Mixed marine and lacustrine input to an oil-cemented sandstone breccia from Brora, Scotland

A rounded cobble collected from a beach near Brora, Scotland contains angular, brecciated sandstone clasts cemented together by crude oil. No oil occurs within the tight, dolomite-cemented clasts, except along fractures. Provenance of the cobble is uncertain, however, the lithology and shape of the clasts suggests that they are fault gouge breccia composed of Old Red Sandstone from the nearby Helmsdale Fault. Oil emplacement, presumably along the fault, cemented the breccia prior to transport and weathering of the cobble. Independent source and biodegradation parameters support mixing of two charges of oil. Like oil from the nearby Beatrice Field, the oil cement contains 24-n-propylcholestanes and β-carotane that indicate marine and lacustrine source rocks, respectively. The oil cement also contains pristane, phytane, residual n-alkanes and a complete series of 25-norhopanes, which indicate mild and heavy biodegradation, respectively. The first charge consisted of lacustrine Devonian oil that was heavily biodegraded at shallow depth. The origin of the second charge is unclear. Unlike Beatrice oil, the oil cement contains oleanane and shows a higher 24/(24+27)-nordiacholestane ratio, suggesting input from a Cretaceous or younger, paralic marine shale source rock. Although Cretaceous and Paleocene source rocks are inferred to exist in various depocenters in the northeast Atlantic margin, subcrops of these rocks offshore Brora are thermally immature. Alternately, the second charge could originate from an effective Middle Jurassic source rock that contains oleanane. This hypothesis is supported by mass spectrometric confirmation of oleanane in an extract from the marine equivalent of the Middle Jurassic Brora coal in the nearby UKCS 11/30-2 well. Oleanane in crude oil is not positive proof of a Cretaceous or younger source rock for the oil.