鄂尔多斯盆地西南部上三叠统延长组长8、长6油层组的沉积体系与物源方向

Depositional systems and provenance directions for the Chang 6 and Chang 8 reservoir groups of the Upper Triassic Yanchang Formation in the southwestern Ordos basin, China.

野外露头剖面的岩石学与岩相组合,沉积特征与相标志,古流向测定,室内砂岩的骨架矿物、重矿物组合及其平面分布规律的研究结果表明:鄂尔多斯盆地西南部上三叠统延长组长8油层组是以线状或点状物源为特征的一套近源快速堆积的冲积扇与扇三角洲沉积体系,形成于盆地由快速拗陷转入逆冲负荷沉降期间。长8沉积期盆地西南部的古水流与物源主要来自盆地西南方向,其次为西北和东南方向。长6油层组沉积期,盆地内部底床下沉作用减缓,湖盆开始收缩,湖盆西岸除北部有少量扇三角洲沉积外,主要为辫状河三角洲沉积;而盆地东北部与东部的沉积作用大大加强,致使在盆地东部形成一系列大型曲流河三角洲沉积体系,至盆地西南部相变为半深湖—深湖相与浊流相沉积。长6沉积期古水流除来自西南、西北和东南方向外,还有来自北东和正东方向的次要物源,它们在盆地西南部悦乐—玄马—板桥—固城—合水一带汇合,使该地带成为混合物源区。

Studies of the petrology, facies assemblages, sedimentary characteristics and measurements of paleocurrent directions shown by typical outcrops, as well as framework minerals and heavy minerals in sandstones and their distribution characteristics indicate that Chang 8 reservoir group of the Upper Triassic Yanchang Formation in the southwestern Ordos basin is a proximal and rapidly accumulated alluvial fan and fan delta depositional system deposited during the transition from rapid depression to thrust load subsidence, which is characterized by point or linear provenances. The main paleocurrent and source direction for the Chang 8 reservoir group in the area was the southwest, while the northwest and southeast directions were subordinate. During the deposition of the Chang 6 reservoir group, braided delta deposits predominated on the western Ordos margin besides some fan delta deposits in the northern part of the western margin, which was because the subsidence of the lake bottom in the inner part of the basin became slow and the lake began to shrink. By contrast, during this period, deposition in the eastern and northeastern parts of the basin was enhanced, thus resulting in the formation of a series of large meandering river-delta depositional systems in the eastern Ordos basin, These meanderingdelta systems were facially transfornled into semi-deep lacustrine to deep lacustrine and turbidity deposits in the southwestern Ordos basin, So during the deposition of the Chang 6 reservoir group, there were also subordinate provenances from the northeast and east in addition to those from the southwest, northwest and southeast. The Yuele-Xuanma-Gucheng-Heshui-Ningxian area in the northwestern Ordos basin became a mixed source area where the southwest, southeast, northeast and east sources converged when the Chang 6 reservoir group was deposited.