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Depositional Setting, Lithofacies, and Pore Networks of the Mississippian Deepwater Barnett Shale Facies in the Fort Worth Basin

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

 

Several basin-center Barnett Shale cores were described to define depositional setting, general lithofacies, and pore network. Mineralogy ranges from pure terrigenous sediment to terrigenous-rich carbonate. Terrigenous sediment is composed of microcrystalline silica with varying amounts of clay, carbonate, pyrite, and phosphate. Fissile shales are rare. These rocks are classified as calcareous to noncalcareous, laminated terrigenous mudstones. Units containing more than 50% carbonate are classified as lime mudstones.

    

Nearly all facies are laminated. Unequivocal burrows are not present. Interbedded shell layers, scour surfaces, ripples, phosphatic nodules and hardgrounds, and calcite concretions are locally common. Fossils include transported bivalves, brachiopods, foraminifera, cephalopods, radiolarians, pelagic algal cysts, and other debris.

 

Sedimentary features suggest that both terrigenous- and carbonate-dominated Barnett facies accumulated in an anoxic basin below storm-wave base. This interpretation is consistent with the complete lack of bioturbation and high TOC content. If the sea bottom were oxygenated, bioturbation would have destroyed laminations. The predominance of laminations is evidence of suspension deposition. Other structures (e.g., graded bedding, ripples) indicate gravity-flow deposition and reworking of sediments by contourites.

    

A thick, deepwater hemipelagic carbonate unit (Forestburg) is locally present in the Barnett Shale. This unit is composed of alternating layers of lime mud and silica/clay mud. Bioturbation is absent, and shell material is extremely rare.

    

Macropores are not apparent, and fractures appear to be cemented. On the basis of capillary pressure and NMR analysis, porosity in the Barnett consists of pores less than a micron in diameter having pore throats generally less than 10 nanometers in diameter.