A Fresh Approach to Understanding the Genesis and Distribution of Widespread Reservoirs at the Basin Edge: A Case Study of Gas-Bearing Sandstone in the Sangonghe Formation of the Jurassic Period, Qianshao Area, Junggar Basin, China

A 20m-thick sandstone drilled in the second member of the Sangonghe Formation in the Qianshao area of the Junggar Basin has been identified to be a high-quality gas-bearing reservoir. However, the recent missions reveal an uncertainty on reservoir distribution and bring high risk to the subsequent gas development, although the gas-bearing reservoir has been encountered by several drillings and proved to be widespread. To investigate the accurate distribution and reveal the evolution of the reservoir, we reconstructed the stratigraphic framework and depositional stages by high-revolution sequence stratigraphic method, effectively identified the gas-bearing reservoir by seismic attributes refusion and seismic stratigraphic slices, and finally mapped the distribution strata distribution at different stages. The results show that the landward onlap phenomenon in the seismic data indicates a retrogradational stratigraphic structure during the water level rise rather than the equal-thickness stratigraphic pattern under stable water conditions. According to retrogradational stratigraphic structure, the real driver for the widespread characteristic of gas-bearing reservoir is the backward staggered movement of multi-stage sand-bodies during the transgressive process, rather than single-stage deposition. The fusion of seismic Acoustic Impedance and Main frequency attributes provides an effective survey to identify the accurate distribution of the reservoir, by which interpretation of a stratigraphic slice long the vertical centerline of the reservoir extracted in the multi-attribute fusion survey can present the distribution and evolution of the three stages on only one map. Achieved by the appropriate interpretation on the map, two sandy debris flow bodies were identified to be deposited on the bottom successively during the first two transgressive stages, they were completely staggered in the source direction with a clear boundary between well Qs9 and Qs7.