| The Xinchang tectonic zone is located on the west side of the Xiaoquan-Fenggu structural belt in the western Sichuan depression.With the uplift of the longmenshan tectonic belt and the influence of multistage tectonicadjustment,multiple source-reservoir-cap assemblage and gas bearing formations have formed since the late Triassic,which show a great potential for future exploration.In this study,we focus on the member 2 of the Xujiahe Formation,component of Xinchang tectonic zone,analyzing the characteristics of both compaction of reservoirs and the tight fluid,discussing the evolution of hydrodynamics and gas migration,thus uncovering the distribution characteristics of gas&water,and the main controlling factors.Combined with the history of hydrocarbon generation,diagenesis and hydrocarbon accumulation,we summarize the accumulation process of gas reservoir and thereby predicting the favorable exploration area of tight sandstone gas.The drawn conclusion are as follows:(1)The member 2 of Xujiahe formation is dominated by secondary dissolution pores.Mechanical compaction and cementation,a destructive digenesis process,exert a significant effect on reservoir porosity,heterogeneity and densification.Feldspar dissolution and subsequent tectonic fracture development improved physical properties of reservoir;(2)The mid-late Jurassic is a key period in the evolution process of member 2 of Xujiahe formation gas reservoir.The gas-bearing reservoir zone formed before that period is a favorable zone for exploration.Stratum water of the Xu 2,which has the feature of fluid source of underlying marine strata,has undergone strong enrichment and metamorphism.The condensate water appearing in the mining process,which reflects the current stagnant hydrodynamic characteristics and the special gas-water distribution principle in new site area,is an important evidencefor gas accumulation and preservation;(3)The The main force controlling the migration of gas in the micropores pores of the Xu 2 reservoir is the capillary pressure,and the slippage effect has a relatively great influence on the gas seepage.The reservoirs are mainly consist of bound water and the main gas accumulation space of Xu 2 has a characteristic of low water-phased relative permeability,high gas-phased relative permeability,irreducible water saturation,high capillary pressure and low formation water salinity;(4)There are four types of water outlet in Xu 2 which are ascending type,descending type,steady type and mixed type determined by the degree of fracture development,water connectivity and energy difference etc The dynamic curves of both production and the water-air ratio can reflect the strength of gas-water migration.By controlling the pressure and pressure conditions reasonably,it is possible to achieve the purpose of controlling the productivity of the reservoir;(5)The Xu 2 formation reservoir enter the stage of hydrocarbon generation and expulsion after the middle diagenetic stage,oil and gas migration occurred through advantage migration channels,where favorable traps will accumulate to form gas reservoirs.From the mesodiagenite to late diagenetic stage,the natural gas migration and accumulation after densification of reservoirs.The late tectonic movement produced a large number of fractures,which obviously improved the reservoirs,thus forming a tight gas reservoir eventually in the context of high-quality reservoirs in the Xinchang field;(6)The main factors controlling the distribution of gas and water in the Xinchang field tectonic belt are reservoirs and fractures.The favorable reservoir areas are mainly distributed in estuarine bars andfault-bearing systems,which have a relatively high fluid potential and topography in the main reservoir-forming period(Late Jurassic Eocene to Early Cretaceous). |
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