Organic Origin of Petroleum.

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Figure 3

In the case of bitumen and petroleum, chemical balance also is maintained as each matures in response to intensifying temperature and pressure burial conditions (Tissot and Welte 1984, IV-2, -4). For example, some bitumen or petroleum molecules take in hydrogen and evolve toward smaller, more open-chain saturated (paraffinic) molecules while other molecules yield hydrogen and evolve toward larger, less saturated (aromatic and asphaltic) compounds. This leads us to another fundamental requisite for petroleum. A favorable postdepositional maturation history is essential for carbonaceous organic source material to generate hydrocarbons.

The general chemical balance that is maintained overall within the petroleum generative system during maturation finds direct use in exploration geochemistry. By determining what is the present chemical relationship between kerogen and bitumen, or within bitumen and petroleum, we can, for example, estimate state of productivity in source rocks and level of maturation in petroleum.

Physical alterations Modifications in physical properties of organic materials parallel the chemical transformations that occur along the organic matter-petroleum pathway. Kerogen color observed in transmitted light, for example, darkens as kerogen matures and yields bitumen (e.g., Staplin 1969; Burgess 1974). As maturity increases, kerogen also develops a more structured internal lattice in many components which increases its reflectance in incident light (Teichmuller 1958; Ammosov and Tan 1961). Bitumen and petroleum, in turn, decrease in optical activity as maturation increases (Louis 1968). This decreased optical rotation observed in polarized light is interpreted to indicate organic molecules breaking down and losing inherited biotic source characteristics.

Artificial generation Petroleum like products can be generated from organic matter. When subsurface conditions are simulated in the laboratory, as when organic matter is subjected to elevated temperature/pressure conditions, the products generated are grossly similar to bitumen and petroleum. For example, the addition of heat in a closed system, or pyrolysis, can produce both gaseous and liquid hydrocarbons from modern and ancient organic materials (e.g., Barker 1974; Bailey 1980). Once again, the relationship between organic matter and petroleum is demonstrated. Such artificial generation of bitumen is another evaluative tool of exploration geochemistry.

To summarize to this point, scientists have documented numerous relationships and associations between organic matter and petroleum that confirm the organic origin of petroleum. These relationships and associations form the bases for exploration geochemistry's concepts, principles, and interpretations.

Properties of Petroleum

Petroleum is commonly categorized according to two sets of characteristics:

· chemical properties

· physical properties

For example, many disciplines that deal with petroleum primarily as an organic substance commonly subdivide it into related chemical compound types (e.g., hydrocarbon versus nonhydrocarbon compounds; paraffin content; presence of sulfur-bearing compounds). Others that deal with oil and gas primarily as a market product, are more apt to categorize petroleum on physical bases (e.g., gas versus liquid; distillation ranges; API gravity). Exploration geochemistry as a discipline deals with both perspectives. Those who use geochemical programs and data have to integrate both chemical and physical properties and must recognize how the two can but need not be directly related.

Gross Character of Petroleum

Petroleum is composed of two principal fractions ( Figure 1 ):

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Figure 1

· hydrocarbon compounds

· nonhydrocarbon compounds

The first of these consists of molecules that are combinations of hydrogen and carbon atoms only, hence their name. The molecules of the other fraction also contain high percentages of hydrogen and carbon atoms; nonhydrocarbons, however, also have additional atoms bound into the molecules. The most common of the additional atoms, or heteroatoms as they are designated, are nitrogen, sulfur, and oxygen. Because these three elements are the most common heteroatoms, the names NSO compounds, or just NSOs, are applied in some analytical labs and exploration offices to all or a portion of the nonhydrocarbon fraction. In this discussion, we use the more generic term "non-hydrocarbons" for that fraction of petroleum containing heterocompounds.

The nonhydrocarbon fraction can have molecules which contain heteroatoms besides nitrogen, sulfur, and oxygen. Typically these heteroatoms consist of metals such as nickel, vanadium, magnesium, and copper. In some heteromolecules the metals represent trace elements originally taken in by biotic processes. In others, metals have been incorporated in organic compounds as a result of postdepositional alteration and migration processes.

Volumetrically, hydrocarbons generally make up more than 70% of liquid petroleum, that is crude oil, as it is withdrawn from subsurface reservoirs (Tissot and Welte, 1984); nonhydrocarbon compounds make up the remainder ( Figure 2 ).

Principal Subdivisions of Petroleum

Hydrocarbon and nonhydrocarbon fractions can be subdivided into four chemically distinct groups based on molecule types present ( Figure 1 ,

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Figure 1

General classification system used for petroleum); the four major subdivisions are (1) saturated hydrocarbon molecules called alkanes, (2) unsaturated hydrocarbon molecules called arenes, (3) small nonhydrocarbon molecules called resins, and (4) large nonhydrocarbon molecules called asphaltenes.

Because arenes (ar for aromatics - fragrant - and enes for unsaturated) in petroleum consist entirely of aromatic-series compounds, the series term aromatics is generally substituted for the group term arenes in geochemical data dealing with petroleum. This discussion follows that convention and for the four petroleum groups uses the terms

· alkanes

·