Estimating exhumation from seismic stacking velocity data : a case study from the Slyne Basin, offshore NW Ireland

Abstract

Exhumation describes the vertical upward displacement of rocks due to the removal of overburden material. In this study, seismic velocity profiles from 2D and 3D seismic reflection datasets have been inverted to constrain the distribution and the magnitude of exhumation within the Slyne Basin, offshore NW Ireland. The method has already been successfully applied to 2D datasets from offshore Britain and Africa; this study is the first attempt to extract exhumation estimates from 3D seismic data. Inversion of 3D seismic velocity data yields a continuous map of exhumation across the entire 3D footprint. Exhumation estimates in this study from 2D seismic sections agree with estimates from co-located 3D data. However, there is greater scatter in the 2D-derived exhumation estimates, most easily seen at seismic line ties. This scatter in the 2D measurements arises because 2D seismic stacking velocities are less well constrained than 3D velocities. Together, the 2D and 3D seismic stacking velocity profiles can be used to estimate exhumation patterns on spatial scales > 10 km with an accuracy of ± 200 m. Many of the variations in the calculated amount of exhumation are correlated with geological structures, suggesting confidence in the results. The margins of the Slyne Basin have undergone about 1-1.5 km more erosion than the centre of the basin during formation of the Jurassic-Miocene composite unconformity. Inversion-related anticlines in the centre of the basin have undergone a few hundred metres more erosion at their crests than at their flanks. There is good agreement between 2D and 3D seismic-derived exhumation estimates and existing exhumation estimates using traditional techniques applied to borehole data, such as apatite fission track analysis, vitrinite reflectance or sonic log modelling. Overall, the results show that regional exhumation can be mapped in hitherto unprecedented detail using good quality seismic stacking velocity data. There is strong potential for applying this method to hydrocarbon research. As long as the velocity field of a given area is well constrained and understood, the routine presented in this dissertation can quickly process large amounts of data and produce maps of the distribution of exhumation. Exhuma¬tion events can have a major effect on the maturity and quality of potential hydrocarbon occurrences within a sedimantary basin, while exhumation results can be integrated into reservoir modelling.