and vertical variations in composition or texture within a formation are com- 

 mon. Thin sections often provide a critical means of correlation where other 

 routine methods fail. Inasmuch as thin sections of cuttings can be prepared 

 quickly, they can be used for routine formation identification during drilling. 



Lithofacies Studies 



Probably the most extensive use made of thin sections in subsurface geology 

 at present is in lithofacies investigations. Many geologists believe that even 

 subtle lithologic changes have profound influence on oil occurrence. A careful 

 study, then, of these changes not only results in better control for selection of 

 wildcat locations, but may actually aid in a development drilling program in 

 areas having abrupt facies changes. 



A word of caution is appropriate in this regard. Lithofacies maps are only 

 as reliable as the data on which they are based. Useless and even detrimental 

 maps can result from inaccurate analysis or from careless or inconsistent nomen- 

 clature in rock classification. Care should be taken to see that studies are done 

 accurately and that, for the map area, the elements of description and nomencla- 

 ture are consistent. The number of different elements that can be used in litho- 

 facies studies is too great for consideration here; however, some of the more 

 common general elements may be outlined. 



Cement Versus Matrix 



The kind of interstitial material in a detrital sediment might have an im- 

 portant bearing on the migration and accumulation of fluids. Two significantly 

 different kinds of interstitial material are chemical material or cement, and 

 detrital material or matrix. The conditions by which cement or matrix are pro- 

 duced are profoundly different and most likely have an influence on the origin 

 of petroleum. The shifting in relative proportions of cement and matrix, either 

 laterally or vertically, might prove a useful criterion in a given area. Plotting 

 these data might reveal trends that correlate with better producing characteristics 

 or, in some other way, increase development or exploration prospects. 



Amount of Interstitial Material 



The amount of interstitial material in a sediment is a very important ele- 

 ment to consider. It not only largely determines porosity and permeability but 

 may have genetic significance, which in turn may correlate directly with petro- 

 leum occurrence. Detrital interstitial material is always contemporaneous with 

 deposition of the major fraction, whereas chemical interstitial material may be 

 either contemporaneous or post-depositional. The movement and accumulation 

 of fluid within rocks then, must be affected profoundly by the amount and rela- 

 tive age of interstitial material. 



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