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Detailed Sequence Stratigraphic Framework of the Middle Devonian Geneseo Formation of New York, USA: Implications for Unconventional Reservoir Quality and Distribution


The Middle Devonian Geneseo Formation and lateral equivalents in the Northern Appalachian Basin constitute shale-gas plays with promising economic potential. Mudstone properties within the Geneseo are highly variable, and reflect an overall shallowing trend that corresponds to the westward progradation of the Catskill delta. High-resolution stratigraphy has allowed differentiation of genetically related packages, comprised of distinct lithofacies, with characteristic physical, biological, and chemical attributes. Correlation of this succession was conducted at the parasequence scale, and includes detailed descriptions of multiple drill cores and surface exposures, as well as subsurface mapping. Isopach maps were constructed to identify thickness trends and lateral variations of mudstone properties, and the Geneseo Formation has been differentiated into two discrete depositional sequences with three lithostratigraphic units (the Lower Geneseo, Fir Tree, and Upper Geneseo members). The Lower Geneseo Member overlies the Tully Formation, and where the latter is absent, its basal contact is marked by a pyritic-phosphatic lag (the Leicester Pyrite Bed; MFDLS). The Lower Geneseo is an organic-rich dark gray to grayish black mudstone succession with aggradational to progradational parasequence stacking patterns (HST). The Fir Tree Member unconformably overlies the Lower Geneseo, displays progradational-aggradational-retrogradational parasequence stacking patterns (LST and TST), and consists of silt-rich calcareous mudstones rich in auloporid tabulate corals, ostracodes, and small brachiopods. The Upper Geneseo displays aggradational to progradational parasequence stacking patterns (HST), and consists of dark gray silty mudstones and muddy siltstones with abundant wave/current ripples, graded beds, and evidence for extensive reworking and erosion. Reactivation of basement structures and syndepositional faulting appears to have strongly influenced accommodation during deposition of the Geneseo Formation. In particular, the N-S trending Clarendon-Linden Fault System seems to have acted as a western sediment barrier during Geneseo Time. Through the development of a fully integrated sequence stratigraphic framework that incorporates surface and subsurface data, reservoir quality and distribution in the Geneseo Formation can be evaluated away from sample control, and thus enhance the potential for future economic success of unconventional resource plays in this interval.