四川盆地下古生界油气地质条件及气藏特征

采用地表与地下地质相结合，应用有机和无机地球化学分析技术刻画了四川盆地下古生界油气地质条件，分析了下古生界气藏特征，揭示了古油藏与气藏相互耦合关系，初步建立了下古生界油气成藏模式。下寒武统和下志留统是四川盆地下古生界的2套高品质烃源岩，平均厚度约为100 m，两者分布区域具有显著继承性，存在川南、川东及鄂西3个生烃中心。储层主要为中上寒武统洗象池组,中下奥陶统南津关组及宝塔组，下志留统石牛栏组或小河坝组；基质孔隙度小于1%者占70%～80%，但局部层段发育岩溶孔洞、裂缝型及礁灰岩等优质储层，有效孔洞率可达4%～11%。下古生界发育有上奥陶统-下志留统泥质岩、中下寒武统膏质岩2类区域性盖层，其主要分布在川南地区和川东北地区。四川盆地下古生界天然气显示主要分布在乐山-龙女寺古隆起区，层位繁多。天然气干燥，非烃含量高，多为原油裂解气，川东、川南气源分别主要为志留系烃源岩和寒武系烃源岩。四川盆地下古生界油气成藏共经历了如下过程：油气生成→古油藏（捕获成藏）→古油藏调整(跨层运移)→古气藏(高温裂解)→古气藏遭受破坏或形成现今气藏(构造调整)。古油藏原油裂解气是下古生界现今气藏天然气的主要来源，古油藏对四川盆地现今天然气藏的成藏和分布具有重要的控制作用。

Geological Conditions and Gas Reservoir Features in Lower Paleozoic in Sichuan Basin

By means of surface and subsurface geological combination research,we use the organic and inorganic geochemistry to describe the geological conditions of lower Paleozoic gas formation and gas reservoir characteristics in the Sichuan basin,reveal mutual coupling relations between the destroyed oil pool and gas reservoir,and establish the model of lower Paleozoic petroleum accumulation.Both lower Cambrian and lower Silurian are two sets of high-quality source rocks in the Sichuan basin,with an average thickness of about 100m.Their distributions show a significant inheritance,with three centers of hydrocarbon-generating intensity like as southern Sichuan,eastern Sichuan and western Hubei.Several gas layers in the lower Paleozoic sequence in the Sichuan basin exist,including the middle-upper Cambrian Xixiangchi Formation,lower and Middle Ordovician Nanjinguan Formations and Baota Formations,and lower Shiniulan Formation(called Xiaoheba Formation in east Sichuan basin).The matrix porosity of reservoir less than 1% accounts for 70%-80%,but the high quality reservoirs such as karstic,fracture,and reef limestone reservoir with the effective porosity of 4%-11% are locally developed.The upper Ordovician,lower Silurian mudstones and the middle-lower Cambrian gypsum rocks as two types of regional cap rock,are mainly distributed in the southern and northeast parts of Sichuan basin.The destroyed oil pools and natural gas suggests the potential gas field in lower Paleozoic reservoir would be mainly distributed in the Leshan-longnvshi paleo-uplift areas.The Paleozoic natural gas mostly derived from oil cracking has high dry coefficient,and high content non-hydrocarbon gas.The natural gas in the eastern and southern Sichuan basin is sourceed from the Silurian and Cambrian source rocks,respectively.The process of gas accumulation in lower Paleozoic is as follows: oil generation→early oil pool(accumulation) →oil pool adjustment(transformational migration)→early gas reservoir(oil cracking) →gas reservoir destroyed or kept in present-day(tectonic adjustment).Oil cracking in early oil pools are main source of present lower Paleozoic gas reservoirs,and the early oil reservoir has importantly controlled accumulation and distribution of present-day gas reservoir.