Lithofacies of the Devonian Marcellus Shale In the Eastern Appalachian Basin, U.S.A.

Abstract

The Marcellus Shale is an important hydrocarbon source rock and an unconventional reservoir whose origin has engendered much recent debate among geologists. Our analysis of the formation in West Virginia and adjacent States aims to identify and describe the several lithofacies in the eastern Appalachian basin and to interpret their depositional setting. The study is based on field, hand-sample, and microscopic observations, supplemented by X-ray diffraction, thermogravimetric, and gamma-ray-scintillometer data. We identify five major lithofacies: (1) calcitic coarse mudsto ne, quartz-silty and sparsely fossiliferous; (2) skeletal wackestone–packstone limestone , exhibiting a low to moderate faunal diversity; (3) calcitic carbonaceous medium mudstone , containing a high organic-carbon content and low-density, low-diversity fossil community; (4) siliceous carbonaceous fine mudstone , exceedingly radioactive with a high organic-carbon content and usually barren of benthic fossils; and (5) argillaceous coarse mudstone , quartz-silty, micaceous, and kaolinitic. Most layers of Facies 4 mudstone—the quintessential black shale of the Marcellus—formed in a stagnant, anoxic environment beneath a stratified water column and starved of terrigenous sediment. Yet, some layers of this same facies plus the other closely associated mudstones and limestone record markedly different conditions. Rhythmic organic-rich and -poor laminae indicate periods of a stratified water column followed by mixing of the water body. Rip-up clasts, graded bedding, and small-scale cross-bedding resulted from moderate storm action down to the sea floor. Alternating styliolinid-rich and -poor laminae point to a repeated disruption of the thermocline and the recycling of bottom nutrients up to the surface water. The communities of benthic organisms and reduced organic-carbon contents reflect times of oxic–dysoxic bottom water. Widely varying thorium/uranium geochemical ratios suggest that the redox potential in the depositional environment shifted frequently between anoxic (Th/U 4.0). Finally quartz-silty, micaceous, and kaolinitic mudstones demonstrate a considerable influx of terrigenous coarse mud. Our research argues for a relatively shallow marine setting in which the sea floor everywhere in the study area remained above storm-wave base, the thermocline was intermittent being tied to a seasonal stratification–mixing cycle, the redox state of the benthic zone varied considerably, and the supply of detrital silt was at times quite high.