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Abstract: Sedimentologic and Diagenetic Controls on Aquifer Properties, Lower Cretaceous Edwards Carbonate Aquifer, Texas: Implications for Aquifer Management

S. D. Hovorka, A. R. Dutton, S. C. Ruppel, Joseph Yeh

The three-dimensional distribution of water in the Edwards aquifer was assessed using a core and log-based study. Porosity distribution reflects both depositional fabric and subsequent diagenesis. Vertical facies stacking patterns influence the depositional porosity as well as dolomitization and diagenetic porosity modification. Subtidal facies deposited during sea level highstands are generally undolomitized and exhibit low porosity (5-10%); platform grainstones typically have high depositional porosity and significant solution enhancement (20-42% porosity). Dolomitized subtidal facies in tidal-flat-capped cycles have very high porosity (20-40%) because of selective dolomite dissolution in the freshwater aquifer. Porosity in gypsum beds is high in some areas because of dissolution an collapse, but low where gypsum was replaced by calcite cement. Low-energy subtidal and evaporitic units in the Maverick basin have porosity generally less than 15%. The overlying basinal packstones and grainstones have solution-enhanced porosities of 25 to 35%.

Mapped average porosity also shows nonstratigraphically controlled variation. Diagenesis associated with fluctuations in water chemistry near the saline-freshwater interface may explain one high-porosity trend. Other complex patterns of high and low porosity are attributed to structurally and hydrologically controlled porosity enhancement and cementation.

Three-dimensional mapping of porosity trends provides data for improved aquifer management. Only about 3% of the maximum stored water lies above the water table at which natural spring flow is diminished. An average specific yield of 42% in the unconfined aquifer is determined from total porosity, changes in the water-table elevation, and changes in estimated

recharge and discharge. Average storativity of 2.6 × 10-4 in the confined Edwards is estimated using average porosity and barometric efficiency calculated from comparing water-level hydrographs and atmospheric pressure changes.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994