Page 100 - The Petroleum System From Source to Trap

94 Peters and Cassa

Bitumen in rocks is that fraction of the organic matter pressure, and salinity) . Oxic water (saturated with
that is soluble in organic solvents. Small amounts of oxygen) contains 8.0-2.0 mL Oz/L HzO (Tyson and
bitumen originate from lipid components in once-living Pearson, 1991). Dysoxic water contains 2.G--0.2 mL Oz/L
organisms, but most is generated by cracking (thermal H20, suboxic, 0.2-0.0 mL Oz/L H20, and anoxic water
dissociation) of the kerogen. Lipids are oil-soluble, water­ lacks oxygen. When referring to biofacies, the corre­
insoluble organic compounds, including fats, waxes, sponding terms are aerobic, dysaerobic, quasi-anaerobic, and
pigments, steroids, and terpenoids, that are major anaerobic.
precursors for petroleum (Peters and Moldowan, 1993). Below the 0.5 mL Oz/L H20 threshold, the activity of
Petroleum is a complex mixture of gas, liquid, and multicellular organisms as agents in the oxidative
solid hydrocarbons and nonhydrocarbons occurring destruction of organic matter is severely limited
naturally in the earth (Magoon and Dow, Chapter 1, this (Demaison and Moore, 1980). Anoxic sediments are
volume). The term hydrocarbon is commonly used in the typically thinly laminated (distinct alternating layers <2
petroleum industry to indicate crude oil or natural gas. mm thick) because of the lack of bioturbation by
In the chemical sense, hydrocarbons are compounds burrowing, deposit-feeding organisms. Pederson and
containing only carbon and hydrogen. Nonhydrocarbons Calvert (1990) contend that anoxia is less important than
contain elements in addition to hydrogen and carbon. primary productivity in determining quantities of
For example, NSO compounds contain nitrogen, sulfur, organic matter preserved . However, Peters and
or oxygen, and porphyrins contain metals such as Moldowan ( 1 9 93) stress the effect of anoxia on the
vanadium or nickel. For this volume, petroleum, oil and quality rather than quantity of organic matter preserved,
gas, and hydrocarbons, used without modifiers, have that is, anoxia favors preservation of all organic matter,
similar meanings. including hydrogen-rich, oil-prone organic matter. This
may explain the positive relationship between petroleum
Depositional Environment source rocks and the faunal, sedimentologic, and
geochemical parameters indicating anoxia.
Descriptions of oils or source rocks using the terms
marine or terrigenous are unclear without specifying Alteration of Organic Matter
whether these terms refer to provenance (origin) or
depositional environment. Geochemists commonly use Diagenesis refers to all chemical, biological, and
these terms to refer to organic matter derived from physical changes to organic matter during and after
marine and land plants, respectively, whereas geologists deposition of sediments but prior to reaching burial
usually refer to marine or terrigenous depositional envi­ temperatures greater than about 60°-80°C. The quantity
ronments. For example, when geologists refer to a and quality of organic matter preserved and modified
"marine" sedimentary rock, they are discussing deposi­ during diagenesis of a sediment ultimately determine the
tional environment, not provenance of the mineral petroleum potential of the rock (Horsfield and
grains. Likewise, a geologist might equate a marine Rullkotter, Chapter 10, this volume).
source rock with marine depositional conditions, Catagenesis can be divided into the oil zone, which
although the included organic matter or kerogen might corresponds to the oil window, where liquid oil genera­
be of marine, terrigenous, or mixed origin. For similar tion is accompanied by gas formation, and the more
reasons, the meaning of the terms marine oil, lacustrine oil, mature wet gas zone, where light hydrocarbons are
or terrigenous oil is unclear without further explanation. generated through cracking and their proportion
Misunderstandings can occur because a marine oil might increases rapidly (Tissot and Welte, 1984). Wet gas (<98%
be ( 1 ) generated from land plant organic matter methane) contains methane and significant amounts of
deposited in a marine environment, (2) generated from ethane, propane, and heavier hydrocarbons. The gas
marine organic matter, or (3) produced from a reservoir window corresponds to the interval from the top of the
rock deposited in a marine environment. Rather than just wet gas zone to the base of the dry gas zone.
"marine" oil, it must be specified whether the oil is Metagenesis corresponds to the dry gas zone where dry
derived from a source rock deposited under marine gas is generated (2.0-4.0% Ro). Dry gas consists of 98% or
conditions or from marine organic matter. more of methane (Tissot and Welte, 1984). Dry gas is also
Various factors play a role in the preservation of found as deposits of bacteriogenic (microbial) gas
organic matter, notably the oxygen content of the water generated during diagenesis of organic matter by
column and sediment (oxic versus anoxic), primary methanogenic bacteria under anoxic conditions (Rice and
productivity of new organic matter by plants, water Claypool, 1981).
circulation, and sedimentation rate (Demaison and Thermal maturity refers to the extent of temperature-­
Moore, 1980; Emerson, 1985). time driven reactions that convert sedimentary organic
For ancient sediments, the oxygen content of the matter (source rock) to oil, wet gas, and finally to dry gas
overlying water column is unknown, but it can be inter­ and pyrobitumen. Thermally immature source rocks
preted from the presence or absence of laminated or have been affected by diagenesis without a pronounced
bioturbated sediments and organic matter content in the effect of temperature (<0.6% Ro) and are where microbial
sediment (Demaison and Moore, 1980). The oxygen gas is produced. Thermally mature organic matter is (or
content of water is determined by availability and solu­ was) in the oil window and has been affected by thermal
bility of oxygen (which depends upon the temperature, processes covering the temperature range that generates