Analysis of low-resistivity oil pay and fluid typing method of Chang 81 Member, Yanchang Formation in Huanxian area, Ordos Basin, China

Abstract

The Chang 81 Member, Yanchang Formation in Huanxian area, Ordos Basin, China, is a tight sandstone dominated reservoir with low porosity and low permeability. The contribution of pore fluid to well logging response is greatly reduced, and the resistivity of oil layer and water layer are close, which bring great difficulties to the fluid identification and logs interpretation. In this paper, we first analyzed the genetic mechanism of the low-resistivity oil pay layer in terms of reservoir petrological characteristics, irreducible water saturation and formation water salinity. The high content of clay and complex pore structure resulting in high irreducible saturation which leads to additional conductivity; and the formation water salinity has great difference in this area. In order to reduce the influence of the formation water salinity difference on the fluid identification, we separated the study area into normal oil pay region and low-resistivity oil pay region based on the relationship between formation water salinity and formation resistivity. Then, double apparent formation water resistivity difference method, fluid identification factor cross-plot method, full hydrocarbon mud logging and well logging hybrid method and the normal distribution method were established in two different regions, respectively. The comparison of different fluid identification methods show that full hydrocarbon mud logging and well logging hybrid method is the best method for fluid typing of oil layer, oil-water layer and water layer. Double apparent formation water resistivity difference method is not effective to distinguish the oil-water layers from the water layers and oil layers, and the normal distribution method is not applicable for distinguishing the low-resistivity oil pay. In the case study, the above methods are together used to identify the fluids of the reservoirs, and the application result indicates that comprehensive analysis of multiple methods can effectively improve the accuracy of fluid typing.