, from which standard d r i l l i n g muds are prepared, i s a

h i g h l y va r iab le c l a y cons is t i ng dominantly of sodium- and/or calcium

montmori l loni te, bu t which may a l so contain, depending on i t s source,

k a o l i n i t e , i l l i t e , saponite and h e c t o r i t e (Patterson and Haydn, 1975).

I 7

i u i t : i i 'i; Montmori l loni te, k a o l i n i t e and i l l i t e a r e common a l t e r a t i o n products i n a c t i v e i

1 : geothermal systems (Browne, 1978) and i n a l t e r a t i o n haloes ( p a r t i c u l a r l y

a r g i l l i c ) around many m i neral deposits (Rose and Burt, 1979) . Attapul g i t e

(palygorski t e ) and s e p i o l i te, used t o prepare salt-water-based and/or h igh

temperature muds (Westin, 1980; Carney and Meyer, 1976) are much more

r e s t r i c t e d i n na tura l occurrence (Patterson and Haydn, 1975) and therefore

less l i k e l y t o be mistaken f o r const i tuents o f t h e . d r i l l e d rock.

D r i l l i n g mud weight ing agents, which are added p r i m a r i l y t o con t ro l

formation pressure and decrease caving, are prepared from a v a r i e t y of heavy

minerals which a1 so occur as hydrothermal a1 t e r a t i o n products.

f requent ly used weight ing agent i s b a r i t e which, although r a r e l y repor ted from

major ac t i ve geothermal systems (Browne, 1978), i s a common ore and gangue

,mineral i n hydrothermal vein deposits. Other mud weight ing mater ia ls l i k e l y

t o be confused w i t h a l t e r a t i o n minerals i n d r i l l e d rock are used much less

commonly than ba r i t e . They inc lude ilmenite/magnetite, hematite, galena,

(which may contains spha le r i t e as an impur i ty) , and c a l c i t e o r limestone.

The most

Los t - c i r cu la t i on mater ia ls , added t o d r i l l i n g f l u i d t o r e s t r i c t o r

prevent f l u i d l o s s t o pores, f ractures, f a u l t s and s o l u t i o n c a v i t i e s

encountered du r i ng d r i l l 1 ng , are general l y 1 ow-densi ty organic mater i a1 s

u n l i k e l y t o be v i sua1 l y m i staken f o r hydrothermal a1 t e r a t i o n products.

organic mater ia ls inc lude nut and seed hu l l s , hardwood and cane f i be rs , ground

paper, wool, and a l f a l f a . Two o ther l o s t - c i r c u l a t i o n addi t ives, however,

processed mica and cellophane f lakes, may mimic indigenous mica i f c u r s o r i l y

examined. Impure processed mica ( p a r t i c u l a r l y ve rm icu l i t e ) a d d i t i o n a l l y may

conta in subs tan t ia l amounts o f such impur i t i es as c h l o r i t e , p lagioclase,

These

quartz and r u t i l e .

D r i l l i n g muds and mud addi t ives, r e a d i l y

cons t i tuents dur ing b inocular o r petrographic

even more troublesome i n analyses o f cu t t ings

34

confused w i t h na tura l rock

microscopic examination, a re

by X-ray d i f f r a c t i o n and atomic

absorpt ion spectrophotometry. Dypvik (1981) has analyzed most o f the common

muds and add i t i ves by one o r both o f these methods.

on ly d r i l l i n g mud clays, b u t a lso many dominantly organic add i t i ves (such as,

for example, walnut h u l l s ) produce s t rong X-ray peaks which e a s i l y could be

confused with patterns generated by natura l const i tuents of the d r i l l e d

rock.

He has shown t h a t no t

/J

,

He .has a l s o shown t h a t many add i t i ves conta in s i g n i f i c a n t amounts o f

m e t a l l i c t r a c e elements. Bar i te, f o r example, may be enriched not only i n

s t ront ium (as expected), but a l so i n s i l v e r , lead, z inc and manganese. As

previously discussed, such spurious geochemistry i n c u t t i n g s could lead t o

erroneous conclusions about t h e nature o f and distance from a geothermal

resource o r m i neral deposi t . Most metals abraded from d r l l l s t f i n g s , casing, and downhole cementing

assemblies, i n t l u d i n g s t e e l , brass, bronze, monel , lead and aluminum can be

recognized i n cu t t i ngs as fo re ign debr is by t h e i r c h a r a c t e r i s t i c d u c t i l i t y and

tendency t o form discrete, cu r led shavings and peels. Steel , however, r e a d i l y

rusts, p a r t i c u l a r l y i n samples s tored under moist condit ions. The r u s t

s t rong ly resembles n a t u r a l l y occurr ing earthy t o waxy g o e t h i t i c t o hemat i t i c

" l imon i te " (Hulen, 1978) which, by contrast with steel, cannot be removed from

c u t t i n g s magnetical ly.

Rust i n cu t t i ngs commonly can be d is t inguished from natura l ly -occurr ing

l i m o n i t e by consider ing the depth of t h e sample r e l a t i v e t o t h e base o f

ox ida t i on as i n te rsec ted i n the borehole.

t h i s zone i s l i k e l y t o be rust , although i s o l a t e d pockets of l i m o n i t e may be

Earthy i r o n oxide i n a sample below

l o c a l l y present i n deeply ox id ized f a u l t s o r f r a c t u r e zones. Rust der ived

from s tee l and i r o n w i l l a l s o tend t o occur on ly on chip surfaces whereas

indigenous i r o n oxides may a lso occur i n f ractures, along c r y s t a l boundaries

b c

L

and i n the i n t e r i o r s o f mineral grains.

SUMMARY AND CONCLUSIONS

. D r i l l cut t ings, i n s p i t e o f t h e i r disadvantages as borehole rock samples

r e l a t i v e t o core, can provide valuable in format ion about a subsurface resource

i f c a r e f u l l y sampled and nterpreted. I n t e r p r e t a t i o n o f cu t t i ngs from

geothermal and mineral exp lo ra t i on boreholes can be complicated by i n t r i c a t e

igneous and metamorphic l i t h o l o g y , s t r u c t u r e and hydrothermal a l t e r a t i o n as

wel l as severe contamination. These complications c m be minimized by

c m b i n i n g c r i t i c a l sc ru t i ny o f c u t t i n g s w i t h a f u l l understanding o f both

l o c a l (and reg iona l ) geology and t h e e n t i r e range o f l i k e l y borehole

contaminants. 10 be most e f f e c t i v e i n character iz ing a resource, c u t t i n g s

i n t e r p r e t a t i o n should be used i n conjunct ion w i t h a l l o ther methods o f

subsurface invest igat ion, p a r t i c u l a r l y geophysical we l l logging.

ACKNOWLEDGEMENTS

We g r a t e f u l l y acknowledge the generous f i n a n c i a l support o f t he U. S.

Department o f Energy, D iv i s ion o f Geothermal Energy and the valuable

suggestions and ca re fu l c r i t i c a l reviews o f Dennis Nielson, Jon Ze is lo f t , Mike

Wright, Jim S t r i ng fe l l ow and Mike B u l l e t t .

prepared by Dor is Cul len and Connie Pixton.

Un ive rs i t y o f Utah Medical I 1 l u s t r a t i o n s Department. The manuscript was

Drafted i l l u s t r a t i o n s were

Photographs were produced by t h e

organized and prepared by Ho l l y Baker.

36

ci I I - u u

I ‘

1

r”!

,

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37

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I

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' 0

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