A COMPARISON OF SEQUENCE STRATIGRAPHY AND MINERALOGICAL VARIATIONS ASSOCIATED WITH TOTAL ORGANIC CARBON IN THE MARCELLUS FORMATION: WASHINGTON COUNTY, PENNSYLVANIA
Luker, Austin 1985-
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A recent surge of interest has arisen concerning the Devonian organic-rich black shales native to the Appalachian Basin of the east coast of the United States and their potential as gas producers The Marcellus Shale Formation is part of the middle Devonian Hamilton Group (380 Ma) and is one of ten extensive black shale units in the Appalachian Basin deposited as part of a cyclic repetitive progression of three distinct rock types consisting of organic-rich shales, coarser clastics (silty shales, siltstones, and sandstones), and carbonates (Roen, 1984; Lash and Engelder, 2009). Gas production from these shales is widespread; with high versus low production rates not only controlled by the gas content in the shale, but also largely by the mineral content of the rock that makes the rock more conducive to fractures remaining open. It is hypothesized that the ability of shale to fracture is controlled by the amount of silica and/or calcite in the rock, and that the variability in the occurrence of those minerals can be predicted by sequence stratigraphy. The construction of a sequence stratigraphic model on a basinwide scale over the state of Pennsylvania began with a correlation of 821 wireline well logs. Then, to tie the working sequence stratigraphic model to mineralogy within individual zones of rock, analysis of 24 rotary sidewall cores was conducted using qualitative x-ray diffraction to determine the mineralogy of each sample. This study determined that the mineralogy of the organic-rich shales within the Marcellus Formation can be predicted by sequence stratigraphy, and also found there to be a relationship between mineralogy and total organic carbon (TOC). Now that a relationship is found between mineralogy, TOC, and its location within a sequence stratigraphic framework specific zones may be identified in a predicable manner within certain sequences that are likely more productive than others.