From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
Bureau Seminar, March 17, 2006
Predicting Fracture Porosity Evolution in Dolostone
Fracture growth and diagenesis interact to create and destroy fracture porosity in dolostones and thereby affect fluid-flow properties in these important hydrocarbon reservoir rocks. We have observed dolomite crystals that bridge fracture walls in several dolostones. Similar quartz-bridge morphologies in fractured sandstones have been replicated using geometric crystal growth models that consider anisotropies in growth rates associated with crystallographic orientation, euhedral versus noneuhedral nucleation surfaces, and the influence of repeated episodes of crystal breakage during incremental fracture opening. Our analysis of detailed SEM-CL images from fractured dolostones suggests that similar linked mechanical and chemical processes could cause the formation of dolomite bridges.
To evaluate this hypothesis we developed a geometric crystal growth model for dolomite fracture fill and evaluated the ability of the model to replicate microstructures within dolomite fracture cement. We assumed equivalent growth rates along each euhedral growth face (as suggested by SEM-CL images, which show little growth anisotropy) and 20 times faster growth rates on noneuhedral versus euhedral surfaces on the basis of analogy with quartz (comparable data are lacking for dolomite). Several model runs were completed with different fracture-opening rates. The results were compared with natural fracture morphologies, producing the anticipated replication of bridges and lining cement morphologies. We also found that modeled dolomite bridges commonly have rhombic shapes even when they are subjected to multiple crystal breakage episodes. In addition, the crystal growth model predicts morphologies that are analogous to features in SEM-CL images that superficially appear to be caused by crystal dissolution.