Stratigraphic architecture, depositional systems, and reservoir characteristics of the Pearsall shale-gas system, Lower Cretaceous, South Texas

Abstract

This study examines the regional stratigraphic architecture, depositional systems, and petrographic characteristics of the South Texas Pearsall shale-gas system currently developed in the Indio Tanks (Pearsall) and Pena Creek (Pearsall) fields. The Pearsall Formation was deposited as a mixed carbonate-siliciclastic system on a distally steepened ramp over a period of 11.75 million years. It was deposited between maximum floods of two second-order sequences and contains at least five third-order cycles. Up to three Oceanic Anoxic Events (OAE 1-A, Late Aptian Regional Event, and OAE 1-B) figure prominently in the deposition of the Pearsall sediments, and during these intervals, depending on the location within the Maverick Basin, sedimentation rates were between 0.5 and 2 cm/ky. Facies in the Pearsall section arise from interactions between pre-existing topography, oxygenation regime, eustatic sea-level fluctuation, and depositional processes. In the Pearsall Formation, OAEs affected depositional environments and resulting facies patterns during several time periods. The OAEs occurred in association with transgressions but not necessarily in concert with them. Outer ramp OAE facies are siliciclastic-dominated, TOC-rich, and little-bioturbated. Conversely the outer ramp facies deposited under normally oxygenated paleoenvironmental conditions tend to be carbonate-rich, TOC-poor, and are more prominently bioturbated.