Ch. 33] CARBONATE-FORMATION WATERS 629 



modified the original opening to a large extent. Figure 7 likewise shows 

 the influence of fracturing, but in this instance a large amount of sec- 

 ondary ground-water solution is shown by the lacy pattern of the 

 plastic model. This sequence of pictures effectively illustrates the pro- 

 gressive action of ground-water solution in the improvement of car- 

 bonate porosity and permeability. 



All three of the preceding suggested methods of study are largely 

 dependent on satisfactory recovery of samples representative of the 

 reservoir bed. The use of conventional coring methods leaves much 

 to be desired, as recoveries are usually not complete and the core loss 

 occurs in the more porous rock. By use of a diamond-studded core 

 head, recoveries approaching 100 percent are now possible in carbon- 

 ates where the chert content is not too high. Improved design and tech- 

 nique may eventually make it possible to core chert formation with 

 recoveries approaching 100 percent. 



THE CHERT PROBLEM 



Almost all carbonate rocks contain varying amounts of interbedded 

 primary and secondary chert. Inasmuch as chert is so common in car- 

 bonates, we might expect to know more about conditions of carbonate 

 deposition if we were in a position to know the environment in which 

 chert was formed. The deposition of chert is in itself enough of a 

 problem to justify a special program of research; moreover, owing to 

 the intimate association of chert with limestone and dolomite, a pro- 

 gram of carbonate research could not be considered complete without 

 investigation of chert in carbonate deposits. 



Generally speaking, chert may be formed by (1) chemical precipita- 

 tion of colloidal silica, (2) the release of silica by bacteria, and (3) dep- 

 osition from silica-saturated ground waters. Most of the techniques 

 for carbonate research could be equally well employed on the chert 

 problem. 



As chert is a very important part of many carbonate rocks, it gives 

 rise to many economic problems. The presence of chert not only adds 

 to the drilling problem, but also sometimes the associated limestones 

 and dolomites do not develop porosity, and it is the fractured chert 

 that actually forms the reservoir. 



CARBONATE-FORMATION WATERS 



A study of the chemical characteristics of carbonate-reservoir waters 

 should receive attention as part of a well-rounded program of funda- 



