Main controlling factors and genetic mechanism for the development of high-quality reservoirs in the mound-shoal complexes on the platform margin of the intra-cratonic rift: A case study of the fourth member of the Dengying Formation in the eastern limb of Deyang-Anyue intra-cratonic rift, Sichuan Basin, China

Reservoirs of large platform margin mound-shoal complexes of the fourth member of Dengying Formation (Deng 4 Member) are developed in the margin of the Deyang-Anyue intra-cratonic rift in the Sichuan Basin and it is the main pay horizon of the Anyue gas field. A clear understanding of the reservoir genetic mechanism of the mound-shoal complexes is the key to predicting the distribution of high-quality reservoirs and guiding the deployment of exploration. Based on the data of drilling, seismic, outcrop, and analytical data, this paper analyzes the reservoir characteristics and genetic mechanism of the mound-shoal complexes at the margin of the Deng 4 Member and obtains three new understandings: (1) Platform margin mound-shoal reservoirs are developed on the margin of Deyang-Anyue intra-cratonic rift in Sichuan Basin. The mound-shoal complexes are mainly composed of algal mounds and bioclastic shoals in multiple stages. The reservoir space is mainly dissolution pores, caverns, and fractures, with low porosity and low permeability in general. (2) The reservoir can be divided into three types, i.e., the fracture-dissolution pore type, the dissolution pore type, and the matrix pore type, and the reservoirs of fracture-dissolution pore type are high-quality reservoirs; the development of reservoirs is mainly controlled by the platform margin mound-shoal complexes, the penecontemporaneous interstratal karst, and two-stage weathering crust karstification as well as multi-stage disruptive actions; the upper part of the reservoir in the same stage is good, and the reservoir at the top of Deng 4 Member is good. (3) Before the Himalayan movement, the reservoir forming and the environments of the two platform margin mound-shoal complexes were the same, and the characteristics of the formation of the reservoir were similar. The Himalayan movement led to a great difference in the current buried depth of the reservoir, resulting in a host of fractures and retaining a host of dissolution pores and caverns. The whole platform margin mound-shoal complexes have large-scale reservoirs developed and have a good exploration prospect. The results enrich the theory of ancient and deep carbonate reservoir forming and its genetic mechanism and provide the geological basis for the deployment of exploration.