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Petroleum System--A Classification Scheme for Research, Resource Assessment, and Exploration

Leslie B. Magoon

A petroleum system includes all those elements that are essential for an oil and gas deposit to exist in nature. The basic elements include a petroleum source rock, migration path, reservoir rock, seal, and trap. All these elements must be placed in time and space such that a petroleum deposit can occur.

A petroleum system can be identified in terms of oil-source correlation at three levels of certainty: proved(.), hypothetical(!), or speculative(?). A proved identification includes those petroleum systems where successful oil-source rock correlations are obtained. A hypothetical petroleum system is one where oil or gas deposits can be shown to be genetically related and where source rocks can be geochemically identified, but no geochemical correlation presently exists. In a speculative petroleum system only the geological evidence supports a relation between the petroleum deposit and its source rock. I propose that a petroleum system name should include the source rock and major reservoir rock and the level of certainty, e.g., Beluga-Sterling(!).

After a petroleum system is identified and named and the level of certainty is established, it must be classified as either a purebred or hybrid system. A purebred petroleum system includes those systems whose elements are all deposited in a single sedimentary basin during a single tectonic cycle; a hybrid petroleum system requires at least two superimposed sedimentary basins and tectonic cycles. For example, the Beluga-Sterling(!) petroleum system, a series of biogenic gas deposits, is a pure-bred system in the Cook Inlet basin, Alaska, where the source and reservoir rock were deposited in a single sedimentary cycle. The Tuxedni-Hemlock(!) petroleum system is a hybrid system also in the Cook Inlet basin where the source rock was deposited in a forearc marine basin and separated by a egional unconformity from the reservoir rock and overburden that were deposited in a nonmarine basin.

By classifying petroleum systems using these and other geologic criteria, various elements of each system can be more clearly identified, compared, and contrasted to determine their efficiency relative to the generation, migration, and trapping of hydrocarbons. Characterizing the efficiency of each petroleum system and of each element within it will help focus research, resource assessment, and exploration programs.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.