Abstract: The Role of Convective Geothermal Systems in the Generation, Migration, and Entrapment of Oil

Jeffrey B. Hulen

Modern convective geothermal systems (for example, at Yellowstone National Park) and their fossil equivalents, epithermal mineral deposits (like McLaughlin, California) have traditionally been considered poor petroleum prospects. The concentrated heat which drives these systems is commonly viewed as a negative influence: in other words, any oil initially present in or generated by these systems is quickly degraded to a useless carbonaceous residue. This is true in extreme cases, but numerous examples from the Great Basin, the northern California Coast Ranges, and elsewhere suggest that under certain circumstances, geothermal systems can efficiently generate, transport, and entrap significant quantities of producible petroleum. B.R.T. Simoneit has shown that oil can be hyd othermally generated in an "instant" of geologic time. It follows, then, that away from geothermal systems' high-temperature centers (or in wholly moderate-temperature systems), the encompassing shallow thermal anomalies can distill large volumes of oil from otherwise immature hydrocarbon source rocks. Transport of this newly-generated oil is enhanced by the buoyant upwelling of heated aqueous fluid. Porosity for oil transport and storage is created or increased by hydrothermal dissolution of rock-forming or secondary silicates and carbonates. Finally, geothermal "self-sealing" -- the deposition of secondary minerals (especially silica and clay) at the margins of a system -- can provide a very effective hydrocarbon seal. Lee Allison has noted the coincidence of igneous intrusions and oil reservoirs in Nevada. It is suggested that here and elsewhere, both igneous-related and amagmatic geothermal systems in otherwise favorable settings should be viewed as prime petroleum exploration targets.

AAPG Search and Discovery Article #90959©1995 AAPG Rocky Mountain Section Meeting, Reno, Nevada