Oil production and the diet of worms

Oil geologists now have a new villain to worry about – the digestive processes of the marine worm.

Clay minerals are the bane of an oil geologist’s life. They sit in pore spaces and block the necks of communication between them, so reducing both porosity and permeability – the two essential characteristics of an oil reservoir, which holds the precious black fluid, like a sponge.

Now, researchers based in Liverpool University have shown that these troublesome minerals are crucially affected by passing though the guts of marine worms that burrow through sediments as they build up on the seabed. The results are published in this month’s issue of the Geological Society of London’s flagship research publication, the Journal of the Geological Society (Notes 1, 2)

Conventional wisdom has it that the clays in muddy sandstone sediments originate partly from the incorporation of faecal pellets along with the sand grains. But the Liverpool team thought that the action of passing unstable detrital minerals through worm guts might actually have a role. They decided to test the hypothesis.

The scientists performed controlled experiments in laboratory tanks. They made their own sediment using one part mature silica sand, one part ground Welsh slate (to provide inherently unstable detrital minerals) and one part pulverised leaves (as food for the worms). The sediment was then placed in two lab tanks, and seawater added. One tank was then populated with lugworms – whose guts had first been cleaned out to avoid contamination.

The scientists then performed X-ray diffraction analysis on the worm casts thrown up after the lugworm were introduced, and compared their mineralogy with the original sediment they had made. The original sediment contained no clay minerals – just the mica minerals muscovite and chlorite (from the ground-up slate). Because slate is formed from pre-existing sediments buried deep in the Earth, these minerals are stable at high pressures and temperatures and unstable at surface. They tend, by natural weathering, to degrade into clay minerals.

The contents of the faecal casts proved to be distinctly different from the sediment in the control tank, which did not alter in composition during the 20 weeks of the experiment. The ratio of muscovite to chlorite changed. New minerals began to form, and the diffraction peaks of the original minerals broadened – showing their crystal structure had become markedly degraded. The researchers ascribe this acceleration of mineral breakdown to the acidity of the worms’ digestive juices.

“The digestive processes of the lugworm clearly encourages the growth of authigenic clays – clays that form after the sediment has formed” says lead author Duncan McIlroy. “These, on burial, are the precursors for clay mineral cements, filling pores and blocking communication, impairing the quality of the rock as reservoir.”

The results show that attempts to predict rates of mineral weathering that do not take account of the presence or absence of burrowing animals are likely to inaccurate. Also, certain environments favoured by worms, such as shallow shelf sediments, stand a higher chance of being unsuitable reservoirs than, for example, the sediments of deepwater marine fans in cold seas with rapid sediment accumulation from land areas undergoing hot, subaerial weathering.

Full text available: Go to reference URL for a PDF of the paper