thin-section data on signs of metamorphism could prove very helpful in an ex- 

 ploration program. 



Reservoir Studies 



In recent years, encouraging results have focused considerable attention 

 on reservoir engineering. The increasingly important problem of recovery is 

 one involving the characteristics of the reservoir. In view of the fact that oil 

 reservoirs are in rocks and that the best way to study rocks is in thin section, it 

 follows that efficient study of reservoirs must include study of thin sections. Al- 

 though quantitative analysis of porosity and permeability is better accomplished 

 by other means, thin sections provide a means of studying other equally im- 

 portant features of the reservoir, such as type of porosity, composition as affect- 

 ing wettability, and texture as affecting tortuosity. 



Porosity patterns in rocks are very complex, more complex than is generally 

 realized. Although the main types of porosity are intergranular, fracture, and 

 cavity, there are many different variations in each; therefore in evaluating po- 

 rosity of carbonate rocks especially, one must consider each rock individually. 

 In phaneritic detrital sediments, porosity is normally intergranular. In aphanitic 

 detrital sediments and chemical sediments, porosity is normally of the fracture 

 or cavity type. A highly indurated sandstone, however, may have the same kind 

 of porosity as chemical rocks. A thorough treatment of porosity in petroleum 

 reservoirs is given by Levorsen (1954). 



Determination of the kind of porosity is important because porosity de- 

 termines to a large degree the response of a rock to treatment in completion or 

 secondary-recovery practices. Many good examples of the part thin sections can 

 play in solving these problems are given in a recent paper by Waldschmidt, 

 Fitzgerald, and Lunsford (1956). In their summary they state, "The importance 

 of the phases of petrographic research here described can not be overemphasized, 

 especially as related to the selection and improvement of treating processes, 

 initial and remedial, and also as related to the estimation of reserves, secondary 

 recovery problems, and problems of stratigraphy." 



Depending on the degree of lithologic detail and accuracy desired, several 

 thin sections from the same sample may be required. Slices should be cut in 

 different directions to portray the three-dimensional aspects of the rock. The 

 original spaces and grain relationships may be made more conspicuous by im- 

 pregnating the specimen with dyes. 



Two of the most important factors in fluid transfer through rocks are : ( 1 ) 

 the attraction between rock and fluids, and (2) the surface area of the channels 

 along which the flow takes place. Both of these factors must be considered in 

 reservoir analysis. For given pressure-temperature conditions, the attraction be- 

 tween rock and fluid depends on their respective compositions. The fluids may 

 be readily analyzed, but thin-section study is required to identify the minerals 



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