358 Subsurface Geologic Methods 



The sorting is dependent on the proportion of fine minerals present, such 

 as silt or shale, or the proportion of large grains, such as large frosted 

 quartz grains commonly found in the Whitehorse section of the Permian 

 Basin. The amount of cement also largely determines the porosity which 

 is at a maximum in free unconsolidated sands and minimum in quartzite. 

 The permeability is largely determined in the same manner as porosity, 

 but here the size of grain and the sorting are most important. 



Oil stain and odor are very important and much experience on the 

 part of the sample examiner is required to estimate these accurately. 

 Dry gas will not stain sandstone, but wet gas may show a very light tan 

 stain. Oil is commonly darker, but very high-gravity oil shows no more 

 stain than the gas. Porous sand, containing no stain whatsoever may be 

 suspected of carrying water. In examining samples of oil stain, it is 

 very convenient to have a fluoroscope, which may detect very light stains 

 of high-gravity oil which are not obvious in ordinary light. Best results 

 in detection of oil stains are obtained with mercury vapor lamps emit- 

 ting light, with wave lengths ranging from 3,300 to 3,800 Angstrom units. 

 Lamps giving light of shorter wave lengths, such as the quartz tube, cause 

 fluorescence of oil, but also cause much mineral fluorescence in the 

 sample, which may confuse the observer. 



Use of Cuttings Descriptions in Study of 

 Limestone Reservoirs 



Limestones and dolomites form important oil reservoirs, and cuttings 

 descriptions furnish one of the chief sources of information concerning 

 them. There are two chief types of porosity in these rocks. The first is 

 intergranular porosity which consists of openings between the crystals, 

 oolites, or other discrete particles of the rock which in its geometry is 

 similar to sandstone porosity. The second is fracture porosity (or fora- 

 menular porosity of Bulnes and Fitting ^) which consists of large open- 

 ings through otherwise solid masses, such as fractures and vugs. 



The intergranular porosity is easily observed under the binocular 

 microscope where the tiny openings may commonly be seen connected 

 with each other. The larger openings are not ordinarily visible in their 

 entirety under the miscroscope but are indicated by irregular surfaces 

 lined by crystals which have been formed in a comparatively large cavity. 

 These large openings are much more difficult to detect than the smaller 

 pores and ordinarily they have gone unnoticed unless indicated by the 

 drilling-time or the manner of drilling. Permeability in the fractures or 

 foramenular porosity is almost impossible to determine under the micro- 

 scope, since there is no means of knowing how far apart the walls of the 

 large openings originally were. 



* Bulnes, A. C, and Fitting, R. U., Jr., An Introductory Discussion of Reservoir Performance of 

 Limestone Formations : Petroleum Development and Technology, 1945, Petroleum Division, Trans. Amer. 

 Inst. Min. Met. Eng., vol. 160, pp. 181-201. 



