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The AAPG/Datapages Combined Publications Database
Five types of petroleum-producing configurations are present in the Middle Ordovician Trenton Limestone around the Lima-Indiana field in northwestern Ohio. The first, an anticlinal trap, is present along the Findlay arch. Closure is provided on top of the Trenton by regional carbonate bank buildup, folded with and capped by the Utica Shale. The second, a faulted anticline, is present along the Bowling Green fault in Wood, Hancock, and Lucas Counties. A high-angle reverse fault along the crest of the Findlay arch juxtaposes dolomitized Trenton rock with the overlying Utica Shale. This configuration has accounted for significant oil and gas production. The third trap type consists of an updip facies change from Trenton Limestone to Utica Shale, with draping of the thickened shale over the Trenton Limestone. The fourth type, in the Michigan basin, is the fracture systems and dolomitization in the Albion-Scipio trend. The fifth, and less well documented, is a porosity trap in dolomitized upper parts of the Trenton. Dolomitization may function in two ways, both as a prerequisite to formation of sufficiently porous reservoir rock with other trapping mechanisms. Other Trenton fields are not accounted for by these five configurations.
Stratigraphic and structural cross sections from the top of the Trenton to the Knox unconformity, as well as structural and isopach maps of the area, show a major carbonate buildup of the Trenton in a northeast-southwest trending arc in northwestern Ohio. A broad carbonate platform with wedge-top dolomitization in an island environment is postulated as an alternative to a regional erosional unconformity between the Trenton Limestone and Utica Shale.
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