Sedimentological Research On The Reefs Reservoirs Of Changxing Formation In Eastern Sichuan

Reef-beach is a special type of carbonate sediments, which has peculiar hydrocarbon pore volume and is an excellent carbonate reservoir. Reef-beach reservoirs are widely distributed all over the world, and they are also common in our country. For example, a series of reef gas reservoirs have been founded in Changxing Formation of Upper Permian, eastern Sichuan Basin. However, the prediction of reef-beach reservoirs has always been the difficulty of hydrocarbon exploration. The most reef gas reservoirs so far have been founded by random drilling, while the successful prediction has been few. Therefore, the research on the origin and distribution rule of reef-beach reservoirs in Changxing Formation is of theoretical and practical significance to improving the prediction successs rate.Guided by the theories of carbonate sedimentology, sedimentary dynamics and reservoir geology, the paper made a sedimentological research on the reefs reservoirs of Upper Permian Changxing Formation in eastern Sichuan, and got the following progress:A carbonate platform depositional model of the reefs was builded, and the plane distribution of sedimentary facies were drawn. Based on it, the paper made a further study on the distributional characteristics of each sedimentary facies, and got the distribution and geometry of reefs, and its relation with the reservoir.A detailed geological survey of Changxing reefs was carried out in eastern Sichuan and its adjacent areas, and a detailed analysis was also made of the platform margin reefs and intra-platform reefs. According to the reefs location and the scale of their development, the reefs in the study area were divided into4types. The paper summarized the growth stage of the4reefs types, and respectively builed the corresponding evolutionary model, which greatly enriched the research contents of Changxing reefs in eastern Sichuan.The paper made a integrated analysis on the controlling factors of reefs development in the study area, and presented for the first time that Paleogeomorphology and regional synsedimentary structures were the main controlling factors of reefs distribution, eustasy change of paleosea-level controlled the shape and scale of reefs, troughs of different natures and positions determined the types of platform margin reefs, and the major cause of reefs extinction was the frequent and rapid eustasy of sea-level on a large scale at the end of the Permian. Besides, different depositional models of the reefs were summarized in Changxing Formation, eastern Sichuan. Moreover, a statistics research on the reefs reservoir properties was conducted, which made it clear that dolomitization was the essential factor of the reefs reservoir development in Changxing Formation, and then distributive models of platform margin reefs and intra-platform reefs were established.Finally, the paper summarized the characteristics of reefs in Changxing Formation, estern Sichuan. And on the basis of reefs distribution and its forming conditions, a prediction of the Reef-beach reservoir in the study area was made.