Shale Wedges and Stratal Architecture, Barnett Shale (Mississippian), Southern Fort Worth Basin, Texas
Published:January 01, 2012
Rachael M. Monroe, John A. Breyer, 2012. "Shale Wedges and Stratal Architecture, Barnett Shale (Mississippian), Southern Fort Worth Basin, Texas", Shale Reservoirs—Giant Resources for the 21st Century, J. A. Breyer
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A thick shale section cored in the EOG Resources Gordon saltwater disposal (SWD) well in the southern Fort Worth Basin contains six different lithologies. Gamma-ray readings on well logs can be used to distinguish the Barnett Shale (Mississippian) from the overlying Pennsylvanian shales and to divide the Barnett Shale into upper and lower units referred to informally as the Barnett A and Barnett B. Laminated silty claystone to mudstone is the dominant lithology in the Pennsylvanian shales above the Barnett Shale. The relative abundance of this lithology decreases downward in the core. It makes up a significant part of the Barnett A, but only a minor part of the Barnett B. A dark claystone shows the opposite trend, decreasing in relative abundance upward in the core. Sponge spicules are the most common silt- and sand-size grains in both the laminated claystone to mudstone and the dark claystone. Thin shell layers and phosphatic intervals are also found throughout the core. Shell layers are more common in the Pennsylvanian shales. Phosphatic material is most abundant in the Barnett B at the base of the core. Claystones and mudstones, lacking sponge spicules, but containing significant amounts of silt-size quartz are found only in the Barnett B. Isopach maps show that the Barnett B is part of a large shale wedge that prograded into the central and southern parts of the Fort Worth Basin from the northeast and that the Barnett A is part of a smaller shale wedge that prograded from east to west across Johnson County. The upper wedge onlaps and dies out against the flank of the lower wedge. The distribution of lithologies in the Gordon SWD well can be related to the position of the well site on the shale wedges. The site was far removed from areas of active sedimentation during the deposition of the Barnett B and closer to the main sources of sediment and areas of sedimentation during deposition of the Barnett A.
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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.