Shales and Imposters: Understanding shales, organics,

and self-resourcing rocks

Manika Prasad OCLASSH and RockAbuse Labs

(with material stolen from various sources: e.g., M. Batzle, U. Kuila, S. Zargari, J. Havens, P. Dechongkit, O. Akrad, K.

Milliken, Q. Passey)

Current and Potential Shale Plays in USA

Curtis & Schwochow, 2008

3

Indian Scenario

• Initial (basic) Assessment

– Sedimentary basins

– Sediment thickness

– Prospective beds

– Stratigraphy

– First well: mud logs

– Basic log analysis

– Shale analysis

4

OUTLINE

• What is a shale?

• What is Organic Matter or Kerogen?

• Organic Maturity / Porosity

• Brittleness

• Effective Stress

5

Justification to Study Shales

• Definitions and Rock Models – Shale / Organic Matter or Kerogen / Porosity

• Building Rock Physics Models – Texture and Heterogeneity

– Elastic Properties: Modulus; Anisotropy

• Spatial detection of sweet spots – Hydrocarbon generation: Storage versus Transport

– Brittle: Static to Dynamic Conversion!

• Change in petrophysical properties with maturation – Primary oil or gas production pathways

6

Shale Deposits!

7

Green River Shale Gillsonite

Monterey Shale

Clay Mineral?

Clay Size Fraction?

8

Carbonates: mud and ooze? E.g., Niobrara fm

Siliclastic: Diatomaceous mud: E.g., Monterey fm

Organic-rich: have kerogen; often no or very little clays

Is “SHALE” a misnomer?

Must Shales have Clay?

9

Evolution of Shale Definitions

• Grain size description (before XRD)

• Mineralogic description (after XRD)

• Geologic Description

• Engineering (geomechanical) description

• Petroleum definition

Inexact descriptive

Grain size based

Mineralogy based

Ignorance based

Geological description

Classification of Shales?

Fine-grained

Sediment

SRR

Marl

Silty shale; Shale silt

Shale

Shale

Family

Marl

Siltstone

Claystone

Shale

Mudstone

YES

NO

Organic-rich “Shale” family Self Resourcing Reservoir Rocks (SRR)

Shale family

YES

NO

Fissility?

Organic-rich?

11

Some of the terms applied to fine-grained sedimentary rocks:

There is no consensus on shale classification.

“Most people begin with some variant of a textural classification and then resort to a morass of terms variously directed to composition, grain source, depositional process, and diagenesis.”

Slide from Kitty Milliken, BEG

Classification of Shales?

12

Porosity Reduction in Inorganic Shales

Bjørlykke (1998) 13

1. Shallow (70 – 100°C): Mostly mechanical compaction

2. Deeper (> 100°C): Mostly chemical compacction

Porosity Reduction in Organic Shales?

• Does porosity reduce in organic matter?

• Does porosity develop in organic matter?

• What is average pore size in organic matter?

– Where does nothing end and something start?

14

Organic Matter

/ Kerogen

15

Keros = wax; Kerogene = generating wax

Kerogen

• Keros = wax; Introduction to geology debated

• Kerogene = generating wax

– Used to describe kerosene produced from cannel coals

• OR

– Used to describe the OM of a Scottish oil shale that produced a waxy oil upon distillation

What is Kerogen

• Defined by solubility:

Organic Matter insoluble in organic solvents

• Defined by petroleum:

Organic Matter capable of producing petroleum

• Extraction method alters kerogen properties: physical, compositional, and structural!

Can be mixed with other insoluble OM: tar,

asphaltene, bitumen!

Total Organic Budget

Shale Oil 5.1011 t

In Oil Shales 1012 t

Geological burial (natural generation) Pyrolysis (artificial generation)

Total Kerogen 1015 t

In coal 1013 t

In Org. rich Sh.

