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Journal of the Geological Society

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Discussion on mechanisms and controls on the formation of sand intrusions

Penparc, Cwmystwyth SY23 4AG, UK (e-mail: davidmd.james@ virgin.net) BP Exploration, Chertsey Road, Sunbury-on-Thames, Middlesex, TW16 7LN, UK (e-mail: jollr1@bp.com) Department of Earth Science & Engineering South Kensington Campus, Imperial College London, London SW7 2AZ, UK (e-mail: l.lonergan@ic.ac.uk)

The first 20% of the full text of this article appears below.

D. M. D. James writes: I congratulate the authors on a very clear and comprehensive account of a widespread and complex phenomenon of increasing economic importance. However, presumably for didactic ease, they have made some simplifications that I feel are counterproductive for full physical understanding.

It is not entirely clear from the paper that sand intrusions require a concatenation of source/seal properties that on the face of it are very rare. These are that the source sand bed must have a negligible grain cohesion typical of burial at very low vertical effective stress (not shallow depth, necessarily) and that the seal must have effectively negligible permeability, typical of burial at very high vertical effective stress (and often associated with high cohesion). In most basins sands that can be fluidized are interbedded with ‘leaky‘ shales of high permeability (for shales) that do not sustain high overpressure during normal compaction. However, such high overpressure is vital because the intrusion is powered by a pressure decline at source that provides the mechanical energy to strain elastically the intrusion walls and to drive the ascent of the fluidized sand against frictional resistance. This cannot happen if fracture takes place when fluid pressure reaches a value merely equal to that of the horizontal stress (as illustrated by the authors in their figs 6 and 8) as this lies on the closure gradient of any fracture, not its inception gradient. How then can the requisite overpressure for fracture initiation and propagation be attained at the shallow depths normally associated with low vertical effective stress?

Of the obvious ways around this problem, the development of zero permeability diagenetic or clathrate seals seems unlikely to be a universal solution. More likely, and not mentioned in the paper, is that the source pressure builds up at such a high . . . [Full Text of this Article]