Analysis of four cores from Crane and Upton Counties, Texas, provides a basis for interpreting the origin of calcium sulfate minerals in Guadalupian-age carbonates. The San Andres and Grayburg Formations along the eastern margin of the Central Basin platform consist of basinward prograding supratidal to shallow marine carbonate facies. These units have undergone dolomitization and frequently contain large amounts of gypsum and anhydrite. Carbonate deposits are periodically interrupted by terrigenous clastic influx.

Investigation began with hand sample observation of core from Gulf Oil Corp. wells in the Dune and McElroy fields of Crane and Upton Counties. To define diagenetic fabrics better, core analysis was augmented by the preparing and observing 200 thin sections. Electron microscopy was performed on selected samples to provide a three-dimensional view of the nature of calcium sulfate pore fill and intergranular cement. In order to understand the relationship between dolomite and anhydrite/gypsum, detailed chemical analysis was conducted with the x-ray diffractometer and the electron microprobe.

Calcium sulfate minerals occur in a variety of forms in the San Andres and Grayburg Formations. The complex interrelationships between gypsum and anhydrite indicate multiple stages of dissolution, reprecipitation, hydration, and dehydration. The result of these processes is the occurrence of anhydrite and gypsum in the following forms: (1) nodules, (2) fracture fill, (3) pore fill/replacement, (4) poikilitic cement, and (5) a variety of gypsum and anhydrite intergrowths. The following occurrences of calcium sulfate minerals are evidence for their secondary origin: (1) replacement of carbonate grains and matrix, (2) pore filling along with pore filling of voids lined with secondary dolomite, (3) anhydrite filling of fractures connecting anhydrite nodules, and (4) occurrence of anhydrit nodules along stylolites.

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