Lithologic Characterization of Middle Devonian Marcellus Shale, Appalachian Basin and its Implication for Organic-Rich Mudstone Deposition, Diagenesis and Shale Gas Exploration
Zhou, Jie; Rush, Patrick; Sridhar, Angarai; Miller, Randy
The Middle Devonian Marcellus Shale is fine-grained, pyritic and organic-rich silty mudstone widely distributed in New York State, Pennsylvania, and West Virginia. In the last decade, it has been extensively studied and drilled due to its great natural gas potential. However, most previous geologic work has been conducted on the outcrops from peripheral regions of the basin. This study provides a thorough lithologic characterization of the Marcellus Shale based upon the conventional cores across the basin with an aim to shed light on the deposition and diagenesis of organic-rich mudstones. Marcellus Shale displays remarkable lithologic variation and heterogeneity regionally and stratigraphically, and can be thoroughly characterized through the analysis of its compositional constituents and textural elements. First, the compositional constituents consist of transported framework grains, authigenic minerals, and matrix. Framework grains include detrital siliciclastic silt grains and minor amounts of skeletal fragments and coarser volcanogenic grains. Authigenic minerals, including carbonate concretions, pyrite nodules, barite nodules and mineralized algal cysts, are also common and locally concentrated. These minerals are predominantly early diagenetic products and strong indicators of depositional environment. The matrix comprises clay minerals, silica and carbonate cements with varying amounts of organic matter. Second, the textural elements consist mainly of lamination, bioturbation, clay pellets, deformation and fracturing, and micropores. Marcellus Shale is characterized by massive to faintly laminated texture due to slight differential concentration of clastic silt or organic matter, scarce bioturbation, sparse to common clay pellets, and common nanopores, especially in macerated organics. Fracturing and deformation are increasingly common in the shale towards the east side of the basin (orogenic front). Five major lithofacies, including more than ten sedimentary facies, have been differentiated. Lithological variation in Marcellus is controlled by depositional processes as well as diagenetic overprinting. The Marcellus Shale was deposited in a low energy, mostly anoxic to euxinic depositional environment within a deeper part of the foreland basin with episodic dysoxic disturbance. The organic-rich mudstone facies results from high organic productivity and exceptional preservation, and displays supreme reservoir properties and gas richness.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013