The effect of hydrocarbon saturation on resistivity distribution and CSEM response

The controlled-source electromagnetic (CSEM) method can be used to map resistivity differences in the sub-surface with the purpose of identifying hydrocarbon-filled reservoirs prior to drilling exploration wells. In this study, 15 exploration wells in the Norwegian Sea were analysed to investigate the relationship between reservoir properties, hydrocarbon saturation, resistivity and, subsequently, CSEM response. Two discovery wells (Linerle and Falk) with similar reservoir properties but different reservoir resistivity were further compared and evaluated through forward CSEM modelling to identify possible causes of the observed contrasting resistivities. The results demonstrate that hydrocarbon saturation, which can significantly affect the resistivity of hydrocarbon-filled reservoirs, may change rapidly over short distances, Controlling factors include (1) oil density and viscosity, (2) reservoir quality and (3) the location of the reservoir relative to the transition zone associated with the oil–water contact (OWC). High oil density and poor reservoir quality commonly result in a thicker saturation transition zone than low oil densities and high permeability. If the reservoir section is located within the transition zone, the average resistivity can be significantly reduced. Low average resistivity in the Linerle reservoir is found to be caused by low hydrocarbon saturation as a combined result of high-density oil, reservoir heterogeneity and that the reservoir section is situated within the oil–water transition zone above the OWC. Higher resistivity in the Falk reservoir which has similar thickness and burial depth is attributed to better reservoir properties (i.e. higher permeabilities), lighter oil and the location of the reservoir section well above the saturation transition zone associated with the OWC. The ability to understand and predict key fluid and reservoir parameters is critical for appropriate application of the CSEM technology in hydrocarbon exploration.