Oil and gas are fluids but our prospecting methods have been based mainly on observations of the reservoir rocks. Little use has been made of measurements of the physical and chemical properties of their fluid contents.
There is now general agreement that petroleum originates in a very disperse state and is segregated into oil pools during the compaction and lithification of the sediments. The mechanism of this process is little understood, but it may be caused by the expulsion of pore water from the sediments and involve a capillary filtering action. If the routes of water migration and the location of capillary filters could be predicted, much more use could be made of our available geological data in predicting likely places for oil to accumulate.
Usually the oil remains where it was originally trapped. Sometimes, however, the regional dip may be reversed and the oil will then run to the other end of the aquifer that contains it. In mountainous areas, reservoirs are severely disturbed and often breached so the oil can run out, which is the principal cause of surface oil seeps. In such areas an effort should be made to locate the sites of the original accumulations, and then consider where the oil might now be.
