Fault Picking Strategies using Seismic Data, and the Effects on Fault Seal Analysis: A Case Study from the Horda Platform, with Implications for CO₂ storage

Michie, E.A.H.¹, Mulrooney, M. J.¹& Braathen, A.¹

Department of Geosciences, University of Oslo, Sem Sælands Vei 1, Oslo 0371, Norway.

Significant uncertainties occur through varying methodologies when interpreting faults using seismic data. These uncertainties are carried through to the interpretation of how faults may act as baffles/barriers or increase fluid flow. How fault segments are picked when interpreting structures, i.e. what seismic line spacing is specified, as well as what surface generation algorithm is used, will dictate how detailed the surface is, and hence will impact any further interpretation such as fault seal or fault growth models. We can observe that an optimum spacing for fault interpretation for this case is set at approximately 100 m. It appears that any additional detail through interpretation with a line spacing of ≤50 m simply adds further complexities, associated with sensitivities by the individual interpreter. Hence, interpreting at a finer scale may not necessarily improve the subsurface model and any related analysis, but in fact lead to the production of very rough surfaces, which impacts any further fault analysis. Interpreting on spacing greater than 100 m often leads to overly smoothed fault surfaces that miss details that could be crucial, both for fault seal as well as for fault growth models.

This contribution is a case example showing how different picking strategies influence analysis of a bounding fault in terms of CO₂ storage assessment. This is an example from the Horda Plaform: the Smeaheia potential storage site, 20 km East of Troll East. This is a fault bound prospect, and hence this bounding fault is required to have a high seal potential and a low chance of reactivation upon CO₂ injection, increasing the pore pressure.

Uncertainty in the seismic interpretation methodology will follow through to fault seal analysis, specifically for analysis of whether in situ stresses combined with increased pressure through CO₂ injection will act to reactivate the faults, leading to up-fault fluid flow / seep. We have shown that changing picking strategies significantly alter the interpreted stability of the fault, where picking with an increased line spacing has shown to increase the overall fault stability. Surprisingly, differences in picking strategy show little influence on the overall fault seal (i.e. shale gouge ratio) of the fault.