Reservoir Characteristics and Main Control Factors of Conglomerate Reservoir of El₃ in the Northwest Steep Slope Zone of Weixinan Depression

Based on core observation, thin section examination, scanning electron microscopy, X-ray diffraction analysis and other experimental means, the reservoir performance, diagenetic characteristics and main control factors of conglomerate reservoir quality are studied in the third member of Liushagang Formation of northwest steep slope zone in Weixinan depression, combined with physical properties, organic matter thermal evolution, burial history and logging data. The results show as follows: (1) The main rock type of study area is litharenite, and the main lithologies are glutenite, conglomeratic sandstone and other coarse grained sandstones, with varying reservoir space types such as intragranular dissolved pores, residual intergranular pores, and mold pores. The physical properties, characterized by low porosity-ultra low permeability, medium porosity-low permeability, medium porosity-permeability and high porosity-permeability, show strong heterogeneity. (2) Continuous compaction is the major reason for the deterioration of physical properties. Carbonate cementations and authigenic clay minerals filling result in the loss of some primary pores, and the sites with high argillaceous content are characterized by early strong compaction. The medium strong dissolution of coarse grained sandstone is conducive to the development of secondary pores, and micro-fracture is one of the most important flow passages in deep glutenite reservoirs. The fan-delta subaqueous distributary channel in the third member of Liushagang Formation of the northwest steep slope zone develops thick layers of glutenite and conglomeratic sandstone with low content of matrix and weak compaction. It is a high-quality reservoir development zone in the study area. The compaction of sublacustrine fan is strong, while the micro-fracture provides the flow passages for organic acidic fluid migration. The secondary dissolution of the conglomerate reservoir improves the reservoir performance and is relatively favorable reservoir for the deep sandstones.