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Geologic Characterization of Johnson County, Texas


The Fort Worth Basin (FWB) in north-central Texas has seen a surge in shale-gas drilling and associated hydraulic fracturing in the Barnett Shale. The FWB is a structural basin bounded by the Ouachita Thrust Zone to the east and the Muenster Arch to the north. Disposal of most of the fracturing (frac) fluids and subsequent produced water from the wells is into a deep carbonate formation, the Ellenburger dolomite, which underlies the Barnett Shale. This disposal appears to be associated with seismic activity. An attempt is being made to model the disposal of these fluids into the Ellenburger and develop methods to predict the most likely places of seismic activity. A working model could assist in mitigating the strength of seismic activity through providing optimum disposal rates, optimum disposal well siting, and prediction of potentially imminent seismicity. To develop models to simulate the disposal of frac fluids, a geologic framework of the area is required. This framework includes the distribution, thickness, and depth of major geologic units, and the distribution of existing fractures and faults in the area. To develop this geologic framework, data from nearly 300 wells within and adjacent to Johnson County were reviewed along with over 220 geophysical logs obtained from the Texas Railroad Commission and IHS. These data show that the depth and thickness of major units generally increases eastward toward the Ouachita Thrust Zone on the east side of Johnson County. The principal unit of interest, the Ellenburger, ranges from 5,600 to 10,000 ft in depth and its thickness increases from about 2,400 ft to the northwest to 3,400 ft near the Ouachita Thrust Zone. These increases are in agreement with the geologic literature. Structure contour maps in conjunction with the cross sections indicate potential fault zones that trend north-northeast near the Ouachita Thrust and east-northeast in the western part of the county. The faults near the thrust correspond with faults in Ewing (1990) and Elebiju (2010). Potential faults to the west more closely follow faulting presented in Turner (1957), which are slightly different from those presented by Ewing. The data compiled gives a starting point for modeling the injection of frac fluids and the potential effects of this on nearby faults and fractures. These data can be used to create seismic models to assist regulators in mitigating seismic activity due to frac fluid disposal.