Abstract The UK Oil & Gas Authority carried out post-well failure analyses of exploration and appraisal wells in the Moray Firth and the UK Central North Sea to fully understand the basis for drilling the prospects and the reasons why the prospects failed. The data consisted of Tertiary, Mesozoic and Palaeozoic targets/segments associated with 97 wells drilled from 2003 to 2013. Seal was the primary reason for failure followed by trap, reservoir and charge. Root causes for failure were a lack of lateral seal, the absence of the target reservoir and the lack of a trap. The main pre-drill risk was not accurately predicted in over one-third of the cases and a third of the segments were targeted on the basis of perceived Direct Hydrocarbon Indicators. This study identified a number of interpretation gaps and pitfalls that ultimately contributed to the well failures. These included poor integration, improper application of geophysics, lack of regional play context, and absent or ineffective peer review. Addressing these gaps in a comprehensive and systematic way is fundamental to improving exploration success rates.

Abstract The Goldeneye gas–condensate field lies in the Moray Firth Basin of the North Sea and illustrates the potential for further field life after the normal end of production. It was discovered in 1996 in a three-way dip-closed structure in the Lower Cretaceous Captain Sandstone. Five development wells were drilled from a single production platform and first gas was produced in 2004. The field pressure decline indicated partial aquifer support and no compartmentalization. Approaching the end of production, the opportunity arose to propose Goldeneye as a store for CO 2 . The cap-rock seal was capable of containing gas and the removed hydrocarbons left a significant volume that could be refilled without raising pressures above original conditions. The field's good reservoir properties were favourable for injection, the wellstock and infrastructure were modern, and CO 2 sources were available close by. The different requirements of a storage project called for a detailed understanding of the overburden to guard against possible leak paths and to identify secondary containment. Furthermore, greater understanding of the aquifer was needed as it limits storage volumes through its impact on reservoir pressures. Updated interpretation, analysis and modelling demonstrated that Goldeneye is an excellent potential CO 2 storage site, which gives it a possible second span of life helping to offset UK CO 2 emissions.

Abstract: Folds associated with normal faults are potential hydrocarbon traps and may impact the connectivity of faulted reservoirs. Well-calibrated seismic reflection data that image a normal fault system from the Inner Moray Firth basin, offshore Scotland, show that folding was preferentially localized within the mechanically incompetent Lower–Middle Jurassic pre-rift interval, comprising interbedded shales and sandstones, and within Upper Jurassic syn-rift shales. Upward propagation of fault tips was initially inhibited by these weak lithologies, generating fault propagation folds with amplitudes of c. 50 m. Folds were also generated, or amplified, by translation of the hanging wall over curved, convex-upward fault planes. These fault bends resulted from vertical fault segmentation and linkage within mechanically incompetent layers. The relative contributions of fault propagation and fault-bend folding to the final fold amplitude may vary significantly along the strike of a single fault array. In areas where opposite-dipping, conjugate normal faults intersect, the displacement maxima are skewed upwards towards the base of the syn-rift sequence (i.e. the free surface at the time of fault initiation) and significant fault propagation folding did not occur. These observations can be explained by high compressive stresses generated in the vicinity of conjugate fault intersections, which result in asymmetric displacement distributions, skewed towards the upper tip, with high throw gradients enhancing upward fault propagation. Our observations suggest that mechanical interaction between faults, in addition to mechanical stratigraphy, is a key influence on the occurrence of normal fault-related folding, and controls kinematic parameters such as fault propagation/slip ratios and displacement rates.

Abstract A seismic stratigraphic analysis constrained by well and wireline log data has been undertaken on the Paleocene and earliest Eocene succession of the South Buchan Graben (Quadrants 20 and 21), Outer Moray Firth Basin (OMFB). Two principal sequences have been described relating to two regressive/transgressive second-order cycles of relative sea-level change. The Maureen, Andrew, Glamis Tuff and Balmoral sandstone members are expressed as a stacked set of lowstand basin floor fans separated by mudstone intervals representing four cycles of third-order relative sea-level change. The Sele and Balder formations contain both basinal and shelfal packages as an expression of two cycles of third-order relative sea-level change. The Forties Sandstone Member is deposited within highly mounded, levee-confined channels downlapped by a prograding slope succession with well-defined clinoforms and deltaic topsets attributed to the Dornoch and Beauly formations. The individual parasequences of the prograding wedge are related to higher-order eustatic fluctuations with incision and slope fans, attributed to the Cromarty Sandstone Member, deposited during periods of relative sea-level lowstand. It is demonstrated that through the integration of lithostratigraphic, seismic geomorphological and sequence stratigraphic analyses an understanding of depositional environments and the distribution of facies within them can be obtained. The identification of basinal and slope features with reservoir potential, along with an understanding of their chronostratigraphic relationship to sealing facies, play an important role in regional play fairway mapping and risk analysis in this area and beyond. Future prospectivity within mature basins, such as the OMFB, relies on subtle stratigraphic traps typical of lowstand systems tracts, where the main risk is associated with reservoir quality and containment.