Abstract: Depositional and Diagenetic Controls on the Distribution of Porosity in the Lower Cretaceous Fall River Sandstone

Greg P. Anderson, Wendy J. Harrison

The 22 million barrel Buck Draw Field, is the only significant Fall River oil accumulation in the southwest part of Wyoming's Powder River Basin. Since its discovery in 1983, approximately 100 wells have been drilled to depths exceeding 11,000 feet (3,350 meters) exploring for another Buck Draw-type accumulation. These wells consistently encountered sandstones with low porosity and permeability, resulting in no additional Fall River discoveries.

By integrating cores, E-logs, petrographic, and seismic data within a sequence stratigraphic framework, Buck Draw Field is recognized as a fluvial deposit in an estuarine setting. This field is located at the intersection of a fluvial valley-fill system and the Cretaceous Epeiric Sea. Prolific production is encountered at the seaward: termination of a fluvial channel in a bayhead-delta complex.

This relative position allowed the syndepositional invasion of marine waters into the intergranular pore spaces of the porous fluvial sandstones. The mixing of marine, and iron-rich fluvial waters enabled chlorite clay coats to form over the framework grains of these sandstones during early diagenesis. The clay coats inhibited silica cementation during later diagenesis, thereby preserving porosity. High energy channel deposits landward of the estuary generally lack chlorite clay coats. Pervasive silica cementation has destroyed these potential reservoirs. Additional Buck Draw-type accumulations should be found by drilling the untested, seaward termination of other Fall River fluvial channels.

AAPG Search and Discovery Article #90959©1995 AAPG Rocky Mountain Section Meeting, Reno, Nevada