Improved representation of sub-wireline scale lithological heterogeneity in geological models

Abstract

Siliciclastic reservoirs often comprise lithological heterogeneity at mm-to cm-scales in the form of fine laminations of intraformational baffles. Such heterogeneities enhance capillarity and are often enriched in clay minerals such as chlorites. These properties could significantly influence subsurface fluid flow and fluid rock reactions. However, these heterogeneities are difficult to represent in geological models primarily because they exist at a scale which is below the resolution of conventional wireline logs. This paper presents an approach for deducing such heterogeneities using a combination of core plug and wireline log data to derive rock types capturing variations in intraformational baffles. The rock types were further populated in geological models to honour a nested representation of heterogeneity ranging from seismic to core plug scale. We also present a new upscaling method to ensure that cm-scale discrete rock properties are not lost to averaging. The study shows that property models derived using the conventional geo-modelling approach could overestimate porosity by about 20–30% for the sections with cm-scale laminations. Additionally, wireline log derived permeability values were found to be inconsistent with heterogeneity in fine laminations as observed in core plugs. The proposed approach attempts to overcome this issue by upscaling permeability in a way that honours variations in intraformational baffles.