Abstract

In this age of 3D seismic, computer processing of both seismic and subsurface data, E-mail, faxes and modems, we thought it a good idea to pass on to young geoscientists and others who have not looked at the rocks a few hints about drill cutting analysis. The technique of viewing the cuttings with transmitted, polarized light gives the geologist a better idea of rock type, and thus a clue to the depositional environment.

Sample Study: Each sample is screened through a strainer with approximately 1.5 mm mesh, which allows a uniform cutting size to be studied. The large particles were not examined. In many of the samples, particles down to powder size also go through the screen, and these were excluded from examination unless there was no other material to look at. It is preferable to look at similar size cuttings because these are coming directly off bottom and are believed to arrive at the shaker, and in the sample bag, at a uniform time. Thus, they are the best representative of the lithology for the interval sampled. After screening, the sample is placed in a glass dish and studied in reflected, blue filtered light with 15X magnification under the binocular microscope. An estimate of the percentage of each component is made. The carbonate portion is then viewed with transmitted, polarized light which allows rock type classification using Dunham (1962) terminology (see Fig. 1) The thin cuttings are actually translucent and rock particles plus cement can be estimated in the transmitted, polarized light. Some of the cuttings were treated with acid to determine lime content, and sometimes viewed under water with transmitted light to make particle identification. Occasionally the use of transmitted and reflected light in combination gives a better idea of porosity.

Both interpretative and quantitative descriptions were made. Since the depositional environments are estimated from the Dunham rock type, one must know his classification, and what it means.

In dolomites, much of the original texture is destroyed with the recrystallization. Nevertheless, thin sections sometimes show phantom outlines of the original grains. Looking at the thin cuttings with polarized, transmitted light gives similar results.

Estimates of porosity in any recrystallized rock is quite difficult, but in dry cutting examination some types of porosity are visible, i.e., intercrystalline, pin point, and vuggy.

Ten San Andres samples were selected in the Phillips Coates #1-A, Pecos County. The cuttings were studied using the binocular microscope. A small portion of each sample was made into a thin section which was examined using the petrographic microscope for a more precise lithologic description and porosity measurements. The results were compared and reasons for discrepancies discussed.