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