AAPG Annual Convention and Exhibition

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Characterization of Faults on the Flanks of the Llano Uplift for Application to Understanding Seismogenic Faults in the Subsurface of the Fort Worth Basin


Beginning in about 2008 the U.S. southern mid-continent has experienced a dramatic increase in the rate of seismicity including the Dallas-Ft. Worth metropolitan area. This increase has been attributed by many researchers to the activation of geologically-dormant faults by elevating ambient pore fluid pressure by brine injection thereby driving susceptible faults to a condition of rupture. These ruptures have occurred mainly in the crystalline basement and also within the overlying injection intervals along faults that have generally not been documented previously. Assessment of the hydraulic continuity within the disposal zones and the connection into basement, presumably via faults and their damage zones, is of paramount importance in assessing the dynamic storage capacity of disposal intervals and under what conditions that capacity might be exceeded. Therefore our understanding of the geometric, kinematic, and hydraulic characteristics of these faults, and how they are influenced by in situ geomechanical conditions must be improved. The Llano Uplift of central Texas exposes Precambrian igneous and metamorphic basement, an abbreviated Paleozoic section including units equivalent to the Ellenburger Fm, and 50+ faults of the Llano fault zone (LFZ) which are geologically continuous to the NE with seismogenic faults of the Ft. Worth Basin (FWB). The LFZ formed in Pennsylvanian time when the region was flexed in the footwall of the encroaching Ouachita tectonic front which was synchronous with the development of the FWB. Faults of the LFZ are NNE- to ENE-striking, normal to normal-oblique, generally offset the Pc/Pz unconformity, and have throws of up to 1 km. Associated with the LFZ, previously mapped at 1:25,000 scale, are numerous smaller faults exposed in road cuts suitable for quantitative characterization including ortho-rectified 3D models obtained via areal drone. We will present the results of our fault characterization which includes construction of a 3D model of the LFZ as informed by structural analysis and outcrop-scale models. We will discuss fault length, height, attitude, mechanical stratigraphic controls, areal and volumetric density, fault zone properties, assessment of horizontal and vertical hydraulic continuity, assessment of how these faults would mechanically behave if subjected to representative in situ conditions of the FWB, and how they inform our understanding of earthquake sequences in the FWB.