Simpson Group Facies: Interpretations from the McKee Member Formation, Pecos County, Texas

Authors: Rebecca H. Jones and Stephen C. Ruppel

Venue: West Texas Geological Society Symposium, in Midland, Texas September 10-12, 2008. http://www.wtgs.org [external site]

Abstract: Middle Ordovician Simpson Group rocks were deposited in greenhouse conditions during a period of overall sea level rise and marine transgression following an extensive hiatus and development of an irregular karst topography on Lower Ordovician Ellenburger carbonates. Simpson Group production comes from three sandstone units in the middle of the group, each sourced from eroding cratonic highs and transported to the margin by eolian and/or fluvial processes. These sandstones, the Connell, Waddell, and McKee members, occur at the base of the Oil Creek, McLish, and Tulip Creek formations, respectively, from oldest to youngest.

Simpson sandstones represent lowstand and/or early transgressive deposits of three third-order sequences; non-reservoir shales and clay-rich carbonates, deposited during maximum flooding, and normal marine and restricted carbonates, deposited during highstand, comprise the balance of the sequences. McKee Member facies, as observed in the C.C. Canon 3-10 core from Pecos County, Texas, are consistent with this general depositional setting. The cored interval contains interbedded and mixed siliciclastics and carbonates, not just sandstone as the nomenclature suggests. Siliciclastic facies include sandstones with a bimodal distribution of silt and medium-grained sand. Sand grains are subrounded to rounded and 200-400 µm in size. These rocks are locally burrowed and/or contain dolomite or calcite cement. Rounded, mature grains are consistent with distant sediment sources and eolian or fluvial transportation mechanisms, cementation supports a margin setting, and burrowing would be expected during relative sea level fall. Silty, fossiliferous wackestone, siltstone, and silty dolostone are common accessory facies.

Related NETL Project
This presentation is related to the NETL project DE-FC26-04NT15509, "Integrated Synthesis of the Permian Basin: Data and Models for Recovering Existing and Undiscovered Oil Resources from the Largest Oil-Bearing Basin in the U.S." The objectives of the project are twofold: (1) to produce a detailed, comprehensive analysis and history of Paleozoic depositional and reservoir systems in the Permian Basin, and (2) to create spatially integrated databases of depositional, stratigraphic, lithologic, and petrophysical properties for selected reservoir plays and stratigraphic horizons. These objectives will be undertaken and completed sequentially during the 3 years of the project. The overall objective is to provide Permian Basin operators with (a) outcrop and subsurface reservoir specific data, data syntheses, and models to be applied to geological, engineering, and completion-based redevelopment of existing reservoirs, and (b) a detailed regional stratigraphic framework for applying such models to new exploration targets.

Project Contacts
NETL – Virginia (Ginny) Weyland (Virgina.Weyland@netl.doe.gov or 918-699-2041)
Bureau of Economic Geology, University of Texas at Austin – Stephen C. Ruppel (stephen.ruppel@beg.utexas.edu or 512-471-1534)