Pore Distribution in the Ordovician Shale of the Utica/Point Pleasant Sub-Basin

Michael Murphy, Jeff Daniels, David Cole, Julie Sheets, and Susan Welch
Subsurface Energy Materials Characterization and Analysis Laboratory (SEMCAL), School of Earth Sciences, The Ohio State University, Columbus, OH, [email protected]

Shale and mudstones are potential reservoir seals for geologic sequestration of carbon dioxide. When rich in organic matter, they are critical in the creation and distribution of hydrocarbons. The objective of this investigation is to characterize and image pore networks in shale. The techniques used in this study characterize porosity in small samples at the nanometer to micron scale. Linking pore types to depositional environments can help to scale up laboratory measurements. Pore types were classified by their size, shape, and connectivity, and then analyzed in relation to facies distributions in the Utica/Point Pleasant sub-basin. The sub-basin is an Ordovician feature in the Midcontinent United States flanked by carbonate platforms to the northwest and southwest, and the Taconic foreland basin to the east. Core samples from six wells were used to investigate porosity in the Utica Shale, the Point Pleasant Formation, and the Logana Member of the Lexington Limestone.

Mercury intrusion porosimetry and gas sorption techniques were used to estimate total pore volume, pore size distribution, connectivity, and capillary breakthrough pressure. Total organic carbon was measured with an elemental anamlyzer. A scanning electron microscope with Quantitative Evaluation of Minerals by Scanning Electron Microscope (QEMSCAN) software was employed to image pores and for quantitative analysis of mineralogy, texture, and porosity. Several samples were analyzed with a dual beam focused ion beam scanning electron microscope (FIB-SEM). The FIB-SEM analysis produced 3D representations of pore networks and organic matter distribution. Initial results indicate that pore size, shape, and connectivity vary with the distributions of clay and carbonate matrix, but are most strongly correlated with the distribution of organic matter. These differences correspond to location within the sub-basin and to stratigraphic position. Most of the connected pores are nanometer scale and associated with with organic matter, pyrite, and fossil fragments. The pores in organic matter appear to be related to thermal maturity.

AAPG Search and Discovery Article #90154©2012 AAPG Eastern Section Meeting, Cleveland, Ohio, 22-26 September 2012