The non-marine shallow-middle gas reservoirs(with depth ranging from 500 m to 3000 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.