Tuesday 11 March 2014

Do styolites really present a barrier to fluid flow?

Styolites are pressure structures formed in limestones under stress. They tend to form thin layers which can continue laterally for great distances, forming planar layers along which the limestone has been recrystallized, apparently as a result of pressure-liquefaction, and which is thought to be denser than the surrounding matrix. These styolite layers have generally been assumed to present a barrier to fluid flow (i.e. the movement of liquids or gasses through the limestone), though this has never actually been tested experimentally.

In a paper published in the journal Geology in January 2014, Michael Heap, Patrick Baud and Thierry Reuschlé of the École et Observatoire des Sciences de la Terre at the Université de Strasbourg and Philip Meredith of the Department of Earth Sciences at University College London describe the results of a series of experiments on segments of limestone from boreholes from the Jurassic Oxfordian and Dogger Limestones, to evaluate whether the presence of styolites did in fact reduce the permeability of the limestone.

Styolite layer in a borehole section from the Dogger Limestone. Heap et al. (2014).

Heap et al. found no evidence that the styolites did in fact present a barrier to fluid flow, in fact in some of the samples the presence of styolites appeared to facilitate an increased rate of fluid flow within the limestone. Examination of the styolites using CT Scanning suggested that only the thinnest styolite layers appeared to be denser than the surrounding matrix, with thicker styolite layers being significantly less dense than the matrix, and therefore more permeable to fluid flow. While the thinnest layers were denser than the matrix, and therefore could potentially have presented a barrier to fluid flow, these very thin layers were not constant in thickness, and in places thinned to a zero thickness (i.e. they were patchy, and had gaps), so that they did not present an impenetrable barrier to fluid flow.

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