Petroleum Geoscience and Geophysics Chapter 4

CHAPTER 5

• CLASTIC RESERVOIR

ROCKS

Reservoir Geology

• Deals with the origin, spatial distribution, and

petrological characteristics of reservoirs.

• Utilizes information from sedimentology,

stratigraphy, structural geology, sedimentary

petrology, petrography, and geochemistry to

prepare reservoir descriptions.

• Direct observations of depositional textures,

constituent composition, principal and accessory

minerals, sedimentary structures, diagenetic

alterations, and pore characteristics provide the

foundation for reservoir descriptions.

Reservoir Geology

• The goal of such interpretations is to formulate

geological concepts to guide in predicting

reservoir size, shape, and performance

characteristics.

• Reservoir characterization; like reservoir

geology, deals with physical characteristics of

the reservoir.

• It differs from geological description in that data

on petrophysics and fluid properties are

included.

Reservoir Geology

• Sandstone and limestone (including dolomite)

are the most common reservoir lithologies.

• The main reasons to study clastic and

carbonate reservoirs and aquifers are to learn

more about how to find, extract, and manage

the oil, gas, usable water, or other resources

they contain.

Clastic Reservoir Rocks

• (i) Sandstones

• Environments: Coastal/shelf marine, fluvial,

sub-aerial.

• Composition: Grain Size: – framework fraction:

particles 63 to 2000μm in diameter.

• Mineralogy:

• – Quartz (SiO2) dominant mineral -- 50 - 60%

framework.

• • monocrystalline form - single large grains.

• • polycrystalline – chert nodules.

Quartz

Polycrystalline (P)

Monocrystalline (M)

P

M

P

M with overgrowth

(formed during

diagenesis) P&S, Fig. 5.8

Sandstones Reservoir Rocks

• – Feldspars (AlSi3O8) second most abundant

mineral - 10-20% of the framework

• less stable than quartz; alters to clays

• Alkali (Potassium -K) Feldspars (orthoclase,

microcline), Plagioclase.

• – Clay Minerals < 5% matrix

• – Accessory Minerals - < 1 to 2%

• micas (muscovite, biotite)

• heavy minerals (zircon, rutile, magnetite,

pyroxenes, amphiboles).

Feldspars

Feldspar

crystal

Blue = pore space (crystal largely dissolved during deep burial)


• Mineral Cements:

• – Silicate (SiO2) based cements (mainly

Quartz).

• – Carbonate (CaCO3) based cements


• Sandstones Classification:

• Provides information about:

– Provenance (source rocks from which components

derived)

– Transport processes

• Concept of maturity:

Physically mature

– All grains well rounded/ spherical

– All grains same size

– No matrix


Chemically

mature;

• All grains are

quartz

Stable Grains 100% = HIGHLY MATURE

Matrix

100%

Unstable

Grains

100%

Sandstones Classification:

- Folk’s classification

Sandstones Classification

- Pettijohn’s classification


- Pettijohn’s classification

• based on QFL triangles

• uses matrix %

• no simple scheme for physical maturity

• needs thin section -- rarely possible in hand specimen


• Sandstones Classification:

• Arenites - grain supported, well sorted

sandstones (<15% matrix).

• 1. quartz arenite

extensive chemical weathering - product of

multiple recycling, mature

Marginal marine facies (beach)


• 2. arkosic arenites (>25% feldspar)

Abundant feldspar, micas – low maturity

Poorly sorted, angular grains

limited chemical weathering - either very cold

and dry climate, or rapid erosion and

deposition


• 3. lithic (rock fragments) arenites

limited chemical weathering - mountainous

region, rapid transport

alluvial fans, or other fluvial environments

Laminations, cross-bedding possible.


• 4. Wackes - abundant matrix, poorly sorted

(>15% matrix)

• Deep water facies – waning turbidity current.

• a. quartz wacke, feldspathic wacke

• b. lithic (rock fragments) wacke

• c. graywacke

matrix rich sandstone of any composition

very hard, and dense – undergone deep

burial


• Examples of sandstone reservoir

rocks. (A) clean, well sorted

sandstone, (B) angular,

feldspathic sandstone, and (C)

argillacious, very poorly sorted

sandstone.


• Photomicrograph of a quartz arenite under ordinary

light. Simpson Group, Ordovician, Oklahoma, USA.


• Photomicrograph of a greywacke under polarized light.

Jurassic, UK North Sea. Note the poorly sorted texture

and abundance of matrix and twinned feldspar..


• Photomicrograph of quartz wacke under polarized

light. Carboniferous, Chios, Greece. Note the poorly

sorted texture and abundance of matrix.


• Photomicrograph of arkose under polarized light.

Torridonian, Precambrian, Scotland. Note the

abundance of twinned feldspar and the better sorted

texture.

Conglomerates

• A coarse grained siliclastic rock with a muddy

or sandy matrix.

• Associated with High Energy environments:

• – mountains, margins-fans, glacial, turbidity

current.

• Composition:

• • Grain Size:

30% gravel size (>2mm in diameter) rounded

clasts.

