Geoff Thyne1, Jos Okkerman2, Tone Pedersen3, Kristian Soegaard3
(1) Colorado School of Mines, Golden, CO
(2) Norsk Hydro ASA, Bergen, Norway
(3) Norsk Hydro ASA
Carbonate-cemented zones were examined in the Oseberg Field, North Sea as part of an improved reservoir model. Unlike the adjacent Veslefrikk Field, the carbonate cement in the Oseberg does not constitute a significant fraction of the reservoir volume, but does form local flow barriers. Based on geophysical data, carbonate cemented zones are very spatially restricted in lateral and aerial extent. The zones are usually about 1 meter thick (mean=1.25 m, stdev=1.13 m) up to a maximum of 6 meters thick. Some wells have only a single zone, while others have multiple zones. The zones occur mostly in the lower third of the reservoir, near the major fault zones in the northern and southern parts of the Field. Thin sections show the calcite cement is poikilotopic, filling all porosity in the center and decreasing in abundance toward the edge. There is no evidence of pre-cursor carbonate. Detailed isotopic analyses of a single zone show systematic changes and suggest that cement formed over a relatively short period. The elemental, isotopic, fluid inclusion, and paragenetic data suggest a late origin for calcite cement. The position of the cemented zones in the reservoir is not directly related to sedimentary structures such as edges of deltaic lobes, facies or lobe boundaries or channel bases, but is related to the major faults. Accurate prediction is difficult given the lack of correlation with internal sedimentary structure and deltaic architectural elements.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado