Page 365 - Petroleum Geology

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where 5" is the vertical component of deviatoric stress in the diapir, and h is
the mean height of the diapir .(in units consistent with the weight density).
The total stresses are all compressive. As a diapir grows, so the deviatoric
stress increases, tending to accelerate the growth of the diapir. But in mud-
stone diapirs, all the quantities in eq. 15.5 change with time. In particular,
the mudstone tends to compact, and eventually mechanical equilibrium will
be reached. The effect of these forces depends on the size of the forces and
the time rate of their application, and on the mechanical properties of the
rocks on which they act.
While on the topic of densities, it is worth noting that a large gas, or even
oil, accumulation constitutes a local density anomaly within the sedimentary
sequence that may encourage structural growth. A sandstone with 30% poro-
sity, for example, has a mass density:
(2650 * 0.7) + (1040 - 0.3) = 2167 kg m-3
where it is water-saturated. Within the gas accumulation, assuming 20% water
saturation, the bulk density may be:
(2650 . 0.7) + (1040 . 0.3 X 0.2) t (200 . 0.3 X 0.8) = 1965 kg m-3

100 150 us/ft
1 I t
b=1200m
0

km
6.1 0.6 02
Fig. 15-7. The shale-transit-time plot of mudstone in a well in Borneo indicates a density
inversion below about 1350 m, down to nearly 2000 m. (Data courtesy of the Royal
Dutch/Shell Group.)