High-resolution sequence stratigraphic characteristics of Jurassic System in the central part of Junggar Basin

The prospecting degree is low,and the subtle reservoirs are very common in the central part of Junggar Basin.The principles and methods of high-resolution sequence stratigraphy were used to predict the distribution rules of the Jurassic reservoir.Based on drilling data and seismic calibration results,the Jurassic System was divided into four long-term base-level cycles bounded by unconformity and sedimentary transform surfaces.There are mainly three types of short-term base-level cycles,which control the formation of coastal-shallow lagoon sheet sand,mouch bar and subaqueous distributary channel sediments of braided stream delta front.According to stacking patterns of short-term cycles and calibration of logs restrained by key interfaces,a log interpretation model for the Jurassic medium-term base-level cycles was established,and 14 medium-term cycles were distinguished.Under the control of dynamic variation of the accommodation-sediment ratio,the favorable reservoirs were usually developed during the rising stage of the medium-term base levels.The sandstones deposited during descending stage are laterally interconnected,while the lithologic reservoirs can not be formed easily in this stage.The drilling data confirm that the mid-lower part of SQ3 sequence is the main interval for development of lithologic reservoirs.