Risk Mitigation through Porosity and Thickness Determination of Paleogene Injectites in the Norwegian North Sea

Abstract

A quantitative interpretation (QI) study is presented using broadband 3D seismic data over a license block in the Norwegian North Sea. The exploration target under analysis is a Paleogene injectite prospect at a depth of ~1200-1300m bmsl. Injectites are sandstones that have been ejected from underlying parent sandstones into a neighbouring formation under the impact of fluid and pressure. They are a proven play for the Palaeocene in the Norwegian North Sea and could also present a possible hydrocarbon (HC) play for the Upper Eocene/Oligocene. The QI workflow included extensive fluid replacement modelling (FRM) for Eocene parent sandstones in nearby wells. FRM scenarios comprised fluid variations, perturbations in porosity, degree of cementation (a well-known phenomenon within the injectites) and injectite thickness. Seismic forward modelling was performed pre-stack in order to predict AVO effects. In a further step regional porosity-acoustic impedance (AI) trends, depending on depth and fluid content were established for the injectite parent sandstones. These trend functions were used to calculate seismically derived porosities. The different HC-thickness modelling scenarios and the porosity-AI trends are the basis of the volumetric determination of the aforementioned prospect.