Source Rocks: The Origin of Petroleum

Hydrocarbons and Kerogen Types

Oil-prone kerogens generally consist of more than 65 percent exinite and amorphous particles (Figure 1). Kerogens composed of 65 to 35 percent of oil-prone components generate mostly condensate and wet gas. Kerogens with less than 35 percent oil-prone components yield dry gas if dominated by vitrinite and are barren if dominated by inertinite.

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FIGURE 1 Kerogens composition

There are four types of kerogens, two of which are oil-prone Table 1. Type I kerogens are rich in algal components of exinite and form in lacustrine or marine environments. These kerogens are derived primarily from lipids and tend to be rich in saturated hydrocarbons. Type II kerogens come from mixed marine sources, are mostly amorphous and result from the decomposition of phytoplankton, zooplankton and some higher-order animals. Type II kerogens produce napthenic- and aromatic-rich oils, and yield more gas than do Type I kerogens. In contrast, Type III kerogens are coaly, rich in vitrinites and tend to generate dry gas, rather than oil. Oils derived from Type III kerogens are mainly waxy crudes derived from exinites and amorphous constituents. Type IV kerogens are extremely rare and, because they are derived primarily from inertinites, their hydrocarbon yields are very low.

Kerogen TypeOriginOrganic Constituents
l Algal Algae of marine, lacustrine, boghead coal environmentsMostly algal components of exinite (alginite); some amorphous material derived from algae
II Mixed MarineDecomposition in reducing environments, mostly marineAmorphous particles derived mostly from phytoplankton, zooplankton, and higher organisms; also some macerals from these groups
III CoalyDebris of continental vegetation wood, spores, leaf cuticle wax, resin, plant tissue)Mostly vitrinite; some exinite (not algal) and amorphous decomposition products
IV InertFossil charcoal and other oxidized material of continentalMostly inertinite; some amorphous vegetation decomposition products
Table 1: Kerogen types, their origins, and their organic constituents

Sedimentary rocks contain mixtures of kerogen types. Oil shales contain dominantly Type I kerogens. Coals and near-shore clastic source rocks found in deltas contain primarily Type III kerogens. Many marine source rocks have either Type I or, more commonly, Type II kerogens.

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