Geological Society of America 1998 Toronto Meeting Organic Geochemistry Division Session NATURAL GAS

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An Integrated Approach for Recent Challenges of
Geochemistry From Empirical to First Principle
Yongchun Tang
Power Environmental Energy Research
Center California Institute of Technology
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Research 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
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The Empiric ApproachGenetic Characterization of
Gases and Recognition of Complex Formation
Histories
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Objectives

• 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)

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Fundamentals
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
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Kinetic Isotope Fractionation Is Also Temperature
Dependent
Small Fractionation
Large Fractionation
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Fundamentals
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)

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In General, Isotope Fractionation Increases with
Bond Energies
Tang Y et al GEOCHIMICA ET COSMOCHIMICA ACTA 64
(15) 2673-2687 AUG 2000
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A 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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Measured at Trail Ridge
Isotope Difference d13CPropane - d13CEthane
Indigenous vs Migrated Gases
a
(Ro)
(Ro)
0.5
0.9
1.3
1.7
2.1
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Kerogen Type Specific C1 and C2 Isotope
Correlation
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MRTN 15H291 12/04/01
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Using Kerogen Type Specific C1 and C2 Isotope
Correlation to Determine Where Gas Come From
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New 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
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Conclusion

• 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

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Thermal 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
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A 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
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TSR reaction mechanism

• Oil CaSO4 H2O

No Reaction Observed under Laboratory Condition
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No Catalytic Effect for CaSO4 (also No H2S
Observed)
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A Sulfate Ion is Stable
A sulfate ion has a perfect tetrahedron (Td)
symmetry, rendering an extra stable energy.
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Direct Interactions of SO4-2 with Olefin
Replacing alkane with Olefin does not improve the
sulfate reduction.
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Symmetry-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)

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H2S generation from CaSO4 in the presence of
MgCl2
paraffin CaSO4? MgCl2H2O 300C-575C_at_ 20C/hr
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Other Controlling Factors Qualitative Prediction
Solution PH Ionic Effect H2S Auto-acceleration Min
eral Surface Oil Compositional Effect Role of
Sulfur
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Activity of Aqueous Sulfate Species during
Seawater Heating
Bischoff and Seyfried, 1978
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Prediction 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
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The 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?

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First-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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Conclusions
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
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Financial Support
Department of Energy NETL BP-Amoco Chevrontexaco
ExxonMobil Saudi Aramco Shell International Shell
USA ConocoPhillips Anardarko China National
Petroleum Company