Characteristics and main factors controlling the Jurassic gas reservoirs in the middle section of the Western Sichuan Depression

In the middle section of the Western Sichuan Depression, lithologic and tectonic-lithologic gas reservoirs are widespread in the Jurassic.These reservoirs feature multiple layers, local enrichment, no obvious gas/water boundary, low temperature and overpressure.Accumulation conditions in the Jurassic indicate that gases were mainly sourced from the Lower Jurassic and preserved in the Upper Jurassic.The fourth and fifth members of the Xujiahe Formation provided important source rocks for natural gases.Vertical pathways connections between source rocks and reservoirs were the factors controlling natural gas accumulations.The physical properties of the Jurassic sandstones varied both horizontally and vertically.The development of favorable reservoirs also influenced the enrichment of natural gases.A large amount of vertical fractures which extended directly to the earth surface were found in the front of the Longmen Mountain and on both sides of the Longquan Mountain.They destroyed the neighboring cap formations, and damaged the sealing capacity of the Jurassic sand bodies in the adjacent slope area.Effective traps were necessary for natural gas enrichment in Jurassic.