THE EVALUATION AND PREDICTION OF CRETACEOUS SILICICLASTIC RESERVOIR IN WUSHI SAG, TARIM BASIN, WESTERN CHINA

Wushi sag is an elongated sedimentary sag developing on the Paleozoic basement during Mesozoic to Cenozoic in northwest of Tarim Basin. Although previous researchers has already done some work in Wushi reservoir, the complicated tectonic evolution, great changes in palaeogeography and large amount of reservoir layers result in rough studies on Cretaceous sedimentary system. Therefore, we did a further and detailed research on sedimentary characteristics, sequence division, and reservoir evaluation of Cretaceous in Wushi sag. Based on the data of four outcrop sections and five wells, we conclude that the facies of the studied area is Mesozoic continental facies, including alluvial fan, fan delta, alluvial plain, shallow braided river delta, braided river, meandering river, and shallow-semi deep lake. Combined with thin sections, the main reservoir rock type is siliciclastic rock, which is mainly lithic sandstone, and the main space type of reservoir is dissolution pore, only a small amount of fracture pores develop. Through the analysis of profile sections, the thickness of drilling mudstone and the proportion of sandstone and conglomerate, we divided the studied area into six sequences and analyzed the distribution of sedimentary facies in the sequences. According to the geological data of well logging, well drilling as well as core analysis, a predictive model for porosity and permeability of the reservoir in the studied area was established. Based on authigenic minerals, clay minerals, S% in the I/S interlayer, the temperature of the inclusions and the types and characteristics of diagenesis, the stages of diagenesis were divided. Through comprehensive analysis, it is predicted that Mesozoic high quality reservoirs developed in the lower plain and leading edge of all kinds of delta at the margin of depression. The advantaged lithofacies in reservoir space is sandstone supported by particles with clay filled. Shushanhe formation and Kelamayi formation have superb performance of storing and accumulating hydrocarbon, developing in class II reservoir with low porosity and moderate permeability in parts. This study would provide a directive guidance for the exploration of hydrocarbon in this area in the future.