oa Triassic Skagerrak reservoirs, Heron Cluster, Central North Sea

The Heron Cluster fields form part of the Eastern Trough Area Project (ETAP), an integrated development with BP of a total of seven fields. The Heron Cluster comprises: Heron (discovered in 1988), Egret (1985) and Skua (1986). These form subsea tiebacks to a Central Processing Facility located over the Marnock Field. The main reservoir within the cluster is the Triassic Skagerrak Formation. The fields are classified as HPHT reservoirs, with initial pressures and temperatures of 9,300-12,900 psi and 300-350 F respectively. The Skagerrak Formation is a largely terrestrial succession deposited by terminal fluvial systems in a broadly semi-arid climatic regime. The section in the Heron Cluster area can be subdivided into an upper, more channel-dominated interval and a lower, poorer quality, more unconfined fluvial section, bounded below by the largely playa Marnock Shale and above by the lacustrine Heron Shale. These widespread shales may be time equivalent to the Rot Halite and middle Muschelkalk respectively, and record the expansion of playa, marsh and lacustrine environments marginal to these marine flooding (and evaporitic) events. The intervening coarsening-upward, splay to channel succession of the Skagerrak defines the large-scale progradation and expansion of terminal fluvial fans in response to increased hinterland run-off. Internally the Skagerrak reservoir is dominated by terminal splay deposits, arranged into cycles bounded by a hierarchy of shales which locally form laterally persistent, effective barriers to vertical flow and which locally compartmentalise the reservoir. The Skagerrak reservoirs in the Heron Cluster appear to have the following common characteristics: good lateral connectivity of channel belt facies in the upper section, but poor to zero vertical connectivity; large faults become 'leaky' with sufficient pressure drawdown, and there appears to be no aquifer support. The short term production behaviour of these reservoirs is not representative of their longer term behaviour, and in particular, short term well tests indicate a level of compartmentalisation which does not materialise during production.