The Mixing And Gas Pool Effect Of The Fluid In The West Sichuan Foreland Basin

This article discussed the migration and variation of fluid from the Upper Triassic and Jurassic series in a compact sandstone-type natural gas field, West Sichuan Foreland Basin, based on the concentration of and the relationships among Na++K+ , Ca2+ , Mg2+, Cl-, TDS, the water chemistry characteristic parameter(rNa+/rCr-, 100δrSO42-/rCl-, r Mg2+ /rCl-, r Mg2+/r Ca2+), stratum water mould, pH value, hydrogen-oxygen isotope characteristic of the formation water ,as well as the carbon- oxygen isotopes of the minerals(cements and fissure veins of the Upper Triassic Formation). The result demonstrates the original fluid in the Upper Triassic series was derived from the series itself (marine-land sediments which are the source strata of hydrocarbon), mixes with atmosphere precipitation and forms the mixing fluid. The fluid ,in the Jurassic system with natural gas pools, has the similar chemistry and isotopic component, and possesses the same evolution trend, indicate that it probably originates from the Upper Triassic series and mix with the infiltrating fluid inside the Jurassic system. By the relationship between δ D, δ 18O, Br / Cl and Cl's consistency, explain that the film percolation effect is not the major factor controlling oil gas water in the Jurassic system. The evidence that the fluid comes from the down earth's crust and mantle did not found by studying fluids ( water, glued thing and natural gas ) in the basin. The natural gas in the basin is chiefly moved with water. Under the upper pressures belt or minute super pressures of the Upper Triassic series and Jurassic system interface formation , the fluid in the Upper Triassic series moved along stratum interface, fault and crack network thoroughfare, formed " secondary gas pool " in the Jurassic system.