Geochemistry

UNCONVENTIONAL RESERVOIRS

Hamid Emami-Meybodi

Universidad de Los Andes

June 2023
Outline 2

Organic Matter

• Type and source

Maturation Process and Thermal Maturity

• Maturation and TOC, porosity, gas content

• Vitrinite Reflectance

Organic Content

• TOC determination

• Rock-Eval Pyrolysis

• Geochemical calculations
Petroleum Geochemistry 3

Why Do We Care?

• Confirming pay type (oil vs. gas) at a given maturity.

• Finding bypassed pay and predicting pretest, pre-completion oil quality.

• Identifying sweet spots for oil/gas production.

• Predicting gas yields (SCF/ton).

o Gas content is directly proportional to TOC, kerogen type, and maturity.

• Predicting calorific value (BTUs) of gas.

o BTU content of gas is inversely proportional to thermal maturity.

• Predicting GOR values.

o GOR is directly proportional to thermal maturity.
Organic Matter 4

Composition and Type

• OM has complex chemistry; composed of arrays of compounds including:
o Proteins (53%C, 7%H, 22%O, 1%S, 17%N): polymers of amino acids
o Lipids (76%C, 12%H, 12%O): substances that are soluble in fat solvents
o Carbohydrates (44%C, 6%H, 50%O): sugars and their polymers (i.e. cellulose)
o Lignin (63%C, 5%H, 32%O, 0.1%S): aromatics with phenolic and carboxyl groups

Petroleum ~ 85%C, 13%H, 0.5%O, 1%S, 0.5%N (composition by weight)

• OM chemistry is a strong function of source (and subsequent diagenesis):

o Bacteria: 50% protein, 10% lipids, 40% nucleic acid
o Phytoplankton: 50% protein, 40% carbohydrate, 5-25% lipids
o Higher plants: 30-50% cellulose, 15-25% lignin, 3% protein

• Chemistry of plants and organisms can vary widely:

o Higher plants: higher percentage of lignin and cellulose
o Aquatic organisms: made up primarily of lipids, proteins
o Lipids: main contributors to liquid hydrocarbons
Organic Matter 5

Composition and Type

• Source of OM (and subsequent diagenesis) determines chemistry, giving rise to

different types of kerogen.

• Kerogen is insoluble in organic solvents, and is a solid, composed of the high

molecular weight fraction of sedimentary OM.

• Bitumen is soluble in organic solvents, and can be a solid or a liquid.

• Soluble fraction contains free HC, whereas insoluble fraction can be converted to HC.

• Kerogen type can be distinguished by the relative abundance of elemental C, O, H;

and the relative abundance of these elements determines what products are
generated upon maturation.
 Organic Matter 6

Hydrocarbon Source
Total rock
Organic matter Minerals

Organic matter

Bitumen (Soluble in organic solvent) Kerogen (Insoluble in organic solvent)

Other
(Pyrobitumen)
Bitumen
Kerogen

Saturated HC Aromatic HC Other (Resins, asphaltene)

Petrography Liptinite/Exinite Vitrinite Inertinite (Maceral

groups)

Rock evaluation Type I Type II Type III Type IV

Hydrocarbon potential Oil prone Gas prone Dead carbon

Light oil, condensate prone

Organic Matter 7

Kerogen Type
The van Krevelen diagram is commonly used
• Type I (H/C>1.2; O/C~0.5) to assess origin and maturity of kerogen.
hydrogen rich derived from zooplankton,
phytoplankton, microorganisms (bacteria) and
the lipid rich components of higher plants.
Type I
• Type II (1.2<H/C<1.4; O/C~0.1)
Increasing
intermediate composition, derived from Type II burial
mixtures of degraded/partly oxidized remnants
of higher plants and marine phytoplankton.
Type III
• Type III (0.7<H/C<1.0; O/C~0.2)
hydrogen poor but oxygen rich, derived from
Type IV
lignin-cellulose from higher plants.

• Type IV (0.4<H/C<0.7; O/C~1.5)

hydrogen poor/oxygen rich, derived from
charcoal, fungal bodies, and decomposed OM.

(Modified from Flores, 2014)

Maturation Process and Thermal Maturity 8

Thermal Maturity of Kerogen

• During burial, kerogen transforms (irreversibly) in response to biologic (early, some time
later) and thermal (later) processes.
Type I & II Type III
1) Diagenesis: involves low temperature reactions
that are biologically-mediated. Microbes break
down the least resistant compounds but more

Increasing burial and temperature

resistant OM survives. Mainly biogenic CH4.

