GEOLOGICAL STRUCTURES 



FAVORABLE TO THE 



ACCUMULATION OF PETROLEUM 



By W. L. Watts 



Geologist, Mining Engineer, Chemist and Assayer 



I have been asked to say something on oil deposits that will make 



their nature and manner of occurrence plain to readers who have not 



studied petroleum geology. Therefore, I have drawn a few diagrams 



of simple structures which will elucidate the subject. It must be 



borne in mind, however, that in nature, modifications of the type 



forms are more frequently met with than are the simple structures. 



Petroleum is a mineral very widely distributed in the crust of the 

 earth but it requires special conditions to accumulate in valuable 

 quantities. These conditions are: The existence of sufficiently por- 

 ous rock to act as storage reservoirs and enclosing strata sufficiently 

 impervious to prevent the escape of water, gas and oil ; also a geolog- 

 ical structure favorable to the arrangement of water, gas and oil 

 according to their relative specific gravity. 



Figures 1, 2, 3, 4, 5, 6 and 7 show geological structures favorable 

 to the process of segregation and arrangement mentioned. 



ANTICLINAL STRUCTURE 



Figure 1 represents an anticlinal fold somewhat cut down by 

 erosiong; strata AA and BB are formed of porous sand containing 

 water, gas and oil enclosed by impervious strata of shale and lime- 

 stone; in stratum BB, the gas, having the least specific gravity, has 

 forced its way through the strata containing the water and oil, and 

 accumulated in the crown of the fold, and as the specific gravity of 

 oil is less than that of water and greater than that of gas the oil has 

 accumulated between the water and the gas. In strata AA and A' A', 

 water has driven the oil into the truncated ends of the strata. If 

 the oil has an asphaltic base there would probably be a bed of asphal- 

 tum, a tar spring or an exposure of bituminous rock, where strata 

 AA and A'A' crop out at the surfice. 



An oil with an asphaltic base is an oil which, after its lighter con- 

 stituents are driven off by heat, leaves a residue of asphaltum. If 

 the oil had a paraffin base the outcrop of strata AA, A.' A', would 

 probably show a seepage of oil, or an exposure of dry oil sand. An oil 

 with a paraffin base is one which after distillation leaves a residue of 

 paraffin. In some instances the water may have driven all or nearly 

 all the oil out of strata situated like AA, A' A', but in Fig. 1 we will 

 suppose that this is not the case. 



It is evident that the wells shown in Fig. 1 must differ greatly as 

 to results. Wells tapping the broken strata AA, A' A', are not likely 

 to be as productive as those tapping the unbroken strata BB; well C 

 is drilled through into the water, well E will start off as a very good 

 well having a long back of oil sand to draw from but its life will 

 be short for as the oil is extracted the water will rise and capture the 

 well ; well X will have struck oil in stratum AA and water in stratum 



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