The non-marine shallow-middle gas reservoirs(with depth ranging from 500 m to 3000 m)in western Sichuan depression are near-normal to tight sandstone of distal facies.These reservoirs are mainly distributed in the Jurassic and Cretaceous formations where large-to medium-sized gas fields,such as Xinchang,Luodai,Xindu,Majing,and etc,are harbored.The shallow-middle reservoirs in western Sichuan depression feature in well-developed reservoirs,favorable transportation conditions,ideal matching of reservoir distribution and structure configuration,and good sealing.Sedimentary micro-facies reservoirs are mainly composed of delta front sill-like sand,delta front distributary channel sand,estuarine sand bar,and sublacustrine fan sand.The reservoirs typically show low wave impedance and large wave impedance difference with surrounding rocks.The seismogeological conditions are rather unfavorable in western Sichuan depression,where the surface is covered by unconsolidated pebble and sandy soil beds with thickness ranging from several meters to 200 meters,resulting in severe high frequency attenuation and strong interference waves,such as surface wave,acoustic wave and refraction wave,and low S/N ratio.Therefore,a successful gas reservoir recognition in the study area involves the application of high-precision 3D seismic data acquisition in areas covered by unconsolidated pebble layers,3D seismic processing with gas bearing seismic responses preserved,reservoir seismic attributes extraction and analysis based on forward modeling,and reservoir spatial delineation and multi-disciplinary integrated evaluation.The exploration and production of the shallow-middle gas reservoirs in the study area show that seismic exploration plays an important role in delineation of reservoir distribution,description of the spatial geometry of the reservoirs,inverse modeling of the reservoir parameters,study of sedimentary facies and heterogeneity,determining prolific gas-bearing zone,and recognition of gas-bearing properties.
