Lithologic and Stratigraphic Variation in a Continuous Shale-gas Reservoir: The Barnett Shale (Mississippian), Fort Worth Basin, Texas
Published:January 01, 2012
John A. Breyer, Philip J. Bunting, Rachael M. Monroe, Michael B. Steed, 2012. "Lithologic and Stratigraphic Variation in a Continuous Shale-gas Reservoir: The Barnett Shale (Mississippian), Fort Worth Basin, Texas", Shale Reservoirs—Giant Resources for the 21st Century, J. A. Breyer
Download citation file:
Shale reservoirs are continuous accumulations in which the same formation commonly serves as the source, reservoir, and seal for commercial accumulations of natural gas. Intrabasinal differences within continuous accumulations account for the indistinctly bound areas of better gas production termed sweet spots by operators. Generally similar sets of facies have been recognized in the Barnett Shale in the Fort Worth Basin by all recent workers. Dark mudstone to claystone with a matrix of clay minerals and cryptocrystalline quartz is the most common depositional facies in the Barnett Shale. Two predominantly calcareous depositional facies are next in abundance: argillaceous lime mudstone and skeletal argillaceous lime packstone. A variety of minor depositional and diagenetic facies are also present. The abundance and distribution of facies change with geographic location within the basin and stratigraphic position within the Barnett Shale. The most obvious example of this is the relative abundance of calcareous depositional facies in the northern part of the basin compared with their relative scarcity in the central part of the basin. All of the major facies recognized in the Barnett Shale have high concentrations of organic matter. The variation in facies is greater than the variation in organic matter content. The location of sweet spots with higher production rates within the Barnett Shale may ultimately be explained by the distribution of facies that respond differently to various completion procedures. As the play matures, it is likely that a detailed understanding of the geology, especially the distribution of facies, will become increasingly important in selecting well locations, intervals in which to land laterals, and which fracture stimulation techniques to use.
Figures & Tables
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.