Reservoir Resistivity Modelling for CSEM Interpretation and Model Building

In controlled-source electromagnetic (CSEM) interpretation and model building, generating accurate resistivity models of the Earth is essential. When hydrocarbons are present, this process requires developing reservoir resistivity models, which typically involve linking expected hydrocarbon saturation to reservoir resistivity at the field scale. In predrill scenarios — where no well logs or core data are available — field-scale resistivity expectations must be derived through modelling. However, these models often rely on simplifying assumptions, such as constant reservoir properties combined with well-established resistivity-saturation relationships. While these relationships are valid at the core or log scale (cm to m), their applicability at the field scale (m to km) is questionable. This paper examines the impact of these simplifying assumptions and modelling techniques, with a focus on the consequences of neglecting hydrocarbon saturation variability. We explore the relationship between sediment resistivity and water saturation across scales, from core and log to field scale. Our findings highlight the significant role of saturation variability, driven primarily by pressure build up with height and reservoir property variations. To address these challenges, we propose a workflow that accounts for the main sources of variability, enabling the upscaling of core-scale relationships to the field scale. We illustrate our proposed methodology using an example from the North Sea.