Abstract

Disposal of hydraulic fracturing flowback and produced water into Ordovician and Cambrian formations of the Fort Worth Basin (FWB), coupled with an increase in observed seismicity in the Dallas-Fort Worth area, necessitates an understanding of the geologic character of these disposal targets. More than 2 billion barrels (Bbbls) of wastewater have been disposed into the Ordovician Ellenburger Group of the FWB over the past 35 years. Since the implementation of the TexNet Earthquake Catalog (1 January 2017), more than 20 earthquakes of local magnitude ML2.0 or greater have been detected in the area, with depths ranging from 2 to 10 km (approximately 6500–33,000 ft). The cited mechanism for inducement of these earthquakes is reactivation of basement faults due to pore pressure changes, either directly related to proximal disposal or due to disposal volume buildup over time. Here, we present a stratigraphic and petrophysical analysis of FWB disposal targets and their relation to basement rocks. The Ellenburger consists of alternating layers of limestone and dolomite, with minor siliciclastics above the basement toward the Llano Uplift. Matrix porosity averages <5 porosity units (p.u.), with higher porosity in dolomitic layers than in limestone. Dolomite dominates at the top of the Ellenburger, which was exposed at the end of both the Lower and Upper Ordovician. Where crystalline basement rocks are penetrated, the composition ranges from granitic to chlorite-bearing metamorphosed lithology. The basement-sediment interface is frequently marked by increased porosity. An updated map of structure on top of basement indicates elevations ranging from outcrop at the Llano Uplift to more than 12,200ft (3.7km) subsea toward the northeast. The disposal zone pore volume is estimated from thickness and porosity maps and ranges from <0.1 to >0.60 billion barrels per square mile (Bbbl/mi2).

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