Abstract:

In recent years，large-scaIe gas fields have been found in the Kala 2 area of the Kuqa depression，and show well prospects in gas and oiI exploration.Basjqke Formation and Baxgai Formation are the main resefvoirs．The researches for the causes of their formation can establish scientific predicting system；therefore have a profound significance．Based on the analyses of tectonic，the source of the deposits，the microstructure of the sandstone，the paIaeontoIogical species，the trace elements，the granularmetric character，the sedimentary structure and system，we can see that the strata are under arid—oxidizing climate during their formation，that the lithology of the reservoirs show a Iow—grade compositional maturity and structural maturity character, that the strata are a typical sedimentary structure and sedimentary sequence from bottom to top．It is concluded that the Cretaceous strata should be included in a braided delta，meandering river and lacustrine sedimentary system．One of the most important characters in Upper Cretaceous strata is that granularmetric character shows a transitional peculiarity，which is not only different to fluvial sediment，but also different to alluvial fan，all of which constitute a manifest character of the Cretaceous strata.

Key words: the Kuqa depression, Cretaceous, fan delta, braided river