四川盆地桐湾期不整合面的侵蚀成因地质新证据

New geological evidence for erosive genesis of Tongwan unconformities in the Sichuan Basin

四川盆地震旦纪桐湾期不整合面具有多幕演化和复杂成因机制等争议,目前主要存在2种端元模式:大陆构造变形成因与全球海平面升降侵蚀成因。为了进一步明确扬子陆块新元古代桐湾期不整合面的成因及其对油气的控制作用,为下一步油气的勘探指明方向,基于野外露头和薄片观察、地震资料和地球化学等研究,综合揭示了四川盆地桐湾期不整合面特征、成因机制及其地质意义。研究结果表明:①桐湾期不整合面以扬子陆块台地相区灯影组二段/灯影组三段间、灯影组四段/下寒武统底界间2幕平行不整合面为典型特征,具有平行不整合和超覆不整合的结构特征;②桐湾期不整合面附近碳氧同位素呈现明显负漂移特征,灯影组四段顶部δ^(13)C值减小到-2‰以下、δ^(18)O值减小至-10‰~-12‰以下,但灯影组二段/灯影组三段间同位素负漂移相对较弱;③桐湾期平行不整合面可能为全球新元古代末期Vingerbreek冰期和Baikonur冰期事件海平面下降成因相关的侵蚀型不整合面。结论认为,桐湾期不整合面发育类型及其分布范围不仅控制着灯影组不整合面储层和下寒武统筇竹寺组含油气系统油气运移特征,而且新元古代末桐湾期构造运动期次与性质、不整合发育特征对四川盆地震旦系灯影组油气成藏具有重要意义。

The multi-episode evolution and complex genetic mechanism of the Sinian Tongwan unconformities in the Sichuan Basin are controversial,and there are currently two main end-member models,i.e.,intracontinental deformation genesis and global eustasy erosion genesis.In order to further determine the genesis and petroleum control effect of the Neoproterozoic Tongwan unconformities in the Yangtze Continental Block and point out the next petroleum exploration directions,this paper comprehensively analyzes the characteristics,genetic mechanisms and geological significance of the Tongwan unconformities in the Sichuan Basin based on outcrop and thin section observation,seismic data and geochemical analysis.And the following research results are obtained.First,the Tongwan unconformities are represented by two parallel unconformities between the second and the third member of Dengying Formation(Z_(2)dn^(2)/Z_(2)dn^(3))and between the fourth member of Dengying Formation and the bottom boundary of Lower Cambrian(Z_(2)dn^(4)/Є_(1)q)in the platform facies area of Yangtze Continental Block,presenting the structural characteristics of parallel nonconformity and overlap unconformity.Second,the carbon and oxygen isotopes near the Tongwan unconformities show the significant characteristics of negative shift.Theδ^(13)C andδ^(18)O values at the top of Z_(2)dn^(4) decline to less than-2‰and-10--12‰,respectively,while the isotope negative shift between Z_(2)dn_(2) and Z_(2)dn_(3) is relatively weak.Third,the Tongwan parallel unconformity may be an erosive unconformity whose genesis is related to the sea-level drop during the Vingerbreek and Baikonur glacial events at the end of the Neoproterozoic all over the world.In conclusion,the development type and distribution range of Tongwan unconformities control the reservoir at the Dengying Formation unconformity and the oil and gas migration characteristics of Lower Cambrian Qiongzhusi Formation petroleum system,and the stages and properties of Tongwan tectonic movement at the end of Neoproterozoic and the development characteristics of unconformities are of great significance to the Sinian Dengying Formation hydrocarbon accumulation in the Sichuan Basin.