Log Evaluation For Extra Low-Permeability Reservior Of Triassic Yanchang Formation In Weibei Oilfield

Weibei oilfield is located in the southern margin of Ordos basin,and across the Yishanslope and the Weibei uplift which are the first-order tectonic units. The Mesozoic oil systemgrew in the oilfield,which is mainly Triassic Yanchang formation. The hydrocarbonaccumulation condition is good and the resources is rich.Yanchang formation is the purposelayer of the exploration and development layer of Weibei oilfield.The geologicalcharacteristics of Weibei oilfield are complex,and logging in oil and water layer was notsignificantly different,so the difficulty caused by oil and water layer identification,resulting inthe rate reducing of the oil-water layer logging coincidence interpretation.Changes oflithology in the area are uniform,and rocks are mainly fine sandstone and feldspar lithicsiltstone.The content of clay is high, the development of porosity is low, and permeability isextremely low.According to the geological characteristics of Weibei oil field,it is proposed toidentify the reservoir parameters by using high resolution array induction logging curveinversion.Set Weibei oilfield formation model as three parameters formation model.Directfitting function are used to get the formula of radial integral geometric factor.Array inductionlogging inversion is that to use three formation parameters to fit six logging datavalues,resulting in the three formation parameters,such as,undisturbed formationresistivity,invasion zone resistivity and invasion radius,providing a good explanation base forthe subsequent logging interpretation work.The formation parameters is used to calculate theformation water resistivity Rw.Studying the variable value m and applying the Simandouxformula are to establish precise water saturation model of the formation.The suitable shalecontent model,porosity model and permeability model for the ultra-low permeability reservoirof Weibei oilfield were established.Compared with the core analysis data,we found that theparameters calculated by the models are relatively accurate.Array induction logging inversioncurve Rt and Rxo are applied to divide the formation permeability.According to the Rt andRxo amplitude difference,we can judge the fluid property of the reservoir.The cross plot ofRt-Rxo with porosity and Rt-Rxo with shale content also are established respectively to judgethe fluid property of the reservoir well.