UNDERGROUND MIGRATION OF OIL AND GAS 
185 
reservoir is a prime essential to wise exploitation of petroleum 
accumulation. Ifj on the other hand, the reservoir is only 
partly filled with water, oil should be accumulated above the 
water surface, unless the pores are of capillary dimensions (for 
oil) in which case it will be broadly disseminated through that 
portion of the reservoir which is devoid of water. Hence, 
knowledge of the water conditions and the porosity of the reser- 
voir rocks are likewise of first importance to the petroleum en- 
gineer. Lateral migration does not necessarily require an up- 
ward component, especially if it is due to gas expansion, but, 
because of the general prevalence of water in a more or less 
static condition within the strata from which oil and gas are 
obtained, it probably has nearly everywhere proceeded in an 
obliquely upward direction. Its results in concentrating oil 
and gas in certain parts of the reservoirs will depend chiefly upon 
their shape. 
