FACTORS CREATING AN OIL FIELD 717 



amount of original oil in the rocks. In tiie case of movement down the 

 dip across a great number of local structures the amount of oil pushed 

 on ahead would be relatively small, due to its having been previously col- 

 lected from point to point. But the ideal condition for a big field is 

 that of a large closed structure lying relatively far, but not too far, down 

 the dip of good oil-bearing beds, without other areas of catchment close 

 above it, and having a long plunging axis extending along the strike line 

 so as to subtend large areas of catchment. Such a field should also in- 

 clude a thick porous sand. 



Four important Factors 



Throughout the Mid-Continent region it is evident that the original 

 supplies of oil and gas were localized. Splendid structures in some dis- 

 tricts fail to furnish commercial fields, due to lack of sufficient material 

 from which oil had its orig-in. Generally speaking, there are four factors 

 that must work together in harmony in this region in order to make an 

 oil field in any particular district. They must be : 



1. A porous bed or ^^sand'^ in which oil may be absorbed. 



2. A suitable structural condition to permit concentration at a 



particular locality. 



3. Presence at one time or another of more or less water as the 



accumulating agent. 



4. Petroleum originally diffused through the rocks within a rea- 



sonable area surrounding the structure. 



The commercial value of an untested structure in the Mid-Continent 

 region depends not alone on its size and form, but also on the relation it 

 bears to these other conditions. Some local folds do not produce fields 

 because of the absence of sands; others show dry sands containing very 

 small quantities of oil, but indicating that no water body has lain within 

 or moved through them to concentrate the oil. Others show sands with 

 salt water alone, there having been no oil or gas for it to concentrate 

 within the structures. Carbonaceous shale beds are usually of local ex- 

 tent, and Just so the original supplies of petroleum and gas in the rocks 

 were confined to certain districts. In most cases oil and gas have been 

 moved in limited and disconnected bodies. Local structures that lie in 

 districts where there are good open beds of persistent sandstone contain- 

 ing an abundance of salt water and lying adjacent or near to thick beds 

 of persistent, black carbonaceous shale (such as the Bartlesville sand in 



