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Geologic Controls on Production of Natural Gas from Devonian Shale in Appalachian Basin: a Preliminary Report

Patrick H. Lowry, R. Michael Peterson

A geologic study of a 16-mi2 area surrounding the second Gas Research Institute Comprehensive Study Well (CSW), drilled in the Tariff quadrangle, Calhoun County, West Virginia, has revealed geologic features that appear to influence the production of natural gas. Drilling in the area of the CSW No. 2 well had previously shown the presence of an antiformal structure--the Chestnut Ridge anticline. Along this structure, isolated wells are highly productive (initial potential greater than 1 MMCFD).

Examination of well logs from 34 wells showed irregularities in structural and isopach contour patterns. Structural contour anomalies are interpreted to be possible faults. Construction of isopach maps of the lower Huron interval revealed faultlike relationships among the lower Huron Member of the Ohio Shale, the Java Formation, and Angola Member of the West Falls Formation.

Both the irregularities on the structural surface and the thickening can be explained by a northwest-southeast-trending normal fault and a north-northeast-south-southwest-trending reverse fault that cuts the normal fault. This faulting appears to be localized at the nose of the Chestnut Ridge anticline. The normal fault may not be a distinct fault but rather a grabenlike structure or two or more faults. The sense of movement on this fault zone is generally down-to-the north, with the fault dipping at a high angle to the southwest. The structural characteristics indicating the reverse fault may also be a group of faults rather than a single fault. The displacement of the reverse fault is down-to-the-west and the fault appears to dip steeply to the east. Preliminary mapping indicated th t the reverse fault has offset the trace of the normal fault in a left lateral movement.

Contouring the initial production data from the wells in the study area indicated a decreasing initial potential with distance from the faults. Wells very close or cut by the faults tend to have the highest initial potentials and may also produce fluids. Wells away from the fault have lower initial potentials and do not produce fluids.

AAPG Search and Discovery Article #91031©1988 AAPG Eastern Section, Charleston, West Virginia, 13-16 September 1988.