Conglomerates

Conglomerates

1% of all sedimentary rocks.

High energy environments - mountains,

margins-fans, glacial.

Composition:

Grain size – 30% gravel size (>2mm in

diameter) rounded clasts.

Conglomerates

• Classification:

Orthoconglomerates consist primarily of

framework grains and <15% matrix.

Paraconglomerates have a matrix of sand and

finer clasts and are matrix-supported.

Diamictite is another term for a

paraconglomerate, and is often used to denote

glacial rocks.

Shales

• • LOW ENERGY Environments;

• – Deep-quiet water

• – Abundant fine sediment

• • Composition:

• – Grain Size:

• • silt and clay (< 63 μm)

• – Mineralogy:

• • fine grain quartz

• • clay

Shales

• • Classification:

• 1. siltstone (>66% silt)

• 2. mudstone (<66% silt,

• >33%clay)

• 3. claystone: (>66% clay)

Pelagic

clay

silt

Pore Space Properties

• The basic framework of a sandstone reservoir

is formed by the sand grains between which the

pore space may or may not contain interstitial

fine material and/or cement.

Framework of reservoir sand with

interstitial clay and cement.


• The amount of this intergranular pore space or

porosity is controlled primarily by sorting of the

sediment.

• The porosity of a reservoir rock is defined as

that fraction of the bulk volume of the reservoir

that is not occupied by the solid framework of

the reservoir.

b

p

b

grb

V

V

V

VV


• Grain textures are the chief factors that control

the porosity and permeability of sediments in

siliciclastic settings.

• These include:

• 1) grain size distribution (mean, median, and

sorting),

• 2) shape (sphericity),

• 3) packing,

• 4) composition, and

• 5) cementation.


• In sandstones, porosity is controlled primarily

by sorting, cementation and, to a lesser extent,

by the way the grains are packed together.

• Porosity is at its maximum for spherical grains

but becomes progressively less as the

angularity of the grains increases because such

grains pack together more closely.

• However, porosities of packed sands show a

general decrease as sorting becomes poorer.


• This is because the smaller grains partially fill

the interstices between the larger grains.


• Packing is the mutual spatial relationships

between grains. Close packing reduces

porosity and permeability.


• Packing:

• Cubic

arrangements:

47.6% - low

packing.

• Rhombus

arrangements:

• 26%.


• Porosity Ranges:

• Sand and gravel 20-50 %

• Till 10-20 %

• Silt 35-50 %

• Clay 33-60 %

• Clastic sediments typically 3-30 %

• Limestone <1 to 30 %

• Basalt 1-12 %

• Tuff 14-40 %

• Pumice - 87 %

• Fractured crystalline rock 1-5 %

• Unfractured crystalline rock ~0.1 %


• Permeability is a measure of the ability of a fluid

or a gas to cross a network of pores.

• Measured in Darcy (D, or mD)

• A measure of the degree of interconnectedness

of pores.

• Permeability depends primarily upon the size,

shapes and extent of the interconnections

between individual pores (pore-throat diameter)

rather than the size of the pores themselves.


• What influences the throughput of a fluid (Q)

through a porous solid?

– Length, l

– Fluid viscosity, μ

– Cross-sectional area, A

– Pressure difference, Δp

• lTherefore: permeability (proportionality

constant), k

p

l

AkQ

41

• The general darcy’s equation is:

dL

dPk

A

Q

Q

L

A

P1 P2

Q = flowrate (cm3/sec)

k = permeability (darcy)

A = cross section area (cm2)

= fluid viscosity (cp)

P = pressure (atm)

L = length (cm)

42

• 1 darcy is defined as the permeability that will permit a fluid of 1 centipoise viscosity to flow at a rate of 1 cubic centimeter per second through a cross sectional area of 1 square centimeter when the pressure gradient is 1 atmosphere per centimeter.

Q

L

A

P1 P2

Q = 1cm3/sec

A = 1cm2

= 1 cp

P = 1atm

L = 1cm

Find k ?


• Experimental data show a marked decrease in

permeability as grain size decreases and as

sorting becomes poorer.

• Composition:

• The amount and kind of clay, as well as

distribution throughout the reservoir rock, has

an important bearing on liquid permeability,

whereas a small amount has little effect on

porosity.


• Three general types

of dispersed clay in

sandstone reservoir

rocks and their

effects on

permeability: (a)

discrete particles of

kaolinite;

• (b) pore lining by

chlorite; (c ) pore

bridging by illite/

montmorillonite.

EaES 350-2 45

EaES 350-2 46

EaES 350-2 47


• Cementation:

• The highly cemented sandstones have low

porosities, whereas the soft, unconsolidated

rocks have high porosities.

• Both permeability and porosity of sedimentary

rocks are influenced by the extent of the

cementation and the location of the cementing

material within the pore space.


• Conclusions:

• Porosity is independent of grain size.

• Porosity is dependent of packing, sorting,

composition and cementation.

• Permeability depends upon the size,

shapes and pore-throat diameter.

• Packing is dependent on depositional and

diagenetic history.

