鄂尔多斯盆地及邻区中—晚二叠世构造-沉积环境与原型盆地演化

Tectonic-depositional environment and prototype basin evolution of Middle-Late Permian in Ordos Basin and adjacent areas

根据钻井、测井、野外露头及地球化学资料,参考盆地周缘构造环境、盆地构造沉降特征及沉积相展布,恢复了鄂尔多斯盆地中—晚二叠世不同时期构造-沉积环境,探讨了盆地性质及演化过程。研究结果表明:①中—晚二叠世鄂尔多斯盆地南缘和北缘经历了洋盆俯冲消减过程,以强烈的挤压构造环境为特征,上二叠统石千峰组沉积期盆地北缘古亚洲洋闭合进入陆陆碰撞阶段,盆地内构造沉降速度缓慢。②中—晚二叠世鄂尔多斯盆地为大型的克拉通内坳陷盆地,中二叠统石盒子组沉积期基准面呈下降趋势,盆地主体延续山西组沉积期南北高、中部低的古地理格局,南华北地区地势相对更低;周缘隆起区快速抬升且范围逐渐扩大,为坳陷盆地内提供物源,盆地内发育冲积扇、冲积平原、三角洲和浅湖相,向南发育残存潮坪—潟湖,呈南北向展布;晚二叠世基准面趋于稳定,湖相范围扩大,主要发育于盆地南部,潮坪—潟湖环境向南缩小。③研究区油气成藏受控于沉积环境,中二叠世发育的陆相三角洲沉积体系有利于形成储集层和盖层,且和下部煤系地层组成中二叠统良好的生-储-盖组合;上二叠统为氧化干旱的古气候,沉积水体较浅,形成的泥岩厚度小,生烃潜力有限,无法形成大规模气藏。

According to drilling,logging,field outcrops and geochemical data,combined with peripheral tectonic environment,tectonic subsidence characteristics and sedimentary facies distribution characteristics,the tectonic-depositional environment in different periods of Middle-Late Permian in Ordos Basin was restored,and the nature and evolution of the basin were discussed.The results show that:(1)The northern and southern margins of Ordos Basin experienced subduction of ocean basins in the Middle to Late Permian,which was characterized by strong compressive tectonic environment.During the sedimentary period of Upper Permian Shiqianfeng Formation,the northern margin of Ordos Basin entered the stage of continental collision when the Paleo-Asian Ocean closed,and the tectonic subsidence in the basin was slow.(2)The Ordos Basin was a large intra-craton depression basin in Middle-Late Permian,and the base level of Middle Permian Shihezi Formation showed a downward trend.The main part of the basin followed the paleogeographic pattern of Shanxi Formation,which was high in the north and south and low in the middle,and the topography of the southern North China was relatively lower.The sur‐rounding uplift areas were rapidly uplifted and gradually expanded,providing provenance for the depression basin.Alluvial fans,alluvial plains,deltas and shallow lacustrine facies developed in the basin.To the south,the remaining tidal flat-lagoon was developed,showing a north-south distribution.In the Late Permian,with the base level tended to be stable,the lacustrine scope expanded and the tidal flat-lagoon environment narrowed southward.(3)The sedimentary environment in the study area plays a controlling role in hydrocarbon accumulation.The terrestrial delta depositional system developed in Middle Permian is conducive to favorable reservoirs and cap rocks,and form favorable reservoir-reservoir-cap assemblages with lower coal measures in Middle Permian.Due to the paleo-climate of oxidation and drought in Upper Permian,shallow water bodies and thin mudstone,the hydrocarbon generation potential is limited and it cannot form large-scale gas reservoir.