Title: Geological Society of America 1998 Toronto Meeting Organic Geochemistry Division Session NATURAL GAS
1An Integrated Approach for Recent Challenges of
Geochemistry From Empirical to First Principle
Yongchun Tang
Power Environmental Energy Research
Center California Institute of Technology
2Research Areas
Isotope Geochemistry - Natural gas origin,
maturity, biogenic versus thermal - Compound
specific isotope - Mechanism and modeling of
fractionation process Organic-Inorganic
Interaction - Thermal deadline of crude oil
(life of organic species) - Methane and
hydrocarbon oxidation (Sulfate reduction) -
Mineral thermal decomposition (kinetics and
modeling) Hydrocarbon geochemistry -
Biological markers - Oil stabilities -
Biodegration and Bio-CO2 sequestration -
Diamondoid, - Gas hydrate modeling
3The Empiric ApproachGenetic Characterization of
Gases and Recognition of Complex Formation
Histories
4Objectives
- Quantitatively model modeling isotopic
fractionation under geologic condition - Predict origin of gas (thermal verus biogenic
gas) - Predict gas type (from shale, coal)
- Predict gas maturity (deep gas versus shallow
gas)
5Fundamentals
k (CF)1-n (RT/Lh) eDS/R e -DH/RT
Rate Constant Ratio For 13CH4 and 12CH4
Generation
k13/k12 e DDS/R e -DDH/RT
(A13/A12) e -DDH/RT
DDS DS(13C labeled) - DS(12C labeled) and DDH
DH(13C labeled) - DH(12C labeled) _at_ DEzpDDE
6Kinetic Isotope Fractionation Is Also Temperature
Dependent
Small Fractionation
Large Fractionation
7Fundamentals
Some Key Variables of Gas Isotope Modeling
- Needs to determine the change of isotope
fractionation with activation energies
DDE F (Ea(i))
- Needs to determine the precursor isotopic values
which lead to gas generation. For distributed
activation energies, one must determine the
precursor isotopic values of each reaction - d13Coi Y (Ci)
- Needs to determine the entropic contributions of
istopic fractionation (Frequency Factors)
8In General, Isotope Fractionation Increases with
Bond Energies
Tang Y et al GEOCHIMICA ET COSMOCHIMICA ACTA 64
(15) 2673-2687 AUG 2000
9A Linear Correlation Between the Ratios of
Frequency Factor and Fractionation Constant (DDE)
Is Generally Observed
Tang et al GEOCHIMICA ET COSMOCHIMICA ACTA 64
(15) 2673-2687 AUG 2000
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11Measured at Trail Ridge
Isotope Difference d13CPropane - d13CEthane
Indigenous vs Migrated Gases
a
(Ro)
(Ro)
0.5
0.9
1.3
1.7
2.1
12Kerogen Type Specific C1 and C2 Isotope
Correlation
23
MRTN 15H291 12/04/01
13Using Kerogen Type Specific C1 and C2 Isotope
Correlation to Determine Where Gas Come From
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15New Scheme for Natural Gas Generation and Its
Isotope Fractionation
Source Rock
Bitumen
Gas d13C -45 -30
Asphaltene NSO (KANSO)
Gas d13C -60 -20
Kerogen
KANSO
degradable Hydrocarbons
Light Hydrocarbon
Gases from secondary Oil Cracking
Primary cracking of Gas (and oil) mainly from
Asphaltene and NSO
Low Temp Catalysis
S
Early Gas Microbially or inorganically catalyzed
cracking d13C -70 -50
P
E
16Conclusion
- Using advanced modeling technology, one can
obtain many quantitative information for
geochemical process, such as isotope
fractionation. - Mathane from thermalgenic origin can have
isotopic light signal (such as 50 per mil) - Kinetic modeling can provide valuable prediction
of (1) mathane generation temperature, and (2)
source of gas generation
17Thermal Sulfate Reduction
Business Interest Thermal deadline of crude oil
(life of organic species) Methane and hydrocarbon
oxidation (Sulfate reduction) Scientific
Interest Fundamental Reaction Mechanism Controllin
g Factors Kinetics Timing TSR reactions
18A Major Challenge of Oil/Gas Product Due to H2S
Issue (Some well known H2S-rich hydrocarbon
Basins)
Dalan Formation Iran Permian
Nisku Formation W. Alberta Basin Dev. - Carb.
Tarim Basin China Ordo-Silurian
Khuff Formation Abu Dhabi Permo-Triassic
Big Horn Basin Wyoming Permian
Great Australian Bight Quaternary
Smackover Fm. Mississippi Int. Salt Basin Jurassic
19TSR reaction mechanism
No Reaction Observed under Laboratory Condition
20No Catalytic Effect for CaSO4 (also No H2S
Observed)
21A Sulfate Ion is Stable
A sulfate ion has a perfect tetrahedron (Td)
symmetry, rendering an extra stable energy.
22Direct Interactions of SO4-2 with Olefin
Replacing alkane with Olefin does not improve the
sulfate reduction.
23Symmetry-Breaking of the Sulfate Ion
- Formation of Contact Ion Pairs
- On the Solid Surfaces
- High H Concentration (Sulfuric Acid)
- Initiators to Reduce Sulfates Oxidation State
- Bacterial Sulfate Reduction (BSR)
24H2S generation from CaSO4 in the presence of
MgCl2
paraffin CaSO4? MgCl2H2O 300C-575C_at_ 20C/hr
25Other Controlling Factors Qualitative Prediction
Solution PH Ionic Effect H2S Auto-acceleration Min
eral Surface Oil Compositional Effect Role of
Sulfur
26Activity of Aqueous Sulfate Species during
Seawater Heating
Bischoff and Seyfried, 1978
27Prediction of pH Conditions for CaSO4 MgCl2
Systems
Model results for C8 0.56 CaSO4 5.6
MgCl2 NaCl
(1n)Mg SO4 H2O (l) ? (2n)H
(n)Mg(OH)2MgSO4(1-2n)H2O
28The Dilemma of TSR Simulation
- MgSO4(aq) is the dominant form of sulfate in
formation waters at typical reservoir conditions - Under laboratory conditions (gt300 C) bisulfate
is the dominant sulfate species - Increasing Mg concentration leads to lower pH
- Simulating TSR in the laboratory under
geologically realistic conditions may be
impossible - Can we apply first principals calculations
(Quantum Chemistry) to understand TSR reactions?
29First-Principal Transition State Calculation
Transition Barrier Calculation E0 Gas-Phase
(GP) EZPE GP ZPE GT GP ZPE T Gsolv GP
ZPE T Sol
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33Conclusions
A combined experimental-theoretical-geological
approach is a Must-Have for some complicated
geological modeling, in particular kinetic
modeling and extrapolation Potential key TSR
process might involve MgSO4 ion pairs H2S can
initiate the TSR reaction TSR will react faster
toward condensate hydrocarbons
34Financial Support
Department of Energy NETL BP-Amoco Chevrontexaco
ExxonMobil Saudi Aramco Shell International Shell
USA ConocoPhillips Anardarko China National
Petroleum Company