Research On The Mechanism Of The Anomalously High Porosity And Permeability Zone In Ch8Formation In Jiyuan Area, Ordos Basin

Ordos Basin reserves a huge amount of hydrocarbon resource and is a veryimportant oil and gas producing area in China. In Ordos Basin, Ch8formation haslong been studied, explored and exploited as a typical compact reservoir. However,several anomalously high-porosity and-permeability belts have been discovered withthe maximum of porosity and permeability reaching over20%and dozens of mD,respectively, while only10%and around1mD were observed in other regions. Andsuch anomaly reservoirs are located mainly in Ch82formation, the lower positon ofCh8. Obviously, a further study on its mechanism is required for future exploration.The presence of the anomalously high porosity and permeability has attractedmuch attention. But current research usually focus on provenance or its delta system.Research on reservoir characteristics and diagenesis were mainly conducted on Ch6and upper formations. In this thesis, I am going to analyze and discuss its mechanismbased on diagenesis, sedimentary facies and reservoir characteristics.As well known to all, the distribution of sedimentary facies, in macrocosm,controls the distribution and properties of reservoir. And good reservoirs alwaysestablish in distributary or underwater distributary channels and mouth bars. Ch82consists of front delta and delta plain. In this study, micro sedimentary facies in wellswere analyzed according to core description and well logs, and the lateral variation ofthem were recognized by setting up cross sections. Then the distribution map of sandswas achieved.The composition of the reservoir rocks is a important factor that affects thepetrophysical properties of the reservoir, determining not only rock strength, but alsothe subsequent diagenesis. In this study, petrology was analyzed based on cores andsections. A prevalence of quartz was observed, followed by feldspar and rock cutting.Clasts were well sorted. Diagenesis is another factor affecting petrophysical properties. It directly occursin pores and throats. According to their effects on properties, they are categorized into1) Constructive,2) destructive, and3) preservative diagensis. thin sections, SEM,EPMA，C/O isotopes and inclusion analysis were conducted on diagenesis. Thedestructive compaction and pressure solution, the constructive dissolution, andpreservative chlorite rim/coat were identified. And further, four types of diagenesisfacies were determined: namely, chlorite rim dominated facies (type1), the best interms of porosity and permeability; the compaction and pressure soluation dominatedfacies (type2); the calcite cementation and compaction dominated facies (type3), andcalcite cementation dominated facies (type4), the worst reservoirs in all.Reservoir characteristics is one critical part when discussing reservoir, in whichthe properties and pore structure can be even seen as two direct reflection of reservoirrocks. Analysis on petrology, petrophysics and pore structure and development werecarried out through thin sections SEM and mercury injection. It was concluded thatCh82is made mainly of residual intergranular pores and dissolution pores with goodconnectivity. Mercury injection curves were usually uplifted without an apparentplatform. And reconstruction of pore development attributed the low porosity andpermeability to the impetuous diagenesis after sedimentation.In particular, we have analyzed the reservoir characteristics of the anomalouslyhigh-porosity and-permeability area. Comparing with normal ones, it was suggestedthat the high porosity and permeability was formed because of1) its compositionstrong enough to resist upper load,2) location in positive sedimentary facies and3) aseries of appropriate diagenesis.