Upper Permian Carbonate Buildups and Associated Lithofacies, Western Hubei-Eastern Sichuan Provinces, China

ABSTRACT In the Upper Permian Changxing Formation in western Hubei and eastern Sichuan, China, three types of carbonate buildups have been recognized, including platform-margin reefs, organic mud mounds, and patch reefs. Carbonate buildups in the Changxing Formation have yielded natural gas in the well J-7 area of the Jiannan gas field in western Hubei and in well B-1 on the Dashigan structure in eastern Sichuan. Distribution and nature of these buildups were controlled by movement of basement blocks along paleorift zones, such as the Anping-Xianfeng rift, that experienced resurgent Late Permian movement. For example, one of these buildups, the linear platform-margin Jiantianba reef, formed along the upthrown western side of this rift zone, and thin starved-basin deposits accumulated on the downdropped eastern side of the zone. Paleotopographically high blocks west of the rift became sites of carbonate platforms upon which patch reefs developed in the Late Permian. Carbonate mud mounds developed along gentle margins of the platforms. Calcareous sponges, tabulozoans, hydrozoans, and algae, the latter including Archaeolithoporella, are the dominant reef-forming or baffle-forming organisms in all three types of structures. The Changxing Formation in the area is composed of five lithofacies, including (1) black, thin-bedded, widespread, basinal limestones, (2) light-gray, massive, areally limited boundstones, grainstones, wackestones, and breccias of buildup and fossiliferous carbonate platform facies, (3) dark-gray, medium-bedded limestone, (4) light-gray, structureless, areally limited dolostone, and (5) thin, interbedded shales of probably volcanic ash origin. These facies are arranged in sections that show (1) completely basin facies through the formation in the east, (2) basin facies grading upward to shallow platforms and buildups and then to submerged shelf facies in intermediate areas, and (3) basin facies grading upward to submerged shelf facies successions in western areas. Platform-margin reefs have only a few meters of overlying Permian strata, whereas Permian rocks over the patch reefs on the platforms typically are 10 to 50 m thick. Permian strata over mud mounds that formed on gentle slopes may be as much as 100 m thick. Dolomitization is most extensive in the platform-margin reefs and these contain the best hydrocarbon reservoirs. Dolomitic reservoirs occur most commonly in the upper 50–80 m of these reefs. Dolomitized reef rocks also occur in the patch reefs where the altered zone may be up to 50 m thick. Mud mounds typically have little dolomite and poor reservoir potential.