XCT Analysis of Decreasing Porosity in the Trenton Limestone of Williams County, Ohio

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The Ohio State University

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The Trenton limestone is found in the Michigan and Appalachian Basins, beneath the UticaPoint Pleasant and Antrim shales. The Trenton is of Ordovician age and consists of a fine, lightgrey to dark-brown matrix, stylolites, and secondary dolomite. In Williams County, Ohio, the Trenton limestone displays a decreasing neutron porosity in well log data, and this thesis investigates the porosity decrease. Four boxes of Trenton limestone from Core 3256 were obtained from the Ohio Department of Natural Resources. The Trenton spans a depth of 2267.2 to 2497.8 feet. For this thesis, the following intervals were scanned: 2268–2270 feet (Box 208), 2290–2300 feet (Box 211), 2310– 2320 feet (Box 213), and 2330–2340 feet (Box 215). The core is light grey, and significant oil staining and mineralization are present. The core contains large vugs, many of which are filled with dolomite crystals. This core was scanned using an X-ray computed tomography (XCT) scanner to provide density data in the form of Hounsfield units (HU), and to visualize internal structures and vug frequency. The scans produced average HU values that ranged from +2600 to +2800. Scan images displayed vugs concentrated from 2290–2300 feet, mineralized zones containing dolomite crystals, shale laminations, and fractures in cores. I have used well log data as a comparison to HU values from the XCT scanner to examine the Trenton porosity which continually decreases with depth from 10% to 1% by 2323.5 feet. The porosity may decrease because there is increased mineralization with depth, a decrease in the amount of vugs with depth, or the limestone matrix may change to an increased amount of magnesium (dolomitization); if this is the case, more magnesium-rich rock should occur near the top of the core and decrease with depth. I find that while mineralization and vuggy porosity decrease with depth, the dolomitized zone is the significant factor. The porosity is decreasing as a function of core density and matrix type, as the Trenton limestone transitions from the upper dolomitized zone to a calcite-rich limestone.



XCT scanning, Dolomitization, Trenton limestone, Core analysis, Well logging