Late anhydrite cements mark basin inversion: isotopic and formation water evidence, Rotliegend Sandstone, North Sea

Sullivan, M. D., Haszeldine, R. S., Boyce, A. J. , Rogers, G. and Fallick, A. E. (1994) Late anhydrite cements mark basin inversion: isotopic and formation water evidence, Rotliegend Sandstone, North Sea. Marine and Petroleum Geology, 11(1), pp. 46-54. (doi:10.1016/0264-8172(94)90008-6)

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Abstract

The Lower Permian Rotliegend Sandstone in the Leman Field today contains formation waters which are extremely saline (165 530–238 531 mg/l), with cation/chloride ratios indicating mixing of Zechstein marine evaporite brines with meteoric water. Ancient Rotliegend formation waters, inferred from the analyses of cements, were similarly a mixture of marine brine and meteoric water (δ18O= +0.3 ‰ SMOW; °D= −16 ‰ SMOW). The strontium isotopic compositions of present day formation waters (87Sr/86Sr= 0.7107) are also consistent with mixing between late Permian marine brines (87Sr/86Sr= 0.707) and radiogenic strontium from a detrital source (87Sr/86Sr= 0.720–0.730). Isotopic analyses of texturally late intergranular and fracture-filling anhydrite suggest waters very similar to the present day formation waters. δ34S values of anhydrite cements (+10‰ CDT) indicate that sulphate in the Rotliegend originated from the stratigraphically overlying Zechstein evaporites. The (87Sr/86Sr)150 ratios of anhydrites (0.7080–0.7113), however, suggest variable mixing of strontium sourced from the overlying Zechstein evaporites with more radiogenic strontium derived from diagenetic reactions in the underlying Carboniferous shales. The oxygen isotope compositions of anhydrite reflect precipitation temperatures between 120 and 140°C from waters with a δ18O composition similar to the present day formation waters (approximately 0‰ SMOW). These are the maximum inferred burial temperatures for the Rotliegend Sandstone in the Leman Field and would have occurred at maximum burial depths of 3.5–4 km (11 500–13 100 ft) during the Late Jurassic-Early Cretaceous at the start of basin inversion. These data indicate an unusual, short-lived, catastrophic event of cross-formational flow, through fractures which were tectonically induced by basin inversion. Pore fluids may have escaped from the overlying and underlying formations into the lower pressure Rotliegend Sandstone regional aquifer to ‘drain’ this part of the basin.

Item Type:Articles
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Boyce, Professor Adrian and Fallick, Professor Anthony
Authors: Sullivan, M. D., Haszeldine, R. S., Boyce, A. J., Rogers, G., and Fallick, A. E.
College/School:College of Science and Engineering > Scottish Universities Environmental Research Centre
Journal Name:Marine and Petroleum Geology
Publisher:Elsevier Ltd.
ISSN:0264-8172
ISSN (Online):1873-4073

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