Edwards Aquifer Ground-Water Resources: Geologic Controls on Porosity Development in Platform Carbonates, South Texas
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The Edwards aquifer of south-central Texas is a prolific but heavily developed water resource. Amount and distribution of water in the aquifer are related to development of porosity in highly heterogeneous, complexly altered, platformal to shallow basinal carbonate rocks of the Cretaceous Edwards Group. Uplift and faulting of the Edwards Group along the Balcones Fault System have contributed to the geologic complexity of the aquifer. We assessed distribution of water in the Edwards aquifer by means of a core- and log-based stratigraphic study that included 200 neutron and resistivity logs and 300 porosity and permeability plug analyses. Resource evaluation utilized Stratamodel Stratigraphic Geocellular Modeling software to interpolate and quantify log data throughout the region. Calculated average porosity of the Edwards aquifer is 18 percent. Estimated total waterfilled pore volume of the Edwards aquifer within the study area is 173 million acre-feet. Only 3 percent of this total water lies in the traditionally used part of the aquifer between the highest and lowest recorded water levels. Spatial distribution of water in the aquifer is strongly related to the geologic factors involved in porosity development. Depositional fabric, Cretaceous diagenesis, and late diagenesis during development of the fresh-water aquifer are the major geologic contributors. High-frequency subtidal-supratidal cycles, the fundamental depositional unit in San Marcos platformal facies of the Edwards Group, exerted a strong influence on porosity evolution in the east part of the study area. Two relative-sea-level-controlled sequences corresponding to the Kainer and Person Formations are defined by cycle stacking patterns. Edwards Group rocks in the Maverick Basin, also cyclic but entirely subtidal, have lower average porosity than the platformal facies. Vertical facies stacking influences the amount of diagenetic modification of porosity. Dolomitized subtidal facies beneath stacked tidal-flat cycles have extremely high porosity (as much