Reservoirs Characteristics And Well-logging Evaluation Of The Low Porosity And Low Permeability Reservoirs Of Chang 6 Member,Triassic Yanchang Formation In Zhidan Area,Ordos Basin

The Low Porosity and Low Permeability Reservoirs of Chang 6 Member in Zhidan Area has characteristics of poor physical properties,complex pore texture,strong heterogeneity and so on.By core observation,traditional thin section,scanning electron microscope and nuclear magnetic resonance,the sedimentary environment,reservoir geological characteristics,reservoir heterogeneity and diagenesis are analyzed.Then the logging interpretation model of reservoir physical properties and oil-bearing are determined,based on core calibration,four-property ralationship of reservoir and logging facies.According to oil testing data and production data,the lower limits of effective reservoir are calculated.Verified by many methods,the optimum logging interpretation model of reservoir physical properties is established.The main conclusions and acknowledgements are as follows:(1)Sedimentary microfacies of Chang 6 Member in study area are identified.Chang 61 sub-member belongs to delta front subfacies including underwater distributary channel and interchannel microfacies,While Chang 62 sub-member,Chang 63 sub-member and Chang 64 sub-member are all shallow lake subfacies,which comprises shallow lake mud microfacies and beach bar microfacies.(2)Fine-grained arkose with off-white and pale grey-grey color is the main rock type of Chang 6 Member,which has low compositional maturity.The most common pore types are intergranular pores and dissolved pores which are mainly composed of feldspar dissolved pores,followed by micropores and intercrystal pores.Because of going through many kinds of diagenesis,the reservoir has poor physical properties and strong intra-layer,inter-layer and plane heterogeneity.(3)Four-property relationship of reservoirs suggests high natural gamma ray exists in Chang6 reservoir;The method of SP-spontaneous potential and acoustic time overlay can qualitatively evaluate the quality of reservoir.Dual induction resistivity has high sensitivity to oil-bearing property of reservoir.(4)Compared with Wyllie's equation,AFF equation has higher accuracy to calculate reservoir porosity of Chang 6 Member in study area.(5)The lower limits of effective reservoir of Chang 6 Member are determined.The lower limit of porosity in Chang 61 sub-member is 7%,while that in Chang 62 sub-member,Chang 63 sub-member and Chang 64 sub-member is 5%.The lower limit of permeability in Chang61 sub-member is 0.3×10-3?m2,which is higher than that in Chang62 sub-member,Chang63 sub-member and Chang 64 sub-member,with the value of 0.2×10-3?m2.The lower value of oil saturation are all 35%.