PATCHEN, DOUGLAS, MICHAEL E. HOHN, AND RON MCDOWELL, West Virginia Geological Survey, Morgantown, WV; and RICHARD SMOSNA, JIM BRITTON, LI ZHENG, and XIANGDONG ZOU, West Virginia University, Morgantown, WV

ABSTRACT: Anatomy of a Lower Mississippian Oil Reservoir, West Virginia, United States

Several lines of evidence indicate that the oil reservoir in Granny Creek field is compartmentalized due to internal heterogeneities: an analysis of initial open flows vs. year completed and well location; mapping of initial open flows and cumulative production; and the nonuniform behavior of injection pressures and rates in waterflood patterns.

The Big Injun sandstone includes an upper, coarse-grained, fluvial channel facies, and a lower, fine-grained, distributary mouth-bar facies. The bar facies is the main reservoir, and can be subdivided into crest, distal, and proximal subfacies. Low original porosity and permeability in the poorly sorted channel facies was reduced further by quartz cementation. In contrast, chlorite coatings restricted quartz cementation and preserved porosity and permeability in the proximal bar subfacies.

Small, low-amplitude folds plunge northeastward on the flank of the main syncline in which the field is located. These minor structural highs seem to match areas of high initial open flows and cumulative production.

High production also occurs where the distal and marine-influenced, proximal mouth-bar subfacies pinch out against at least a few feet of the relatively impermeable channel facies. Lower production is associated with (1) thin areas of proximal mouth-bar subfacies; (2) a change from marine to fluvial dominance of the bar facies, which is accompanied by a reduction in porosity and permeability; and (3) loss of the less permeable channel facies above the porous reservoir sandstone, due to downcutting by regional erosion that produced a post-Big Injun unconformity.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.