Geology of the Mississippian Barnett Shale-Gas Play in Texas: Refional Setting, Sedimentology, and Pore Networks
R. G. Loucks
Bureau of Economic Geology, University of Texas, Austin, TX, [email protected]
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. The Barnett interval comprises a variety of facies but is dominated by fine-grained (clay- to silt-sized) particles. Four general lithofacies are recognized on the basis of mineralogy, fabric, biota, and texture: (1) laminated to nonlaminated siliceous mudstone; (2) laminated argillaceous lime mudstone (marl); (3) skeletal, argillaceous lime packstone, and (4) phosphatic-rich mudstone to grainstone. Each facies contains abundant pyrite and phosphate (apatite). Much of the pyrite appears to be micron-sized framboidal pyrite that was precipitated in a euxinic water column. The phosphate formed in a slope environment and was transported into the deeper basin by gravity-flow processes. 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 taken place over a 25-Ma period, and despite the variations in sublithofacies, sedimentation style remained remarkably similar throughout this span of time.
The pore network in the Barnett Shale consists predominantly of nanometer-scale pores (nanopores). Carbonaceous grains host the majority of nanopores with many of these grains containing hundreds of nanopores. Other nanopores are found in bedding-parallel wisps of largely organic matrix material. The nanopores within grains result from devolatilization of the organic material during hydrocarbon maturation. Median pore diameters vary from grain to grain, but a typical diameter is ~100 nm with a general range of 5 nm to 500nm.
AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009