Temperature (oC)
2) Catagenesis: progressive diagenesis in response
to thermal exposure (time and temperature).
Occurs at 40-150°C. Kerogen increases in carbon
content and decreases in volatiles (H and O).
Significant amounts of thermogenic CH4 and CO2.

3) Metagenesis: C-C bonds broken to generate

CH4, remaining heavy hydrocarbons cracked to
(Modified from Flores, 2014) Hydrocarbon generated (l/kg)
CH4 (only dry gas).

Overall reaction example: 𝑅𝑅𝐶𝐶𝐶𝐶2 𝐶𝐶𝐶𝐶2 𝐶𝐶𝐶𝐶3 + 4𝐻𝐻2 𝑂𝑂 → 𝑅𝑅 + 2𝐶𝐶𝐶𝐶2 + 2𝐶𝐶𝐶𝐶4 + 5𝐻𝐻2
Maturation Process and Thermal Maturity 9

Vitrinite Reflectance

• Vitrinite reflectance index is the most commonly used method for determination of
coal rank and shale thermal maturity.

• Vitrinite is a common constituent of organic-rich source rocks.

• Vitrinite-rich rocks tend to be prone to gas generation.

• As the maturity of the organic matter increases,

vitrinite becomes more reflective.

(Modified from Hackley, 2012)

Maturation Process and Thermal Maturity 10

Vitrinite Reflectance

• Vitrinite reflectance (%Ro) is measured as the percentage of light reflected from the
sample, calibrated against a material which has ~ 100% reflectance (e.g., a mirror):

Thermal maturity %Ro

Immature < 0.6
Oil 0.6 ─ 1.1
Wet gas 1.1 ─ 1.4
Dry gas 1.4 ─ 3.2
Gas destruction > 3.2

• Limitations
o is vitrinite present in the rock (for shales)?
o is vitrinite correctly identified (for shales)?
o is the sample polished properly?
o is the photomultiplier correctly calibrated?
o are any extraneous light sources affecting the reading by the photomultiplier?
Maturation Process and Thermal Maturity 11

Thermal Maturity of Coal (Coalification)

(Modified from Flores, 2014)

Maturation Process and Thermal Maturity 12

Coal Properties Analysis

• Proximate analysis
o Determination of the overall composition (i.e., moisture, volatile matter, ash, and
fixed carbon content).

• Ultimate analysis
o Absolute measurement of the elemental composition (i.e., carbon, hydrogen,
sulfur, nitrogen, and oxygen content).
Maturation Process and Thermal Maturity 13

Proximate Analysis for Coal

• Proximate analysis provides relative abundance of moisture, ash volatiles and fixed
carbon content in coal (ASTM procedure).

As received

Surface moisture
Inherent moisture
Air dried

Ambient conditions
Dry

Volatile matter
105 -110oC Solid fuel
(CH4, H2, CO, CO2, N2) Dry, (C, H, O, S, N)
volatile
free
930 - 970oC Ash
Inert atmosphere (Nitrogen)
Combusted at 750oC
Oxygen atmosphere

Fixed Carbon = 1 – (moisture + volatile + ash) / total

Maturation Process and Thermal Maturity 14

Proximate Analysis for Coal

• Common basis from reservoir

engineering perspective are
“as received” and “dry and
ash free” (daf).

(Modified by Folger, 2014, from Ward, 1984)

Maturation Process and Thermal Maturity 15

Proximate Analysis and Coal Rank

%carbon %volatile specific energy % in situ

Rank Stages
(daf) matter (gross in MJ/kg) moisture

wood 50 > 65 - -

peat 60 > 60 14.7 75

brown coal (lignite) 71 52 23 30

sub-bituminous 80 40 33.5 5

high-volatile bituminous 86 31 35.6 3

medium-volatile bituminous 90 22 36 <1

low-volatile bituminous 91 14 36.4 1

semi-anthracite 92 8 36 1

anthracite 95 2 35.2 2
Maturation Process and Thermal Maturity 16

Products of Kerogen Maturation in Shales Type I&II

• Products type and composition dependent on:

Oil
o Meteoric (snow, rain) flow of water prone

o Hydrocarbon migration
Gas
o OM type prone

o Degree of maturation Type IV Type III

Biodegeredation
Biogenic
Kerogen Bitumen Oil (Dry) Gas

Secondary
cracking
Source of gas
Thermogenic 1) Thermogenic (Kerogen
(Wet/Dry) Gas cracking, bitumen
Source rock cracking, oil cracking)
processes 2) Biogenic
Dead Carbon
(Modified from Jarvie et al., 2007)
Maturation Process and Thermal Maturity 17

Total Organic Carbon (TOC) in Shales

• TOC is amount of carbon bound in organic compounds of a rock (total carbon

minus total inorganic carbon).

