--> Abstract: Mississippian Barnett Shale: Lithofacies and Depositional Setting of a Deepwater Mudstone Succession; #90063 (2007)

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Mississippian Barnett Shale: Lithofacies and Depositional Setting of a Deepwater Mudstone Succession

 

Loucks, Robert G.1, Stephen C. Ruppel2 (1) The University of Texas at Austin, Austin, TX (2) Bureau of Economic Geology, Austin, TX

 

The Mississippian Barnett Shale of the Fort Worth Basin is a classic “shale-gas” system, in which the rock is source, reservoir, and seal. In the producing area, Barnett rocks are not shales but are nonbioturbated, laminated carbonate and siliceous mudstones. Barnett strata were deposited in a deeper water foreland stratified basin characterized by poor circulation with the open ocean. For most of the basin's history, bottom waters were euxinic, facilitating preservation of organic matter resulting in a rich source rock. This rock contained abundant microframboidal pyrite that was precipitated out of a euxinic water column. The Barnett strata, comprising a variety of lithofacies, are dominated by fine-grained (clay- to silt-sized) particles, many of which are peloids. Three general lithofacies are recognized on the basis of mineralogy, fabric, biota, and texture: (1) laminated siliceous mudstone (2) laminated argillaceous lime mudstone, and (3) skeletal, argillaceous lime packstone. Each facies contains abundant pyrite and phosphate. Carbonate concretions, an early diagenetic product, are also common. All biota within the basin were transported from the shelf or upper oxygenated slope by hemipelagic mud plumes, dilute turbidites, and debris flows. Biogenic sediment was also sourced from the shallow, oxygenated water column. We suggest that water depths may have been as great as 400 ft to 700 ft. These depths seem required for below storm-wave base and stratified water-column conditions. Barnett deposition is estimated to have taken place over a 25-Ma period, and despite variations in sublithofacies, sedimentation style remained remarkably similar throughout this span of time.

 

AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California