Banff Field, UK Central Graben – evaluation of a steeply dipping, fractured chalk reservoir

The Banff Field is a steeply dipping raft of fractured Late Cretaceous and Danian chalk on the flank of a salt diapir structure on the western margin of the Central Graben in the UK North Sea. Paleocene sands draped over the raft and a carbonate cap rock provide additional reservoir potential. The unusual geometry of the reservoir is a result of a complex structural history. Generally positive relief and minor pulsed growth through the Cretaceous and Lower Tertiary resulted in a condensed sequence of Cretaceous and Tertiary sediments over the structure. Contractional rejuvenation and active diapirism in the mid-Miocene deformed and fractured the thinned overburden to produce a tilted raft with reservoir dips generally over 45°. A vertical oil column of over 950 m (3000 ft) is present. Hydrocarbon migration began during the Oligocene, preserving good porosity in the chalk matrix during subsequent Late Miocene burial. Seismic imaging difficulties caused by the steep dips and uncertainties over reservoir performance and connectivity led to a phased development plan. Phase 1 involved a six month early production system (EPS) via two imaginatively designed production wells, during which time 5 × 10 6 BBL oil were produced. The two wells were drilled from the crest of the raft down to the oil water contact. Each well encountered over 1000 m (3300 ft) of oil-bearing chalk reservoir and provided important geological and reservoir performance data. Seismic data have been reprocessed using 3D pre-stack depth migration to improve image quality and lateral positioning. Geophysical, geological and biostratigraphical data have been integrated with the dynamic performance from the EPS to assess the controls on reservoir quality. The bulk of the reserves, and highest productivity occur in the Late Cretaceous Tor Formation. Chalk porosity ranges from 15–35% but matrix permeabilities are low, generally less than 5 mD. High productivities (up to 40 000 BOPD) achieved during the EPS are attributed to specific zones within the Tor chalks which have a favourable combination of chalk lithofacies (including local reworking to produce chalk debris flows), open fractures, depth of burial, diagenetic history and stratigraphy. Sand units within the Ekofisk chalk may have a significant impact on fluid flow. In situ stress is expected to exert a major (favourable) influence on waterflood performance. An effective phased development programme has resulted in an improved understanding of the complex controls on chalk reservoir performance in preparation for Phase 2 development. First oil is planned for mid-1998 and ultimate reserves are expected to be in excess of 60 × 10 6 BBL oil.