四川盆地西南缘山地复杂构造区页岩气富集模式及勘探启示:一个页岩气新区

Shale gas enrichment model and exploration implications in the mountainous complex structural area along the southwestern margin of the Sichuan Basin:A new shale gas area

四川盆地南部页岩气是目前中国海相页岩气勘探开发最为成功的地区,为了进一步开拓页岩气的新战场,寻找新的页岩气储量、产量接替区块,需要不断丰富、完善和建立不同构造背景下的页岩气评层选区标准。前期的勘探实践表明,四川盆地西南缘山地复杂构造区页岩含气性差异大,系统研究分析该区页岩气富集规律对于寻找页岩气勘探新区具有重要的意义。为此,基于四川盆地西南缘山地复杂构造区5口井的实钻资料,在对龙马溪组页岩露头和岩心、岩屑有机地球化学、物性、含气量、生烃史和孔隙演化史开展系统实验和分析的基础上,探讨了页岩含气性的主要控制因素和页岩富集模式,为一个页岩气新区的诞生奠定了坚实的地质基础。研究结果表明:①四川盆地西南缘山地复杂构造区龙一段1亚段页岩平均TOC为3.02%~4.97%,R_(o)为2.38%~3.37%,局部富集区平均总含气量可达4.62 m^(3)/t,属于优质页岩;②生烃史研究表明,页岩具有“慢热低熟”特征,生烃时间晚,热演化速率慢,现今成熟低,晚期扩散时间短,为页岩气的富集创造了有利条件;③孔隙演化史揭示页岩孔隙演化可以划分为孔隙锐减、孔隙减少、有机孔形成、孔隙保存、有机孔消亡和岩溶作用等6个演化阶段,有机质孔及有机碳含量是页岩含气量最直接的控制因素;④有利陆棚沉积相带利于形成规模储集空间和有效孔隙,不同构造样式和构造部位保存条件的差异性控制了不同孔隙演化阶段,山地复杂构造区页岩气具有水平分带、差异富集的特点,“慢热低熟”和“构造缓抬”有利于页岩气的长期富集与保存。结论认为,四川盆地西南缘山地复杂构造区具有“沉积控源、成岩控储、构造控保”的页岩气富集模式,该成果为寻找页岩气新区、页岩气新战场提供了思路和参考。

Marine shale gas in the southern Sichuan Basin is the most successful area of shale gas exploration and development in China.In order to open up new shale gas fields and search for new shale gas reserves and production replacement blocks,it is necessary to continuously establish and complete the standards on shale gas reservoir evaluation and area selection under different structural settings.The early exploration practice shows that shale in the mountainous complex structural area along the southwestern margin of the Sichuan Basin varies greatly in the gas bearing property,so systematical analysis and study on the shale gas enrichment mechanisms in this area is o f great significance to searching for new shale gas exploration areas.Based on drilling data of 5 wells in the mountainous complex structural area along the southwestern margin of the Sichuan Basin,the main factors controlling the shale gas bearing property and the shale gas enrichment model were discussed based on the systematical experiments and analysis of the outcrops,cores and cuttings of Longmaxi Formation shale from the aspects of organic geochemistry,physical property,gas content,hydrocarbon generation history and pore evolution history,so as to lay a solid geological basis for the birth of a new shale gas area.And the following research results were obtained.First,the shale of the first submember of first Member of Longmaxi Formation(S_(1)l_(1)^(1))in the mountainous complex structural area along the southwestern margin of the Sichuan Basin has an average TOC of 3.02%-4.97%and R_(o) of 2.38%-3.37%,and the average total gas content in local enrichment zones is up to 4.62 m^(3)/t,so it is classified as quality shale.Second,the detailed studies on hydrocarbon generation history indicate that the shale has the characteristics of"low thermal evolution rate and low maturity".It is characterized by late hydrocarbon generation,low thermal evolution rate,low current maturity and short late diffusion time,which are favorable for shale gas enrichment.Third,the pore evolution history reveals that shale pore evolution can be divided into six stages,i.e.,sharp pore reduction,pore reduction,organic pore formation,pore preservation,organic pore dissipation and karstification,and organic pore and TOC are the most direct control factors of shale gas content.Fourth,favorable shelf facies belt is conducive to the formation of large-scale reservoir space and effective pores,and diversities of preservation conditions under different structural styles and at different structural positions control different pore evolution stages.The shale gas in the mountainous complex structural area is characterized by horizontal zoning and differential enrichment."Low thermal evolution rate and low maturity"and"slow structural uplifting"are beneficial to the long-term enrichment and preservation of shale gas.In conclusion,the mountainous complex structural area along the southwestern margin of the Sichuan Basin has a shale gas enrichment model of"sedimentation controlling source rocks,diagenesis controlling reservoirs and structure controlling preservation".This research result provides idea and reference for searching for new shale gas areas and fields.