The Late Permian organic reefal oil pool section in Panlongdong, Xuanhan, Sichuan

The Panlongdong organic reefs are developed in the middle and upper parts of the Upper Permian Changxing Formation, with a thickness of about 80 m. The organic reefs consist dominantly of light gray massive spongy bafflestone and spongy skeletal limestone. The reef-building organisms are composed of room-type sponges, polys, bryozoans and so on. The reef-attached organisms comprise brachiopods, pelecypods and gastropods. The frame-builders make up about 30 to 50%. The fillings are made up of micritic calcite, with a content of 50 to 70%. Abundant pore spaces are well connected, and almostly filled by calcite crystals. The occurrence of abundant asphalt in the organic reefs and the strata above and below them has disclosed the migration of oil and gas. Two phases may be identified for the filling processes of the asphalt:the one is the filling into organic frameworks, intergranular pores, sponges and other organic coelomata such as brachiopods; the other is the filling into paulopost fissures and secondary pores. In this phase, the asphalt is widespread in the organic reefs and their cap rocks (including spararenite and dolostone). The asphalt occurs on a large scale in this phase, and the asphalt-containing strata amount to about 120 m in total thickness.