VARGO, ANA G., RONALD R. McDOWELL, and DAVID L. MATCHEN, West Virginia Geological and Economic Survey, Morgantown, WV; and JAMES Q. BRITTON, West Virginia University, Morgantown, WV

ABSTRACT: Reservoir Heterogeneity within the Big Injun and Squaw Sandstones, Granny Creek Field, West Virginia

The Lower Mississippian Big Injun and Squaw sandstones form the hydrocarbon reservoirs in the Granny Creek field. The field, located in Clay and Roane counties, is approximately 6 sq mi in area. Examination of 15 cores, 22 core analyses, and approximately 400 wire-line logs (gamma ray and bulk density) shows that the Big Injun can be separated into an upper, coarse-grained, and a lower, fine-grained sandstone.

Due to an Early to Middle Mississippian age unconformity, the coarse-grained sandstone of the Big Injun ranges from zero feet in thickness in the northeast portion of field to 30 feet in the south. The coarse-grained sandstone contains as many as three zones of high bulk density, creating probable impediments to fluid migration. Core and thin-section analyses from wells with only one density barrier show this barrier is due to an increase in quartz cement and a decrease in porosity.

The fine-grained sandstone is 15 to 40 ft thick, and thins to the northeast due to the unconformity. Cores and thin-section analysis show that high density zones are associated with an increase in calcite and siderite cement and a decrease in porosity. In the fine-grained sandstone, these high bulk density zones form horizontal, inclined, and irregular pods of impermeable sandstone. At a 400-ft well spacing, pods may be confined to a single well or encompass as many as 25 wells, creating linear and irregular barriers to fluid migration. These pods increase the length of the fluid flow path and may divide the reservoir into discrete compartments.

The Squaw sandstone (6 to 32 ft thick) is fine-grained and occurs approximately 30 ft below the Big Injun. It is not affected by the unconformity

and is variable in thickness due to shale interbeds. The sandstone in the northern part of the field contains abundant shale intertongues resulting in an irregular basal contact. Discontinuites in the Squaw sandstone are due to interbedded shale.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.