The Development Of The Brent Field-A Complex Of Projects

Abstract

Introduction Well 211/29-1 was the most northerly offshore well in the world when it discovered the Brent Field in Aug. 1971. It was drilled in 470 ft of water ca 100 miles east of the Shetland Islands (Fig. 1) by Shell Expro, the operator for the Shell/Esso joint venture in the U.K. sector of the North Sea. Now, 7 years later, not only the Brent Field itself, but an entire complex of projects both on and offshore - perhaps the most difficult offshore activity ever undertaken - is well under way. DISCOVERY WELL RESULTS AND GEOLOGICAL PICTURE Well 211/29-1 found an oil column of 197 ft with an apparent oil-water contact in an 800-ft-thick Middle Jurassic sand body, now known as the Brent sands. Four wireline formation tests (FIT's) were carried out in the well, indicating that the sands had high (up to severed darcies) permeability and contained oil of 39 deg. API with a GOR of 1200 scf/bbl. Saturation pressure was 4200 psig, well below the reservoir pressure of some 6000 psig, so the reservoir appeared quite undersaturated. Appraisal of the discovery was not possible until 1972, as at that time rigs could not operate in the severe winter conditions of the northern North Sea. Initial development, therefore, was planned only on the basis of the limited data from the discovery well, a detailed seismic evaluation, and the regional geological picture. The subsurface picture of the field at the time of completing Well 211/29-1 showed the reservoir to he on a westerly dipping, monoclinal flank with closure to the east provided by a major fault zone. The Middle Jurassic sand and the major fault system were seen to be unconformably overlain by an Upper Jurassic organic-rich shale of Kimmeridgian age. This unconformity, call the X-unconformity, was the horizon that could be mapped regionally by seismic. Postdepositional movement of the major fault blocks Postdepositional movement of the major fault blocks had folded the X-surface, providing the seismic lead for the prospect. Closure to the north and south was conjectural at that time, but major east-west faults cutting the X-unconformity were identified on seismic. Thus, an oil-bearing structure some 16 km long was mapped. Reference to the excellent paper by Bowen will show the stratigraphy and both regional and local geology as interpreted at that time. The field is located in the Viking Graben. This is the northern extension of the over 1000-km-long rift system of the Viking and Central Grabens. The evolution of thus rift system was described fully by Ziegler. Quoting from his summary, "Development of the North Sea rift system started during the Triassic and dominated the paleographic setting of the area during the Jurassic and Cretaceous. The evolution of the North Sea rift is related to the development of the Arctic-North Atlantic rift zone. The latter reached the stage of crustal separation in early Tertiary, at which time the North Sea rift became inactive." It is of interest to note that outcrops on the southeastern coast of Greenland show remarkable similarity in both structure and stratigraphy to features found in the Viking Graben, though they are separated now by some 1500 km. It appears that the two areas were indeed in geographic proximity up until the time of sea-floor spreading, which began in Permian times. Permian times. The North Sea rift system experienced rapid differential subsidence during Triassic and thoughout Jurassic times. Sedimentation occured as a series of regressive-transgressive cycles. During Bajocian-Bathonian times, in particular, the up to 300-m thick Brent sands-were deposited. These now form an outstanding hydrocarbon reservoir. "Minor rifting pulses preceded a regional transgression during the Callovian and Oxfordian that led ultimately, during Kimmeridgian, to the development of deep water troughs in the Viking and Central Graben. Throughout the Northern as well as parts of the Central North Sea, the Kimmeridgian is parts of the Central North Sea, the Kimmeridgian is represented by organic rich shales that constitute the main oil source rock in these areas."