20 



T. 0. Bosu-ortJi — Outlines of Oilfield Geology. 



2. Undenuded Folds. — In general, water, oil, and gas are all present 

 in the porous oil-bed, and under the action of gravity arrange tliera- 

 selves vertically in order of density. Hence the gas will occupy the 

 crests of anticlines, whilst the oil will be found in the limbs of the 

 folds resting on the water, which will occupy the remaining space 

 below (Fig. 2a). 



Fig. 2. Oil accumulated in undenuded folded beds, a = bed saturated with 

 water ; 6 = bed saturated with water up to a level shown byjlotted line ; 

 c = bed containing no water. Oil shown black ; gas under pressure shown 

 by small circles ; water shown by horizontal lines. 



The heights of the oil will depend upon two factors : it will be in 

 equilibrium under — 



(1) The gas pressure generated — keeping it down. 



(2) The hydrostatic pressure — keeping it up. 



If there is no gas, oil will occupy the crest of the anticline. And if 

 there is no water, oil will occupy the syncline. The wells must be 

 placed accordingly. 



A certain amount of gas is present in most fields, and in some cases 

 there is such great quantity that it is exploited independently by 

 wells on the anticlinal crests, as in some of the Pennsylvanian and 

 West Virginian Fields and the Surakhany gas-field near Baku ; often 

 a single well will yield ten million cubic feet of gas per day, and 

 when the well is closed a pressure of thirty to forty atmospheres may 

 be registered. Water usually is present, and therefore the oil is 

 found in quantity only at or near the crests of the folds. Sometimes, 

 however, the porous rock contains only sufficient water to saturate it 

 up to a certain level, in which case the oil is not forced so far up 

 towards the crest whether there be much gas pressure or no (Fig. 2b). 

 The Gordon sand in Pennsylvania is an instance.' 



Oil in synclines is not common and rarely is present in commercial 

 quantity, but occasionally in the absence of water considerable 

 quantities have accumulated (Fig. 2c). Instances are cited in the 

 Californian Fields. 



The shapes of oil-bearing folds are very varied. Some are 

 ' Bull. No. 318, U.S. Geol. Surv., 1907, p. 16. 