• TOC is reported in weight percentage (gram organic carbon/gram rock x 100).

• Used to evaluate the quality of source rock and to estimate adsorbed gas.

• Corresponds to the original kerogen content.

• Certain elements (H, N, O, S) present in kerogen but not in TOC.

Quality TOC (wt%)

Poor < 0.5
Fair 0.5 ─ 1
Good 1─2
Very good 2─4
Excellent >4
Maturation Process and Thermal Maturity 18

Question: In an exploration project area, the average annual temperature is 67°F, and
geothermal gradient is 1.9°F/100 ft. The project is in a structurally simple basin that, to date,
has experienced slow, continuous subsidence since basin initiation and only minor
deformation that resulted in simple fold structures. You have drilled a vertical exploration well
for an anticlinal trap in a sandstone reservoir identified from seismic data. In the exploration
well on the crest of the anticline, top of the reservoir sandstone was encountered at 21,113-ft
drilling depth, base was at 21,317-ft depth, and net sand thickness was 180 ft. Source rock
for the reservoir was a 300-ft thick shale that direct underlies the reservoir and has 3.5%
Type I kerogen. When drilling the well, you also encountered three coal beds having net
thickness of 45 ft between the depths of 2,230 and 2,297 ft.

a) Is the reservoir most likely a conventional or tight sand reservoir?

b) Is the sandstone reservoir most likely an oil or gas field?
c) The kerogen concentration of the source rock is or is not sufficient to charge the
reservoir.
d) Have the coals most likely generated or not generated significant volumes of
thermogenic methane?
Maturation Process and Thermal Maturity 19

TOC and Kerogen Type

• Distribution of TOC will vary by kerogen type.

Oil/bitumen-free TOC (wt%) Convertibility

(Daly and Erdman, 1987)
Type I Generative OC Nongenerative OC 80%

Type II Generative OC Nongenerative OC 50%

Type III Generative OC Nongenerative OC 20%

• Knowing kerogen type, restoration of TOC at full conversion (only TOC remaining is
the spent organic carbon that has no remaining potential to generate petroleum):

TOCo = TOCpd / (1 – convertibility)

Maturation Process and Thermal Maturity 20

TOC and Kerogen Type

Question: What is TOCpd for a fully converted sample with TOCo = 10 wt% for

a) Type I:

b) Type II:

c) Type III:
Maturation Process and Thermal Maturity 21

TOC: Generative vs. Nongenerative Organic Carbon

• TOC for a given kerogen type, e.g. type II

Oil/bitumen-free TOC (wt%) (gram OC/gram rock x 100)

Generative Organic Carbon Nongenerative Organic Carbon

o Accounts for petroleum o Does not accounts for

generation. petroleum generation.

o Accounts for any organic o Accounts for adsorptive

porosity development. capacity development.
Maturation Process and Thermal Maturity 22

Maturation and TOC

• OM converts to oil and gas.

• Dead carbon (nongenerative OC) increases slightly.
• Meanwhile oil also cracks to gas.

TOC (Original)

Immature Generative OC Nongenerative OC

Gas Oil Generative OC Nongenerative OC

Gas Oil Generative OC Nongenerative OC

Mature
Maturation Process and Thermal Maturity 23

Maturation and TOC

• When kerogen matures and produces gas and oil, the amount of kerogen will
decrease, whereas TOC remains constant until the generated hydrocarbons
expelled to other reservoirs.

TOC (Original)

Immature Generative OC Nongenerative OC

Gas Oil Generative OC Nongenerative OC

Mature

Mature Gas Oil Gas Oil Generative OC Nongenerative OC

Carbon in expelled TOC (present day)

or lost petroleum
Maturation Process and Thermal Maturity 24

Maturation and Organic Porosity Development

• As OM is converted to petroleum, pores (organoporosity)

are created that are filled with petroleum.

