John P. Graham¹

(1) Lander University, Greenwood, SC

ABSTRACT: Incised Valley Reservoirs And Enhanced Exploration Potential In The Lower Cretaceous Muddy Sandstone, Northern Denver Basin

The Lower Cretaceous Muddy Sandstone is the major hydrocarbon-producing reservoir in the northern Denver basin. Based on sedimentary structures, ichnofacies, geophysical logs, and sediment distribution patterns, the Muddy Sandstone reservoirs can be partitioned into six informal members (MS-1 to MS-6). These six informal members can be assigned to three major depositional systems defined by bounding discontinuities.

Fluvial sandstones (MS-3) deposited in incised valleys are bounded by a lowstand surface of erosion that marks the initiation of the third, or upper, depositional system. Sandstones above this lowstand surface of erosion have consistently higher porosity and permeability values than do those below the unconformity. The fluvial sandstones are separated from estuarine sandstones (MS-2) by a bayline surface. Once the valleys filled, nearshore-beach sandstones (MS-1) were deposited above a ravinement surface that prograded across the basin. Facies, unconformities, burial depth, and subsequent ground water flow have controlled the porosity and permeability of these reservoir sandstones. Mapping these regionally extensive informal members using this revised stratigraphic framework rejuvenates the exploration potential of Cretaceous reservoirs in a basin previously thought to be mature.

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