The Sedimentary Background and Sandstone Construction of Shallow Water Delta in Bashijiqike Formation of Cretaceous,Tarim Basin

The thickness of Bashijiqike formation of Cretaceous in Tarim Basin is over 300 meters thickness. And there has developed large-scale braid river delta deposit of lacustrine with the area of about 2000 square kilometers. It is... [ view full abstract ]

The thickness of Bashijiqike formation of Cretaceous in Tarim Basin is over 300 meters thickness. And there has developed large-scale braid river delta deposit of lacustrine with the area of about 2000 square kilometers. It is a significant series of strata for trillion cubic natural gas. The research is based on outcrop, seismic, experimental analysis data and sedimentary evolution. And the conclusions can be drawn that the pre-sedimentary setting of braid river delta deposit in Bashijiqike formation was a relatively stable basin structure phase, the paleoclimate was in relatively drought period, the lacustrine was in relieved depositional setting which was formed in leveled up the rugged geography process. The mud cracks in underlying formation and the shallow water trace fossils indicate the sedimentary water is shallow. And the shallow water, the seasonal lacustrine level fluctuation and the provenance supply during the construction period lead vertical accretion and frequent horizontal migration of distributary channels. The features of relieved shallow water lacustrine generate that the sandstone microfacies are mainly distributary channel. The ratio of sandstone is high(80%-90%), the mudstone barrier bed is thin and short(thickness 0.5-1m, length 50-100m). Therefore, the reasons of thick delta deposition thickness and the horizontal continuous of Bashijiqike formation of Cretaceous in Tarim Basin are wide-gentle slope structure setting, relatively shallow water, seasonal lake transgression and regression, abundant provenance supply and frequent migration of channels.