Jurassic sedimentation in the Outer Moray Firth took place under a changing tectonic regime, accompanied by regional transgression in the late Jurassic. An early phase of tectonism is recognised, accompanying the collapse of the Central North Sea Dome. This Mid-Cimmerian phase of tectonism is characterized by North-South, Viking trend, faults and reactivated Northeast-Southwest Caledonian faults. These predominantly Bathonian age faults controlled subsidence and deposition of the Middle Jurassic Rattray Volcanics Formation. The Rattray Volcanics are unconformably overlain by the Sgiath and Piper Formations, of Oxfordian to early Kimmeridgian age, deposits of a northward prograding delta that continued to be affected by movement on Mid-Cimmerian faults.A dramatic change in tectonic regime occurred during the Kimmcridgian, possibly concurrent with the major Eudoxus -Zone transgression, that heralded the onset of Kimmeridge Clay deposition. During this Late Cimmerian rift-phase, sedimentation took place under conditions of active extension controlled by northwest-southeast, Witch Ground Grabcn trend, faults. The Kimmeridgc Clay Formation is a typical syn-rift sequence with sands deposited on the downthrown sides of rotational fault blocks, whose crests were commonly eroded. Studies of ammonites recovered from cored sequences have refined the interpretation of timing of tectonic and transgressive events and have also highlighted anomalies in correlation between ammonite and dinocyst zonation schemes. 259 260 S.A.R. BOLDY & S. BREALEY c ~ ~o c i . © 7 i o~ w | z 0 .= .o e~o .=.C)
Regional seismic and sequence stratigraphic analysis of the Browse Basin identified a new Late Cretaceous play fairway involving ponded turbidite systems deposited within confined basins. This work highlighted the potential for isolated sandstone reservoirs in the Middle Campanian sequence of the Caswell Sub-basin. Extensional faults were expected to provide vertical conduits for charge from underlying Early Cretaceous source rocks.The play concept was tested by the drilling of two exploration wells in 2001. The Carbine prospect was a potential stratigraphic trap involving deposition of turbidite sandstones within a localised basin set up by slumping in an intra-slope setting. Carbine–1 penetrated a 77 m thick section of high quality, 100% net-to-gross sandstone but failed to encounter hydrocarbons.A similar ponded turbidite model was invoked for the Marabou prospect although in this case the confined basin was controlled by pre-existing topography at the toe of the slope. The trapping mechanism for Marabou was largely stratigraphic although a small area of anticlinal closure was present. Marabou–1 penetrated 102 m of good quality sandstone with elevated gas readings over the uppermost 22 m. Borehole problems prevented the acquisition of wireline logs or testing but it appears likely that the well penetrated a sub-commercial hydrocarbon column restricted to the four-way dip closure.The well results confirmed the presence of ponded turbidite systems with excellent reservoir characteristics. Further work is required, however, to address the critical risks associated with hydrocarbon migration and updip seal. Nevertheless, ponded turbidite systems remain attractive exploration targets particularly in basins where updip seal is assisted by structuring and where the reservoirs are intercalated with prolific source rocks.
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