The reservoir sandstones of the Lower Cretaceous Muddy Sandstone, the major hydrocarbon-producing reservoir in the northern Denver basin, are restructured into six informal members (MS-1 to MS-6) based on sedimentary structures, ichnofacies, geophysical logs, bounding discontinuities, and sediment distribution patterns. Bounding discontinuities defining allostratigraphic units are used to assign these six informal members to three major depositional systems that may be used to redefine hydrocarbon reservoirs, leading to new exploration targets in this "mature" basin.
The lowermost depositional system consists of MS-6 and MS-5 informal members representing a deltaic system overlain, in part, by lowstand bar environments deposited during a relative sea level fall. The upper boundary of the lowermost depositional system is marked by a flooding surface generated by a regional transgressive event. Brackish to open-marine shales and sandstones of MS-4 were deposited above this discontinuity and represent the second depositional system.
A lowstand surface of erosion marks the lower boundary of the third depositional system. Fluvial sandstones of MS-3, deposited in incised valleys, are separated from MS-2 estuarine sandstones by a bayline surface. The nearshore-beach MS-1 complex was deposited above a ravinement surface that prograded across the study area when the valleys filled. Eventually, the Muddy Sandstone was buried by the marine shales of the regenerated, Cretaceous seaway.
The informal members of this revised stratigraphy can be traced throughout the basin and represent an enhancement of the traditional "J" sandstone stratigraphy that has become outdated. Once the Muddy Sandstone is organized into the revised stratigraphic framework, porosity and permeability data can be reanalyzed in terms of each unique sandstone package defined by bounding discontinuities. Mapping the regional distribution of MS-1 to MS-6 sandstone packages may lead to renewed exploration potential for the Muddy Sandstone in the northern Denver basin.