Implications of Linearly Correlated Oxygen and Hydrogen Isotopic Compositions for Kaolinite and Illite in the Magnus Sandstone, North Sea

Abstract

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.

Indicators of hot fluid migration in sedimentary basins: evidence from the UK Atlantic Margin

“…Stable isotope data suggest that these fluids are most likely a mixture of ancient meteoric water (Mesozoic-Tertiary) and marine porewaters modified by interaction with volcanic material. Kaolinites equilibrated at temperatures of around 80 C Data from Glassman et al (1989), Fallick et al (1993) and McAuley et al (1994). These results have several implications for the development of sandbodies and source rocks in basin settings that have experienced hot fluid migration.…”

“…In addition to the relationship between morphology and temperature of precipitation discussed above, previous isotopic studies of kaolin in sandstones have also demonstrated a direct correlation between morphology and isotope chemistry. In a compilation of North Sea data, Osborne et al (1994) noted that low temperature 'vermiform' kaolinite was characterized by high O values and strongly depleted D. Similar kaolinite compositions were noted in upper Jurassic reservoir sandstones from the Magnus Province, east Shetland Basin (Fallick et al 1993). Higher temperature polytypes of kaolin, including samples described as kaolinite (McAuley et al 1994), 'blocky' or 'intermediate' kaolinite (Glassman et al 1989) and dickite (McAuley et al 1994) (Fig.…”

“…It has already been noted that the fluids with which the kaolinite equilibrated were not of magmatic origin. Isotopic calculations (Glassman et al 1989;Fallick et al 1993;McAuley et al 1994) predict that kaolinite of mean WOS composition would precipitate in equilibrium with meteoric waters which have a composition of approximately 18 O= 4‰, D= 21‰ at 80 C to 90 C. These temperatures are consistent with literature observations that blocky kaolinite precipitates at temperatures in excess of 70-80 C. When fluid compositions for all WOS samples (with the exception of GW-01-05) are plotted, it is apparent that they lie on a linear trend depleted in 18 O relative to meteoric waters ( Fig. 10) and, therefore, cannot have precipitated directly from unmodified present-day meteoric water compositions.…”

“…A strong positive linear correlation between O and D in kaolinites from the Magnus Sandstone, North Sea, was interpreted by Fallick et al (1993) as evidence for precipitation of kaolinite from pore fluids of varying isotopic composition at a single temperature. They suggested that a two-phase mixture comprising of present-day formation waters and modified Cretaceous meteoric water.…”

“…If the WOS formation water composition is to represent one component of the calculated fluid mixing line, it must be depleted in 18 O relative to typical formation waters (e.g. Fallick et al (1993) 18 O=+2‰) and, more importantly, depleted in 18 O relative to the present-day MWL. Fallick et al (1993) O component of a two-phase mixing line.…”