Edwards Aquifer Ground-Water Resources: Geologic Controls on Porosity Development in Platform Carbonates, South Texas

Focusing on the highly developed Edwards aquifer, which supplies water for a large area of south-central Texas, this report presents a three-dimensional model of the porosity distribution in the aquifer, which quantifies the spatial distribution of water resources and provides information that can be used in ground-water flow and transport models. The study provides a detailed assessment of the amount of water in the Edwards aquifer as of 1996 on the basis of a large data set and a reproducible methodology. 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 as 45 percent) because of dolomite dissolution. Calcite cements derived from evaporite diagenesis and resulting from marine flooding occlude porosity in grainstones. A high-porosity area that shows the influence of the hydrologic setting is found in southern Medina and southwestern Bexar Counties on both sides of the fresh-water-saline interface known as the bad-water line.The quantitative, three-dimensional model of the porosity distribution in the aquifer presented in this study quantifies the spatial distribution of water resources and provides information that can be used in ground-water flow and transport models. Additional studies built upon the data set collected for the porosity study have been used to better quantify the permeability structure of the aquifer for input into numerical hydrologic models. Future numerical hydrologic models are the tool needed to better understand and predict aquifer responses to situations of public interest such as the changes in recharge rate, water levels, pumpage, and spring flow.