Abstract

Buck Draw Field has cumulative production of over 22 million barrels of oil; it is the only significant Lower Cretaceous age, Fall River oil accumulation in the over-pressured envelope of the southern 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, resulting in no additional Fall River discoveries.

Two lowstand surfaces of erosion can be identified overlying shallow marine sandstones in Fall River core. In paleo-low areas, fluvial valley-fill sediments are recognized in core above the upper unconformity surface. There is sufficient core control to recognize a facies change from porous, fluvial deposits in Buck Draw Field, to non-porous estuarine facies immediately north of the field. Core and well control place the prolific Buck Draw channel within a bayhead delta complex in a drowned valley.

This relative position allowed the syndepositional invasion of marine waters into the intergranular pore spaces of the porous fluvial sandstones. The resultant mixing of magnesium-rich 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, and preserved porosity. Fluvial deposits to the south of the estuary generally lack chlorite clay coats, consequently, pervasive silica cementation has destroyed these potential reservoirs. Dry holes immediately north of Buck Draw Field display an apparent fluvial channel character on electric logs, but these wells did not encounter fluvial deposits. They penetrated marine and estuarine valley-fill sediments that overlie the two regional unconformity surfaces. Additional Buck Draw-type accumulations may be found by focusing exploration within the bayhead delta complexes of drowned Fall River valleys.