The Douglas Oil Field

Abstract

Abstract The Douglas Field, with an estimated STOIIP of 225 MMBBL and on stream in February 1996, is the first oil field to be developed in the offshore East Irish Sea basin. The field structure consists of three tilted fault blocks formed during extensional faulting in Triassic-early Jurassic times, and later readjusted by contractional movements during Tertiary inversion. The oil is trapped in the Triassic Helsby Sandstone Formation, which comprises moderate- to high-permeability aeolian and fluvial sandstones. The reservoir depth is shallow (2200 ft) with a maximum oil column of 330 ft. The sandstones can be divided into several laterally extensive units based on vertical facies variations. Although reservoir quality is principally controlled by primary depositional processes, a complex diagenetic history is recognized, with various phases of silica and carbonate cements. Authigenic clay minerals are not important, but bitumen is extensive in limited areas of the field, causing significant permeability reduction. The hydrocarbon filling history of the field was complex, with the occurrence of at least two phases of oil generation and migration. The Douglas Field contains a relatively ‘dead’ oil with a low GOR, thus 11 production and seven water injection wells are planned. Although the crude is classified as sweet it contains low levels of mercaptans and H 2 S, which will be removed during processing offshore. First oil is planned for December 1995 with peak oil production reaching 40000 BPD.