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

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 meandering-delta systems were facially transformed 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.