M.S. Wilson¹, T.S. Dyman², V.F. Nuccio²

(1) Consultant, Evergreen, CO
(2) U.S. Geological Survey, Denver, CO

ABSTRACT: Is there a Basin-Center Gas Accumulation in the Upper Cretaceous Forbes Formation in the Sacramento Basin?

The U.S. Geological Survey (USGS) in cooperation with the U.S. Department of Energy is re-evaluating the resource potential of basin-center gas accumulations in the U.S. Basin-center gas accumulations are characterized by abnormal reservoir pressures, presence of gas in extensive low-permeability (tight) reservoirs, and lack of conventional gas-water contacts.

In 1995 the USGS estimated 0.9 TCF of undiscovered, technically-recoverable gas in two conventional gas plays in Forbes Formation sandstones in the Sacramento basin. Recent detailed evaluation of geologic and production data has provided new insights into the nature of these Forbes plays and also into the previously unrecognized potential for a basin-center gas accumulation.

The Upper Cretaceous Forbes Formation is a shale-rich depositional system containing discontinuous turbidite sandstone reservoirs. At least twenty-seven gas fields have been discovered in the Forbes, mainly in traditional structural and stratigraphic traps with distinct gas-water contacts.

The gas in most Forbes reservoirs is methane-rich and has migrated upward from deep in the basin. Such a deep basin kitchen may be represented by the Delta depocenter, a deep, fault-bounded, wrench sub-basin in the southwestern part of the Sacramento basin where the Forbes and the underlying Dobbins Shale are buried to more than 18,000 feet. In relatively shallow conventional traps, Forbes reservoirs have gas-water contacts. The lower Forbes and Dobbins are overpressured throughout the central and southern parts of the basin where little apparent potential exists for the possibility of basin-center gas. A deep basin-center gas accumulation, however, might exist in the Delta depocenter, but few wells have been drilled into the Upper Cretaceous interval and a hypothetical accumulation is highly speculative.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado