PETERS, KEN E.¹, MIKE J. CLUTSON², and GARY ROBERTSON^3
1Mobil Technology Company, Dallas, TX
²Mobil North Sea Limited, Aberdeen, Scotland, UK
³Mobil New Exploration and Producing Ventures, Dallas, TX

Abstract: Mixed Marine and Lacustrine Input to an Oil-Cemented Sandstone Breccia from Brora, Scotland

A rounded cobble from a beach near Brora, Scotland, contains angular, brecciated sandstone clasts cemented by crude oil. No oil occurs within the tight, dolomite-cemented clasts, except along fractures. Provenance of the cobble is uncertain, but lithology suggests that the clasts are fault gouge composed of Old Red Sandstone from the nearby Helmsdale Fault. Oil emplacement 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 b-carotane and 24-n-propylcholestanes that indicate lacustrine and marine source rocks, respectively. The oil cement also contains pristane, phytane, residual n-paraffins, 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. 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 immature. Alternately, the second charge could originate from an unusual Middle Jurassic source rock that contains oleanane.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas