ABSTRACT: Hydrodynamic Flow in Lower Cretaceous Muddy Sandstone, Bell Creek Field, Montana

TSENN, MICHAEL C., Texas A&M University, College Station, TX

Lower Cretaceous Muddy sandstones form a stratigraphic trap at Bell Creek field, Powder River basin, Montana. Two elongated barrier-island reservoir sandstones partly overlap in the center of the field. Average porosity and permeability of 30% and 5300 md are shown in wells where the reservoir sandstone is thicker than 20 ft. Lagoonal barrier has average porosity and permeability of 23.4% and 61 md.

A potentiometric surface map was constructed for the field area, based on drill-stem test interpretations, and exhibits fluid flow to be mainly downdip. At the downdip margin of the field, a local reverse gradient exists. This local reversal may be caused in part by a downward pressure gradient from the overlying Mowry Shale. The potentiometric surface for oil shows the oil accumulation is present in the low-potential area. Although this low-potential volume also includes a part of the barrier, oil may be excluded from this area by capillary pressure.

The average potentiometric gradient of 25.3 ft/mi (4.8 m/km) across the field yields a tilt of the oil-water contact of 120 ft/mi (22.7 m/km). The inclination traps a hydrodynamic oil column of 516 ft (157 m), whereas the capillary pressure difference due to lateral permeability change can account for only 38 ft (11.6 m) of oil column. The total calculated oil-column height is 554 ft (169 m), which agrees well with the observed oil-column height of 486 ft (148 m). More than 90% of the total oil column was trapped by regional, downdip hydrodynamic flow. The results of this study suggest that downdip flow can be a significant trapping mechanism in stratigraphic accumulation.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)