Late Albian rudist buildups of the Edwards Formation in central Texas: A shallow reservoir analog study

Late Albian rudist buildups of the Cretaceous Edwards Formation (Fredericksburg Group) occurring widely in central Texas, capped by younger dolostones, form important reservoir analogs for highly productive oilfields in the Middle East and are partially exposed in spillways and gullies near Georgetown and Belton in Texas.;Four varieties of mounds, some with amalgamated growth stages, are identified based on outcrop and GPR studies: (i) Biohermal caprinid mounds -- High porosity and permeability. (ii) Biohermal toucasid mounds -- Low porosity and permeability. (iii) Biostromal caprinid-toucasid-radiolitid mounds -- Porosity and permeability unknown. (iv) Biostromal toucasid mounds -- Moderate porosity and permeability.;Petrography reveals that the degree of dolomitization has a strong control over the evolution of porosity-permeability within the buildups. Biomoldic pores, vugs, intercrystalline pores, fracture pores, and intracrystalline pores within dolomites are common, with many demonstrating solution enlargement by meteoric waters. Leaching of the calcite cement surrounding the dolomite crystals in the matrix has produced high degrees of intercrystalline porosity, eventually enhancing permeability within the mound depositional facies. The evolution of this very local platformal sequence was not uniform and changed from progradational in the lower section to increasingly aggradational upwards.;The GPR is capable of imaging subsurface mounds with a vertical resolution of ∼0.2 m. An optimal sounding frequency of ∼250 MHz and moisture content of less than ∼20% is suggested for best results on shallow subsurface GPR surveys at Lake Georgetown Spillway or in similar ancient carbonate environments.;Results of laboratory dielectric measurements suggest that within carbonates, dielectric contrasts improve with a decrease in sounding frequency and/or an increase in moisture content; and the relationship between dielectric permittivity and moisture content may be represented by 3rd order polynomial equations. Amplitude decays of the backscattered radar signal correlate to moisture distribution with high-amplitude zones coinciding with moisture-saturated rocks.