Main controlling factors of ultra-deep sweet spot reservoir development in northern section of Baochu Slope, Xihu Sag, East China Sea Basin

The Y area in the Xihu Sag of East China Sea Basin has superior conditions for hydrocarbon accumulation, with the T oil-gas structure already discovered, demonstrating significant exploration potential. Drilling confirms that the ultra-deep sweet spot reservoirs are key to hydrocarbon enrichment in this area. However, under the background of high sand-to-ground ratio, reservoir heterogeneity is strong, and the genetic mechanisms and main controlling factors of high-quality reservoirs remain unclear, which restricts the reserve growth and production enhancement in this region. Based on the analysis of petrological characteristics, storage properties, diagenesis, and diagenetic evolution of the Pinghu Formation and Baoshi Formation in the Y area, this study investigated the genetic mechanisms and controlling factors of deep to ultra-deep sweet spot reservoirs. In the Y area, ultra-deep sweet spot reservoirs were widely developed in the Baoshi Formation. A physical-property inversion zone of the sweet spot reservoirs developed below 4 600 m. Medium- to coarse-grained, thick-bedded channel sandstones with low argillaceous content were the dominant sedimentary facies. The constructive effects of chlorite in preserving pores, secondary dissolution in increasing pores, and overpressure were the main controlling factors for the development of sweet spot reservoirs. Chlorite occurred in three forms: grain coatings, pore linings, and pore fillings.The effective inhibition of compaction and quartz overgrowth by early-stage chlorite coatings was a key factor in the formation of sweet spot reservoirs. During the early diagenetic stage A, acidic fluids generated from organic matter dissolved feldspar to form secondary pores, further improving reservoir properties. Additionally, overpressure could inhibit compaction and form micro-fractures to promote dissolution, also contributing to improved physical properties. Based on the differences in sedimentary characteristics, diagenesis, and diagenetic evolution sequences between the Pinghu Formation and the Baoshi Formation, three reservoir evolution models were established for the study area: sweet spot reservoir evolution model of "early chlorite coating pore preservation-later dissolution pore increase-late overpressure relay pore preservation and permeability increase"; tight reservoir evolution model of "early strong siliceous cementation-later weak dissolution"; and tight reservoir evolution model of "early strong compaction-middle weak dissolution - late iron calcite cementation". The first sweet spot reservoir evolution model provides a foundation for oil and gas exploration of ultra-deep formation in new areas and new fields of the Y area.