The origin of carbonate cements in the Hildasay reservoir, Cambo Field, Faroe-Shetland Basin; clumped isotopic analysis and implications for reservoir performance

Abstract

The early Eocene paralic sandstones of the Hildasay Member of the Flett Formation form the major oil-bearing reservoir in the Cambo Field, located in the Faroe-Shetland Basin. The sandstones locally contain calcite-cemented intervals that vary in thickness from decimetre to over 1 m. A calcite-cemented interval from well 204/10a-5 has been analysed petrographically and using clumped isotopes to determine its mode of formation and potential lateral extent. Petrographic analysis shows the cemented interval to consist of ferroan calcite, with a consistent dull red cathodoluminescence, suggesting a single phase of precipitation. The centre of the cemented interval comprises a finer grained unit with detrital kerogen and early sphaerosiderite, while the rest comprises homogeneous porosity-occluding ferroan calcite. The early sphaerosiderite in the centre is replaced by ferroan calcite with a high δ13 carbon isotopic signature (δ13 CVPDB = 7.53‰) suggesting that it formed during anaerobic methanogenesis or fermentation. Samples from the intermediate zone have a lower δ13 carbon isotopic composition (δ13 CVPDB = 0.72 to −3.68‰). In comparison the outer margin of the cemented unit has an even lower δ13 carbon isotopic signature (δ13 CVPDB = −15.5 to -15.9‰) more typical of a strong aerobic oxidation source. The oxygen isotopic signature of the cements is similar (δ18 OVPDB = −10.9 to −12.2‰). Analysis of the clumped isotopes suggest that the ferroan calcite formed at ∼40–50 °C, from pore waters that were predominantly meteoric in origin (δ18 OVSMOW = −4.8 to −5.7‰). Burial history modelling would suggest that the cements formed at a depth of ∼500–1000 m. Meteoric water was likely to have been introduced during the formation of the mid-Lutetian unconformity ∼45Mya, approximately 10 Mya after the sandstones were deposited. The model proposed is that ferroan calcite precipitation was initiated in the fine lag deposits that contained the kerogen and sphaerosiderite, and then grew outwards. If the model is correct, the cemented units should be restricted to this facies and consequently of limited lateral extent. Consequently, it is likely that the cemented intervals will have a limited impact upon the reservoir performance and are unlikely to act as major barriers to fluid flow.