| Summary: | This paper discusses the exploration and development history and lessons of the large ultra-deep biogenic reef gas field in Yuanba area, where the previous “Kaijiang-Liangping shelf” model provides an insight for the discovery of the biological reef and bank on the platform margin. Systematic analysis of key geological conditions for hydrocarbon accumulation shows that the reef and bank reservoirs in Yuanba area are characterized by row and zonal distribution along “Kaijiang-Liangping shelf”, the bank in early and the reef in late, the reef in the front and the bank in the back, and dominated by lateral accretion and progradation. The major exploration target of high-quality reef-bank reservoirs are developed in Changxing Formation instead of in Feixianguan Formation, Three stages of fractures and dissolution, and dolomitization control the development of the high-quality reservoirs. Two sets of effective source rocks (Permian Dalong Formation and Wujiaping Formation) provide sufficient gas source for the large gas field. Three dimensional migration pathway system composed of micro-faults, micro-fractures and interlayer fissures facilitates the hydrocarbon migration and accumulation. The accumulation model of Yuanba gas field is characterized by the near-source accumulation, three -micro migration pathway system, lithological-stratigraphic reservoirs and tectonics controlling enrichment. According to the complex geological conditions such as ultra-deep reservoirs and multiple pressure systems in Yuanba gas field, several key technologies are developed during hydrocarbon exploration and development, including fine prediction of ultra-deep reef-bank reservoirs and identification of gas and water, fine-scale reservoir description and characterization of complex thin and small reef gas reservoirs, optimal & fast drilling of ultra-deep horizontal well, geo-steering of ultra-deep horizontal well for complex reef and bank. All these technologies resulted in the efficient exploration and development of Yuanba gas field and accelerated the innovation of theoretical technology and methods for ultra-deep reservoirs. Keywords: Yuanba gas field, Ultra-deep, Biogenic reef and bank, Micro-migration pathway system, Identification of gas and water, Optimal and fast drilling
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