Subsurface Logging Methods 359 



Permeability in intergranular porosity can be estimated qualitatively 

 by noticing the size of the pores and their apparent interconnection. In 

 general, the larger the pores the higher the permeability. The converse, 

 however, is not always true. Some dolomites showing very fine porosity 

 are shown by core analysis to be surprisingly permeable. However, one 

 type of porosity is non-permeable almost without exception. This is 

 called pin-point porosity and consists of small isolated holes. Some of 

 these holes contain small amounts of asphaltic material and even may 

 contain live oil and gas, but commercial production is not developed 

 from them. So far, no quantitative results about porosity and permeability 

 have been obtained from ordinary cuttings. However, the coarser reverse- 

 circulation cuttings have been analyzed for porosity and permeability 

 with favorable results. . 



An interesting method of reproducing and visualizing these pore 

 spaces is presented by Nuss and Whitney,^ who impregnated limestones 

 with plastic and then dissolved the limestone with acid, leaving a model 

 of the porosity as a residue. 



Oil and gas stains are ordinarily readily detectable in limestones and 

 dolomites. The heavy sour oils, as found in the Upper Permian, leave 

 a dark brown stain which is unmistakable. Lighter oils of the Lower 

 Permian rocks show good stains, and the very light oils found in the 

 Lower Paleozoic strata leave an extremely light stain which is difficult to 

 detect under incandescent light, but may be seen in white or ultra-violet 

 light. Since most gases carry a small amount of light oil with them, 

 they will show slight stains in limestones. Gas-oil contacts commonly 

 can be recognized accurately by the darkening of the stain at the top of 

 the oil column. Water may be indicated by lightening of the stain and 

 black asphaltic residues in the samples. Many dolomites have a char- 

 acteristic sheen on crystal faces within the water zone. However, a well 

 oil-stained section may produce water upon test. This fact may be at- 

 tributed to later movement of the structure which causes shifting of the 

 water table. 



In concluding the discussion of cuttings description, it should be 

 said that no rigid rules can be given for guidance in describing samples. 

 Each geological province and each geological organization have their 

 own problems which must be worked out individually. Since the strati- 

 graphic sections penetrated by wells are as varied as those on the sur- 

 face, there can be no substitute for experience and judgment on the part 

 of the geologist. Description of samples should never be allowed to de- 

 generate into a mere mechanical process. The better the geological back- 

 ground of the person examining samples, the better will be the descrip- 

 tion. 



' Nuss, W. F., and Whitney, R. L., Technique for Reproducing Rock Pore Space: An 

 troleum Geologists BuH., vol. 31, no. 11, pp. 2044-2049, 1947. 



