Research On Reservoir Characteristics Of Middle Jurassic Qianfoya Formation In The Yuanba Area Of Northeast Sichuan Basin,China

Low permeability and low porosity(lower permeability and lower porosity) oil gasreservoir accounts for a rather high proportion pf domestic oil gas reservoir.However,there has been not yet a suit of perfect theoretic and techniques about how to exploit thelow permeability and low porosity oil gas reservoir at present.The gas field ofYuanba area innortheastSichuan Basin is a typical low permeability and low porosity gas reservoir, and at presentthe specific geological characteristics for which is still not clear. In recent years, It has achieved a major breakthrough in Qianfoya Formation in northeast Sichuan Basin, confirming that Qianfoya sandstone has a large exploration prospects. But because of the broad and complex diagenesis,the sandstone reservoir become very dense. It is difficult to predict the high quality reservoir. Therefore,The research take sedimentation, reservoir geology,logging geology and mineral petrology, geochemistry and reservoir comprehensiveevaluation and other theory method as guidance;using thin sections, scanning electron microscopy, the CL and X-diffraction methods, and clearly identifies sandstone diagenesis and its impact on reservoir quality evolution.And we finally expound various controlling factors of reservoirs and establish the reservoir formation and evolution mode.Thereare five reservoir rock types, including lithic quartz sandstone, feldspar lithic sandstone, carbonate lithic sandstone, silicon lithic sandstone and glutenite. The composition maturity of lithic quartz sandstone is medium low or medium high, the structure maturity of lithic quartz sandstone is high or medium high; The composition maturity of feldspar lithicsandstoneis low, the structurematurity of feldsparlithic sandstone is medium; The composition maturity of carbonate lithic sandstone is low, the structure maturity of carbonatelithicsandstoneismedium high; Thecomposition maturity of silicon lithic sandstone is low, the structure maturity of silicon lithic sandstone is low. The feature of clastic components is rich debris and poor feldspar, interstitial content is mostly carbonate, siliceous and clay minerals, and rock debris is mainly siliceous debris, carbonate debris and variable quartz sandstone debris.Reservoir of Qianfoya formation is mainly distributed in two sections of the second member and the third member of Qianfoya Formation. The feature of reservoir is low permeability and low porosity. Porosity distribution is 1.07%~17.02%, Average is 4.017%,main distribution interval is 2%~5%.permeability distribution is 0.001×10-3~1.51×10-3μm2, average is 0.159×10-3μm2,main distribution interval is 0.001×10-3~0.01×10-3μm2.Reservoir type mainly belongs to porous-fracture type. According to the cause, reservoir spacetype is divided into remaining primary grained pore, secondary pore,hole and crack.And secondary pore can be divided into grained dissolution pore,grained microporous pore,in-grain dissolution pore, in-grain microporous pore, molding pore and oversized dissolution pore. The reservoir type of this area is mostly remaining primary grained pore, in-grain dissolution pore, in-grain microporous pore, molding pore and oversized dissolution pore, which is more for small~microporous, low faces rate.It deposits in shallow lacustrine and shallow lacustrine-semi-deep lacustrine environments, and deposits underwater distributary channel microfacies locally of the Qianfoya Formation in northeast Sichuan. Especially in the presence of a typical lacustrine facies area,sandstone obvious thickeningof front- delta deposition, more conducive to oil and gas reservoir.It also has been established a delta plain-delta front-front delta-Lakeside-shallow lake-semi-deep lake sedimentary evolution model.The complicated diagenesis of Qianfoya reservoir has a profound impact on the performanceof the sandstone reservoir. Compaction is a major factor in the destruction of the area of reservoirporosity,thecementationfurther enhanced reservoir density, the occurrence of dissolution make a great of improvement on reservoir properties.The study area has experienced early diagenetic stage,the middlediagenetic stageA period and diageneticstageB. Sedimentation, diagenesis, andtectonic three factors together controlled the reservoir properties.This paper summarizes two important controlling factors of physical properties ofthe reservoir:sedimentationanddiagenesis.Sedimentationofreservoir essentially effects reservoir rock type and structure component features. The type and intensity of latter rock diagenesis is controlled by the characteristics of rock. Diagenesis effects the evolution and pore throat structure characteristics of reservoir space. And diagenesis ultimately determines the stand or fall of physical properties and the distribution of the present reservoir. Through the previous researchachievements and extensive testing analyzing data, we believe diagenetic stage is in high evolution level, overall in B period of middle diagenesis. Denudation in the late period of early diagenetic and early period of middle diagenetic, cataclasis in the late period of middle diagenetic play an important role in reservoir improvement.