Abstract: Basin Center Oil and Gas Fields in the Rocky Mountain Area

Fred F. Meissner

A number of oil and gas fields have been found in the deep centers of several Rocky Mountain Laramide-age basins. Historical production from these fields has beef significant, and considerable effort is currently taking place toward enhancing recovery, extending limits and establishing new areas of similar production. Examples of known accumulations include the Altamont oil field, Uinta basin, Utah, the Blanco/Basin gas field complex of NE New Mexico and SE Colorado, and the Wattenburg oil/gas field complex of the Denver basin, E-Central Colorado.

All basin-center fields area associated with mature source rocks. The species of hydrocarbons (oil vs gas) present is controlled by both the stage of maturity achieved by the source rocks and the types of kerogen present in them multiple source rocks and vertical migration paths have resulted in multiple reservoir complexes. Reservoirs of Cretaceous and Lower Tertiary age are dominantly sandstones, but also include carbonates, shales and coals. Sandstones and carbonates generally have low matrix permeabilities that are often enhanced by natural fractures.

Reservoir pressures characterizing basin-center oil/gas accumulations are either anomalously over- or under-pressured. The pressure state may he related to a cycle of hydrocarbon generation and migration. Bottom-water contacts are not found. Pervasive areas of hydrocarbon saturation are present in all rocks capable of capillary entry. These areas grade laterally updip into water-saturated reservoir zones, with a transition corresponding to either a discontinuity in capillary matrix entry pressure or to the limit of open fractures. Basin center accumulations may be the result of dynamic migration bottlenecks rather than the presence of classical static hydrocarbon "traps."

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994