UK Central North Sea diapir fields: quantification of a previously un-imaged trap via a perched roof flap model, with welded evolution, stress and hydrocarbon containment insights

Central North Sea (CNS) Eocene/Paleocene and Late Cretaceous (P/C) reservoirs host high relief hydrocarbon fields flanking piercing salt diapirs and lower relief fields where un-pierced. This study aimed to predict the un-imaged trap relief on a steep diapir flank – the Isolde prospect – identified on poor seismic data. Analogous CNS diapirs have 2D limb lengths consistent with P/C strata being perched roof flaps; formerly contiguous roof strata prior to piercement marked by Eocene apparent onlaps. Consequently, top Paleocene relief varies with regional distance across the trap, enabling calibrated prediction of c. 900m of un-imaged relief up-dip of the shallowest well on structure, largely confirmed by subsequent 3D reprocessing. A late-stage evolutionary understanding reflecting common characteristics of better-imaged diapirs supports the perched flap model. Limited evidence of Cenozoic salt withdrawal and indications of regionally synchronous structural events reflect episodic, minor active contraction of welded diapirs, driven by fluctuating far-field stresses rather than by halokinesis or substantial contraction. Hydrocarbon fill patterns and in situ stress data illuminate controls on hydrocarbon phase and column height, with high trap relief favouring large oil columns. Limited borehole breakout data indicate radial minimum horizontal stress, unlike common near-diapir stress assumptions, with implications for drilling, production and storage.