THE UPPER DEVONIAN ALEXANDER SILTSTONE IN DODDRIDGE AND RITCHIE COUNTIES, WEST VIRGINIA: STRATIGRAPHY AND PETROLEUM GEOLOGY

The Upper Devonian Alexander siltstone is an informally named drillers’ unit that occurs throughout western New York and Pennsylvania and extends south through West Virginia. In Doddridge and Ritchie counties in northwestern West Virginia the Alexander occurs within the Angola Shale Member of the West Falls Formation and lies stratigraphically below the more prolific Benson sandstone reservoir. The purpose of this study is to determine the stratigraphic architecture of the Alexander to acquire a better understanding of the petroleum geology of the unit in the study area.

The gamma-ray signatures of the Alexander indicate three distinct siltstone intervals referred to here as the lower, middle, and upper Alexander. Each siltstone interval consists of interbedded siltstone bundles and thin shale layers; these siltstone intervals are separated by thicker intervals of Angola shale. The gamma-ray signatures indicate that each siltstone interval has a sharp base with the abrupt appearance of siltstone bundles and a sharp or gradational upper boundary with the siltstone bundles fining-upward into intervals of Angola shale. The lower, middle, and upper Alexander are interpreted to represent distal depositional lobes in the lower fan environment within the fine-grained Upper Devonian turbidite system. The lobes are thickest in the southern/southeastern portion of the study area and thin to the west/northwest. The laterally extensive, stacked siltstone bundles were deposited from turbidity flows and the thin, interbedded shale layers were deposited from suspension between subsequent turbidity flows.

The log porosity of the Alexander siltstone intervals ranges from zero to eighteen percent within the study area, with the majority of the measurements ranging from zero to fourteen percent. The highest log porosities occur within the lower Alexander in laterally extensive porous zones and coincide with areas where the lower Alexander strata are thickest. Available production data indicate that hydrocarbon production is from the lower Alexander and that production is primarily related to porosity in the study area. The hydrocarbon traps are primarily stratigraphic; however, the gently-plunging Arches Fork Anticline creates a structural high where hydrocarbons likely accumulate.