鄂尔多斯盆地及邻区早二叠世构造-沉积环境与原型盆地演化

Tectono-depositional environment and prototype basin evolution in the Ordos Basin during the Early Permian

鄂尔多斯盆地下二叠统致密砂岩气近年来取得重大突破,发现一系列大型致密气田。研究该时期的构造-沉积环境与原型盆地是探究鄂尔多斯盆地及邻区演化过程和油气进一步勘探的基础。本文运用最新的钻井、野外露头等地质资料,结合前人的研究成果,重建鄂尔多斯盆地及邻区早二叠世各沉积时期的构造-沉积环境,探讨原型盆地性质及演化过程。早二叠世构造-沉积环境受北缘古亚洲洋和南缘勉略洋俯冲的影响,以活动大陆边缘为特征。太原组在晚石炭世的古地势基础上继承性发育,基准面呈上升趋势,以广阔的潮坪-潟湖环境为主,见小型潮控三角洲。山西期是鄂尔多斯盆地海陆过渡的关键时期,海水快速从盆地东南部退出。山西组沉积早期,受区域性挤压作用影响,形成以三角洲体系为主的海陆过渡体系,盆地东部残存潮坪-开阔台地;晚期基准面整体先上升后下降,东部残存海相沉积环境向南退去。山西组沉积期后,盆地主体进入陆相沉积阶段。早二叠世盆地以稳定的沉降为特征,体现了稳定克拉通性质。由此可见,早二叠世盆地性质表现为克拉通内拗陷的特征,周缘构造环境、湖盆的差异沉降以及古隆起、古拗陷的发育对该时期的沉积充填演化具有重要影响,控制着沉积中心向盆地东部扩展。

Major breakthroughs have been made in tight-gas sandstone exploration in the Lower Permian of the Ordos Basin with the discovery of a series of large tight-gas fields in recent years. Studying the tectono-depositional environment and prototype basins of this period is fundamental to understanding the evolutionary process of the Ordos Basin area, and is helpful for advancing oil and gas exploration in this region. Using the latest borehole and outcrop data and combining previous research results, we reconstructed the regional tectono-depositional environment in the Early Permian. Furthermore, we explored the properties and evolution of the prototype basin.The tectono-depositional environment of the Early Permian is characterized by active continental margins, impacted by subduction of the Paleo-Asian Ocean in the northern margin and Mianlue Ocean in the southern margin. The inherited Taiyuan Formation developed on the basis of the paleo-terrain of the Late Carboniferous, and the base plane indicates an upward paleogeography in this period dominated by a broad tidal flat and lagoon environment, with small tide-dominated delta formed in the north and south of the basin. The Shanxi period was the key period of sea-land transition in the Ordos Basin as seawater quickly retreated from the southeastern part of the basin. During the early depositional stage, the Shanxi Formation, influenced by regional compression, formed sea-land transitional system dominated by delta system, with residual tidal flat-open platform in the eastern part of the basin;in the late stage, base plane first rose and then fell, and the residual marine facies receded southward. After deposition of the Shanxi Formation, continental sedimentation began. The basin was characterized by stable subsidence in the Early Permian, with stable cratonic properties. Thus the Early Permian basin had the characteristics of an intra-cratonic depression. Peripheral tectonic environment, differential subsidence of lake basins, and development of paleo-uplift/depression had a significant impact on the evolution of sedimentary filling during this period in controlling the expansion of the deposition center to the east of the basin.