四川盆地川中隆起带震旦系—二叠系天然气地球化学特征及成藏模式

Geochemical characteristics and accumulation pattern of gas reservoirs of the Sinian-Permian in central Sichuan uplift zone,Sichuan Basin

四川盆地川中隆起带震旦系—寒武系探明天然气储量超过万亿立方米,发现中二叠统栖霞组气藏,洗象池组、茅口组、玄武岩组、长兴组等层系获工业气流,不同层系气藏天然气地球化学特征存在明显差异。基于大量新获气井天然气组分、碳同位素、氢同位素等分析实验数据,系统研究了川中隆起带天然气成因及成藏特征,明确了下一个万亿立方米级重点勘探领域。研究结果表明:(1)震旦系、寒武系和二叠系天然气均是以烃类气体为主且干燥系数大于0.997的干气,属于原油裂解气。绝大多数二叠系天然气C_(2)H6含量、δ^(13)C_(2)、δ^(2)HCH_(4)与源自下寒武统筇竹寺组烃源岩的寒武系天然气相似;上震旦统灯影组天然气C_(2)H6含量低、δ^(13)C_(2)重、δ^(2)HCH_(4)轻;GS19栖霞组气藏天然气C_(2)H_(6)含量高、δ^(13)C_(2)轻、δ^(2)HCH_(4)轻。C_(2)H_(6)与δ^(13)C_(2)差异主要与成熟度有关,δ^(2)HCH_(4)主要受原始母质沉积水介质盐度影响。(2)安岳气区与太和气区聚集了不同演化阶段的原油裂解气,其中,安岳气区天然气δ^(13)C比太和气区重,主要聚集原油晚期阶段的裂解气,太和气区聚集早期—晚期裂解气。(3)震旦系—二叠系气藏具有单源和双源聚集模式。寒武系气藏和GS19栖霞组气藏为单源聚集模式,烃源岩分别为筇竹寺组和龙马溪组,灯影组气藏和其他二叠系气藏为双源供烃成藏模式,除均有筇竹寺组烃源岩贡献外,分别还有震旦系烃源岩和二叠系烃源岩的贡献。研究认为,不同时代烃源岩贡献比例大小是造成不同层系气藏天然气地球化学特征差异的主要原因,太和气区多层叠置连片的优质储层紧邻烃源灶或处于古油藏范围,具备万亿立方米级资源规模,勘探潜力大。

The natural gas resources are abundant in central Sichuan uplift zone,with proven reserve of more than one trillion cubic meters of the Sinian-Cambrian,gas discovery from the Middle Permian Qixia Formation,and commercial gas flows from Xixiangchi Formation,Maokou Formation,Xuanwuyan Formation and Changxing Formation.The geochemical characteristics of various gas reservoirs vary greatly.A large number of experimental data of natural gas composition,carbon isotope and hydrogen isotope are analyzed from newly drilled gas wells to systematically study the genesis and accumulation characteristics of gas reservoirs in central Sichuan uplift zone,so as to identify the key exploration areas with gas reserve of trillion cubic meters.The study results show that:(1)The gas reservoirs of the Sinian,Cambrian and Permian are dry gas mainly composed of hydrocarbon gas with drying coefficient of greater than 0.997,which is crude oil pyrolysis gas.The geochemical characteristics of C_(2)H_(6)content,δ^(13)C_(2)andδ^(2)HCH_(4)of the Permian gas reservoir is similar with those of the Cambrian gas reservoir generated by source rock of the Lower Cambrian Qiongzhusi Formation.For example,gas reservoir of Qixia Formation in GS19 is characterized by high C_(2)H_(6)content,lightδ^(13)C_(2)and lightδ^(2)HCH_(4),while gas reservoir of the Upper Sinian Dengying Formation has low C_(2)H_(6)content,heavyδ^(13)C_(2)and lightδ^(2)HCH_(4).The difference of C_(2)H_(6)content andδ^(13)C_(2)is mainly related to maturity,and that ofδ^(2)HCH_(4)is mainly affected by water salinity of the parent sediments.(2)The gas reservoirs in Anyue and Taihe areas are crude oil pyrolysis gas in different evolution stages,in which the former is mainly in the late stage,while the latter is in the early-late stage.Theδ^(13)C_(2)of gas reservoir in Anyue area is heavier than that in Taihe area;(3)There are two types of accumulation pattern of the Sinian-Permian gas reservoirs,i.e.,single-source and dual-source.The Cambrian gas reservoir and Permian Qixia gas reservoir in GS19 are single-source accumulation pattern,with source rocks of Qiongzhusi Formation and Longmaxi Formation,respectively.The Sinian Dengying gas reservoir and other Permian gas reservoirs are dual-source accumulation pattern,which are supplied by source rock of Qiongzhusi Formation,and supplemented by the Sinian and Permian source rocks respectively.The study indicates that the difference of geochemical characteristics of different gas reservoirs is mainly caused by the contribution ratio of source rocks of different geological ages.The superimposed and contiguous high-quality reservoir in Taihe area is closely adjacent to source rocks or within the range of paleo oil reservoir,which has a resource scale of one trillion cubic meters,showing great exploration potential.