Abstract

Anhydrite diagenesis greatly influences San Andres reservoir quality at the Willard unit of Wasson field. The largest effect is anhydrite cementation. Up to 20% of the reservoir volume is anhydrite cement. Most of the cement occludes the large pores, so the effective pore system has significantly smaller pores and throats than would be expected from depositional fabric. Peritidal fenestral fabrics and grainstones are selectively cemented, whereas skeletal wackestones and packstones are partially cemented, but they retain effective pore systems. Late anhydrite dissolution, especially around diagenetic anhydrite nodules, locally enhances porosity and permeability.

The distribution of anhydrite can be explained by the anhydrite paragenesis. Some nodular anhydrite replaces primary sulfate nodules developed within and below peritidal sequences. Most nodules replace carbonate as a byproduct of regional, Late Guadalupian dolomitization. Pressure dissolution affects anhydrite nodules more than dolomite. Anhydrite dissolved from the nodules provides the source for burial anhydrite cement. Anhydrite cement is very coarsely crystalline and poikilotopic. It selectively occludes large pores while many nearby small pores remain uncemented. This fabric is consistent with nucleation-controlled growth controlled by slow supply of dissolved anhydrite, low degree of supersaturation, and slow overall crystal growth. Large pores are more likely to have nuclei on their larger surface areas, so they are more likely to be cemented.

Late Cenozoic anhydrite dissolution is present in the Willard unit, but it is not as common as it is on the Central Basin Platform. Dissolution enhances permeability, so a feedback between dissolution and flow causes relatively narrow areas of anhydrite dissolution surrounded by larger areas with little or no evidence of dissolution.