Abstract: Plate Tectonics, Organic-Matter Type, and Basin Evaluation for Petroleum Potential

By

Colin Barker

The concepts of plate tectonics have been used to classify sedimentary basins in terms of physical characteristics, such as location on the plate and geothermal gradient. A complete classification must include the amount, type, and distribution of organic matter because this is the material that generates petroleum. Organic matter can be divided into that which grows on the land surface (terrestrial) and that which grows in water (aquatic). This is an economically important because aquatic materials produce normal crudes whereas terrestrially derived materials produce gas and waxy oil. Transport of terrestrial organic matter to areas of deposition depends on surface relief because this controls drainage patterns. The association of transported terrestrial organic matter with clastic sediments makes deltas the most gas-prone depositional environment. Organic materials are not distributed uniformly in deltas because terrestrial organic matter has its highest concentration nearshore and aquatic material is produced in large amounts offshore. This separation and distribution lead to gas fields near paleoshorelines and oil farther out. As the delta progrades, terrestrial organic matter is deposited over the previously deposited aquatic organic matter, producing a vertical sequence from gas to oil in the delta. Sediments on subducting plates should show the same vertical sequence of terrestrial over aquatic material because the slow transport of sediments toward the source of terrestrial organic matter causes its concentration to increase as the subduction zone is approached. The opposite trend (i.e., aquatic over terrestrial) is found on pull-apart margins where the oldest sediments were formed in a continental rift rich in terrestrial organic matter and were overlain by sediments containing increasing amounts of aquatic organic matter. The distribution of organic-matter types exercises primary control over the distribution of oil and gas.

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