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Forward Seismic Modelling of Exhumed Clastic Intrusions to Improve Understanding of Architecture and Impact of Large-Scale Injectites in the Subsurface

Abstract

Seismic forward modelling integrates reservoir petrophysical properties with outcrop architectural geometries, and is therefore crucial in applying subseismic-scale detail and understanding to seismic-scale interpretation of hydrocarbon reservoirs. Here, four synthetic seismic sections have been constructed using outcrops from the Karoo Basin, South Africa and physical rock properties from a North Sea Tertiary discovery, Northern North Sea. The synthetic seismic was then compared to seismic data from the North Sea, after which several sets of different geometries were identified as being common throughout both data sets. The integration of outcrop panels, well log data, forward seismic modelling and subsurface seismic sections allows these common types of seismic expression to be related back to the original outcrop data. A model was created for a detailed understanding of the injectite geometry at outcrop scale based on six “injectite morphologies”: Type 1 low-angled bowl, Type 2 Anastomosing injectite, Type 3 Bifurcated injectite, Type 4 Abrupt step, Type 5 Composite intra-bowl injectites and Type 6 Vertical dykes. In reflection seismic data, the identification of injectites can be ambiguous, which impacts the construction of geological models. The integrated approach here has developed recognition criteria that will help to discriminate between clastic injectites and the parent sands. Furthermore, the results helps to bridge the gap between outcrop and seismic data by indicating that injectite architecture is largely scale invariant, which permits outcrop-scale knowledge to be used in seismic scale interpretation. Injectites are increasingly being identified in the subsurface, and the specific targeting of clastic injectite complexes as hydrocarbon reservoirs, meaning the approach taken in this study could be used in many other basins around the world to reduce uncertainty in the role of injectites.