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西湖凹陷位于东海陆架盆地东部拗陷中部，经近２０年的勘探，已证实是个富含凝析气的凹陷。该凹陷分为５个二级构造带，沉积地层以第三系为主，主要烃源岩为始新统平湖组煤系。凝析气藏富集在凹陷的西部斜坡带和中央褶皱背科带南段，前者已发现２个油气田和３个含油气构造，凝析气产自平湖组砂岩；后者已发现４个含气构造，主要产气层为渐新统花港组中部砂岩。区内的油气富集主要受３个因素控制：①区域构造发展及局部构造形成期与生、排烃期在时空上的配合；②具有利的储盖组合；③在构造应力场中造成断层的封闭性和开启性。勘探证明，西湖凹陷必将成为我国海上的重要产气基地。

RESERVOIR FORMATION CONDITIONS AND DISTRIBUTION LAW OF NATURAL GAS IN XUHU SAG OF EAST CHINA SEA

Xihu sag is located in the central part of east depression of east China sea continental shelf basin. The 20 years exploration proves it a sag enriched with condensate gas. The sag can be divided into 5 secondary tectonic belts, the main sedimentary formation is Tertiary and the main source rock is Pinghu coal measures of Eocene. Condensate gas pools are enriched at the west slop and the central folded anticlinal belt of the sag, till now, 2 oil gas fields and 3 hydrocarbon-bearing structures tested from sandstone of Pinghu formation have been discovered at west slop; 4 gas-bearing structures have also been discovered at the southern sector of central folded anticlinal belt, but the main gas producing pay are the sandstones in the middle part of Huagang formation, Oligocene.The oil and gas enrichment is mainly controlled by 3 factors: (1)Regional tectonic development and good cooperation in the time and space of the generation, discharging and accumulation of hydrocarbon;(2)favourable reservoir-seal assemblage;(3)the closing and opening of fault in tectonic stress field.