Abstract

Several basins in the central-western U.S.A. have been found to contain large accumulations of oil or gas in their deeper synclinal bottoms. These unusual accumulations are characterized by:

• completely hydrocarbon-saturated reservoirs covering a large area

• overpressure or underpressure

• no basal water contact; updip contact with regional water saturation

• low matrix porosity and permeability, generally enhanced by fracturing

• association with mature source rocks that are either actively generating or have recently ceased generation.

The ubiquitous nature of reservoir saturation associated with mature source rocks indicates an area of supercharge and high migrational impedance.

Conventional "trapped" accumulations are generally found along updip migration paths, suggesting that deep-basin accumulations leak excess hydrocarbon charge. Although updip limits of deep-basin accumulations may be controlled by capillary-entry or fracture-opening pressure, they may also reflect dynamic "back-up" in a region where generation rate exceeds that of migration. The accumulations may not actually be "trapped", and may dissipate with time.

Overpressures characterizing some deep-basin accumulations are produced by kerogen volume and phase changes associated with conversion of kerogen to hydrocarbons. Underpressures found in other deep-basin accumulations may result from water imbibition into previously overpressured reservoirs that are losing hydrocarbons due to continued migration away from the basin bottom. As long as over- or under-pressure due to these mechanisms exist, unorthodox "deepbasin type" accumulations may be present.

Because of their depth, synclinal position, and relatively recent recognition in a few basins, their significance has not been widely recognized. Prediction of such accumulations may lead to refined quantification of petroleum systems and to significant discoveries in unexplored areas.

Examples of petroleum systems containing deep basin-type accumulations will be presented.