Abstract The Brent Field was the first discovery in the northern part of the North Sea, and is one of the largest hydrocarbon accumulations in the United Kingdom licence area. There are two separate major accumulations: one in the Middle Jurassic (Brent Group reservoir) and one in the Lower Jurassic/Triassic (Statfjord Formation reservoir). The Brent Field lies entirely within UK licence Block 211/29 at latitude 61°N and longitude 2°E; the adjacent Brent South accumulation extends into Block 3/4A. The water depth is 460 ft. The Brent Field discovery well was drilled in 1971, and was followed by six further exploration and appraisal wells. Seismic data over the Brent Field has been acquired in four separate vintages.The latest acquisition in 1995 allowed detailed mapping of the complex eastern margin of the field for the first time. The Brent Field is developed from four fixed platforms (Alpha, Bravo, Charlie, Delta) installed between 1975 and 1978. Production commenced in 1976 and, for the first 22 years of field life, the platforms provided production, water injection and gas injection facilities for both the Brent and Statfjord Formation reservoirs. The Brent South accumulation is produced via the Brent Alpha platform, through sub-sea tie-backs and extended reach wells. In 1992, the decision was taken to depressurize the Brent Field to recover an additional 1.5 TSCF of gas and 34 MMSTB of oil, extending the field’s life by 5-10 years. In January 1998, water injection into the main field was stopped and depressurization of the field initiated. As of January 2000, a total of 220 platform wells and three sub-sea wells (173 producers, 50 water injectors) have been drilled in the Brent Field. The original oil/condensate-in-place is currently estimated at 3.8 MMMSTB, and the estimated original wet gas-in-place is 7.5 TSCF. Total ultimate recovery for all reservoirs is expected to be 1988 MMSTB oil and condensate and 6000 BSCF gas. Cumulative oil and net gas production, as of 1st January 2000, was 1875 MMSTB oil and 4196 BSCF gas. This paper summarizes the current understanding of the field based on acquisition of new 3D seismic data, 130 new wells, detailed structural and sedimentological modelling, development of the complex crestal part of the field and finally, the initiation of an extensive brown field re-development project to depressurize the reservoir.

The uniformity of rare-earth element (REE) patterns in clastic sedimentary rocks, due to the low solubility and short residence times of REE in the oceans, provides overall average upper crustal compositions for these elements. Effects of weathering, diagenesis, and metamorphism are minor. Local provenance is recorded in first-cycle sediments, but is rapidly erased with sediment maturity. The REE patterns in Archean sedimentary rocks indicate that the Archean crust was not highly evolved. It appears to have been dominated by basaltic and Na-rich granitic rocks (tonalites and trondhjemites). The REE patterns in Proterozoic and later sedimentary rocks indicate a major episodic break at the Archean/Proterozoic boundary. This is consistent with a change to a more differentiated upper crust, dominated by granodiorites, with negative Eu anomalies. These are inferred to result from intra-crustal melting, during which Eu is retained in a plagioclase-rich lower crust. Detailed descriptions of the change in REE patterns at the Archean/Proterozoic boundary are given for the Huronian (Canada) and Pine Creek Geosyncline (Australia) successions. Evidence from these successions, the Hamersley basin (Australia), and the Pongola (South Africa) sequence indicates that the change in upper crustal composition was not isochronous, but extended over a period from about 3.2 to 2.5 Ga ago. Abundance tables are given for the Proterozoic continental crust, upper and lower crusts, and the Archean crust. A model for continental crust evolution suggests that the evolution of the present crust began in the Archean and that most of the volume of the crust was formed from the mantle between 3.2 and 2.5 Ga. This was followed closely by intracrustal melting which produced the Proterozoic upper crust.