Classification of Porosity

• Primary Porosity

• The porosity preserved from

deposition through lithification.

• 1. Intergranular or interparticle:

voids between grains, i.e.,

interstitial voids of all kinds in all

types of rocks.


• 2. Intercrystalline: voids

between cleavage planes of

crystals, voids between

individual crystals, and voids in

crystal lattices.

• 3. Bedding planes: voids of

many varieties are concentrated

parallel to bedding planes.


• 4. Miscellaneous sedimentary voids:

• (i) voids resulting from the accumulation of

detrital fragments of fossils, (ii) voids resulting

from the packing of oolites, (iii) vuggy and

cavernous voids of irregular and variable sizes

for at the time of deposition, and (iv) voids

created by living organisms at the time of

deposition.


Intergranular porosity Intragranular porosity

Microporosity

Intergranular porosity (X)

in limestone

Biomoldic porosity

Intercrystalline porosity

(X) within dolomite Cavernous porosity


• Secondary Porosity: The porosity created

through alteration of rock (diagenesis and

catagenesis), after the deposition of sediment.

• 1. Solution porosity: channels due to the

solution of rocks by circulating warm or hot

solutions; openings caused by weathering,

such as enlarged joints and solution caverns;

and voids caused by organisms and later

enlarged by solution.

http://www.glossary.oilfield.slb.com/Display.cfm?Term=rock


• 2. Dolomitization: a process by

which limestone is transformed into

dolomite according to the following

chemical reaction:

• 2CaCO3+ Mg2+ CaMg(CO3) + Ca2+

• Because the ionic volume of

magnesium is considerably smaller

than that of the calcium, which it

replaces, the resulting dolomite will

have greater porosity.


• 3. Fracture porosity: openings created by

structural failure of the reservoir rocks under

tension caused by tectonic activities.


• 4. Miscellaneous secondary voids:

• (1) saddle reefs, which are openings at the

crests of closely folded narrow anticlines; (2)

pitches and flats, which are openings formed by

the parting of beds under gentle slumping; and

(3) voids caused by submarine slide breccias

and conglomerates resulting from gravity

movement of seafloor material after partial

lithification.

Diagenesis & Reservoir Quality

• Diagenesis of sandstones

• All changes, physical, chemical, and biological, that

occur in a sediment after deposition and before

metamorphism (<150-200oC).

• These changes happen at sediment-water interface

and after burial.

• Two important processes

Compaction - decrease in volume, largely by

squeezing out of water

Cementation - introduction of chemical

precipitates between grains


• Muds can be compacted because grains are ductile

(flexible) and can pack easily.

• Sands are not easily compacted because they are

supported by grain-to-grain contacts

• The diagenetic history of a sandstone is controlled

principally by the chemistry of the pore fluids that

have moved through its pore system. The main

factors that determine mineral precipitation or

solution are:

the chemistry of the sediment, and

the composition, concentration, Eh, and pH of the

pore fluids.


• Although many reactions occur during sandstone

diagenesis, only a few are of major importance in

sandstone cementation and porosity evolution:

those that control the precipitation of silica,

carbonate, and clay minerals.

• Clay minerals are similarly sensitive to pH.

Kaolinite tends to form in acid pore waters, whereas

illite develops in more alkaline conditions. Siderite,

glauconite, and pyrite are all stable under reducing

conditions.


• SEM images displaying morphological features of

grain-replacing, disordered kaolinite that has been

transformed partly into well-ordered kaolinite.


• Optical micrographs of grain-coating, infiltrated

clay layer saround sand grains.


• SEM images of

smectitic clays, which

have a honeycomb

crystal shape and are

common in

sandstones rich in

volcanic rock

fragments.


of Malay Basin

• Porosity in the sandstones at Angsi, Besar,

Duyong, Duyong Barat, and Sotong varies with

facies and diagenesis.

• It ranges from 7 to 27% in thin sandstones (less

than 10 m) and from 14 to 24% in thicker

channelized sandstones. Porosity loss is due to

compaction and quartz overgrowths and

ferroan-calcite cement.


of Malay Basin

• Composition of clays in the massive sandstones in

the north and northeast of Malay basin varies from

predominantly kaolinite to a mixture of kaolinite

(60%), smectite-illite (17), and chlorite (23%).

• Authigenic kaolinite exhibits a high crystallinity, with

crystals ranging up to several tens of micrometers.

They occur as an alteration product of feldspar; as

booklets or aggregates on grain surfaces filling

pores and associated with authigenic quartz.


of Malay Basin

• Kaolinite booklets (KA) on smectite-illite (SL) -

coated framework grain. Tiong-5, 2312.8 m msl.

SEM photograph. Scale = 10 µm.

• Assignment No. 1

• Title : DIAGENESIS AND

RESERVOIR QUALITY EVOLUTION

OF SANDSTONES

• Assignment No. 2

• Title : DIAGENESIS OF

CARBONATE RESERVOIRS

Petroleum Geoscience and Geophysics Chapter 4

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Transcript of Petroleum Geoscience and Geophysics Chapter 4