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Abstraet--Authigenic kaolinite and illite are important diagenetic minerals in the Magnus Sandstone, a giant oil reservoir in the northern North Sea. These clay minerals, separated from three wells, show considerable ranges in their oxygen isotopic composition (618OsMow = + 9 to + l 6%o) and hydrogen isotopic composition (fiDsMow = --55 to --105%). The variations in 618 O and 6D are positively linearly correlated with a high degree of statistical significance for both kaolinite and illite:Kaolinite: n = 12; 6D = 6.1 ~180 -169; r = 0.66(>95%) Illite: n = 11; riD = 5.96~80 -159; r = 0.78(>99%).Formation of the clays in a pore fluid of uniform isotopic composition over a range of temperatures appears unlikely. It is suggested that the observed relationships between clay mineral 6~sO and 6D are perhaps best explained by a model of precipitation at more or less constant temperature from pore fluids which varied isotopicaUy across the oilfield. The isotopic composition of the formation waters would then lie along the line: fiDw = 6.2 b'8Ow -50. This is most plausibly interpreted as a mixing line with suggested minimal endmembers at (btsO, ~D) values of (+4, -24) and (-4, -76). The first of these represents reasonable isotopic values for Magnus Sandstone formation waters. Although fi~80 of the second is compatible with an evolved Cretaceous meteoric water, its 6D value is difficult to understand in the context of the model.

In the Miller Field, diagenetic quartz abundance, isotopic compositions and salinities of quartz-cementing fluids display a distinct pattern which is related to the structural depth of the reservoir sandstones. Quartz cement volumes increase from the crest of the field (average 6.0±1.5%) towards the flanks of the field (average 13.2±2.1%) and directly reduce reservoir porosity. By integrating petrographic observations with results of fluid inclusion measurements and O isotope analyses of diagenetic quartz, the pattern of quartz cementation is seen to be related to the reservoir filling history. Oil filled the crest of the reservoir first and prevented extensive quartz cementation. At greater depth in the reservoir oil zone, quartz overgrowths continued to precipitate until inhibited by the developing oil column. Oxygen isotope compositions of diagenetic quartz imply that quartz cement continued to precipitate in the water zone of the reservoir up to the present day.

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