Back-Reef Depositional Environments in a Lower Cretaceous (Sligo) Shelf-Margin Complex: Insights into Ultradeep Reservoir Preservation and Controls on Stacking Patterns in an Outer Platform Setting

Abstract

The Early Cretaceous Sligo and Stuart City shelf-margin trends in Texas have been a subject of interest for over 60 years, attracting attention both because of their striking complex reef lithologies and their continued potential for oil and gas exploration. Numerous gas reservoirs have been discovered since the late 1950s, with a major push for revitalization and expansion occurring in the 2000s. The shelf-margin complexes originated as steep-walled reefs rimming the Comanche Platform. They evolved into a narrow (<6 mi) highly heterogeneous belt of banks, islands, shoals, and tidal flats with an associated highly productive shallow-water carbonate factory. Most characterization efforts for these systems have focused on known reservoirs in South Texas; very little work has investigated the margins outside of this region, particularly with respect to the Sligo shelf margin. This investigation presents the first well-documented long stratigraphic section for the Sligo margin in East Texas, which serves as an initial step in understanding this major shelf-margin complex. Facies are mainly associated with back-reef to back-reef lagoon environments and exhibit significant heterogeneity on the basis of variations in water depth and wave energy, with facies including argillaceous carbonate mudstones, oncolitic packstones, rudist biostromes, reef-derived grainstones, ooid grainstones, and algal laminate bindstones. Four depositional sequences are delineated based on broad changes in facies and stacking patterns related to water depth and energy regime, yielding new insight into controls on facies partitioning in a stable long-term shelf-margin complex. The pore network, which is limited to micropores, is largely facies-controlled and is related to the presence of originally Mg–calcite allochems such as miliolids and Lithocodium/Bacinella. As East Texas has significant structural complexity, the ultradeep buried Sligo Formation is an underexplored area with the potential for undiscovered reservoirs related to structural features. Geologic characterization of this core is an important step in understanding some of the porous lithofacies present within the Sligo shelf margin in East Texas.