6
Hybrid shale containing organic-rich and
Matrix & Organic Porosity (%)

5 organic-lean intervals

1
Organic-rich shale
0
0 1 2 3 4 5 6 7 8
TOC (wt%)

(Modified from Jarvie, 2012)

Maturation Process and Thermal Maturity 25

Maturation and Gas Adsorption

• Gas sorption capacity increases by OM.
• In comparison with inorganic matrix, organic matter has high adsorptive capacity.
• As OM more matures the gas capacity increase due to higher nonopores
(organoporosity).
• The sorption capacities for kerogen type III > type II > type I.

2.5
Kerogen is 44X more adsorptive than quartz
2.0

Adsorption (scf/ton/psi) Old shale

Sorbed Gas Capacity (cm3/g)
1.5
Quartz 0.029
1.0
Carbonate 0.089

Chlorite 0.128 0.5

Illite 0.160 0
Younger shale

Kerogen 1.293 0 2 4 6 8 10 12 14
TOC (wt%)
(Data from Schettler and Parmely, 1991)
(Modified from Ross and Bustin, 2007)
Maturation Process and Thermal Maturity 26

Maturation and Gas Flow Rate/Natural Fractures

• Example: Barnett Shale, Fort Worth Basin, Texas

(Modified from Jarvie et al., 2007)

Maturation Process and Thermal Maturity 27

Example of Shale Gas Plays

4500

3000
Pressure (psia)

2000

1000

0
0 20 40 60 80 140
Temperature (oC) (Modified from Bustin et al., 2009)
Maturation Process and Thermal Maturity 28

Example of Shale Gas Plays

2.0

1.6

1.2
RO max (%)

0.8

0.4

0
0 4 8 12 16 20 24
TOC (wt%) (Modified from Bustin et al., 2009)
Organic Content 29

TOC Determination
1) Well log interpretations: Passey methodology
o Using sonic and resistivity logging information.
o Sonic log measures of interval travel time in formation.
o Resistivity log measures resistivity of formation.

Logging tool Response in shale gas

Sonic OM increases the apparent transit time of acoustic logs (lower velocity).

OM is non‐conductive. Resistivity increases with the presence of TOC. With

Resistivity
maturation and conversion of kerogen to HC, resistivity increases dramatically.
Organic Content 30

TOC Determination
1) Well log interpretations: Passey methodology

o A graphical porosity–resistivity overlay technique

developed in 1990.

o It allows the organic rich intervals to be identified

and the TOC weight percentage calculated if the
level of maturity (LOM) of the kerogen is known or
can be estimated.

o Baseline is determined when sonic transit time

(∆t, μsec/ft) curve and resistivity (R, ohm-m) curve
directly overlaid or tracked each other. It assumes
that this condition will exist at organic-lean
interval.

(Modified from Passey et al., 1990)

Organic Content 31

TOC Determination
1) Well log interpretations: Passey methodology

%𝑇𝑇𝑇𝑇𝑇𝑇 = Δ𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 × 102.297−0.1688 𝐿𝐿𝐿𝐿𝐿𝐿

𝑅𝑅
Δ𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 = 𝑙𝑙𝑙𝑙𝑙𝑙10 + 0.02 ∆𝑡𝑡 − ∆𝑡𝑡𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏
𝑅𝑅𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏

o The LOM could be calculated from vitrinite reflectance measurement (but it may not be
known in exploration wells!).

(Modified from Cluff and Miller, 2010)

Organic Content 32

TOC Determination

2) Geochemistry core analysis: Rock-Eval Pyrolysis

Free hydrocarbons
(mg HC/g rock)

Generated hydrocarbon
assuming HC contain (mg HC/g rock)
83.33 wt% carbon

0.833 𝑆𝑆1 + 𝑆𝑆2 + 𝑆𝑆4

𝑤𝑤𝑇𝑇𝑇𝑇𝑇𝑇 =
1000
Nongenerative (residual) organic
carbon (mg carbon/g rock)

• S1 and S2 are obtained from pyrolysis analysis of a sample.

• S4 is obtained through oxidation of the sample after pyrolysis.

Organic Content 33

Rock-Eval Pyrolysis

• Consists of a programmed temperature heating (in a pyrolysis oven) in an inert

atmosphere (He) of a small sample (100 mg) to determine free HC and HC- and O-
containing compounds (CO2); Heat samples from 300 to 600°C and then cool down.

• Based on the pyrolysis of the organic matter, four basic parameters are determined:

1) S1 (mg HC/g rock): amount free HC

600°C
volatilized (300°C).

