BROWN, ALTON A.
In many West Texas Permian reservoirs unaffected by secondary anhydrite dissolution, coarse-crystalline anhydrite occludes a greater fraction of large pores than smaller pores in the same rock sample. Where anhydrite occurs in a void, the pore is typically completely occluded by a single crystal. Most of the anhydrite crystals terminate at the edges of voids and conform to their shape. Crystals do not extend through pore throats except where both pores and throats are exceptionally large (millimeters and tens of microns, respectively).
It is proposed that this crystallization pattern results from nucleation control of anhydrite cementation during burial diagenesis. Low nucleation-site density due to slight supersaturation of anhydrite in the fluid makes it unlikely that more than a single anhydrite crystal is heterogeneously nucleated in most pores. As pore surface area decreases with decreasing radius, pores are less likely to have even a single nucleation site, so small pores are more likely to remain open than large pores. Uncemented pore throats result from surface energy effects, also indicative of a low degree of supersaturation.
To test this hypothesis, thin sections from Wasson field (TX) were point counted to determine the frequency of anhydrite cementation in pores of different sizes. Results support the surface nucleation-control model. Nucleation-site frequency was estimated by fitting probabilistic models to the data. Sparsity of multicrystalline fill of very large pores indicates that anhydrite nucleation and growth continues during burial diagenesis rather than being a short-lived phenomenon.