Ch. 32] CEMENTATION 609 



pressure to a point where little or no oil could have displaced the con- 

 nate water originally present. There is some reason to believe that a 

 rather special depositional environment is required to "winnow" the 

 clay from the sand and provide a good reservoir for oil. 



Cementation 



Most consolidated sandstones contain varying amounts of cement, 

 composed mainly of carbonates and silica, and less commonly of oxides 

 and other minerals. The cement may have been deposited contem- 

 poraneously with the sand, or it may have been introduced later during 

 the process of compaction. 



Certain types of sandstones, notably the Ordovician sands of the St. 

 Peter type in the central and eastern United States and the Tensleep 

 of the Rocky Mountains, were deposited in association with limestone 

 and dolomite. They grade, both laterally and vertically, into sandy 

 limestone. The amount of carbonate thus varies greatly; in some 

 places the rock is pure quartz sand, whereas in others it is 50 percent 

 or more calcite or dolomite. The porosity and permeability vary ac- 

 cordingly. Occasionally the calcareous sandstone has been subjected 

 to erosion and leaching, resulting in a very soft, permeable sand. 

 When reburied under an impervious caprock, as at Oklahoma City, 

 such sands form prolific oil reservoirs. 



Calcite cement is also very common in the absence of limestones 

 and dolomites in the section. Oil very commonly occurs in an alter- 

 nating sequence of gray and dark-gray shale, and thin sandstones and 

 siltstones. As a rule the top and bottom of the sands are tightly 

 cemented by calcite, the central portion alone being permeable. Com- 

 pletely impermeable calcareous siltstones are common. In these it is 

 possible that the calcite was introduced after deposition by circulat- 

 ing waters expelled from the adjacent shales during their compaction. 

 The presence of oil in the coarser parts of the sand may have so reduced 

 their permeability to water that they were not cemented as much as the 

 finer parts. 



Secondary silica cement is also very common in consolidated oil- 

 producing sands. Each grain is frequently coated with a layer of 

 quartz, in optical continuity with the original grain. The deposition of 

 silica apparently begins in the finer interstices and spreads into the 

 larger openings. Thus the finer pores are filled, and the points of 

 contact of the original grains are broadened. The new surfaces bound- 

 ing the larger pores are frequently plane crystal faces. It appears to 

 be possible (although not yet demonstrated experimentally) that the 

 deposition of secondary silica may reduce the porosity considerably 



