Page 344 - Petroleum Geology

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bar
40

30

20

10

Fig. 14-7. Pressure-volume-temperature (PVT) diagram for pure water. Data of Kennedy
and Holser, 1966, table 16-1. Specific volume is in cubic centimetres per gram, or m3
kg-'.

the thermal process increases with depth to the top of abnormal pressures -
the deeper the top of abnormal pressures, the more important the component
due to thermal expansion - and increases as the mechanical process reduces
the porosity and permeability of the mudstone. In brief, the thermal process
may become more important with time.
Chapman (1980) calculated the permeability required in a mudstone to
dissipate the volume of water created by thermal expansion during burial at
the maximum rate known in the U.S. Gulf Coast (500 yrs/m) down the maxi-
mum geothermal gradient known there (36"C/km). A mudstone 500 m
thick, with 20% porosity, would require a hydraulic conductivity of about
10-13 m/s, or an intrinsic permeability of about 5 X cm2 (5 X
md) to dissipate all the water of expansion. This is near the lower limit of
Tertiary mudstone permeabilities measured in Japan (reported in Magara,
1971, fig. 9). Thus the volume of expansion under these extreme conditions
can be dissipated by very small permeabilities, and we cannot assign a major
role to the thermal process. If the thermal process becomes important at
depth, it is only after the mechanical process has largely run its course.