Abstract

Production of oil and gas in the Alwyn North Field, UKCS, commenced in 1987 from Middle Jurassic Brent and Lower Jurassic Statfjord reservoirs. In 1988, a deepened Statfjord producer well (3/9a-N21) detected gas in the underlying Triassic, but testing was not possible at that time. The 3/10b-2 exploration well, drilled in 1992 to the east of the field, tested gas at low rates from a poor quality Triassic reservoir section. In 1995 well 3/9a-N33, a deviated well, was drilled from the Alwyn platform to evaluate the productivity of the deep Triassic. This well penetrated 770 m of Upper Lunde section, which was tested at 1.3×10⁶ m³ per day of gas plus 4400 STB per day of condensate.

The Alwyn trap comprises a westerly dipping fault panel structure. Erosion over the crest of the tilted panel and the eastern-facing scarp slope by the Base Cretaceous Unconformity (BCU) truncates the Jurassic section down to the Upper Triassic. Sealing of the Triassic Main accumulation is by uppermost Triassic shales of the overlying Statfjord Formation and by Cretaceous mudrocks onlapping onto the BCU. Hydrocarbon charging of the Triassic Upper Lunde sandstones is from mature Late Jurassic Kimmeridge Clay source rocks that are juxtaposed against the reservoir along parts of the eastern frontal fault of the Alwyn panel.

The Upper Lunde (of Carnian to Rhaetian age) is a low net:gross reservoir section up to 800 m thick. The sediments are of semi-arid fluvial, floodplain and lacustrine origins, with the best reservoirs occurring in fluvial channel sandstones. The Late Triassic comprises four informal zones from A (top seal), through B and C to D reservoirs, above the Middle Lunde shale. The Triassic reservoir fluids are gas-condensate in the B and C units with an intraformational seal at the C–D boundary, beneath which lies volatile oil in the D unit.

A total of eight further Triassic production wells were added to the AlwynNorth Field by the end of 2001, as the development plan for Triassic Main progressed. These wells contribute in the order of 60% to the Alwyn North daily production.