Research On The Method Of Pre-Stack Gas Detection In The Southwestern Ordos Basin

Commercial gas was produced from He8and Shanl sandstones of Upper Paleozoic Era in Southwestern Ordos Basin. Whether the pre-stack seismic data on loess plateau region is effective to detect gas existence in sandstone is unknown, though this method does work in the desert region of the northern basin. The reservoir geological features, the petrophysical sensitive parameters cross plots, AVO forward models and pre-stack inversion are used to identify gas sandstone from mudstone or dry sandstone. The dissolved pores are the main gas-bearing sandstone reservoir space. Porosity is generally between2%and9%. The reservoir properties of He8sandstones is better than Shanl sandstones slightly. The formation from Shanl to He8sandstones is ultra. So the channel sand body of all wells is unconnected. AVO forward models show that both gas sands of He8and Shanl Members belong to mainly class I AVO anomaly, and partial Shan1gas sand fall into the class II AVO. Gas-bearing sandstone, dry sand and mudstone can be effectively distinguished by low compressive to shear wave velocity ratio on the petrophysical parameters cross plots. The shear wave velocity ratio of He8gas-bearing sandstone is less than1.75and Shanl gas-bearing sandstone is less than1.6. The gas sand distribution, pinch-out and variation in thickness can be better illustrated by low compressive to shear wave velocity ratio(<1.75) and poisson’s ratio (<0.25)on the pre-stack inversion profiles. The product of shear modulus and density (μ×p) has also some effect for identification sandstone gas.The range of gas-bearing sandstone is5～6×1013m-4kg2s-2. Pre-stack elastic parameter inversion is an effective method to detect whether the sandstones contain gas in this region.