Abstract

Historical production data reveals a strong correlation between structure and economic gas accumulations in the Cretaceous Dakota Formation within the San Arroyo Field, southern Uinta Basin. Reservoir sandstones are concentrated along the crests of anticlines, which indicates a structural control. Sandstone distribution, however, fails to fully explain the occurrence of on-structure “dry holes”. Internal heterogeneity and architectural arrangement of lithofacies within channel sandstones provides a mechanism for heterogeneities in porosity and permeability of reservoir sandstones and helps explain hydrocarbon distribution within the San Arroyo gas field. Cathodoluminescense, scanning electron microscopy, and optical microscopy images were analyzed in conjunction with thinsection X-ray diffraction to establish a diagenetic history for Dakota sandstones in two wells. The following diagenetic sequence was inferred for the nonproductive Trapp Springs well: 1) minor compaction; 2) quartz overgrowths; 3) calcite cement; 4) dissolution of calcite, feldspar grains, and quartz overgrowths; 5) authigenic kaolinite; and 6) pyrite growth. The diagenetic sequence for the economically productive San Arroyo #5 well was: 1) compaction; 2) minor quartz overgrowths; 3) minor calcite cement; 4) authigenic kaolinite, 5) minor pyrite. Early, pervasive quartz overgrowths account for the majority of pore-filling cements in the non-productive well but are rare within the productive well. Interstitial clays appear to have prevented the formation of early, pervasive quartz overgrowths and maintained a restricted, yet open pore system within the productive well.