Mississippian shales of the Great Basin: source-rock potential, geographic distribution and tectonic evolution

Abstract

The most important petroleum source rocks in the Great Basin of the western U.S. are Mississippian mudrocks colloquially and collectively known as the Chainman Shale. However, this term actually encompasses two distinct mudrock units, separated by the regional C2 unconformity. Identification, correlation and interpretation of the two units have long been hampered by poor exposure, sparse age control and superficially monotonous appearance. New research presented here, including mudrock mineralogy, petrology, petrophysics, and organic geochemistry, confirms important shifts at the Late Mississippian regional unconformity. We limit the term ‘Chainman Shale’ to mudrocks above the unconformity, and apply the term ‘Gap Wash Formation’ to mudrocks below it. The C2 unconformity also separates units of contrasting source-rock potential. The results presented here are based on more than 1400 analyses, from both publicly accessible and proprietary data. Most important for resource assessment, the Gap Wash has significantly better potential as a hydrocarbon source than the Chainman and may be more suitable for treatment as a shale reservoir. Electric logs in numerous wells show the unconformity clearly, allowing regional correlation. Subsurface data presented here demonstrate that the C2 unconformity extends south into southern Nevada and east into Utah. The geographic distribution and composition of these two mudrocks record the paleogeographic evolution of the western U.S. during Mississippian time. The older, Gap Wash Formation, mudrock was deposited into accommodation space created by the loading of the continental margin during the Antler orogeny. The younger, Chainman Shale, mudrock was deposited into a broad transgressive seaway that subsided in response to resumed continental margin contraction in late Meramecian time. The angular unconformity between them (C2) records Mississippian (Meramecian) tectonism that post-dated the Antler orogeny.

AAPG Datapages/Search and Discovery Article #90266 © 2016 AAPG Pacific Section and Rocky Mountain Section Joint Meeting, Las Vegas, Nevada, October 2-5, 2016