Paleogeographic framework and provenance features during Late Triassic Chang 9 time of the Yanchang Formation, Ordos Basin, China

Abstract

The Chang 9 interval at the lower part of the Upper Triassic Yanchang Formation in the Ordos Basin is a new discovery sequence for strategy replacement in the oil exploration field. By conducting a comprehensive study of outcrop sections, well drilling, drill cores, well loggings, and test data, based on the relevant research achievements of some predecessors, and combined with the analysis of geological background of this basin, it is considered that the Ordos Basin is a broad and shallow lake basin with terrigenous clastic deposition during Late Triassic Chang 9 time. The lake basin had been supplied with multiple drainage systems and provenances. The lake water in this basin is inland freshwater to brackish water. Sedimentary framework is characterized by the development of “a lake surrounded by multiple deltas” pattern. On the basis of paleocurrent analysis, distribution of sedimentary facies and sand bodies, and the quantitative analysis results of rock fragment components and trace elements of Chang 9 sandstone, it is recognized comprehensively that the Chang 9 provenances have a close connection with the basin tectonic evolution and paleogeographic background. The provenance of the Ordos Basin in Chang 9 time can be divided into five provenance systems: i.e., the northeast, north, northwest, southwest, and south provenance systems. Characteristics of rock fragment and trace element assemblages are documented in the individual provenance systems, respectively. It is inferred that terrigenous clastic sediments in Chang 9 interval mainly originate from the old lands around the basin. Parent rocks in these provenances are composed of a variety of rocks dominated by metamorphic rocks.