Paleoenvironments and paleogeography of the Lower and lower Middle Jurassic coal measures in the Turpan-Hami oil-prone and basin, northwestern China

摘要

Details faciles and paleographical analyses of extensive bore-hole and otucrop data from Lower and lower Middle Jurassic oil-prone coal-bearing seqeunces in the Turpan-Hami oil-producing basin of northwest China have led to the reconstruction of a basin-wide depositional model. A total of 20 distinct lithofacies have been identified and grouped into braided fluvial plain, braided delta, meandeering channel delta, and lacustrine depositional systems. Coal-forming swamps occur in each depositional system, but the preferred sites of accumulation are interdelta bay and lower delta-plain environments in the braided delta and meandering channel depositional systems, and it is in these sites that major oil-prone source rocks are located. A series of age-specific, basinwide, paleogeographical maps have been constructed leading to a depositional model for the basin. Results indicate that the basin experienced cyclic flooding to produce swamps and lakes, and that these characterize the deposition of the Lower and lower Middle Jurassic coal measures. Analysis indicate that both the Bogda and Harlik Mountains were uplifted prior to the Early Jurassic, and a lake separated the two mountain regions during the Early and early Middle Jurassic. From these results, it is interpreted that major oil-prone coal sequences are to be found in the western part of the Taibei depression of the basin, and thus, the full economic potential of the basin has yet to be fully realized.