Depositional and structural controls on Triassic reservoir performance in the Heron Cluster, ETAP, Central North Sea

Abstract

The Heron Cluster fields form part of the Eastern Trough Area Project, an integrated development of a total of seven fields. The main reservoir within the cluster is the Triassic Skagerrak Formation (with undeveloped Jurassic Pentland Formation in Heron and Egret). The fields are classified as HPHT reservoirs, with initial pressures and temperatures of 9300–12 900 psi and 300–350°F respectively. Development of these fields initially assumed that the Skagerrak would be well connected and likely to have an active aquifer. However, initial production from Heron in 1998 revealed a dramatic pressure decline, indicating that the reservoir was more compartmentalized than anticipated, with little or no aquifer, necessitating a revision to the reservoir model. The Skagerrak 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. Re-description of the reservoir suggested a setting dominated by terminal splay deposits, arranged into cycles bounded by a hierarchy of shales, which formed laterally persistent, effective barriers to vertical flow. Additional perforations through the Skagerrak section encountered near virgin pressures and restored production profiles to prognosis. Egret came on-stream in 1999 with a single vertical well and again showed a rapid pressure decline, in this case additionally due to fault compartmentalization. However, it was observed that the reservoir was re-charging during shut-in periods, suggesting that the faults were transmissible with sufficient pressure drawdown. With experience from Heron and Egret on vertical compartmentalization and fault transmissibility, Skua was brought on-stream in 2001 with a single horizontal well designed to intersect as much stratigraphy as possible. The Skagerrak reservoir in Skua also exhibits Egret-like recharging across faults. Overall the Skagerrak reservoirs in the Heron Cluster appear to have the following common characteristics: good lateral connectivity, 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 compartmentalization that does not materialize during production.