The Characterization Of Ultra-low Permeability Sandstone Reservoir In Chang6 Member, Tirassic Yanchang Formation In Ordos Basin

The paper chooses Chang6 Member, Triassic Yanchang Formation in Ordos Basin which isthe Ultra-low Permeability Sandstone Reservoir. By observing fine drilling cores, integratedwith regional sedimentary background and log facies, Chang6 Member of the study regionbelongs to delta front and underwater fan.Reservoir sandstones are delta front end and subaqueous gravity flow deposit, whosecharacteristics are fine granularity, high clay content, high plastic debris, salinization of water.The bad depositional environment is the innate factor of dense reservoir. Reservoir petrologicalcharacteristics show low composition maturity and textural maturity. The average content ofplastic mica is 15% among the total rocks content. Grain sizes are mainly very fine sandstoneand siltite, which the average content of lower siltite is 8%.Diagenesis is the key to super- low permeability reservoir. The super-low permeabilitysandstone has entered to the stage A in Chang6 Member in Huaqing Area. Reservoirexperienced various diagenesis including compaction, cementation, replacement and dissolution.Compaction and cementation are harmful to the diagenesis, which leads to lose 70% primaryporosity. Dissolution and disruption increased 3.7% porosity, which improved to some extentflow capability.Integrated with main interstitital types, attitude and diagenesis, four diagenetic facies aredivided, which are chlorite rim diagenetic faces, clay minerals cementing filling diageneticfaces, compact compaction diagenetic faces and carbonate compact cementation diagenic faces.Among them, relative high quality reservoir are sandstones of chlorite rim and clay mineralscementing filling diagenetic faces, but sandstones of compact compaction and carbonatecompact cementation diagenic faces are not reservoirs in research area.Applying limited core diagenetic facies to demarcate logging and based on analyzing logresponse characteristics of different diagenetic faces, log sensitive parameters of differentdiagenetic facies are discerned and plates of log diagenetic faces identification are built. This isan important innovation. Building plates of log diagenetic facies identification is important toapply diagenetic facies theory to petroleum industrialization. Surface diagenetic faces indicatethat monolayer thickness filled with chlorite rim and clay minerals cementing filling diageneticfaces is smaller, generally less than 2m, with lens-shaped distribution in these microfacies, suchas distributary channel of delta front, conduit of underwater fan and branch channel. Foldingparts of high thickness area of chlorite rim and clay minerals cementing filling diagenetic facesare high quality reservoir area.