56' T. 0. Boswortli — Outlines of Oilfield Geology. 



2. Discontinuous inclined Porous Beds. — Where a porous band tliins 

 out, becomes fine-grained, or otherwise loses its porosity before reaching 

 outcrop, gas and oil may aocunmlate at the upper limit (oil at the 

 lower limit if the rock be dry). Examples are known in the Oklahoma 

 and Coalinga Oilfields (Fig. 13), and probably are very common. 



3. Pools resulting from dijferejices in Capillarity and Porosity. — Over 

 large tracts in the United States where oil is mined from strata 

 situated at great depth and almost undisturbed, e.g. parts of the 

 Appalachian Oilfields, oil is distributed sporadicallj' in isolated pools. 

 The pools are discovered and their limits proved by trial ; generally 

 they occur in the most porous parts of highly porous rocks. Probably 



Fig. 13. Oil in inclined discontinuous porous beds, as in Coalinga Field. 

 Length of section, say 1 mile. 



many of these pools of oil have been driven into their present position ' 

 by the complicated movements of waters circuhiting under the guidance 

 of capillary attraction — a force which would vary greatly in different 

 beds, and from place to place in each bed, according to texture, 

 cement, current-bedding, etc. 



Part III. Surface Phenomena of Decarbonization Process. 

 The visible indications of this natural process will now be briefly 

 enumerated. 



Oil-sands at Outcrop. — At outcrop oil-sands, though they have 

 a pitchy smell, are seldom visibly moist with oil, for on evaporation 

 oxidized residues fill up the pores of the rock. Most common are 

 the oils having an asphaltic base, in which case the bituminous 

 residue is very dark, giving tlie rock a deep-brown or black appearance. 

 Although an outcropping oil-rock is generally dark and dry, it is 

 often possible with the pick to break far enough into it to find it 

 oily and of a lighter colour. In coast-sections oil-rocks washed by 

 the tides often present this appearance, and sometimes are so soft and 

 sticky that drops of oil may be squeezed out by hand. In. strata 

 containing light oil with a paraffin base, however, evaporation may be 

 so complete as to leave little or no indication of jietroleum at the 

 outcrop. If, however, the outcropping oil-sands be broken with the 

 hammer, the oil often may be detected by smell, or if water be 

 present, by the oil film which spreads over its surface. 



^ M. J. Munn, " Theories of Gas and Oil Accumulation," Economic 

 Geology, 1909, p. 509 ; also " Anticlinal and Hydraulic Theories", Petroleum 

 Beview, January, 1910. 



