OIL AND GAS LANDS. 119 



portant step in the classification, the productive portions of the field 

 being thus limited to the areas underlain by the petroliferous units 

 and the areas not so underlain being at once disregarded as having no 

 prospective value for oil. 



As it is the structure of the petroliferous strata which largely de- 

 termines the place of acpumulation of oil and gas, it isapparent that 

 a detailed knowledge of the structural conditions is absolutely essen- 

 tial to classification. In the classification of areas whose structure 

 is anticlinal all lands are classified as oil bearing which lie along the 

 axes of the anticlines or which are so located on the flanks that the 

 oil sand underlies them within an arbitrarily chosen limit of depth. 

 This limit may be the depth below which it is estimated that drilling 

 can not be profitably carried because of mechanical difficulties, or it 

 may represent the distance from the axis beyond which it is consid- 

 ered improbable that oil or gas in valuable quantities have accumu- 

 lated. In monoclines similar considerations limit the distance down 

 the dip to which classifications as oil land are carried. In the rarer 

 synclinal accumulations the width of the, zone classified as oil bear- 

 ing depends mainly on what is known regarding the quantity of oil 

 present. 



The thickness and porosity of the reservoir rock are important fac- 

 tors to be considered with regard to the production and life of the 

 wells, but their bearing on classification is subordinate, for classification 

 is concerned with the boundaries of productive areas rather than with 

 estimates of probable production. The continuity of the oil-bearing 

 zone is, however, an important factor in classification, though, un- 

 fortunately, it is one whose exact value can rarely be determined, 

 because of the difficulty of obtaining adequate data on the subject. 

 In many areas of Tertiary rocks, where lateral variations in litho- 

 lo^c character within short distances are to be considered the rule 

 and not the exception, changes in the thickness and extent of the oil- 

 bearing zones must be expected, and although field examination may 

 show the general trend of the variations in a certain region, local 

 variations can seldom be predicted in advance of drilling. A factor 

 of uncertainty is thus introduced, which may now and then residt in 

 an erroneous conclusion and give rise not only to adverse criticism 

 of a particular classification but even to sweeping and unjust con- 

 demnation of the entire method of procedure. 



The quality of the oil obtained in a given field is considered in clas- 

 sification mainly with regard to its bearing on distribution. In 

 general the greater the specific gravity of the oil the slower will be 

 its migration, the less complete its separation from the associated 

 water, and the lower down on the flanks of the folds its resting place; 

 and, conversely, the less the specific gravity the more rapid the rate 

 of migration, the more complete the segregation, and the higher in 



