Oil and Organic Matter in Source Rocks of Petroleum1
Practically all shales and carbonate rocks contain indigenous organic matter disseminated in three forms—1. soluble hydrocarbons, which are similar in composition to the heavier fractions of crude oil found in reservoir rock, 2. soluble asphalt, which is similar to the asphaltic constituents of crude oil, and 3. insoluble organic matter (kerogen), which is pyrobituminous in nature.
Non-reservoir ancient sediments have been found to contain up to 5 times as much oil as that reported from recent unconsolidated sediments off the Gulf and California coasts. A typical ancient petroleum source rock such as the Frontier shale in the Powder River basin of Wyoming, which has yielded millions of barrels of oil to reservoirs in the past, still contains 6 barrels of oil, 20 barrels of asphalt, and about 250 barrels of kerogen per acre-foot.
The distribution of this oil, asphalt, and kerogen within the non-reservoir rocks of a sedimentary basin varies between formations and between different facies of the same formation.