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The ability of mudrock seals to prevent CO 2 leakage is a major concern for geological storage of anthropogenic CO 2 . The long-term performance of a mudrock seal, which provides a natural analogue, in the North Sea Miller oil fi eld has been evaluated. This mudrock seal is immediately above a natural CO 2 -rich reservoir. The paper reports the stable isotopes of carbon from carbonate minerals in the mudrock that have precipitated in contact with CO 2 during 4 km of burial. A well-defi ned linear trend of upward-decreasing δ 13 C traces the progressive penetration of free-phase CO 2 causing dissolution and reprecipitation of carbonate minerals. The CO 2 was emplaced ca. 70‐80 Ma, and has only penetrated 12 m vertically in this case. The infi ltration rate is estimated as ~9.8 ◊ 10 ‐7 g cm ‐2 yr ‐1

When quartz-rich sands are buried and heated, pore space is gradually filled by precipitation of quartz cement from aqueous formation fluids. Here we examine whether the presence of oil in the pore space can retard or halt this loss of porosity by slowing or stopping quartz cementation. The effect of oil fill on quartz cementation is examined by using the distribution of quartz cement in the Brae Formation deep-water sandstone reservoir of the Miller oil field (North Sea). Petrographic data demonstrate that sandstones from the oil zone have much less quartz cement, and more porosity, than sandstones from the water zone. Sandstones in both oil and water zones are compositionally and texturally identical and have been affected by a similar burial history. Kinetic modeling of the cementation process suggests that progressive oil charging has slowed quartz-cement growth rates by at least two orders of magnitude, halting it completely in the most extreme cases. Our data demonstrate that early oil charging in the crestal part of an anticline can preserve porosity in deeply buried sandstones. This knowledge is especially relevant to porosity prediction for petroleum exploration in deeply buried sandstones.

Isotopic analyses of carbon and strontium in diagenetic dolomite and ankerite present in the eolian Lower Permian Rotliegend Sandstone, southern North Sea, show an excellent linear correlation between {delta}{sup 13}C and {sup 87}Sr/{sup 86}Sr (r = 0.93), suggesting that the sources of these elements were interlinked. The isotopic data indicate that Late Permian Zechstein seas flooded the basin and displaced the interstitial meteoric Rotliegent pore fluids, producing early dolomite with predominantly marine bicarbonate ({delta}{sup 13}C = {minus}15{per thousand}) and strontium ({sup 87}Sr/{sup 86}Sr = 0.7076) isotopic signatures. The isotopic values of this dolomite form a trend to merge with the diagenetically later ankerite, which has lower {sup 13}C/{sup 12}C ({delta}{sup 13}C = {minus}4{per thousand}) and more radiogenic strontium ({sup 87}Sr/{sup 86}Sr = 0.7112). The obvious source of low-{delta}{sup 13}C carbon was thermal decarboxylation of organic matter in the underlying Upper Carboniferous mudstones. The authors also suggest that the increasing quantities of radiogenic strontium ({sup 87}Sr/{sup 86}Sr = 0.720) were released when organic acids dissolved silicates (feldspar ) in the same underlying mudstones. The upward movement of fluid or diffusion of ions across formation boundaries, therefore, progressively became the dominant control on carbonate precipitation in the Rotliegend Sandstone.