Bob Loucks

Lecture BL1: Geology of the Mississippian Barnett Shale-Gas Play in Texas:
Regional Setting, Sedimentology, and Pore Networks

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.

Lecture BL2: Origin of Lower Ordovician Ellenburger Group Brecciated and Fractured Strata in Texas;
Paleokarst, Thermobaric, and/or Tectonic?

Several origins have been suggested for the development of large-scale brecciation and fracturing in Ellenburger strata, including karst-related paleocave development and collapse, subsurface thermobaric dissolution and fracturing, and tectonic fracturing and brecciation. Most brecciation and fracturing in the Ellenburger Group resulted from near-surface, meteoric karst that developed at the Sauk Unconformity (~475 Ma before present) and was modified by later karst processes during Pennsylvanian exposure (~280–300 Ma before present). Some areas show a strong overprint of hydrothermal alteration and tectonic fracturing, suggesting all three processes affected Ellenburger strata, but at different times.

Early, widespread brecciation and fracturing in the Ellenburger Group have been well documented to result from subaerial exposure and associated meteoric cave formation and collapse. The collapse, starting at the surface contemporaneous with cavern formation, continued into the subsurface to at least 9,000 ft of burial. Cave formation is evidenced by (1) multiple types of water-lain, detrital cave-sediment fill in cavities and fractures, (2) Upper Ordovician to Mississippian conodonts in the sediment fill, (3) speleothems, and (4) lateral dimensions and patterns of brecciated bodies. Paleocave collapse is the origin of most brecciation and fracturing in Ellenburger strata. Boxwork structures and vugs along bedding planes and higher temperature coarse- to very coarse crystalline saddle dolomite cements are evidence of a passive to aggressive hydrothermal overprint that occurred during the Ouachita Orogeny. Boxwork structure is composed of closely spaced, dolomite-filled fractures on the scale of decimeters to millimeters; the host rock is commonly dissolved, leaving an open dolomite boxwork. Saddle dolomite cements passively filled many voids created by cave processes, but they also appear to have filled some vugs associated with hydrothermal dissolution. Tectonic fractures cutting host rock, lithified breccias, and lithified sediment fills have relatively strong directional patterns in the Ellenburger Group where they have been mapped.

Because deciphering the origin of complex breccia and fracture systems, such as seen in Ellenburger strata, is commonly difficult, the complete paragenesis of a system must first be understood. Paleokarst is common at ancient, composite unconformities, as shown in the geologic record. The paleokarst pore network may therefore form the dominant conduit in these systems, and later fluids, such as subsurface-derived hydrothermal fluids, may migrate through these conduits and modify the strata either passively or aggressively. Passive modification occurs when hydrothermal cements precipitate in former voids and have no relationship to the origin of the voids. Aggressive modification occurs when fluids create voids and breccias and precipitate hydrothermal cements into these voids. Tectonic fractures can overprint any system at different times, and these events must be worked out by geoscientists first becoming familiar with tectonic and burial histories of individual regions. Therefore, all three processes (karst, thermobaric, and tectonic) have affect the Ellenburger strata.

Loucks, R. G., and Ruppel, S. C., 2007, Mississippian Barnett Shale: lithofacies and depositional setting of a deep-water shale-gas succession in the Fort Worth Basin, Texas: AAPG Bulletin, v. 91, no. 4, p. 579–601.

Loucks, R. G., 2007, A review of coalesced, collapsed-paleocave systems and associated suprastratal deformation: Acta Carsologica, v. 36, no. 1, p. 121–132.

Loucks, R. G., 1999, Paleocave carbonate reservoirs: origins, burial-depth modifications, spatial complexity, and reservoir implications: AAPG Bulletin, v. 83, p. 1795–1834.