The Barnett Shale of the Southern Fort Worth Basin; Comparison of Depositional Setting, Lithofacies, and Mineralogy with Equivalent Deposits in the Northern Basin

Authors: R. G. Loucks and S. C. Ruppel

Venue: 2008 American Association of Petroleum Geologists (AAPG) Annual Convention and Exhibition, San Antonio, TX, April 19-24, 2008 (http://www.aapg.org)

Abstract: Ongoing studies of outcrops and more than 30 subsurface cores from the Barnett Shale along the southern margin of the Fort Worth Basin (FWB) show both similarities and differences from the better known deposits of the northern part of the basin. Like deposits in the north, southern FWB strata consist predominately of in situ argillaceous siliceous mudstone and thinner beds of calcareous mudstone. In both areas, transported gravity-flow deposits, composed of thin-shelled mollusks are common; crinoid-bearing debris beds are found primarily in the south. Carbonate concretions, phosphatic layers, and diagenetic pyrite are common to both areas. Earlier studies of the northern FWB provided strong evidence that the rocks in this area accumulated in a deeper water basin that was dominantly anoxic. Similar conditions are indicated for the southern FWB, although the presence of a crinoid-rich shallow-water fauna and local bioturbation suggest that this area may have been closer to the basin margin. Bioturbation is commonly located beneath event beds, suggesting gravity-flow transport of shallow-water fauna into deeper water, setting up “doomed pioneer” assemblages. The thickness of the Barnett Shale is generally less in the southern FWB than in the north and thickens to the northeast and east (range: 0 – 160 ft). Mineralogical analysis reveals that silica and clay content in the southern FWB are significantly different from the north (32% and 26% versus 21% and 49%, respectively). Total Organic Content(TOC) is similar in both areas, but maturity is substantially lower in the south (average 0.5% Ro).

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 United States.” 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 (Virginia.WEYLAND@netl.doe.gov or 918-699-2041)
University of Texas at Austin – Stephen Ruppel (stephen.ruppel@beg.utexas.edu or 512-471-1534)