(From Vandenbroucke and Largeau, 2007; Durand, 1980)

GAS 2.1011 t

OIL 2.1011 t

ASPHALTS 2.1011 t

1014 t

18

Kerogen Type

Oxidization State and Maceral group

Kerogen Composition HI (mg HC/g TOC)

S2/S3 Main Product Expelled at Peak Maturity

I Anoxic, Hydrogen rich

Amorphous / alginite >600 >15 Oil

II Anoxic, Hydrogen rich

Exinite: (Spores, planktonic debris)

300-600 10-15 Oil

II/III Hydrogen-poor exinite/vitrinite (Land plants)

200-300 5-10 mixed oil and gas

III

Hydrogen-poor vitrinite (Land plants) 50-200 1-5 Gas

IV Hydrogen-poor inertinite: (Fossil charcoal, fungal remains)

<50 < 1 None

Kerogen Types

From Peters and Cassa, 1994

19

Specific Gravity of Kerogen Types

Kerogen type

Sample maturity HI

(mg/gC) Tmax (°F)

Specific gravity

II End of diagenesis 532 414 0.814

II Onset of oil

window 439 438 0.995

II Top of oil window 242 443 1.115

II Wet gas window 22 479 1.518

III Onset of oil

window 250 435 1.295

(From Vandenbroucke and Largeau, 2007)

20

Kerogen Maturation

Durand, 1985

Okiongbo et. al. (2005)

21

22

From Quinn Passey

22

Measuring Porosity and Grain

Density

23

Bakken FiB-SEM Images

Movie courtesy Brian Gorman

24

6 x 6 µm

Kerogen

Pore Size Associations

25

Curtis et al., 2010

Organic Porosity Clay Porosity

4-8 nm ~120 nm

~20 x 280 nm

OM

10 Å = 1 nm = 0.001 mm

Where are the pores? What are the pore sizes?

FESEM ion milled samples

Clay Microstructure

26

• The intra- “aggregate” clay mesopore volume is preserved and shielded from compaction

Higher Clay

Content

Kuila et al. 2012

Conceptual Storage Space

We can “see” this!

28 Passey et al. (2010)

The 50% concept!

Rockphysics of ORR

29

Acoustic Scans- Bakken Shale

62 µm

294 319

122 175

Prasad et al., 2005

Reduce scatter in porosity – elastic modulus relation by accounting for

pore-filling kerogen

y = 30.436e-4.94x

R² = 0.881

0

10

20

30

40

0.0 0.1 0.2 0.3 0.4 0.5

C66 (

MP

a)

Porosity + Kerogen Content

BAKKEN BAZHENOV NIOBRARA

KC_Φ = Φ + 0.4 KC

0

0.5

1

1.5

2

2.5

3

0.0 0.1 0.2 0.3 0.4 0.5

RH

OB

(g

/cc)

Porosity-modified Kerogen Content

BAKKEN BAZHENOV

NIOBRARA WOODFORD

All Others

Density - Porosity plus modified kerogen content correlate better:

accounts for kerogen density.

Prasad et al., 2010

Modulus–Porosity–Kerogen Content

31

Effect of Organics and …Location?

32

Lucier et al., 2011 Increasing TOC

Mudrock Vp-Vs lines

Increasing TOC

Effect of Organics and …Location?

33

Lucier et al., 2011 Increasing Sw

Mudrock Vp-Vs lines

Decreasing Sw

Rock Physics Mixing Models

Ahmadov, 2011 (Data from Vernik and Landis,1996)

Bulk modulus (K) and shear modulus (μ) versus kerogen volume

along with Voigt-Reuss-Hill and Hashin-Shtrikman bounds.

34

Conceptual Rock Model (Ahmadof, 2011)

- Pyrite forming

-Porosity reduction

-Preferred orientation of clays

-Bedding-parallel elongated organic

matter lenses

- Load bearing Kerogen (Prasad 2000)

- Scattered distribution of Kerogen

without reference to original

depositional setting

- kerogen not load bearing and does

not contribute in stiffening the rock

(Ahmadov, 2011)

35

Interrelated Effects

36

-800

-400

0

400

800

4 5 6 7

-500

-250

0

250

500

5 6 7 8 9 10

Am

plit

ud

e (

mV

)

0.39

0.39

0.17

0.17 0.55 0.55

P-waves S-waves

0.0001 0.001 0.01 0.1 1

T2 Relaxation Time [s]

0.001 0.01 0.1 1

Water Layer Size, d [µm]

