Reef-Bank Features and Their Constraint to Reservoirs of Natural Gas, from Permian Changxing Formation to Triassic Feixianguan Formation in Daxian-Xuanhan Area of Sichuan Province, South China

Detail studies of the seismic section and certain wells, such as the Puguang-2, the Puguang-6, and the Maoba Wells, demonstrate that the reef facies from the Permian Changxing Formation to the Triassic Feixianguan Formation are expressed as the lens with medium-strong changing swings and chaotic images in the seismic section. The reef facies is made up of gray limestones and dolomites of both baffling sponge reef and the frame sponge reef. Moreover, the bank facies from the Permian Changxing Formation to the Triassic Feixianguan Formation is represented as the lens with medium-strong changing swings and discontinuity images in the seismic section, which is made up of several petrologic types, that is, the French gray thick-bedded to massive sparitic oolitic dolomite, the sparitic gravel-oolitic dolomite, the sparitic pisolitic-oolitic dolomite, the sparitic bioclastic dolomite, and the sparitic dolarenite. Both the reef facies and the bank facies concomitant are developed and distributed in space along the platform margin, which form a special reef-bank facies zone. This facies zone controls the petrological features, the reservoir qualities, and the spatial distribution of the reservoirs of natural gas in the Daxian-Xuanhan area.