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Core Analysis and Depositional Model for Upper Devonian Benson Sandstone of Clarksburg Gas Field, Harrison County, West Virginia

L. Robert Heim

Since the drilling of the discovery well in 1915, the Benson turbidite sandstone has been an important gas reservoir in north-central West Virginia.

Macroscopic and petrographic analysis of a thrust-faulted 60-ft section of core from this sandstone in the Clarksburg field suggests several subenvironments in a submarine channel system. This interpretation is supported by regional trends and detailed local mapping. Subenvironments include a middle-fan channel, middle-fan interchannel, outer-fan lobe, and basin-plain facies. Oscillatory progradation and lateral migration of this channel system sourced by migrating tectonic delta distributaries are envisioned as the mechanisms responsible for stacking these facies.

Five major lithologies identified in the core are (1) very fine-grained porous sandstone, (2) a very fine-grained calcite-cemented sandstone, (3) massive structureless siltstone, (4) bioturbated siltstone, and (5) dark-gray to black laminated shale. The sandstone of the middle-fan channel facies is the only rock type with reservoir qualities. Although host to a variety of pore-filling cements (ankerite, silica overgrowths, quartz microcrystals, pyrite, limonite, siderite, and calcite), the most important cause of porosity variation in the reservoir sand is the abundance of depositional (dispersed) shale, which ranges from low (1%) to high (7%).

Depositional shale content near the axes of submarine-channel facies is minimized due to the focusing of higher energy conditions here during sedimentation. A variety of log parameters can be mapped to identify this facies. Integration of core data with log analysis and mapping techniques in this well-defined field has improved understanding of the nature of Benson sand deposition and led to an economically successful drilling program, as well as a development strategy that can be used in analogous areas.

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