27
3.1.1 Source Dependence, Generation, Accumulation,
and Alteration of Petroleum
The molecular distribution of petroleum in a reservoir depends on several factors.
This chapter cannot cover all the details of these important factors, and the reader is
referred to one of the great summary books of petroleum/organic geochemistry and
the references therein (e.g., Tissot and Welte 1984; Peters et al. 2005). Briefly, the
origin/source of the organic material (OM) has strong impact on the composition of
expelled oil from source rocks, the OM-containing rock after burial and thermal
alteration. Sources include terrestrial, lacustrine, and marine organic matter and
mixtures thereof, derived from the domains of life which are prokaryotes (eubacteria and archaea) and eukaryotes in different aquatic ecosystems. The extent of preservation of OM depends on the depositional environment during sedimentation.
High sedimentation rates, fine-grained mud, and an anoxic aqueous environment
during OM sedimentation are beneficial for the preservation of OM. These conditions minimize the occurrence of aerobic microorganisms, the fast degraders of
organic material, and their contact time with the OM prior to its settling. In addition,
the redox conditions have an impact on the chemical alteration of OM during sedimentation. After sedimentation, the OM is further altered through chemical, physical, and biological processes during burial up to temperatures of 60 °C (diagenesis)
yielding a complex, high molecular weight polymer named kerogen. When buried
to greater depths (60–150 °C; oil window; catagenesis), heat/thermal maturity
becomes the main OM alteration process transforming the kerogen into petroleum.
At the early oil window stages, large kerogen structures are cracked into smaller
ones leading to the first expelled oil which contains typically larger molecules with
high abundances of heteroatoms in the oil mixture. The average molecular weight
of newly expelled oil from the kerogen decreases with increasing thermal maturity
at later stages in the oil window accompanied with an increase in the proportion of
the hydrocarbon fractions. At the end of the oil window, mainly gas (C1 to C5
hydrocarbons) is expelled from the kerogen. The oil expelled from the kerogen at
the different stages in the oil window migrates through a carrier bed (porous and
permeable layer) to a trap (the reservoir) primarily governed by buoyancy forces.
The accumulated petroleum in the reservoir might be single charged or more likely
a mixture of oil from the same source rock but from various maturity stages of the
kerogen or even a mixture of expelled oil from more than one source rock. In most
cases of petroleum systems, the complexity of the petroleum composition is further
increased by in-reservoir alteration such as biodegradation, gas washing, water
washing, segregation, and mixing of altered petroleum with fresh charged expelled
oil. The most common and in many cases most impactful alteration process affecting the petroleum composition in a reservoir is biodegradation. This slow anaerobic
microbial process at the interface between oil and water occurs over a multimillion
year timescale by degrading preferred small hydrocarbons and leaving a residual
heavy oil (Head et al. 2003; Bennett et al. 2013). In addition, many of the larger
molecules are transformed by partial oxidation processes leading to undesirable
3 Physical and Chemical Properties of Oil and Gas Under Reservoir and Deep-Sea…
3.1.1 Source Dependence, Generation, Accumulation,
and Alteration of Petroleum
The molecular distribution of petroleum in a reservoir depends on several factors.
This chapter cannot cover all the details of these important factors, and the reader is
referred to one of the great summary books of petroleum/organic geochemistry and
the references therein (e.g., Tissot and Welte 1984; Peters et al. 2005). Briefly, the
origin/source of the organic material (OM) has strong impact on the composition of
expelled oil from source rocks, the OM-containing rock after burial and thermal
alteration. Sources include terrestrial, lacustrine, and marine organic matter and
mixtures thereof, derived from the domains of life which are prokaryotes (eubacteria and archaea) and eukaryotes in different aquatic ecosystems. The extent of preservation of OM depends on the depositional environment during sedimentation.
High sedimentation rates, fine-grained mud, and an anoxic aqueous environment
during OM sedimentation are beneficial for the preservation of OM. These conditions minimize the occurrence of aerobic microorganisms, the fast degraders of
organic material, and their contact time with the OM prior to its settling. In addition,
the redox conditions have an impact on the chemical alteration of OM during sedimentation. After sedimentation, the OM is further altered through chemical, physical, and biological processes during burial up to temperatures of 60 °C (diagenesis)
yielding a complex, high molecular weight polymer named kerogen. When buried
to greater depths (60–150 °C; oil window; catagenesis), heat/thermal maturity
becomes the main OM alteration process transforming the kerogen into petroleum.
At the early oil window stages, large kerogen structures are cracked into smaller
ones leading to the first expelled oil which contains typically larger molecules with
high abundances of heteroatoms in the oil mixture. The average molecular weight
of newly expelled oil from the kerogen decreases with increasing thermal maturity
at later stages in the oil window accompanied with an increase in the proportion of
the hydrocarbon fractions. At the end of the oil window, mainly gas (C1 to C5
hydrocarbons) is expelled from the kerogen. The oil expelled from the kerogen at
the different stages in the oil window migrates through a carrier bed (porous and
permeable layer) to a trap (the reservoir) primarily governed by buoyancy forces.
The accumulated petroleum in the reservoir might be single charged or more likely
a mixture of oil from the same source rock but from various maturity stages of the
kerogen or even a mixture of expelled oil from more than one source rock. In most
cases of petroleum systems, the complexity of the petroleum composition is further
increased by in-reservoir alteration such as biodegradation, gas washing, water
washing, segregation, and mixing of altered petroleum with fresh charged expelled
oil. The most common and in many cases most impactful alteration process affecting the petroleum composition in a reservoir is biodegradation. This slow anaerobic
microbial process at the interface between oil and water occurs over a multimillion
year timescale by degrading preferred small hydrocarbons and leaving a residual
heavy oil (Head et al. 2003; Bennett et al. 2013). In addition, many of the larger
molecules are transformed by partial oxidation processes leading to undesirable
3 Physical and Chemical Properties of Oil and Gas Under Reservoir and Deep-Sea…