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ABSTRACT: Exploration Applications of Geochemistry in the Midland Basin, Texas

Wallace G. Dow, Suhas C. Talukdar, Leslie Harmon

Reservoirs, source rocks, and crude oils were studied at Pegasus field on the eastern flank of the Central Basin platform. The field is a faulted anticlinal structure and produces oil and gas from seven geologically complex reservoirs ranging from the Ordovician Ellenburger to the Permian San Andres formations. A better understanding of the petroleum systems present should lead to improved exploration and development drilling opportunities.

Good to excellent-quality, mature oil-prone source rocks occur at numerous horizons between the Permian Spraberry and Ordovician Ellenburger formations. Oil-rock correlations indicate three major petroleum systems: Ordovician sources for oil in Ordovician, Silurian, and Devonian reservoirs; Mississippian to Pennsylvanian sources for Pennsylvanian reservoired oils; and Permian sources for oils in Permian reservoirs. The Ordovician to Devonian system experienced peak oil generation, extensive vertical oil migration, and in-reservoir oil maturation in Triassic time; the Mississippian-Pennsylvanian system reached peak oil generation with limited vertical oil migration in Jurassic time; and the Permian system is just reaching peak oil generation and has

had little or no vertical oil migration. The total amount of oil available to charge the field is several times the oil in place, and all available traps were filled to capacity. This implies substantial accumulations remain undiscovered in subtle stratigraphic and combination traps in the Pegasus field area. The same is probably true throughout the Midland basin.

Integrated studies with geological, geophysical, engineering, and geochemical input can provide valuable exploration information on local as well as regional scales. Pegasus field examples include fault-block isolation, reservoir segregation, and waterflood or gas cycling efficiency. Such studies may also contribute information leading to lateral and vertical field extension wells. Extensive databases in mature basins offer unequaled opportunities to understand how petroleum systems work and how this information can be used to increase reserves.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990