618 imbt. CARBONATE POROSITY PROBLEMS [Ch. 33 



millimeter in diameter) to supercapillary (tubular openings greater 

 than 0.5 millimeter in diameter). Theoretically, the subcapillary 

 openings are not important in the production of oil, because the 

 molecular attraction of the solid for the liquid is so great that the 

 liquid tends to remain in the void. In capillary openings, the con- 

 tained liquids move slowly and only under hydrostatic force that ex- 

 ceeds the molecular attraction of the liquids for the walls of the open- 

 ing. Liquids contained in supercapillary openings move freely and 

 obey the ordinary laws of hydrostatics. Therefore it is with the open- 

 ings of capillary and supercapillary size that we are primarily con- 

 cerned. 



The shapes of openings in calcareous rocks are many and varied. 

 Some are tabular or sheet-like, as for example the space between cleav- 

 age surfaces of crystals, joints, and fractures. Other openings may be 

 spherical or ellipsoidal as shown by the common pinpoint porosity, 

 where the voids are due to the removal of oolites by solution, giving 

 rise to oolicastic-type porosity. Under certain conditions much of the 

 cementing material around the oolites is removed by solution to pro- 

 duce a carbonate rock composed of spherical oolites whose porosity 

 does not differ too greatly from that exhibited by a well-rounded sand 

 in sandstone. In some oolitic rock there is strong evidence that the 

 void space between the oolites has never been filled by cementing mate- 

 rial, and the resulting rock is comparable to a sandstone in its man- 

 ner of deposition. The solution of fossils such as ostracods, fusulinids, 

 brachiopods, or gastropods creates openings that represent molds of 

 these remains. Very often openings found in limestones or dolomites 

 are extremely irregular in shape and may be described as cellular in 

 character. 



In some instances, the origin of the resulting porosity can be read 

 from the shape of the openings; however, the secondary forces of 

 solution frequently alter the original openings to such an extent that 

 the primary characteristics are lost. 



In studying the formation and development of porosity, it is con- 

 venient to consider it in terms of origin or primary character and of 

 the changes that have taken place as a result of secondary forces. 

 Primary porosity consists of those openings formed at the time of 

 deposition of the rock or shortly thereafter at the time of lithifica- 

 tion. Such primary openings may also be formed long after rock 

 deposition as a result of various earth movements which produce fault- 

 ing, jointing, and fracturing. Essentially, secondary porosity is pro- 

 duced by chemical solution of the rock, or deposition, molecule by 

 molecule, of soluble salts in existing voids. 



