Print this pageThe Physics of Petroleum Expulsion/Source 
Rock
Compaction:
Solution to an Old Controversy in Petroleum Geology
MICHELSEN, JOHAN K., GANJAVAR KHAVARI KHORASANI, and JULIO DIAZ
Petroleum expulsion and related overpressure in surrounding sediments occur,
due to mechanical instability of the source rock matrix, when a significant
volume fraction of the rock matrix, i.e., kerogen is transformed into a low
viscosity petroleum fluid (typically >1-10 percent by volume of the rock). The
expulsion is driven by gravity (density contrast between the rock matrix and the
free petroleum) and the fluid potential gradients between the free petroleum in
the source rock and fluids in surrounding sediments. The rock matrix behaves as
a viscous fluid, of which bulk and shear viscosities depend on maturity and the
self-generated petroleum porosity. The matrix compacts at a rate depending on
the rate of petroleum desorption from kerogen, the viscosities of the rock
matrix, the viscous forces between the rock matrix and petroleum fluid, and the
capillary properties and fluid pressure of the water wet sediments surrounding
the source rock. The physical model presented reproduces observed features of
real source rocks, and we will demonstrate how spatial variation in kerogen
quality and quantity controls the dynamic behavior of the system, including
porosity/pressure shock-wave generation/propagation. Petroleum dikes and sills
are explained as instabilities, originating from the intersection between
porosity waves and heterogeneities in rock
properties. Most processes previously
suggested as responsible for expulsion, are shown to be of subordinate nature.