MATCHEN, DAVID L., and ANA G. VARGO, West Virginia Geological and Economic Survey, Morgantown, WV

ABSTRACT: Stratigraphic and Diagenetic controls on Reservoir Heterogeneity, Rock Creek Oil Field, West Virginia

Rock Creek field is located in southwestern Roane County and is approximately 24 sq miles in area. Primary production is frog the Big Injun sandstone (Lower Mississippian). Analysis of 10 cores (9 cores from one CO2 flood area) shows the Big Injun sandstone to be a fine-grained sand. The cores show two features that may cause heterogeneity with the reservoir: (1) thin shales (4 inches to 2.5 feet thick), and (2) zones of increased siderite and/or calcite cement (0.5 inches to 4 feet thick). Carbonate cements increase rock density and reduce porosity. These units can be identified using gamma-ray and density logs. Shales are recognized by a kick in both the gamma-ray and bulk density curves; carbonate cement only affects the bulk density curves.

Log analysis reveals two distinct log signatures within the field. Each signature occurs in a particular geographic domain within the field. The first signature (Type 1, southeast portion of the field) could be considered typical for the Big Injun. This signature displays an average of 30 feet of sand, with porosity values ranging from 15% to 20%. The few porosity breaksare due to the siderite cement and shale units. Correlation of these features to the rest of the field show the shale and carbonate zones to be discontinuous. The second signature (Type 2, located in the northwest portion of the field) shows a variable Big Injun interval. This signature displays arc average of 50 feet of sand, but only 20-30 feet of good porosity (15%- 20%). The rest of the unit shows 8%-12% porosity, r less.

Heterogeneity within the Rock Creek field can be found at several stratigraphic scales. At field scale the reservoir can be divided into two domains, based on log signature and core analysis. Within each domain, the presence of interbedded shales and diagenetic zones decrease the amount of available reservoir and increase fluid flow paths.

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