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Abstract

AAPG Bulletin, V. 91, No. 4 (April 2007), P. 579-601.

Copyright copy2007. The American Association of Petroleum Geologists. All rights reserved.

DOI:10.1306/11020606059

Mississippian Barnett Shale: Lithofacies and depositional setting of a deep-water shale-gas succession in the Fort Worth Basin, Texas

Robert G. Loucks,1 Stephen C. Ruppel2

1Bureau of Economic Geology, John A. and Catherine G. Jackson School of Geosciences, University of Texas at Austin, 10100 Bureau Road, Building 130, Austin, Texas 78713-8924; bob.loucks@beg.utexas.edu
2Bureau of Economic Geology, John A. and Catherine G. Jackson School of Geosciences, University of Texas at Austin, 10100 Bureau Road, Building 130, Austin, Texas 78713-8924

ABSTRACT

The Mississippian Barnett Formation of the Fort Worth Basin is a classic shale-gas system in which the rock is the source, reservoir, and seal. Barnett strata were deposited in a deeper water foreland basin that had poor circulation with the open ocean. For most of the basin's history, bottom waters were euxinic, preserving organic matter and, thus, creating a rich source rock, along with abundant framboidal pyrite. The Barnett interval comprises a variety of facies but is dominated by fine-grained (clay- to silt-size) particles. Three general lithofacies are recognized on the basis of mineralogy, fabric, biota, and texture: (1) laminated siliceous mudstone; (2) laminated argillaceous lime mudstone (marl); and (3) skeletal, argillaceous lime packstone. Each facies contains abundant pyrite and phosphate (apatite), which are especially common at hardgrounds. Carbonate concretions, a product of early diagenesis, are also common. The entire Barnett biota is composed of debris transported to the basin from the shelf or upper oxygenated slope by hemipelagic mud plumes, dilute turbidites, and debris flows. Biogenic sediment was also sourced from the shallower, better oxygenated water column. Barnett deposition is estimated to have occurred over a 25-m.y. period, and despite the variations in sublithofacies, sedimentation style remained remarkably similar throughout this span of time.

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