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.