Page 339 - Petroleum Geology

309

Hower, 1970, p. 171; Weaver and Beck, 1971, p. 18). The difficulty with
this hypothesis is that abnormal pressures occur in several areas at depths
much shallower than that of clay-mineral diagenesis. Some of the shallow
occurrences of abnormal pressures were in areas of apparent or evident tec-
tonic activity, such as California, Trinidad and New Zealand, and a tectonic
cause had some adherents.
Smith and Thomas (1971) and Barker (1972) revived the thermal hypo-
thesis. It had long been known that thermal expansion of water with depth
in the subsurface on a geothermal gradient is greater than its compression un-
der the increasing load of overlying water (Versluys, 1932, pp. 924-925)
(Fig. 14-4). Assuming negligible permeability to water, heating of the water
by burial down a geothermal gradient would lead to expansion that would be
resisted by the weight of the overburden. Pressures greatly in excess of over-
burden pressures could theoretically be developed, but they would lift the
overburden and the mudstones would release the excess pressure over the
overburden pressure through fractures. The aquathermal hypothesis, as Barker
called it, dominated the topic for the next decade (and may continue to do
so).
Chapman (1980) found that subsidence at the greatest known rate in the

DENSITY
095 g cm-3 1

5000

11
1.85 cm3 9-' 1
SPECIFIC VOLUME
Fig. 14-4. Density-depth diagram for pure water in the subsurface at normal hydrostatic
pressures and geothermal gradients of 26 and 36'C/km.