Porosity Examples from IAB Field, Coke County, Texas: ABSTRACT

Abstract

IAB field produces oil from an Upper Pennsylvanian (Cisco-Canyon) carbonate buildup situated on the western edge of the Eastern shelf. The reservoir is approximately 6 mi (9 km) long, 2 mi (3 km) wide, and 300 to 1,000 ft (100 to 300 m) thick. The upper surface of this buildup is irregular with several pinnacles, probably caused by subaerial erosion that occurred during drops in sea level. From examination of cores, 10 lithofacies have been identified and divided according to texture, sediment types, fossils, and mineralogic composition. Facies recognized are subtidal normal-marine deposits, ranging from relatively quiet-water skeletal wackestones to relatively high-energy, shallow-water oolitic grainstones. Porosity consists of both primary and secondary types and is controlled by depositional fabric, dolomitization, and freshwater diagenesis. Five main porosity types are (1) primary interparticle and (2) primary intraparticle porosity, both in grain carbonate rocks, (3) secondary leached grain and moldic porosity in grain carbonate rocks, created during a drop in sea level, (4) secondary vug porosity in muddy or very fine-grained carbonate rocks, also created during a drop in sea level, and (5) secondary intercrystalline and vug porosity in dolomites. Muddy carbonate beds had low initial effective porosities because of poor sorting, whereas grain-rich carbonate beds had high initial effective porosities owing to good sorting. Following deposition, sea level probably dropped, and the buildup was exposed to freshwater vadose and phreatic diagenesis. Solution of various grains and simultaneous precipitation of equant calcite cements produced secondary leached porosity and reduced primary interparticle and intraparticle porosity. Porosity preserved during this period of subaerial exposure and freshwater diagenesis was probably later reduced during deeper burial by additional cementation and compaction. End_of_Article - Last_Page 457------------