Identification And Evaluation Of Low Resistivity Reservoirs Based On Conventional Logging Data

Ordos Basin is rich in oil and gas resources.Jurassic reservoir has shallow buried depth,good reservoir physical properties,high single well production,simple reservoir stimulation process and short production cycle,which has considerable development benefits.At present,laws of Jurassic sedimentary evolution and oil and gas accumulation in this basin have been deeply understood through scientific research and production practice.However,a large number of production data confirm that low resistivity pay zones are common in Jurassic sandstone reservoirs in this basin.The electrical characteristics of low resistivity and low logging contrast make this kind of interval easy to be ignored in the process of logging interpretation,which seriously restricts the efficient promotion of oilfield exploration and development.Therefore,taking the Yan’an formation of Lower Jurassic in the southwest of Ordos Basin as the research object,this paper carried out the identification and evaluation of low resistivity pay zones based on conventional logging data,in order to enrich technical means for oilfield exploration and development.This study carried out the research contents of genetic analysis,logging identification and logging evaluation of low resistivity pay zones.The research ideas and methods can be summarized as following: Firstly,based on the study of basic geological characteristics,the main influencing factors of low resistivity pay zones in the study area were revealed through correlation analysis,single factor and multi factor simulation.Secondly,on the basis of clarifying the main control factors of low resistivity pay zones,the corresponding sensitive parameters of reservoir fluid types were constructed,and the appropriate identification methods of reservoir fluid types were proposed around the main causes of electrical characteristics.Thirdly,according to the water saturation calculation models deduced by scholars,focusing on the main influencing factors of low resistivity pay zones in the study area,the formation water resistivity and rock electrical parameters in these models were optimized to improve the applicability of the model to the specific study area.In different stages of this study,some new technical methods were put forward,and the corresponding geological understanding and research results were also summarized,which can be summarized as following: Firstly,it is concluded that the salinity of formation water is the main control factor of low resistivity pay zones in the study area.Through correlation analysis,the correlation coefficient between reservoir resistivity and formation salinity is-0.70,and the absolute value of correlation coefficient with other factors such as irreducible water saturation is less than 0.5.Through single factor simulation and multi factor interactive analysis,the change of reservoir resistivity with formation water resistivity is the most significant in the high-frequency distribution range of various influencing factors.Second,the proposed reservoir fluid types identification methods are suitable for the geological characteristics of the study area.Based on the formation water resistivity calculated by different methods,a series of new parameters were constructed,such as the scale factor of formation water resistivity.These new parameters can amplify the differences among oil layer,oil-water layer and water layer,and improve the logging contrast.Around the newly constructed parameters,the three logging identification methods of reservoir fluid types proposed in this study have an identification accuracy of more than 85% in the study area.Thirdly,after optimizing the relevant parameters in the water saturation calculation models,the quantitative evaluation effect of reservoir oiliness in the study area can be significantly improved.According to the law that cementation index and saturation index are positively correlated with formation water salinity and porosity,the assignment trial calculation method was proposed,and the value and calculation model of rock electrical parameters were obtained based on the corresponding relationship and correlation degree between various parameters.In addition,based on the calculation methods of formation water resistivity,the formation water resistivity required for calculating water saturation was also optimized to a variable value.Based on the research results and understanding of each stage,the macro geological conditions that are easy to form low resistivity and low contrast pay zones were summarized,and the secondary logging interpretation was carried out under the constraints of geological conditions.The relevant low resistivity reservoir identification and evaluation technologies proposed in this paper were applied to the specific practice of oilfield exploration and development.The actual production results have verified the applicability of the corresponding methods and the reference of the workflow,and also realized the rapid transformation from research results to production results.