18 T. 0. BoHicoHh— Outlines of Oilfield Geology. 



With advanciug age both temperature and pressure are increased 

 owing to the deposition of sediments above, and at length the oil 

 and salt water may be driven out and redistributed many times. 



It is a remarkable fact, proved, in the Appalachian Oilfields at any 

 rate, by innumerable deep borings, that as a rule the amount of water 

 now present in the rocks decreases as depth increases. At great 

 depths — several thousand feet — there is little or no moisture, even in 

 porous sandstones, and the rocks are ready to absorb large quantities 

 of water. At more moderate depths there is a little water and that 

 is generally saline, whilst only at comparatively slight depths are the 

 strata thoroughly invaded by fresh water from above. Generally, the 

 further we explore a stratum down the dip the less is the volume 

 and the greater the salinity of the water found. 



How these rocks have lost their original water of sedimentation 

 is not known, but perhaps it was expelled at a time when they were 

 hotter or more compressed. All sedimentary rocks are to some extent 

 porous, and at depths where they are thus dry, capillary action would 

 exert a great influence on the movement of any fluid invading the 

 strata or already imprisoned there. Capillary action has probably 

 played an important part in the process by which the oil has been 

 leached out of the beds of clay and shale. 



The principal forces which have controlled the movements and 

 ultimate position of the oil are — 



1. Vapour Tension {Gas Pr^ss?«Y^).— -Commonly the pressure on the 

 gas and oil is found to be far too great to be due to any hydrostatic 

 pressure communicated to them, and it would seem that additional 

 pressure has been and is being generated in situ. Such a change 

 of vapour tension of the fluid may be brought about by chemical 

 alteration still taking place. Similar changes in vapour pressure, due 

 to chemical or thermal alterations, may have occurred often during the 

 past history of the oil and have disturbed the equilibrium from time to 

 time. The gas pressure, being equal in all directions, simply tends 

 to force the oil out along any line of weakness. 



2. Capillary Action. — This acts diiferentially among the strata and 

 tends to produce motion in particular directions depending on the 

 characters of the rocks. It is probably most effective at depth during 

 the earlier history of the oil. 



3. Gravitation. — In dry rocks gravitation imparts a downward 

 motion to the oil, which seeks the lowest places in the containing 

 stratum. In loet rocks gravitation tends to send oil upwards, for the 

 fluids must arrange themselves in order of density — water at the 

 bottom, oil resting on it, and gas occupying the space above. Where 

 hydrocarbons are entrapped over water the water may impart a con- 

 siderable hydrostatic pressure. The gravitation effects are the most 

 important in determining the present position of the oil in most fields. 



Under the action of these various forces the hydrocarbons have been 

 leached out of clays and shales, and have undergone a very complicated 

 history of movements, to and fro, through and among the rocks. 

 A large amount of hydrocarbons is ever escaping into the atmosphere, 

 whilst always considerable quantities remain temporarily entrapped. 