Partially dried

Dry Kaolinite

Capillary bound water

Slurry

37

Time (µs) Time (µs)

Prasad and Bryar, 2003

Examples of Storage Space

38 Passey et al. (2010)

Barnett shale

Lab Conditions STP

Pore Size 3 nm

Kk/Kp = 102.3

(Pore pressure)

Flow Constants with pore-sizes

39 Kuila , Prasad, and Kazemi, 2012

Flow Constants with pore-sizes

Pore Size 3 nm

Kk/Kp = 0.30

(Pore pressure)

Kuila , Prasad, and Kazemi, 2012 40

Measuring Grain Modulus

41

Theoretical …first principles based

• Create a theoretical model based on first principles

• Collect input parameters – (if necessary, use empirical correlations to derive inputs)

• Compare model with data and recompute

42

Molecular simulation results

Pal-Bathija , 2009

43

Co-locate FiB-SEM & Nanoindent

8 x 8 µm

silt

In collaboration

with Brian

44

Average of Young’s Modulus vs. Soft Material Content – Natural Samples

15

20

25

30

35

40

45

40 50 60 70 80 90 100

Er

(GP

a)

Soft Material Content (2×TOC+Clay Content) (vol%)

2×TOC+Clay Content

45

45

Mechanical properties

46

Modulus of Softer Portion

Kerogen+Clays+ Minerals

Kerogen+Clays+ Bitumen+Minerals

Zargari et. al, 2011

HI

IMPLICATIONS / APPLICATIONS

47

Brittle versus Ductile

Harris et al., 2011

Poisson’s Ratio

You

ng

’s M

od

ulu

s (p

si)

48

Sturm and Gomez, 2009

NESSON STATE 42X-36

49

Deadwood Canyon Ranch

43-28H

Facies C

(10,102 – 10,119 ft)

Facies A

(10,142 – 10,146 ft)

Facies B

(10,119 – 10,142 ft)

Facies D

(10,084 – 10,102 ft)

Black shale facies

(10,146 – 10,192 ft)

Black shale facies

(10,077 – 10,077 ft) Facies E

(10,077 –

10,084 ft)

L. Bakken

M. Bakken

U. Bakken

From

A. Simenson

Three Forks

Lodgepole

Stress and Anisotropy

51

ANISOTROPIC CASE Anisotropic conditions Horizontal stress different:

(Young’s modulus: Eh horizontal, Ev vertical; Poisson’s ratio: σh horizontal, σv vertical). The variation between Ev and Eh gives a different horizontal stress profile. Increased σh in the upper and lower Bakken imply that they will be more effective in hydrofracture containment.

ISOTROPIC CASE Presuming no tectonic stresses and Biot coefficient ≈ 1 and applying isotropic in situ stress equation Almost constant horizontal stress throughout upper, middle, and lower Bakken Fractures are not contained in middle Bakken.

500

1000

1500

2000

2500

3000

3500

0 25 50 75 100

ELECTRICAL RESISTIVITY ohm-meters

BA

KK

EN

SU

BS

UR

FA

CE

TE

PM

ER

AT

UR

E,

F

SHALLOW LOW RESISTIVITY TREND

SUGGESTED TEMPERATURE OF

HYDROCARBON GENERATION AT 1650F

Suggested temperature of Hydrocarbon Generation 165oF or 74oC

Modified from Meissner, 1978;

Hester and Schmoker, 1985

Resistivity of Bakken Shales

52

Why this change? • Wettability • Saturation – where? • Clay – organic

interaction?! Dehydration of clays

In closing: we now have more questions than answers!

59

Curtis et al, 2010

• Porosity: How does total porosity change due to kerogen volume change? How is porosity being generated in the kerogen? Chemically:

dissolution or mechanically: forces of expulsion? Does porosity always exist but is sometimes filled with bitumen? Do pores in kerogen collapse in stress bearing conditions? Is there a porosity generation window?

• Fractures: Conduits for primary migration. Extensional fractures due to overpressure. How extensive are they? How much do they extend? How much are they contributing into production? How are they affecting SRV? Are they as important as tectonically induced fractures?

From S. Zargari