Ankerite cementation in deeply buried Jurassic sandstone reservoirs of the central North Sea

Hendry, J.P., Wilkinson, M., Fallick, A.E. and Haszeldine, R.S. (2000) Ankerite cementation in deeply buried Jurassic sandstone reservoirs of the central North Sea. Journal of Sedimentary Research, 70(1), pp. 227-239.

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Upper Jurassic Fulmar Formation shelf sandstones of the high- temperature, high-pressure Franklin and Elgin Fields (South Central Graben, North Sea) contain abundant disseminated and concretionary ankerite. In contrast, most Jurassic North Sea reservoirs contain only minor amounts of dispersed ankerite. Disseminated ankerite cement in the Franklin and Elgin Fields has a fairly uniform isotopic composition (delta(18)O approximate to -10 to -12.5 parts per thousand PDB, delta(13)C approximate to -3 to -5 parts per thousand PDB). Ankerite concretions have delta(18)O values similar to disseminated cements but a wider range of delta(13)C values (+1 to -5.5 parts per thousand PDB). They also have highly variable intergranular volumes, which (together with the delta(13)C data) are interpreted as a combination of pore-filling cementation and in situ replacement of comminuted bioclastic debris by ankerite. Fluid-inclusion, delta(18)O, and paragenetic evidence suggests that ankerite formed during deep burial (c, 3.5 to 4.5 km, 140-170 degrees C), after the onset of overpressuring, but before hydrocarbon emplacement in the reservoirs. The regionally consistent delta(18)O data suggest that ankerite formed via a temperature-influenced mechanism, and the relatively uniform delta(13)C cement value indicates that organic matter and marine bioclastic carbonate contributed to the dissolved carbon reservoir in constant proportion, This can be explained by calcite dissolution in response to pH decrease during thermal breakdown of organic acids. Such acids were derived from adjacent mudrocks undergoing hydrocarbon maturation and clay-mineral transformations, and are likely to have been transported in pore fluids with Mg2+ and Fe2+. The presence of these cations in solution upon thermal decarboxylation is inferred to have stabilized ankerite at the expense of calcite. A relative paucity of ankerite in other Fulmar Formation reservoirs may reflect different sedimentological compositions (less bioclastic debris) and/or lower burial temperatures (less advanced decarboxylation).

Item Type:Articles
Glasgow Author(s) Enlighten ID:Fallick, Professor Anthony
Authors: Hendry, J.P., Wilkinson, M., Fallick, A.E., and Haszeldine, R.S.
Subjects:Q Science > QE Geology
College/School:College of Science and Engineering > Scottish Universities Environmental Research Centre
Journal Name:Journal of Sedimentary Research

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