Logging Identification Method of Jurassic Low Resistivity Reservoir in Southwest Ordos Basin

Abstract

AbstractThe southwest Ordos Basin is located in Gansu Province, Western China. The regional structure is located in the structural unit named Tianhuan depression. Jurassic low resistivity reservoirs (LRRs) are widely developed in this area. It is difficult to identify the reservoir fluid types by logging. Therefore, this paper studies the logging identification method of LRRs, aiming to provide technical support for the continuous promotion of exploration and development in this area. Based on the analysis of the genesis of the LRRs, it is considered that the controlling factor of the Jurassic LRRs in this area is the high water salinity. With the data of formation water analysis and testing, well testing and production test, and focusing on the main causes of LRRs, the well logging identification of reservoir fluid types is carried out by using cross plot method method. Controlled by many geological characteristics such as the plane difference of formation water resistivity, conventional charts such as cross plot of acoustic interval transit time and apparent resistivity have many restrictive factors, such as scope and conditions of application. According to the control factors of LRRs, the accuracy of reservoir fluid identification is effectively improved by using the cross plot of formation water salinity and reservoir apparent resistivity. Based on the relevant principle of Archie formula, the apparent resistivity increasing rate method is introduced to calculate the apparent resistivity increasing rate of sandstone section. The calculation result has a good correspondence with the type of reservoir fluid. By drawing the cross plot of formation water salinity and apparent resistivity increasing rate of reservoir, the fluid type discrimination and identification accuracy are further improved. In this paper, based on the main genesis of LRRs in southwest Ordos Basin, the logging identification method of reservoir fluid type is studied systematically for the first time. Through the gradual development of the follow-up exploration and development work, the applicability of the new method in this paper has been verified, which ensures the efficient development of oilfield exploration and development work. KeywordsJurassicLow resistivity reservoirFormation water salinityApparent resistivity increasing rate