Genesis of low-resistivity oil pays and a fluid identification method for tight sandstone reservoirs: A case study of the Chang 8 Formation in the Longdong West area, Ordos Basin

The Chang 8 Formation in the Longdong area of the Ordos Basin is a typical tight sandstone reservoir with low porosity and permeability.The sensitivity of pore fluid to logging response is weak, and the resistivity contrast of oil and water layers is low.Based on micro and macro influencing factors of low-resistivity pays in the study area, the fluid identification accuracies of dual formation water resistivity difference cross plot, physical property and array induced resistivity differential cross plot, and joint hydrocarbon logging cross plot methods were compared.In addition, based on the coincidence rate of the cross plots and voting strategy, a comprehensive fluid identification method was developed.The results showed that a high irreducible water saturation and high salinity of the formation water were the main micro factors of low-resistivity pays.The macro factors were mainly related to regional differences in the hydrocarbon expulsion ability of the Chang 7 source rock.Compared with the individual fluid identification methods, the comprehensive fluid identification method had a higher fluid identification accuracy, and the coincidence rate of fluid identification for 30 exploration wells was 83.3%, indicating that the comprehensive fluid identification method is effective at identifying low-resistivity pays, and it is worthy of promotion and application in other similar areas.