Rock physics analysis of reservoir elastic properties often assumes homogeneity of the intra-reservoir shales, and simplifies the cap-rock shale variability. The main reason for simplifying shale variation is lack of data
and suitable analogues; however, such simplification could lead to misinterpretation of seismic amplitudes, including missed opportunities in the exploration phase. Assuming favourable rock physics, seismic analysis in the exploration phase often links bright amplitude areas with high quality reservoir rock or increasing hydrocarbon saturation, while areas with weak amplitudes are considered as poor quality reservoir or water-bearing. Variations in amplitude could however also be caused by variations in the elastic properties of the overlying shale. For example, high hydrocarbon saturated reservoir sands might be overlaid by elastically softer cap-rock shale, resulting in weak seismic amplitudes. This paper describes a detailed shale-variability analysis for the Enfield oil field using rock physics models, and discusses the implications for seismic reservoir characterisation and inversion.