3.1 Porosity and Storativity

There are three types of porosity for the Edwards: matrix, fractures, and karstic (conduits). Hovorka and others (1996) provide a comprehensive set of porosity data using over 300 core plugs and over 200 well-log analyses. The average total porosity was estimated at 18 percent.

Matrix porosity is highly variable vertically from low values (4 to 12 percent) in dominantly subtidal high stand facies to high values (20 to 42 percent) in grainstones and leached, subtidal dolostones. Porosity can approach 45 percent in zones that have had dolomite dissolution. Hovorka and others (1996) show patterns of horizontal porosity gradation.

Fracture porosity is more difficult to assess. Hovorka and others (1998) estimate fracture porosity of 1.41 percent based upon a 30 m-long survey transect. Aquifer-­specific yields are controlled by fracture porosity. Scanlon and others (2001) report specific yield values ranging from 0.001 to 0.06 for fractured areas. Slade and others (1985) estimated a mean specific yield of 0.017 for the Barton Springs segment. Lindgren and others (2004), Maclay and Rettman (1973), and Maclay and Small (1984) give ranges of specific yield (0.025 to 0.200) and storativity (10^–5 to 8×10^–4) for the San Antonio segment.

Conduit (karstic) porosity is also highly variable and difficult to assess directly. Based upon karst feature distributions in outcrops, Hovorka and others (1998) calculated a range in porosity values of 0.5 to 9.43 percent with a median of 2.2 percent. However, it is unclear how well connected the karst voids are in these outcrops and how much they contribute to permeability. The interconnected karstic porosity may be small, but, as in conduits, commonly dominates the flow systems (Worthington, 1999).