琼东南盆地西南部深水古潜山天然气成藏模式OA

Pre-Paleogene granite buried-hill gas reservoirs represent a new frontier for deepwater-ultradeep-water oil and gas exploration in the Qiongdongnan Basin in recent years.Such deepwater buried-hill reservoirs are characterized by typical"young-generation old-storage"features,and their hydrocarbon accumulation his-tory and trap evolution differ distinctly from those of Neogene clastic traps.Based on geochemical analyses of natural gas samples from regional wells,homogenization temperature measurements of fluid inclusions,and reconstruction of hydrocarbon accumulation histories,this study systematically investigates the hydrocarbon migration,accumulation and pooling processes of the buried-hill belt in the Lingnan Low Uplift,southwestern deepwater Qiongdongnan Basin,and establishes a corresponding hydrocarbon accumulation model.Results show that hydrocarbon supply to the W321 buried-hill gas reservoir in the Lingnan Low Uplift is mainly derived from coal-measure source rocks of the Oligocene Yacheng Formation.Compared with middle-shallow Miocene and Pliocene gas reservoirs in the central turbidity channel of the adjacent area,natural gas in this buried-hill reservoir exhibits higher methane content and dryness coefficient,as well as heavier carbon isotopic composi-tions of methane and its homologues.Homogenization temperatures of fluid inclusions in the buried-hill reser-voirs range mainly from 129.6℃ to 135.3℃,corresponding to a single episode of intensive gas charging dur-ing the late stage of the Late Miocene Huangliu Formation to the Pliocene Yinggehai Formation(approximately 5.5 Ma).In addition,major fault activity in the buried hills of the Lingnan Low Uplift ceased by the end of the Miocene Sanya Formation,indicating early trap formation.Such"early charging and early stabilization"buried-hill traps are clearly favorable for natural gas migration,accumulation,enrichment and preservation.In sum-mary,the natural gas accumulation model of the buried hills in the Lingnan Low Uplift can be summarized as follows:natural gas generated from the Yacheng Formation source rocks in the adjacent Ledong-Lingshui Sag migrates laterally along a carrier system composed of structural ridges,source-connected faults and tectonic fractures,and finally accumulates in high-quality weathered crust and fractured reservoirs of Pre-Paleogene granite within buried-hill traps,forming"young-generation old-storage"and"lateral-generation upper-storage(old-storage)"gas reservoirs.The findings of this study provide important guidance and reference for explora-tion evaluation and new frontier expansion of deepwater-ultradeepwater buried-hill oil and gas resources,par-ticularly for the optimization of favorable exploration zones and drilling targets in buried-hill plays.