四川盆地8000m海相超深层天然气成藏条件及有利勘探区带

Accumulation conditions and favorable exploration zones for natural gas in 8000 meters marine ultra-deep strata in the Sichuan basin

超深层(埋深>6000 m)是全球油气勘探的重大领域。当前,四川盆地天然气勘探已经突破8000 m超深层,但是盆地内8000 m超深层领域是否具备规模油气成藏条件、有利勘探区带分布在哪里等重大问题亟待破解。基于烃源岩、有利相带、储集层、圈闭类型、成藏组合等研究,对四川盆地8000 m超深层领域成藏条件与有利勘探区带进行了梳理和评价,得出以下认识:(1)四川盆地超深层领域主要发育于震旦系—中二叠统,且主要分布在环川中隆起带的川西坳陷、川北坳陷、川东褶皱带的深凹地区,其中川西北地区最具勘探潜力;(2)超深层烃源岩明显受凹陷或台内裂陷控制,主要发育陡山沱组、灯影组三段、麦地坪组和筇竹寺组等四套烃源岩,为8000 m超深层油气勘探提供了雄厚的物质基础;(3)震旦系—古生界主体发育(丘)滩孔隙型、岩溶孔洞型和白云岩孔隙型三类基本储层类型,三者相互叠加兼具裂缝改造形成多套规模储层,为超深层油气勘探提供了有利储集条件;(4)多套烃源岩和储层以走滑或逆冲断层相互沟通,主要发育下生上储、旁生侧储两种成藏组合,立体成藏特征明显;(5)岩性圈闭是震旦系—二叠系超深层气藏的主要圈闭类型,大型斜坡背景上发育的背斜或者鼻状构造叠合岩性体形成的构造-岩性圈闭也是重要的勘探对象;(6)8000 m超深层发育六大领域9个有利勘探区带,累计勘探面积超4.5万km^(2),资源量约3×10^(12)~5×10^(12)m^(3)。研究认识对于四川盆地超深层天然气勘探具有重要理论和现实意义。

The ultra-deep layer with depth>6000 m is a major field of global oil and gas exploration.At present,the gas exploration in Sichuan basin has broken through the 8000 m ultra-deep layer,but the scientific questions such as whether the 8000 m ultra-deep field in the basin has the conditions for large-scale gas field accumulation,and where the favorable exploration zones are distributed have not been clearly answered.Based on the study of source rocks,favorable facies zone,reservoir,trap types,and reservoir forming assemblages,the reservoir forming conditions and favorable exploration zones in the 8000 m ultra-deep strata of Sichuan basin were evaluated in detail,and the following conclusions were obtained:(1)the ultra-deep strata of Sichuan basin is mainly developed in the Sinian-Middle Permian,and is mainly distributed in the deep western depression,northern depression and the fold belt in eastern Sichuan.Among them the western depression has the largest exploration potential;(2)the ultra-deep source rocks are obviously controlled by depressions or intra platform rifts,and mainly develop four sets of source rocks,including Doushantuo Formation(Fm),third Member of Dengying Fm,Maidiping Fm and Qiongzhusi Fm,which provide an abundant material basis for 8000 m ultra-deep gas exploration;(3)there are three basic reservoir types in the main body of Sinian-Paleozoic layer,the beach(mound)controlling porous type,karst vuggy type and dolomite porous type,which are superimposed with each other and combined with fracture altered to form large-scale multiple reservoir layers,providing favorable reservoir conditions for ultra-deep gas exploration;(4)many sets of source rocks and reservoirs communicate with each other by strike slip or thrust faults,and mainly develop two kinds of reservoir forming combinations of lower source and upper reservoir,and paraegenic and lateral reservoir assemblages forming obvious three-dimensional reservoir characteristics;(5)lithologic trap is the main trap type of ultra-deep Sinian-Permian gas reservoir in the Sichuan basin,and the structural-lithologic trap formed by anticline or nose structure superimposed lithologic body developed on the background of large slope is also an important exploration object;(6)9 favorable exploration zones in 6 major fields,with a cumulative exploration area of more than 45000 km^(2)are developed in the ultra-deep strata with depth more than 8000 m,and a resource volume of about 3×10^(12)~5×10^(12)m^(3)was predicted.The research of this work is of great theoretical and practical significance for the ultra-deep natural gas exploration in Sichuan basin.