Ch. 33] DYNAMIC PRINCIPLES INVOLVED 617 



success. Natural water-flooding of the McClosky formation in east- 

 ern Illinois represents a successful attempt at secondary recovery. 

 Other programs have attained only moderate results, and many have 

 failed completely. Surely carbonate reservoirs hold more trapped oil 

 than sand reservoirs, oil that cannot be produced by primary produc- 

 ing methods. Why, then, have not more attempts been made to re- 

 pressure the carbonate class of reservoirs? The answer to this ques- 

 tion is manifold and complicated, and there is lack of general agree- 

 ment among geologists and engineers regarding the difficulties of 

 secondary projects in carbonate reservoirs. Basically, most of the 

 difficulty results from a lack of fundamental data pertaining to the 

 formation of carbonate porosity and the character of the resulting 

 rock. Comparatively speaking, determination of the physical aspects 

 of a sandstone reservoir is relatively simple. The thickness of the 

 reservoir can be measured and its lateral extent predicted by deter- 

 mining the area of oil-saturated sand. Differences in effective porosity 

 may be charted by core analyses. Unless a sandstone reservoir has 

 undergone extensive postdepositional change, differences in porosity 

 are mainly of a depositional nature. On the other hand, porosity 

 and permeability in carbonate rocks are largely due to the secondary 

 forces of solution and chemical deposition of moving ground waters. 

 Some porosity is undoubtedly present during early stages of postdep- 

 osition; however, there is rather general agreement among geologists 

 that the main change in the nature of porosity in carbonate rocks 

 occurs as a result of secondary forces. 



There can be little doubt that new petroleum reserves are becoming 

 more difficult and expensive to find. If our petroleum economy can be 

 improved by recovering "trapped" oil, which would otherwise be lost, 

 we have in effect added important discoveries to our general reserve 

 picture. This field of investigation calls for close cooperation of the 

 chemist, physicist, geologist, and engineer. 



DYNAMIC PRINCIPLES INVOLVED 



In a study of porosity we are interested in (1) the shape of the 

 openings, (2) size of the openings, and (3) the origin of openings. 

 The size and shape of the openings can be measured and described, 

 but the origin is more complicated because it involves physical and 

 chemical forces which were operative during geological history. 



The size of openings in calcareous rocks ranges from subcapillary 

 openings (tubular openings less than 0.0002 millimeter in diameter) 

 through capillary sizes (tubular openings greater than 0.0002 to 0.5 



