Sedimentary characteristics and influencing factors of steep slope platform-margin reefs: A case study of Changxing Formation in Kaijiang-Liangping Trough in northeastern Sichuan Basin

Gentle and steep slope platform-margin reefs are well-developed in the Changxing Formation in the Kaijiang-Liangping Trough in the northeast Sichuan Basin. At present， there is less research on the sedimentary structure and evolution model of steep slope platform-margin reefs. The exploration and development of oil and gas reservoirs in the area are restricted. Based on seismic， logging， and drilling data， the internal structure， external profile， and planar distribution characteristics of the steep slope platform-margin reefs of the Changxing Formation were systematically compared. The evolution models and influencing factors of these steep slope platform-margin reefs were discussed. The results show that the steep slope platform-margin reefs of the Kaijiang-Liangping Trough can be divided into fault-controlled steep slope type and sedimentary steep slope type. The external contour of the fault-controlled steep slope platform-margin reef shows a significant symmetry. The deposition thickness is approximately 100-200 m， and there is no lateral migration phenomenon. Moreover， it is mainly distributed in discontinuous and strip-like patterns on the plane. The width of the slope zone at the platform-margin is approximately 1.3-1.9 km. The external contour of the sedimentary steep slope platform-margin reef is asymmetrical. The deposition thickness is approximately 40-120 m， and lateral migration can be observed. It is distributed discontinuously and star-like on the plane. The width of the slope zone at the platform margin is approximately 2.0-2.5 km. The fault-controlled steep slope platform margin reefs have significant differences from the sedimentary steep slope platform-margin reefs in terms of sea level change， paleogeomorphic features， and paleoenvironmental conditions due to the occurrence of synsedimentary fault activities. It leads to obvious differences between the two in terms of external contours， structure， and distribution. Through comprehensive analysis of stratigraphy， paleogeomorphology， and sedimentary facies of the steep slope platform-margin reefs of the Changxing Formation in the Kaijiang-Liangping Trough， the differential evolution models of the three stages of the reef have been established. The adjustment period of Longtan Formation： with the rise of sea level， the environmental conditions are changed， which paves the way for the growth and development of the reef； stable growth period of Changxing Formation： at this time， there are the best environmental conditions for the survival of reef-building organisms， and the reefs develop rapidly； decline period of Changxing Formation： The reef continues to grow and begin to expose the water surface， leading to dolomitization and decline.