Petroleum Geochemistry & Source Rock

​Organic theory

•From the transformation of biomatter.

•The biogenic origin of petroleum is widely accepted on the basis of geochemical studies.

•The deposition of organic-rich sediment is favored by a high rate of production of organic matter and high preservation potential.

•The preservation of organic matter is favored by anaerobic bottom conditions and a rapid sedimentation rate.

​The principal biological components of living organisms are:

•PROTEINS

–More abundant in animals: O, C, N, H

•CARBOHYDRATES

–Occur in both. Cn(H2O)n

–sugars, cellulose, starch

•LIPIDS (Fats)

–Occur in both: C, H, O

–Fats, oils, waxes (e.g. leaf cuticles)

•LIGNIN

–Occurs in plants: complex aromatic ring structures, large molecules

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​THow do living organisms become Organic Matter?

• Organism dies: decay begins

• Complex molecules break down rapidly.

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How then does OM accumulate in the vast quantities needed for identified oil volumes?

• Accumulation of large quantities of living organisms requires Oxygen.

• To facilitate oil generation, the OM produced by the death of these organisms must be preserved.

• Preservation of large quantities of dead organisms requires Anoxia (anoxic environment).

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​2 stages:

•Conversion of OM to kerogen

•Conversion of Kerogen to oil and gas

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Transformation of OM into Kerogen

•As the organic matter is buried, it transforms from basic biological components into new polymeric organic compounds that eventually become kerogen.

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​What is kerogen?

•Insoluble in organic solvents

•Complex mixture of high molecular weight organic materials

•Kerogen is composed of varying proportions of C, H, and O. General composition may be described as: (C₁₂H₁₂ON_0.16)x

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​Type I: algae kerogen LAKE

• “best” oil source

• Lipid-rich

Type II: herbaceous (liptinic) kerogen MARINE

• Good oil source

• Includes zooplankton (sapropelic)

Type III: woody (coaly/humic) kerogen FOREST

• Good gas source

• Rich in humic component

​•The amount of oil generated increases linearly with time

•The amount generated increases EXPONENTIALLY with temperature.

•Thus, TIME IS AN IMPORTANT FACTOR

•The younger the source, the higher the temperature required to generate oil

This Introduced the Concept of the OIL WINDOW

•A range of temperatures through which oil generation can occur

•Below 60°C: sediments are immature

•Above the critical temperature (Approx 120°C): sediments are post-mature

•Assumes that no migration of the oil has occurred (I.e. into lower temperature regimes)