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

Robert G. Loucks and Stephen C. Ruppel
Bureau of Economic Geology, The University of Texas at Austin, Austin, TX

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-baring debris beds are found primarily in the south. Carbonate concretions, phosphatic layers, and diagenetic pyrite are common to both areas. Our 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). TOC is similar in both areas but maturity is substantially lower in the south (average 0.5% Ro).

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas