Carbonate Lithologies of the Mississippian Barnett Shale, Fort Worth Basin, Texas
Published:January 01, 2012
Kitty L. Milliken, Ruarri J. Day-Stirrat, Petro K. Papazis, Christian Dohse, 2012. "Carbonate Lithologies of the Mississippian Barnett Shale, Fort Worth Basin, Texas", Shale Reservoirs—Giant Resources for the 21st Century, J. A. Breyer
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Carbonate-rich lithologies of the gas-producing Upper Mississippian Barnett Shale, Fort Worth Basin, Texas, are diverse and include lithologies with carbonate components that are primarily authigenic, as well as those that have carbonate components dominated by skeletal debris and other allochems such as peloids and intraclasts. Compositionally, carbonate-bearing lithologies of the Barnett Shale (including the informal unit known as the Forestburg Limestone) can be viewed as mixtures of authigenic or allochemical carbonate and siliciclastic sediment derived mostly from outside the basin. With the exception of the Forestburg Limestone, these varied carbonate lithologies dominate only in local zones, at the scale of a hand specimen or thin section, and do not constitute a volumetrically significant part of the gas-producing reservoir rock. Carbonate lithologies are significant, however, for clues they provide on environmental and early diagenetic conditions during accumulation of the Barnett Shale. Carbonate lithologies dominated by skeletal components contain distinct and impoverished marine faunas that are consistent with low oxygenation levels. The generally early timing of carbonate cement precipitation is supported by the reworking of diagenetic carbonate as silt- to sand-size intraclasts, sediment infilling of fractures in cemented beds and concretions, displacive fabrics, and highly random orientations of phyllosilicate grains within carbonate units. In some cases, detrital allochemical carbonates provided nucleation substrates for precipitation of highly displacive authigenic carbonate that was extensively reworked into microspar-size sediment particles. The elemental and isotopic chemistries of authigenic carbonates are consistent with near-sea-floor authigenesis driven by microbial cycling of organic carbon into carbonate minerals under generally reducing and low-temperature conditions.
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Shale Reservoirs—Giant Resources for the 21st Century
In the early 1970s, most exploration geologists in the United States considered subeconomic or marginally economic petroleum resources such as coalbed methane, shale gas, and tight-gas sands as unconventional resources (Law and Curtis, 2002). Tax incentives and federally funded research beginning in the late 1970s helped make these resources economically viable in the last two decades of the 20th century. Economics aside, two important geologic attributes characterize most unconventional petroleum resources (Law and Curtis, 2002). Conventional petroleum systems are buoyancy-driven accumulations found in structural or stratigraphic traps, whereas most unconventional systems exist independent of a water column and are generally not found in structural or stratigraphic traps.
Shale reservoirs are not new. The first commercial hydrocarbon production in the United States was from a well drilled in 1821 in a shale gas reservoir. By 2000, more than 28,000 wells had been drilled in shale gas reservoirs. Rising gas prices and technological advancements in horizontal drilling and hydraulic fracturing associated with the development of the Barnett Shale led to a boom in shale gas development in the early years of the 21st century. Now the exploitation of shale reservoirs is turning to natural gas liquids, condensate, and oil. Far from being isotropic and homogeneous, as once naively envisioned, shale reservoirs are complexly layered accumulations of fine-grained sediment. Geologic variation on scales ranging from that of stratal architecture to that of lamination within individual beds must be understood in order to locate and exploid areas of higher production within shale reservoirs. Shale reservoirs remain largely geologic plays - notmerely lease plays or strictly engineering plays made possible by improvements in drilling and completion technology.