What is a mud diapir and how does it form?
This is an excellent question, and a topic about which there is a lot of literature. In addition to the volume linked above, I can also recommend:

Geological Society of London Special Publication 216: http://sp.lyellcollection.org/content/216/1

Another good place to start is the extensive body of work done by Chris Morley and Pieter van Rensbergen that examines shale diapirs on seismic as well as in outcrop (good outcrop examples are surprisingly rare). Some good papers, in addition to the above two books, include:


Morley, C. K., P. Crevello, and Z. H. Ahmad, 1998, Shale tectonics and deformation associated with active diapirism: the Jerudong Anticline, Brunei Darussalam: Journal of the Geological Society, London, v. 155, p. 475-490.
van Rensbergen, P., C. K. Morley, D. W. Ang, T. Q. Hoan, and N. T. Lam, 1999, Structural evolution of shale diapirs from reactive rise to mud volcanism: 3D seismic data from the Baram delta, offshore Brunei Darussalam: Journal of the Geological Society, London, v. 156, p. 633-650.
van Rensbergen, P., and C. K. Morley, 2000, 3D seismic study of a shale expulsion syncline at the base of the Champion delta, offshore Brunei and its implications for the early structural evolution of large delta systems: Marine and Petroleum Geology, v. 17, p. 861-872.
Barber et al., 1986, Mud Volcanoes, Shale Diapirs, Wrench Faults, and Melanges in Accretionary Complexes, Eastern Indonesia, in AAPG Bulletin.
Stewart, S.A. &amp; Davies, R.J. (2006) Structure and emplacement of mud volcano systems in the South Caspian Basin. AAPGBull., 90(5), 771^786.


If I was to summarise all of the work I have seen over the past 15 years, I'd say that there are three major concepts for shale diapirs:

1) That they are large masses of mobilised shale - essentially big masses of shale, akin to salt diapirs. Note this is starting to fall out of favour, but is still relevant in some places, such as the Niger Delta.

2) Shale diapirs are emplaced in a means more akin to igneous intrusives, undergoing fracturing and stoping of overburden rocks due to their high overpressures. As such, shale diapirs are considered to be more like country rock that has been heavily fractured and invaded by gases and mud. This generates the chaotic seismic character.

3) Shale diapirs may be simple artefacts caused by complex structure. Features interpreted on seismic as diapirs have been drilled into in several places, but these well penetrations typically did not encounter indications of mobile or previously mobile shale. This has led to suggestions that many 'diapirs' may just be zones in which the bedding is at high angle, and/or heavily affected by fault processes and gas, leading to zones of poor seismic resolution.

My personal view is that there is evidence for all three of the above scenarios, depending on the regional basin and tectonic history.

I'm sorry this doesn't really answer the question, but I hope it gives you enough threads to research this in more detail.

Good Luck!

Mark