628 



imbt. CARBONATE POROSITY PROBLEMS 



[Ch. 33 



This is typical of what one might expect to find in a porosity developed 

 by weathering. 



Figures 3 and 4 show plastic models of two different types of oolitic 

 porosity. Figure 3 illustrates typical oolitic porosity, in which the 

 oolites are preserved and the porosity is caused by the void space 

 around oolite in much the same manner as the porosity in a sandstone 

 is formed by the voids between the individual sand grains. Figure 4 

 illustrates oolicastic-type porosity. In this kind of porosity the in- 



Fig. 7. Plastic model of fractured carbonate rock (Devonian), showing secondary 

 ground-water solution. South Fullerton Field, Andrews County, Texas. Magni- 

 fication 10X- (From Imbt and Ellison, 1946, p. 366.) 



dividual oolites are removed by solution, and the resulting rock is 

 actually a mold of the original rock. Generally speaking, oolicastic- 

 type porosity is not so effective as oolitic porosity. Actual core analysis 

 of the rock from which Fig. 3 was prepared shows a porosity of 11.2 

 percent and a permeability of 2,890 millidarcys. Similar tests on the 

 rock material of Fig. 4 shows a porosity factor of 6.7 percent and a 

 permeability of 2.4 millidarcys. Close inspection of Fig. 4 shows much 

 unconnected porosity that is not effective in permitting the free flow of 

 contained liquids in the reservoir. 



Figures 5 through 7 show results of impregnating fractured car- 

 bonates with Catalin. Figure 5 illustrates a fracture model where 

 there has been very little solution by ground-water action, although it 

 is suggested that solution laterals are beginning to develop. Figure 6 

 is still another fracture impregnation showing a ribbon-like pattern, 

 with considerable evidence of solution action which has enlarged and 



