Abstract: Exploring for Naturally Fractured Reservoirs Within a Basin Centered Gas Trap: Piceance Basin

David Decker, Alan Klawitter

The Corcoran, Cozzette and Rollins Sandstone of the Iles Formation were thought to be both laterally continuous and gas bearing within the Piceance Basin interior. Early exploratory drilling did establish lateral continuity. Unfortunately the sandstones were commonly water saturated, casting doubt on the existence of a gas centered accumulation. However, sandstones of the overlying Williams Fork Formation have recently been determined to be gas bearing throughout the basin center.

Thermal maturity modeling indicates that during Late Cretaceous time, burial depth and ensuing high temperatures and pressures caused the onset of active gas generation from the hydrocarbon-rich coal seams interbedded with sandstones. The large gas pulse moving through the section displaced water from the sands and in areas of extreme depth and thick coal deposits caused overpressuring. Subsequent uplift and erosion resulted in the collapse and decay of this dynamic gas system. The stratigraphic section around the basin perimeter has been breached by surface recharge and is typified by normally pressured hydrostatic conditions. Sandstones of the Williams Fork Formation are gas saturated and underpressured at the margins of the basin center. Within the basin center, laterally equivalen sandstones are gas saturated and overpressured; a relict condition from hydrocarbon generation.

The vast gas resource stored within these low permeability sandstones can only be commercially exploited where the reservoirs are naturally fractured. In order to optimize geologic and geophysical methods for locating subsurface fractures, a research program in the Piceance basin sponsored by the Department of Energy, is currently investigating fracture detection methods using advanced geologic, remote sensing and seismic techniques.

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