Stratigraphy, Sedimentology, and Geochemistry of the East Texas Upper Jurassic Smackover Carbonate Ramp Succession

Abstract

The Smackover Formation is a significant hydrocarbon producing formation deposited during the Upper Jurassic in the ancestral Gulf of Mexico. Production is commonly from dolomitized oolitic grainstones present in the Upper Smackover. These Upper Smackover reservoirs are sourced from organic-matter-rich Lower Smackover mudrocks (commonly referred to as the Brown Dense). The upper grainstone section of the Smackover has been the focus of many studies; however, the lime-mud-rich lower portion has been relatively understudied within the East Texas Basin. There is some question as to whether the relatively organic-rich, laminated mudrocks in the lower section were deposited in a deep or shallow-water setting. Without a clear understanding of the depositional setting of the Lower Smackover, its reservoir properties are difficult to assess as an unconventional reservoir and to explore for age-equivalent targets. In this study, we focused on a 550 ft continuous core (Sun Travis Gas Unit No.1) in Van Zandt, TX. The core contains 95% of the Smackover section and the lower part of the overlying Buckner Formation. It covers a spectrum of Smackover facies, ranging from the contentious low-energy organic-rich laminated mudstones to skeletal and oolitic grainstones. This core was previously unworked and has the potential to be the type section for the Smackover in East Texas. The core was analyzed using a binocular microscope and blue fluorescence dyed thin sections. XRF, XRD, and Rock Eval were run on the cores. Selected samples were viewed on the SEM for microtexture and pore networks in the mudstones. Piccolo rebound hammer analysis was used and empirically related to unconfined compressive strength to define mechanical stratigraphy and fracability. Our depositional model is of a seaward deepening ramp where the inner ramp consisted of higher energy, peloidal and ooid packstones and grainstones, a middle ramp composed of packstones (some oncolitic) and an outer ramp (below storm-wave base) consisting of a low-energy, anoxic setting. In this anoxic setting, laminated lime mud and organic-rich laminae were deposited. Some of the laminae appear to be anoxic bacterial mats. Also, some debrites are present consisting of soft-sediment clasts. This continuous core is an excellent reference section for the Smackover Formation in northeast Texas.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018