ABSTRACT

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 (¹⁸O -10 to -12.5o/oo PDB, ¹³C -3 to -5o/oo PDB). Ankerite concretions have ¹⁸O values similar to disseminated cements but a wider range of ¹³C values (+1 to -5.5o/oo PDB). They also have highly variable intergranular volumes, which (together with the ¹³C data) are interpreted as a combination of pore-filling cementation and in situ replacement of comminuted bioclastic debris by ankerite.

Fluid-inclusion, ¹⁸O, and paragenetic evidence suggests that ankerite formed during deep burial (c. 3.5 to 4.5 km, 140-170°C), after the onset of overpressuring, but before hydrocarbon emplacement in the reservoirs. The regionally consistent ¹⁸O data suggest that ankerite formed via a temperature-influenced mechanism, and the relatively uniform ¹³C 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).