Platform Edge Reef and Bank Structure and Depositional Model of Changxing Formation in Panlongdong Section, Xuanhan, Northeastern Sichuan

The Changxing (长兴) Formation has two sequences (sq1 and sq2) in the Panlongdong (盘龙洞) Section, Xuanhan (宣汉), northeastern Sichuan (四川). It belongs to the platform edge facies belt, rich in reefs and banks, which were developed in the highstand system tract (HST) of sq1 and sq2, respectively. During third-order relative sea level dropping, the platform edge reef bank were mainly developed in the overall progradation parasequence sets. The platform edge reefs were developed in three phases, while platform edge banks in two phases. Outcrop observation and microscopic identification for platform egde reefs and banks in the section show that the three-phase-reefs are all composed of reef base and reef core, with string of tube sponge as the major reef-building organisms and fiber sponge, hydrazoan and bryozoans as the minor, while the reef-attached organisms mainly being foraminifera, brachiopod, echinoderm and ostracod. The first-phase-reef rock type is baffling reefs, the second phase baffling reefs and framework reefs assembles, the third phase framework reefs. Typically, the second phase reefs were developed with the maximum sedimentary scale and thickness, with the most abundant reef-building organisms and reef-attached organisms, showing best physical properties for reservoirs. The platform edge banks mainly consists of gravel debris and ooide grains, mostly dolomized. The dolomitizition in the second phase was relatively intense, favoring good reservoirs.