Sequence Stratigraphy and Petrophysics of the Utica Shale and Associated Late Ordovician Strata, Eastern Ohio and Western Pennsylvania
Taylor G. McClain
Department of Geology and Geography, West Virginia University, Morgantown, WV, [email protected]
Approximately 500 well logs and three cores from eastern Ohio, western Pennsylvania, and northern West Virginia are used to construct a depositional model of the Utica Shale and associated Upper Ordovician strata. Previous studies of outcrop data from the Cincinnati Arch and Jessamine Dome recognized a number of Late Mohawkian to Early Cincinnatian third-order sequences deposited in an active foreland basin during onset of the Taconic orogeny. Sequences consist of deepening upward successions of transgressive limestone and shale, recording a period of sustained subsidence and increasing sea-level along the eastern margin of North America. The transition from a carbonate dominated system to a clastic dominated system reflects the collapse and drowning of a widespread carbonate platform. The Trenton/Lexington limestone through Utica Shale comprise the transgressive systems tract (TST) of a large second-order sequence, superimposed with four, smaller scale third-order composite sequences. Third order sequences are regionally correlative, aggradational, and lack low-stand deposits. Sequences are separated by type 3 sequence boundaries that amalgamate with transgressive surfaces and separate underlying highstand system tracts (HST’s) from overlying TST’s. Chronostratigraphic surfaces demonstrate that basinal interbedded lime mudstone, shale, and marl facies of the Logana and Point Pleasant formations are contemporaneous and genetically related to platform limestone on the flanking Trenton and Lexington platforms. Isopach thickness maps of composite sequences indicate significant accumulation of carbonate sediment and build-up of the platforms during the Late Mohawkian, followed by increased clastic sedimentation and basin fill in the cross-strike Sebree trough and Point Pleasant sub-basin during the early Cincinnatian. Intervals with potentially high total organic carbon (TOC) content were identified using sonic/resistivity and density/resistivity log cross plots, and compared to publicly available source rock data. The Logana through Point Pleasant intervals contain the highest amount of TOC in the area of the Point Pleasant sub-basin in eastern Ohio, with the Utica Shale containing the highest amount of TOC along the Trenton platform in northwestern Pennsylvania. These observations indicate a southwest-northeast trending prospective play fairway throughout eastern Ohio and northwestern Pennsylvania.
AAPG Search and Discovery Article #90154©2012 AAPG Eastern Section Meeting, Cleveland, Ohio, 22-26 September 2012