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.