1. Cement is used in wells to set casing, regain lost circulation in highly 

 porous and permeable zones, and in some instances to hold pieces of drilling 

 equipment that have been lost in the hole so that fishing or milling tools can be 

 used more effectively. Cement appears in the samples as a chalky limestone, and 

 it reacts in acid as a limestone. There are few sample men who have not at one 

 time logged cement as chalky limestone. One should check the drilling records 

 for drilling operations mentioned above. Cement is almost invariably peppered 

 with minute black specks. It would be a rare limestone that would have the 

 even distribution of specks so characteristic of cement. 



2. Commercial drilling muds are made from clays, iron oxides, barite, 

 bentonite, and other mineral substances. When samples are improperly washed, 

 the dried mud may be logged as formational materials. Occasionally clays and 

 shales near the well site are ground and used as drilling mud. They may be 

 poorly prepared so that small chips will appear even in the washed sample. 

 Hematite weighting material stains samples red. One should check on the kind 

 of drilling fluid used in the well. 



3. A large variety of substances are used for the purpose of reducing the 

 loss of drilling fluid into porous formations. These materials may not be 

 eliminated in the washing of the samples and might be mistaken for mineral 

 constituents of the rock. Cellophane flakes, for example, might be mistaken for 

 selenite or muscovite. Organic substances, such as cellophane, ignite in a match 

 flame. 



4. Sometimes lubricating oil is inadvertently splashed on cuttings or cores 

 at the drilling rig. It is difficult to distinguish light lubricating oil from crude 

 in small amounts adhering to drill cuttings. Examine freshly broken chips. 

 Extraneous oil will likely remain on or very near the surface; naturally occur- 

 ring oil should be found in the interior of some of the chips. 



5. Stylolitic surfaces in limestones and dolomites appear as black oily 

 films. Crude petroleum occurring in fractures and along bedding planes is 

 quite similar. A match flame will quickly distinguish one from the other. 



6. Up-hole cavings and recirculated cuttings are the microscopist's biggest 

 problem. The danger of relogging caved materials has been discussed. However, 

 there is some danger in the sample examiner becoming too sensitive to the 

 problem. He may develop the tendency to call all recurring lithologies cavings 

 from the depth where the lithology was first described. When in doubt, com- 

 pare the chips directly with the ones logged earlier. There may be some slight 

 differences not apparent from the descriptions, but sufficient to establish the 

 identification. 



7. Samples should be run under uniform conditions of lighting and mag- 

 nification. 



8. Only dilute hydrochloric acid (1 to 7) should be used, but the strength 

 of the acid should be checked frequently. 



37 



