THE SEARCH FOR OIL — LEES 233 



THE NATURE OF OIL FIELDS 



Before continuing with this topic, and certainly before com- 

 mencing an examination of the extent of proved reserves, I shall 

 to a certain extent digress by describing the general nature of oil 

 fields. For many years an acute controversy raged over the origin 

 of petroleum, whether inorganic or organic, but of recent years 

 evidence in favor of organic origin has become overwhelmingly 

 strong. The trouble is that oil, being mobile, does not necessarily 

 remain where it was formed and so allow one to examine the cir- 

 cumstances of its birth, as, for example, is the case with coal. Being 

 a fluid, it can, and often does, migrate considerable distances ver- 

 tically and laterally through the rocks, and one can seldom be quite 

 sure of its original provenance. In general terms it is now thought 

 that oil is formed by slow chemical or biochemical decomposition of 

 the remains of lowly forms of organic life entombed in sedimentary 

 rocks. Circumstances may vary considerably, but marine or estu- 

 arine conditions seem a necessary condition for the fomiation of 

 oil in important quantity. This means that these organic remains, 

 whether seaweeds or sea grasses of whatever form, marine animal 

 elements such as foraminifera or plant life such as diatoms, are en- 

 tombed in the bottom sediments of shallow seas or estuaries under 

 stagnant bottom conditions which prevent complete decomposition 

 before burial. Subsequently this organic material is transformed, 

 perhaps by bacterial action or perhaps by age-long chemical change, 

 into small globules of oil and gas; and in the course of geological 

 time, as the original muds are being compacted into shales or marls, 

 these globules, together with a much gi-eater quantity of original 

 water, are squeezed out and find a lodging in any convenient porous 

 stratum such as a sand lens or a porous limestone. 



In this way a water-bearing porous stratum may carry globules of 

 oil and bubbles of gas, though widely dispersed. The next phase is 

 the concentration of this oil from a state of fine dissemination into 

 what we in our short-term egoism call commercial oil fields. This 

 concentration is primarily brought about by the force of gravity. 

 Oil, being lighter than water, floats, and so the globules trickle up to 

 the top of a given layer of sand, say 50 feet in thickness, until their 

 further upward progress is sealed by an impervious cover of rock 

 such as shale. If this rock layer were itself horizontal, further 

 movement of the oil would then stop, but tiiis is seldom the case. 

 In the course of geologic time the layers of rock become tilted by 

 processes of mountain building, and in front of the major mountain 

 ranges of the world the stratified rocks are thrown into a series of 

 folds or anticlines (fig. 2). The crests of these anticlines become 

 oil traps for oil and gas which float up witliin a porous layer from 