2) S2 (mg HC/g rock): amount of HC

Temperature
generated by thermal cracking of kerogen.

3) S3 (mg CO2/g rock): amount CO2 released Yield Tmax

300°C
from kerogen during low T pyrolysis <390 C.
4) Tmax (oC): temperature at which maximum
S1 S2 S3
rate of HC generation occurs.
Time
Organic Content 34

Rock-Eval Pyrolysis

𝑆𝑆2
• Hydrogen index (mg HC/g C): related to H/C ratio 𝐻𝐻𝐻𝐻 =
𝑤𝑤𝑇𝑇𝑇𝑇𝑇𝑇
and origin/maturity of kerogen.

𝑆𝑆3
• Oxygen index (mg CO2/g C): related to O/C ratio 𝑂𝑂𝑂𝑂 =
𝑤𝑤𝑇𝑇𝑇𝑇𝑇𝑇
and is higher for land plants/inert OM than for
marine/algal OM.

𝑆𝑆1
• Productivity Index: ratio of already generated 𝑃𝑃𝑃𝑃 =
𝑆𝑆1 + 𝑆𝑆2
hydrocarbon to potential hydrocarbon. Also called
transformation ratio.
Organic Content 35

12.0
Rock-Eval Pyrolysis
Type I Type II
10.0
• Kerogen typing may be based on

S2 (mgHC/gRock)
o Visual examination of particulate matter. 8.0
Type III
o Its chemical composition: 6.0

 HI vs. OI plots
4.0
 S2 vs. TOC plots
2.0 Type IV
 HI vs. Tmax
0 0 1.0 2.0 3.0 4.0 5.0 6.0
TOC (w%)
900 900
Type I Type I
800 Oil prone 800
Oil

Condensate
700 700
Type II
HI (mgHC/gTOC)

HI (mgHC/gTOC)
Type II
600 600
Oil/Gas prone
500 500
Gas
400 400 Type II & III
300 300
Type III Gas prone
200 200 Type III
100 Type IV No potential 100
Type IV
0 0
0 50 100 150 200 350 400 450 500 550
OI (mgCO2/gTOC) Tmax (oC)
Organic Content 36

Geochemical Risk Parameters and Assessment

• Minimum and best values for determining whether a low-porosity, low-permeability
shale has high thermal maturity using both visual and chemical maturity parameters:

Ro (%) Tmax (oC) TR (%) HIpd (mg HC/g TOC) Dry gas (%) C20+ (%)
Minimum 1.00 455 80 76 – 100 80 5
Best 1.40 475 95 <50 95 1

Highly converted
shale, potentially high-
Productive well
flow-rate gas system

Non-productive well
Low-level conversion,
low-flow-rate gas system

(Modified from Jarvie et al., 2007)

Organic Content 37

Geochemical Log
• Oil crossover effect: Oil saturation index, OSI = S1 x 100/TOC > 100 mg oil/g TOC.

(Jarvie, 2012)
Organic Content 38

Geochemical log of Laurel #1 well in La Luna shale (1990’s)

Question: Identify oil content and oil crossover zones.

(Jarvie, 2014)
Organic Content 39

Original HI

There are multiple ways to derive the original TOC value in shales from Rock-Eval
data. One way is from HIo. Two ways to estimate HIo:

I. By computation from visual kerogen assessments (Jarvie et al., 2007).

o Average HIo for four kerogen types (averages derived from range of HI in Jones, 1984).

𝐻𝐻𝐻𝐻𝑜𝑜 = %𝐼𝐼 × 7.5 + %𝐼𝐼𝐼𝐼 × 4.5 + %𝐼𝐼𝐼𝐼𝐼𝐼 × 1.25 + %𝐼𝐼𝐼𝐼 × 0.5

II. From analysis of immature samples. (Modified from Jarvie, 2012)

o Distribution of HIo for a marine shale (type II)

database containing immature samples.
Organic Content 40

Geochemical Calculations

For any given HIo, the percentage of generative organic carbon is calculated by:

%GTOCo = 0.085 HIo

• Assume there is 85.0% (varies between 82-88%) carbon in petroleum (hydrocarbon).

Example: if HIo = 400 mg HC/g TOC then

%GTOCo = 0.085 * 400 = 34%

34% of the TOCo could be converted to petroleum (HC)

Organic Content 41

Geochemical Calculations
• Estimating Original TOC from
o HIo: estimated from immature rocks or correlations.
o TOCpd: obtained from Rock-Eval, or Passey method, or etc.
o S1pd (present-day retained free HC): obtained from Rock-Eval.
o S2pd (HC equivalent of present-day GOC): obtained from Rock-Eval.

