Study On Log Evaluation Of Low-Contrast Pay Zones In Triassic In The Western Margin Area Of Ordos Basin

In this paper,the study is focused on the sandstone reservoir of Yanchang Formation in Triassic in the western margin area of Ordos Basin.Through the analysis of the geological structure,log response,petrophysical characteristics and experiments,it is clear that the significant changes in the formation water salinity and the complex pore structure of the reservoir are the main causes for the widespread development of the lowcontrast pay zones.And then the research on different water salinities and the complex pore structure have been carried out.Based on the research,methods for quantitative formation evaluation by well logs of low-contrast pay zones have been achieved.For the study of the complex water salinity,based on the response mechanism of spontaneous potential log and deduced the equations of the membrane potential for shaly sands in saturated water and oil-bearing,the equations and influence factors of the SP deflection have been fully investigated.The results infer that the SP deflection has an effect on formation water salinity,mud filtrate salinity,the cation exchange capacity of shoulder shale,the cation exchange capacity of shaly sands and the saturation.This study has guiding significance for the determination of formation water salinity and the evaluation of saturation.For the study of the complex pore structure,based on the experimental data of the core mercury intrusion method and NMR,the relationship between the capillary pressure curve and NMR T2 distribution has been established by using the model of the classified segmental power function.The results show that the conversion model and the extracted pore structure parameters are effective.This study is significant for the formation evaluation of the permeability of low-porosity and low-permeability reservoirs.Based on the above studies,the log interpretation models of porosity,permeability and saturation,which are suitable for reservoirs,have been established for quantitative formation evaluation of the low-contrast pay zones.