Oil migration makes the difference: regional distribution of carbonate cement delta C-13 in northern North Sea Tertiary sandstones

Carbonate cements in Tertiary reservoir sandstones from the northern North Sea have distinctive carbon isotopic compositions (delta(13)C). Oil migration up faults from deeper structures and biodegradation of oil pools are factors of particular importance in influencing the delta(13)C of carbonate cements in these sandstones. As a result, delta(13)C can be used as an exploration guide to locating the positions of vertical leakoff points from the Jurassic source rocks. The histogram distribution of delta(13)C in these carbonate cements is trimodal, with peaks at around -26, -3 and +12 parts per thousand (ranges -22 to -30, +2 to -10 and +8 to +8 parts per thousand, respectively). Bacterial processes played major roles in determining this distribution, with oxidative biodegradation of oil resulting in carbonate cements with very negative compositions and bacterial fermentation resulting in the positive delta(13)C cements. delta(13)C distribution patterns may be used to differentiate Tertiary reservoir sandstones from Jurassic in the northern North Sea, and these regional carbonate cement delta(13)C datasets allow geologically useful inferences to be drawn From delta(13)C data from new sample locations.