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Based on analysis of outcrop, drilling, logging and seismic data, and geotectonic background, the lithofacies paleogeography and paleokarst geomorphology of the Middle Permian Maokou Formation in the northwestern Sichuan Basin were reconstructed, and the petroleum geological significance of the lithofacies paleogeography and paleokarst geomorphology were discussed. The Maokou Formation is divided into 3 long-term cycles, namely LSC1, LSC2 and LSC3, which correspond to the Member 1, Member 2 and Member 3 of the Maokou Formation, respectively. Controlled by the extensional structure caused by opening of the Mianlue Ocean in the north margin of the upper Yangtze blocks and basement faults produced by mantle plume uplifting, the area had tectonic differentiation in NWW and NE, and sedimentary basement took on episodic settlement from north to south, as a result, the sedimentary systems of Member 1 to Member 3 gradually evolved from carbonate platform to platform-slope-continental shelf. According to the residual thickness, paleokarst geomorphologic units such as karst highland, karst slope and karst depression at different stages were reconstructed. The karst geomorphological units were developed successively on the basis of sedimentary geomorphology. Sedimentary facies and paleokarst geomorphology are of great significance for oil and gas accumulation. The Maokou Formation in northwestern Sichuan has two kinds of most favorable reservoir zone combinations: high energy grain shoal and karst monadnock, platform margin slope and karst slope. Based on this understanding, the planar distribution of the two kinds of reservoir zones were predicted by overlapping the favorable reservoir facies belt with paleokarst geomorphology. The study results provide a new idea and reference for the exploration deployment of the Middle Permian Maokou Formation in the Sichuan Basin.

The resource potential, “fractured” reservoir, reservoir type and stimulation technology of Jurassic tight oil in central Sichuan basin were investigated. Based on the evaluation method of tight oil resource, the amount of oil resources of the five Jurassic layers were estimated at 1.6 billion tons, showing that the Jurassic oil resource potential in central Sichuan Basin is huge. Integrated analysis of static and dynamic data including core-description, thin section examination, well test and exploration show Jurassic reservoirs are fracture-pore reservoir with ultra-low porosity, super-low permeability, rather than single “fractured” reservoir believed before. Jurassic reservoirs are tight oil reservoir with the characteristics of near source charge and continuous distribution. The high-quality hydrocarbon source, reservoir with better physical properties and fracture are the main factors controlling tight oil enrichment. “Horizontal well + volume fracturing” of Shayi Member tight sandstone reservoir and the “horizontal well + fracture acidizing” for the Da'anzhai Member shell limestone reservoir have achieved good stimulation effect. Key words: Central Sichuan Basin, Jurassic, tight oil, resource potential, tight reservoir, enrichment rule, reservoir stimulation

The old Sinian carbonate reservoir in the Leshan-Longnüsi paleohigh was taken as a research object to study the process of gas accumulation in the Sinian reservoir through analysis of gas reservoir characteristics, gas composition, gas reservoir types, accumulation condition and evolution. The results show that the reservoir lithology and type are almost the same in the six gas pools discovered in the Leshan-Longnüsi paleohigh. All the gas reservoirs are characterized by high temperature, ordinary pressure, and intense heterogeneity. The gas reservoir type in different layers and the gas compositions and carbon isotopes in different locations vary obviously. The gas of Sinian Dengying Formation, originated from oil cracking, is mixed gas mainly from source rocks of Sinian Dengying Formation as well as Cambrian Qiongzhusi Formation. The source and reservoir condition, their combination and fluid transporting conditions are favorable, which can determine the gas accumulation and preservation in Dengying Formation. The Sinian gas reservoirs are believed to have been accumulated by the following processes: paleo-oil accumulation, paleo-oil cracking, and gas reservoir adjustment and finalization. There are three processes of gas accumulation in the reservoir, which are influenced by the formation of paleohigh and differential structural evolution in different positions. Key words: Sichuan Basin, Leshan-Longnüsi paleohigh, Sinian System, Dengying Formation, natural gas, accumulation condition, accumulation process

Based on observation of cores and casting thin sections, reservoir fluorescence, hydrocarbon geochemical characteristics and oil production test data, the factors controlling the tight oil accumulation and high production in the Da'anzhai Member in the Jurassic Ziliujing Formation in central Sichuan Basin are studied. The shelly limestone, muddy shell limestone and tight limestone are all oil-bearing in the Da'anzhai Member. The dissolved pores and fractures in shelly limestone and the matrix pores (microfractures, intercrystal pores) in tight limestone are all oil-bearing in large area, which is the reason why oil wells can keep low production for a long period of time. On the basis of the extensive oil-bearing, natural gas from the Upper Triassic Xujiahe Formation provided the major power for oil migration and accumulation in the Da'anzhai low amplitude, water-free tight limestone reservoirs, driving the dispersed oil from the matrix pores into an accumulation, so the oil is more enriched in the western structures of central Sichuan Basin where oil wells are higher in gas-oil ratio. In contrast, in the east of central Sichuan, the Huayingshan major faults generally cut through Da'anzhai Member, allowing gas to migrate to the formations above Da'anzhai Member along fractures to accumulate or dissipate, without gas as driving force, as a result, the oil is less enriched than that in the west part of central Sichuan. Key words: tight oil, central Sichuan Basin, Jurassic Da'anzhai Member, enrichment and high production, gas drive, fault

Since 2011, huge gas fields have been discovered in the Sinian and Cambrian strata in Moxi–Gaoshiti region, central Sichuan paleo-uplift, with the geological reserves reaching more than one trillion cubic meters. The main gas-bearing layers include Deng 2 Member, Deng 4 Member and Longwangmiao Formation. Based on brief review of the exploration history of large gas province in central Sichuan paleohigh, the paper systematically describes the formation conditions and petroleum accumulation patterns of huge gas fields there, and points out the large ancient uplift background, huge net of hydrocarbon supply system, large-scale grain shoal reservoirs and regional reservoir-cap combinations are important material conditions for the formation of large gas fields. In view of the huge depth, high temperature, high pressure and some other complex formation conditions, a lot of efforts have been put into the research on logging, seismic, drilling, reservoir stimulation etc, and a series of accomplishments have been achieved, which work well in the exploration and development of gas in Sichuan Basin. The Sinian and Cambrian strata in the Sichuan Basin have a great potential for petroleum exploration. Key words: Longwangmiao Formation, paleohigh, giant field, strategic discovery, aulacogen, central Sichuan paleouplift