Reservoir Characteristics And Advantageous Area Prediction Of Chang 2 And Chang 3 In Ansai Southern Area Of Ansai Oilfield

Ansai oilfield is a typical ultra-low permeability oilfield,located in the middle and eastern part of northern Shaanxi slope of the Ordos Basin.The main development horizon of the Triassic Chang-6 formation is in the middle and late stages of development.It is particularly important to find new potential horizons for the sustainable development of Ansai oilfield.The results of exploration and evaluation in recent years show that the non-main layers Chang 2 and Chang 3 are widely distributed in southern district of Ansai and have good oil and gas display.Therefore,guided by the theories of sedimentology,structural geology and petroleum geology,this paper studies the sedimentary microfacies characteristics,structural characteristics and reservoir characteristics of Chang 2 and Chang 3 reservoirs in southern district of Ansai by means of core analysis,scanning electron microscopy,thin section identification and mercury intrusion analysis,analyses and summarizes the law of oil and gas enrichment,and combines production performance to carry out advantageous area prediction.The conclusions and understandings are as follows.The top structure of each layer of Chang 2 and Chang 3 is consistent with the westward dipping monoclinic structure in this area,and the slope is relatively gentle.Three nearly East-West nose-like uplift structural belts are developed.The provenances are all northeast.The Chang 3 and Chang 2 periods are in the delta plain subfacies.The distribution of sand bodies in the plane is controlled by the distributary channel microfacies.Fine-grained,fine-medium-grained feldspar sandstone is the main rock type of Chang 2 and Chang 3 layers.The main types of fillings are chlorite,siliceous,dolomite and iron calcite.The coexistence of porous cementation and film cementation is the main type.There are many types of diagenesis,among which mechanical compaction,dissolution and cementation play the main roles.Intergranular pore is the main pore type,followed by feldspar karst pore,while a small number of intergranular pore and debris karst pore are developed.The structure type belongs to micro-pore throat type.The average effective porosity is 12.96%in Chang 2 layer and13.41%in Chang 3 layer.The average permeability is 4.93×10^-3?m² in Chang 2 layers and 4.84×10^-3?m² in Chang 3 layers.The Chang 2 and Chang 3 layers have strong heterogeneity,among which Chang 2₂¹,Chang 2₂² and Chang 3₁² had the best physical properties.Chang 2 and Chang 3 reservoirs are mainly structural-lithologic reservoirs.Oil and gas accumulation is controlled by nose uplift structure,sedimentary facies,diagenesis and reservoir heterogeneity.According to the geological characteristics of the reservoir and combined with the data of sedimentation,structure,petrology,logging and oil testing in this area,the advantageous areas are comprehensively evaluated by multi-factors,and the classification criteria for the evaluation of the advantageous areas are established.Finally,37 advantageous areas are predicted,of which 18 are in class I,14 are in class II and 5 are in area III,which points out the direction for the next production and development.