Rock-physics analysis of clay-rich source rocks on the Norwegian Shelf

Abstract

Rock-physics trends and properties of clay-rich source rocks are investigated in selected wells in the North Sea and Norwegian Sea. Properties can vary significantly because of burial compaction, composition, diagenesis, organic richness, and maturation. The many competing effects can be difficult to disentangle in traditional rock-physics crossplots. However, nearly all the source-rock data in the study are bounded nicely by linear trends that are based on rock-physics models, in acoustic-impedance (AI) versus shear-impedance (SI) crossplots. These reference models serve as a nice screening tool for organic richness and/or maturation level (i.e., hydrocarbon generation and expulsion), regardless of burial depth. The use of rock-physics templates for the early mature to mature stage of clay-rich source rocks is demonstrated by combining a simple basin-modeling approach with a rock-physics model using Backus average in which the organic-rich shale is represented by a transverse isotropic mixture of clays, kerogen, and hydrocarbons. The resulting templates are bounded nicely by reference trends and explain some of the observed trends in the data. However, the local presence of carbonaceous material and diagenesis within the source rock, which have not been accounted for in the rock-physics modeling, might explain why some of the data points have higher impedances and lower VP/VS than the template models. Finally, source rocks with hydrocarbon saturation can cause AVO signatures similar to hydrocarbon-filled sands and therefore represent false positives in exploration.