Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
American Association of Petroleum Geologists Annual Meeting, Dallas, April 18-21, 2004
Poikilotopic Anhydrite Enhances Reservoir Quality
F. Jerry Lucia,
Rebecca H. Jones, and James W. Jennings, Jr.
Anhydrite has long been thought to occlude pore space in dolostones producing a tight, nonproductive carbonate. Although this is true for bedded anhydrite and pore filling anhydrite, our study of Permian dolostone reservoirs of West Texas demonstrates that it is not true for poikilotopic anhydrite. Poikilotopic anhydrite is the most common form of anhydrite in Permian dolostone reservoirs of West Texas. It occurs in patches, commonly with dolomite inclusions, several millimeters to centimeters across and both fills interparticle porosity and replaces the host carbonate. Cross plots of interparticle-porosity and permeability measurements from medium crystalline mud-dominated dolostones and grain-dominated dolopackstones that have considerable amounts of poikilotopic anhydrite tend to plot in the petrophysical class 1 field rather than in the class 2 field as predicted by Lucia (1995). Also, capillary pressure data from these samples show anomalously large pore throats for a class 2 fabric.
These observations suggest that poikilotopic anhydrite reduces porosity but does not reduce pore-throat size. Permeability is fundamentally controlled by pore-throat size and remains near constant whereas the pore filling aspect of patchy anhydrite reduces porosity. This effect causes the data points to shift to the left, from the class 2 to the class 1 field. This effect has been verified by initial results from detailed flow simulations that compare the permeability of a dolostone with and without patches of anhydrite. Therefore, we suggest that poikilotopic anhydrite, and also anhydrite nodules, does not form dense, nonproductive dolomite but actually enhances petrophysical properties.