Depositional and Mineralogical Controls on Organic and Inorganic Pore Distribution in the Lower Cretaceous Pearsall Mudrock System, South Texas

Ko, Lucy (Ting-Wei); Loucks, Robert G.; Ruppel, Stephen C.; Rowe, Harry; Milliken, Kitty L.

An important research goal in mudrock depositional systems is the prediction of the type, amount, and distribution of pores. Primary mineralogy reflects extrabasinal and intrabasinal sediment input and is related to transport and provenance, which may change during deposition. Compaction, cementation, grain dissolution, replacement, and organic matter maturation in response to time, temperature, and pressure are the main controls on pore evolution in mudrocks during burial.

The Pearsall mudrock system is one of the major shale resource plays in the United States and is characterized by its relatively high carbonate content. This study examines the lower Bexar Shale Member of the Pearsall Formation which represents one of the late Aptian regional oceanic anoxic events (OAE). Study of the lower Bexar Shale Member mudrocks provides insights into the understanding of pore types in calcareous siliceous mudstone lithofacies, as well as understanding pore development and loss through burial.

In the lower Bexar Shale Member, mineral assemblages (including kerogen) and detailed major and minor trace elements were investigated from proximal to distal areas to determine the likely controls on the distribution of dominant pore types and associated porosity. Siliceous, argillaceous, and carbonate mudrocks were identified and correlated to fine-scale major-element distribution. Thin sections and samples prepared by Ar-ion cross-section polishing from each lithofacies (partly defined by XRF-based elemental analysis) were investigated using a field-emission scanning election microscope to define pore types, abundance, and distribution.

Initial investigation indicates mineral-related interparticle and intraparticle pores are dominant and intra-organic-matter pores are minor. Previous studies showed that kerogen types change, from proximal to distal area, from mixed Type II and Type III to Type III dominant. Present-day vitrinite reflectance data demonstrate thermal maturity ranges from 0.50 %Ro updip to 1.50 %Ro downdip. Maximum TOC values coincide with lower degrees of environmental oxygenation and range between 2 and 5 wt%. This study contributes to better understanding of the interplay of primary and secondary processes in pore evolution and provides a basis for developing techniques to better predict pore types, abundance, and distribution in mudrocks using integrated data.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013