Depositional System and Development Model for Bashijiqike Formation in Northeastern Tarim Basin

The Cretaceous Bashijiqike formation in northeastern Tarim basin was the product of lacustrine basin margin alluvial fan deposition involving two major sedimentary systems: an alluvial fan-braided fluvial delta depositional system and a fan delta depositional system. The alluvial fan-braided fluvial delta depositional system is composed of an alluvial fan facies,a braided fluvial delta plain facies and a delta front facies from the foothill belt in S-N direction to the basin center. The fan delta depositional system is mainly composed of fan delta front subfacies,which developed near Kuqa depression in the northwest of the study area. When Bashijiqike formation was deposited,the lake was shallow and the climate was arid,which led to deposits with a high sand ratio. During transgression strata deposited in the lake basin margin is thicker than that in the basin center. During regression,in contrast,the basin center experienced more deposition than the margins. Drilling and seismic data show that the northeast of the study area was controlled by single terrigenous depositional source from the paleo-Tianshan mountains in north. Deposition in the southern part of the basin was controlled by the East Kunlun-Altun mountains during transgression and regression. Deposition in central area was controlled by a single sediment source which was gradually supplanted by multiple sources. Therefore,the strata deposited during regressive phase in lake basin center are of good reservoir quality.