SEDIMENTOLOGY AND DEPOSITIONAL SETTING OF THE MISSISSIPPIAN BARNETT SHALE, WISE COUNTY, TEXAS

Several Barnett Shale cores were described to define depositional setting and sedimentation processes. Mineralogy ranges form pure terrigenous sediment to terrigenous-rich carbonate. Terrigenous sediment comprises predominately clay, microcrystalline silica, and minor phosphate. Fissile shales are rare. These rocks are best classified as calcareous to noncalcareous, laminated terrigenous mudstones. Unequivocal burrows were not noted. Common throughout these mudstone are interbedded gravity-flow layers of shell and phosphatic material ranging from millimeters to decimeters. Fossils include transported brachiopod, foraminifera, crinoid, ostracod debris. Ripples, climbing ripples, and scour surfaces are locally present. Calcareous nodules are common; compaction of surrounding terrigenous layers suggests they formed during early burial.

A thick (30 to 40 ft) carbonate unit (Forestburg), which is prominently displayed on wireline logs, is commonly present near the top of the Barnett. This unit is composed of alternating layers of lime mud and silica/clay layers. Bioturbation is absent and shell material is extremely rare.

Sedimentary features imply that both terrigenous- and carbonate-dominated Barnett facies accumulated in an anoxic basin where sedimentation was below storm-wave base. This interpretation is consistent with the complete lack of bioturbation and a high TOC content. If the sea bottom was oxygenated, bioturbation would have destroyed the laminations. Laminations and transported shell beds suggest suspension deposition punctuated by debris flow and dilute turbidite deposits.

Macropores are not apparent. Porosity in the Barnett is presumably developed as nannopores, micropores, and fractures.