The Anatomy of Middle Devonian Marcellus Shale, Appalachian Basin: A Scheme of Mudstone Classification and Its Implications for Shale Gas Exploration

A mudstone classification that incorporates mineralogy and organic content for the gas producing Marcellus Shale is a reflection of depositional processes and diagenesis, and more importantly, is a guide for lateral placement and gas production. A thorough investigation on the Marcellus Shale, including petrography, mineralogy, geochemistry, and petrophysical properties, has been conducted across the Appalachian Basin in Pennsylvania and West Virginia. Five main lithofacies have been recognized for the Marcellus Shale: (1) Argillaceous mudstone (M) with less than 10% calcite (including dolomite) and less than 10% kerogen (all by volume herein), which is the most relevant facies in Upper Marcellus and upper part of Lower Marcellus, (2) Calcareous mudstone (CM) with 10% to 50 % calcite and less than 10% kerogen, which appears as either an integral part of Cherry Valley Member or randomly dispersed as a localized hemipelagic skeletal facies, (3) Organic-rich Mudstone (OM) with more than 10% kerogen and less than 10% calcite, is principal facies of Lower Marcellus and lower portion of Upper Marcellus, (4) Calcareous organic-rich mudstone (COM) with more than 10% kerogen and calcite respectively, occurs as part of the transitional facies from the Onondaga limestone to Lower Marcellus, and (5) Fossiliferous limestone (LS) with more than 50% calcite, which is present either in the Cherry Valley or as nodular concretions. The stratigraphic distribution of these lithofacies is a result of the interplay of sea level, climate, and tectonic activity. Lithofacies change also exists laterally, such as mudstone facies becomes more condensed and organic/pyrite-rich westward in the basin, and similarly, the Cherry Valley Member also shifts from mixed carbonate siliciclastic facies to more pure limestone facies westward.

Generally, each type of lithofacies displays its characteristic rock properties. The organic-rich mudstone facies tend to be gas exploration targets in the Marcellus basin when the thermal maturity is in the gas window. High organic concentration in the mudstone not only ensures ample gas resources, but also generates higher porosity and permeability during hydrocarbon formation, resulting in enhanced storage capacity and deliverability. Intra-organic nano-pores are obviously dominant in volume, bigger in size, and more connective compared to the inter-particle/crystalline pores, which dominates the organic-lean mudstone and calcareous mudstone facies.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California