Abstract
Subsurface biodegradation of crude oil in current oil reservoirs is well established, but there are few examples of ancient subsurface degradation. Biomarker compositions of viscous and solid oil residues (‘bitumen’) in fractured Precambrian and other basement rocks below the Carboniferous cover in Shropshire, UK, show that they are variably biodegraded. High levels of 25-norhopanes imply that degradation occurred in the subsurface. Lower levels of 25-norhopanes occur in active seepages. Liquid oil trapped in fluid inclusions in mineral veins in the fractured basement confirms that the oil was emplaced fresh before subsurface degradation. A Triassic age for the veins implies a 200 myr history of hydrocarbon migration in the basement rocks. The data record microbial biodegradation of hydrocarbons in a fractured basement reservoir, and add to evidence in modern basement aquifers for microbial activity in deep fracture systems. Buried basement highs may be especially favourable to colonization, through channelling fluid flow to shallow depths and relatively low temperatures, and are therefore an important habitat within the deep biosphere.
Highlights
Subsurface biodegradation of crude oil in current oil reservoirs is well established, but there are few examples of ancient subsurface degradation
Biodegradation is widespread in subsurface oil reservoirs, where the oil provides a ready source of food for microbial activity
We investigate the origin of the bitumen by (1) assessment of whether the bitumen shows signatures of biodegradation, (2) if it does, assessment of whether biodegradation was a more recent or ancient feature, and (3) constraining the thermal history of the bitumen, its source rock and the host fracture system
Summary
Subsurface biodegradation of crude oil in current oil reservoirs is well established, but there are few examples of ancient subsurface degradation. Increasing interest in so-called unconventional reservoirs includes a focus on the potential of fractured basement reservoirs, many of which contain heavy oil, such as in North Africa and the Atlantic Margin (Trice 2014). These buried basement blocks (Fig. 1) are typically durable rocks that have been topographic highs when exposed at the surface before burial, and in some countries are named ‘buried hill reservoirs’ (e.g. Tong et al 2012). We examine exhumed fractured Precambrian basement, containing extensive oil residues, in Shropshire, onshore UK, and assess the distribution of microbial activity using organic molecular biomarker evidence for biodegradation. Along with faulting and folding, occurred during the Carboniferous– Permian Variscan Orogeny
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