Mineralogic sensitivity of opening-mode fracturing in limestone and shale

Abstract

Analysis of fracture networks in horizontally bedded limestone and shale in the Eastern Shelf of the Permian Basin of west-central Texas was performed to investigate the lithologic and mineralogic controls on fracture networks. The results show that opening-mode fracturing is sensitive to mineralogy as a primary control and that opening-mode fracturing depends on total clay and calcite percentages. Systematically fractured beds in this study all have >80% calcite, <9% quartz and <7.5% total clay minerals, whereas beds with <80% calcite, >9% quartz and >6% clay minerals are unfractured. Depositional-texture-based lithologic classifications (grainstone, packstone, wackestone, mudstone) based on proportions of carbonate grains and carbonate mud, although extremely convenient and important for interpreting depositional environments of carbonate rocks, are not robust predictors of fracturing. The reason for this is the sensitivity of rock strength to mineralogy – in carbonate rocks, particularly the clay mineral versus calcite content. Our results show that a mineralogic threshold approach may be valuable for opening-mode fracture prediction in limestone and shale.