Background
South Texas oil and gas production in the past decade has focused primarily on horizontal drilling and hydraulic fracturing of the Eagle Ford Formation, which is the richest oil and gas source rock in the region. Many operators have applied similar development strategies to the overlying Austin Chalk, with variable success. Above the Austin Chalk in the producing area lies the Anacacho Limestone, which has received increasing interest for potential horizontal drilling and hydraulic fracturing. The success of modern unconventional production strategies (i.e., horizontal drilling and multi-stage hydraulic fracturing) of Austin Chalk and Anacacho Limestone requires migration of oil and/or gas from the Eagle Ford Formation source rock. Natural fault and fracture networks have been shown to be key pathways for hydrocarbon migration through the Eagle Ford Formation and Austin Chalk in south Texas, and these deformation features are strongly influenced by the mechanical properties of the strata and the stress history. Several south Texas oil and gas producers have noted that the Anacacho and Austin formations are increasingly important in their future development planning, and that there is potential interest in (i) an industry-funded Austin/Anacacho research consortium to understand the mechanical properties and deformation fabrics, and (ii) training that SwRI could potentially provide.
Approach
The Anacacho Limestone outcrops across the Balcones fault system from the Anacacho Mountains southwest of Uvalde to west San Antonio. New mapping by Dr. Thomas Ewing extended Anacacho Limestone outcrop distribution eastward to the western edge of SwRI’s San Antonio campus. These easternmost outcrops of the Anacacho Limestone are more representative of the Anacacho Limestone reservoir rock in the subsurface, due to the absence of syndepositional volcanism that is abundant in the Uvalde area. In this project, we conducted an outcrop-based investigation of deformation in the Anacacho Limestone, including reconnaissance station measurements, drone-based photogrammetry and fracture mapping, and scanline surveying, as well as lithostratigraphic characterization and mineralogic analysis.
Accomplishments
Results show that within the study area, mesoscale deformation in the Anacacho Limestone consists of a primary set of opening-mode fractures that strike NE-SW, a subordinate set of opening-mode fractures that strike NW-SE, and NE-SW-striking normal faults. Opening-mode fracture sets tend to have relatively low intensities on the order of <1 fracture/meter in the Anacacho Limestone away from larger faults. Detailed study of a normal fault with ~10 meters of throw, shows considerably higher fracture intensities including a combination of small-displacement normal faults (shear fractures) and opening-mode fractures within a few meters of the fault. A scanline survey shows combined fracture intensities, including opening-mode and shear fractures, of 6.6 fractures/m within 2.5 meters of the main fault surface, 2.7 fractures/meter from 2.5 to 30 m from the main fault surface, and then further decays to 1.3 fractures/meter at 30 m from the main fault. This 1.3 fractures/meter intensity is similar to background fracture intensities observed and measured in the Anacacho Limestone distant from faults in the area. Project results provide material for new industry training courses and workshops, and the foundation for follow-on research on properties that may influence exploration and production of the Anacacho Limestone as an unconventional hydrocarbon reservoir.