TOCo
Original
GOCo NGOCo

Expelled Gas Oil

NGOCpd
Present-day
TOCpd

NGOCadd (Formed dead carbon

Expelled OC during kerogen conversion)
GOCpd = 0.085(S2pd)
Retained free OC = 0.085(S1pd)
Organic Content 42

Geochemical Calculations
%𝐺𝐺𝐺𝐺𝐺𝐺𝐺𝐺𝑜𝑜 = 0.085 𝐻𝐻𝐻𝐻𝑜𝑜
1) Original GOC in TOCo

2) Present-day NGOC %𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑝𝑝𝑝𝑝 = %𝑇𝑇𝑇𝑇𝑇𝑇𝑝𝑝𝑝𝑝 − 0.085 𝑆𝑆1𝑝𝑝𝑝𝑝 + 𝑆𝑆𝑆𝑝𝑝𝑝𝑝

3) Added dead carbon to NGOC %𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑎𝑎𝑎𝑎𝑎𝑎 = 𝐻𝐻𝐻𝐻𝑜𝑜 × 0.0006

4) Original NGOC %𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑜𝑜 = %𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑝𝑝𝑝𝑝 − %𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑎𝑎𝑎𝑎𝑎𝑎

%𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑜𝑜
5) Original TOC %𝑇𝑇𝑇𝑇𝑇𝑇𝑜𝑜 = 100 ×
100 − %𝐺𝐺𝑇𝑇𝑂𝑂𝑂𝑂𝑜𝑜

6) Original GOC %𝐺𝐺𝐺𝐺𝐺𝐺𝑜𝑜 = %𝑇𝑇𝑇𝑇𝑇𝑇𝑜𝑜 − %𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑜𝑜

%𝐺𝐺𝑂𝑂𝑂𝑂𝑜𝑜
7) Original hydrocarbon (mg HC/g rock) 𝑆𝑆𝑆𝑜𝑜 =
0.085
Organic Content 43

Geochemical Calculations

8) Organo-porosity 𝜙𝜙𝑂𝑂𝑂𝑂 = 𝜙𝜙𝐺𝐺𝐺𝐺𝐺𝐺 + 𝜙𝜙𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁

𝜌𝜌𝑠𝑠𝑠
𝜙𝜙𝐺𝐺𝐺𝐺𝐺𝐺 = %𝐺𝐺𝐺𝐺𝐺𝐺𝑜𝑜 − %𝐺𝐺𝐺𝐺𝐺𝐺𝑝𝑝𝑝𝑝 − %𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑎𝑎𝑎𝑎𝑎𝑎
𝜌𝜌𝑘𝑘𝑘𝑘
𝜌𝜌𝑠𝑠𝑠
𝜙𝜙𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁 = %𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑎𝑎𝑎𝑎𝑎𝑎
𝜌𝜌𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑

9) The bbl equivalent of generated HC per one acre-ft of the source rock:

𝐻𝐻𝐻𝐻𝑜𝑜 − 𝐻𝐻𝐻𝐻𝑝𝑝 𝜌𝜌𝑠𝑠𝑠

𝑉𝑉𝐻𝐻𝐻𝐻,𝑔𝑔𝑔𝑔𝑔𝑔 (𝑏𝑏𝑏𝑏𝑏𝑏⁄𝑎𝑎𝑎𝑎. 𝑓𝑓𝑓𝑓) = 77.58 %𝑇𝑇𝑇𝑇𝑇𝑇𝑜𝑜
1000 𝜌𝜌𝐻𝐻𝐻𝐻
Organic Content 44

Geochemical Calculations
Question: The following data are available for a shale gas formation:
o HIo = 400 mg HC/g TOC o S1pd = 0.86 mg HC/g rock
o %TOCpd = 9.21 wt% o S2pd = 2.94 mg HC/g rock
o 𝜌𝜌𝑠𝑠𝑠 = 2.38 g/cc o 𝜌𝜌𝑘𝑘𝑘𝑘𝑘𝑘 = 1.18 g/cc
o 𝜌𝜌𝐻𝐻𝐻𝐻 = 0.85 g/cc o 𝜌𝜌𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 = 1.35 g/cc

Calculate original TOC, organic porosity, and the bbl equivalent of generated HC per
one acre-ft of the source rock.