Characteristics of mixed sediments and influence on reservoir of Lower Cambrian Longwangmiao Formation, northern Sichuan Basin

Through outcrop observation and thin section identification, the mixed sediments of the Lower Cambrian Longwangmiao Formation in the northern Sichuan Basin are divided into tidal flat facies margin Ⅰ type, intershoal sea facies margin Ⅱ type, shoal facies margin Ⅲ type, open marine punctuated Ⅰ type and open marine punctuated Ⅱ type. The tidal flat facies marginⅠ type with the strongest mixing effect is mainly distributed in the bottom, middle and top of the Longwangmiao Formation. Laterally, the facies mixing sedimentation mainly distributes in the inner ramp which is close to the provenance on the west side, while the punctuated mixing sedimentation mainly distributes in the middle-outer ramp which is far away from the provenance on the east side. The Longwangmiao Formation in northern Sichuan Basin is characterized by NW-SE direction ramp sedimentation under a mixed sedimentation background. The mixed sedimentation is not conducive for the formation of a granular beach. The strong mixed deposits weakened the cementation between grain dolomites, leading to poorer compaction resistance, which is unfavorable for the formation and preservation of primary pores. Meanwhile, quartz has better stability and is difficult to dissolve in an acidic environment, which reduces the reservoir improvement by organic acids during the middle and late diagenetic stages. Therefore, the Lower Cambrian Longwangmiao Formation in northern Sichuan Basin does not have a lot of advantages to form high quality reservoir which is dominated by shoal deposits with obvious dissolution.