Porosity Decrease in the Trenton Limestone of Williams County, Ohio

Abstract

The Trenton limestone is found in the Michigan and Appalachian Basins, beneath the Utica Shale or the Point Pleasant Formation. The Trenton is of Ordovician age and consists of a fine light-grey to dark-brown matrix, stylolites, and secondary dolomite. In Williams County, Ohio, the Trenton has a decreasing neutron porosity in well log data, and we investigate this porosity decrease. We obtained 40 feet of Trenton rock core from Core 3256 from the Ohio Department of Natural Resources (ODNR). The core is light grey, and significant oil staining and mineralization are observed on the outside of the core. 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 and porosity data, and can also be used to visualize internal structures and vug frequency and size inside the core. We will use the geophysical well log data from the well as a comparison to density and porosity data from the scanner, namely, to look at the Trenton porosity which continually decreases with depth. The porosity decrease is curious, as the Trenton in other parts of the region serves as a reservoir for hydrocarbons. The porosity may decrease because there is increased mineralization with depth, a decrease in the amount of vugs (open space) may be observed in the limestone 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. The data will then be examined with a focus on how the basin has evolved over time, in addition to how that might be affecting the porosity decrease in the well-log.

AAPG Datapages/Search and Discovery Article #90373 © 2019 AAPG Eastern Section Meeting, Energy from the Heartland, Columbus, Ohio, October 12-16, 2019