Atoll Reservoir Facies in Pennsylvanian Limestone Higgins Ranch Field, Coke County, Texas
Higgins Ranch field in Coke County, Texas (Eastern Shelf) presently includes thirteen wells which, since 1979, have produced in excess of 1.0 MMBO and 218 MMCFG. Hydrocarbon production in the field is from a thick section of Pennsylvanian (Canyon : Palo Pinto Fm.) limestones that were deposited on an atoll seaward of the Canyon shelf edge. Such small atoll reservoirs in the Permian Basin, although difficult to locate in the subsurface, could be major exploration targets in the future.
Analysis of cores and cuttings samples indicates that the Palo Pinto section in the atoll field consists of stacked sequences of shoal-water limestones overlain by thinner tongues of dark colored shales and argillaceous siltstones. The siliciclastics are thin to medium-bedded, glauconitic and spiculitic, and include graded beds of atoll-derived bioclastic debris. These rocks, interpreted as a forebank slope facies, pass distally into unfossiliferous basinal shales. The reservoir limestones include biograinstones, coral biolithites, biowackestones to packstones, and phylloid algal wackestones. The grainstones and biolithites are high-energy, atoll-margin facies, whereas the micritic and algal limestones represent low-energy, atoll-interior (lagoonal) deposits. Progradation of atoll-marginal facies into the atoll-interior lagoon, which resulted in filling of the lagoon, is recorded in each vertical carbonate sequence. Each such coarsening and shoaling-upward cycle is overlain by forebank slope deposits.
Reservoir formation was related to the establishment of a fresh water lens resulting from partial atoll emergence at or near the end of each phase of lagoonal filling. The reservoir system includes two basic porosity types: primary intra-skeletal pores and secondary biomoldic pores. The existence of such pore systems yields reservoirs of relatively low average porosities and permeabilities (6 to 13% and 0.1 to 0.3 md, respectively). However, permeabilities are locally enhanced by subsequent solution-enlargement of pores and natural fracturing.