Observations of the pressure and composition of interstitial fluids permit more intelligent interpretation of the geological data on the solid rocks in a prospective area. In plains areas, oil-field waters usually are

End_Page 1687------------------------------

static, although regional tilting has shifted the location of some oil fields. In mountainous areas artisan flow occurs, which commonly flushes certain parts of prospective oil-bearing strata. The flushed areas usually can be recognized by a characteristic water composition.

In zones of recent rapid sedimentation such as the Gulf Coast, shale is less compacted than normal and associated lenticular sandstone contains fluids under abnormally high pressure. Similar abnormal pressure is found in front of overthrust mountains because of the rapid addition of stratigraphic sections by tectonic processes. This suggests that during compaction of shale the pore water follows bedding planes and usually does not migrate across the strata or along fault. High pore pressure in shale facilitates large-scale gravity slumping and sliding which are more common than have been supposed.

End_of_Article - Last_Page 1688------------