Organic Geochemistry 30 (1999) 237±248
Mixed marine and lacustrine input to an oil-cemented
sandstone ia from Brora, Scotland1
Ken E. Peters a,*, Mike J. Clutson b, 2, Gary Robertson c
aMobil Exploration and Producing Technical Center, Box 650232, 3033 Irving Boulevard, Dallas, TX 75247, USA
bMobil North Sea Limited, Aberdeen, Scotland, UK
cMobil New Exploration and Producing Ventures, 3033 Irving Boulevard, Dallas, TX 75247, USA
Received 25 June 1998; accepted 1 December 1998
(returned to author for revision 17 September 1998)
Abstract
A rounded cobble collected from a beach near Brora, Scotland contains angular, iated 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 posed of Old Red Sandstone from the nearby Helmsdale Fault. Oil emplacement, presumably
along the fault, cemented the ia 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 b-carotane that indicate marine and lacustrine source rocks, respectively.
The oil cement also contains pristane, phytane, residual n-alkanes and plete series of 25-norhopanes, which
indicate mild and heavy biodegradation, respectively. The ®rst 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 oshore Brora are
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