Patent Number: 052672743
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method of determining the chemical composition of apatite grains contained within rock samples taken from the bore of a well being drilled or from the surface of the Earth. 2. Description of the Prior Art Numerous methods of analyzing the chemical composition of minerals have been used including the currently used method of using a microprobe to focus a beam of electrons onto the surface of a material and causing X-rays to be generated. By carefully monitoring the X-rays emitted during the bombardment of a surface by the electron beam, it is possible to determine the chemical composition of the material being bombarded. Presently, in the oil exploration field, it is desirous to identify apatite grains contained within rocks which are rich in fluorine in order to perform analyses on them for purposes of determining the thermal history of the rock. This is most often accomplished by grinding or crushing the rock sample to obtain sand-sized grains and then separating the grains containing apatite crystals from the surrounding grains of minerals and rock in a multiple stage process using the density and magnetic characteristics of the surrounding minerals. After a multiple step process, the remaining grains of the rock sample consist of a sufficiently high percentage of apatite for purposes of analysis. A representative portion of the remaining apatite grains is then incorporated into an epoxy wafer attached to a petrographic slide, polished to expose internal surfaces, and etched with acid. The epoxy wafer is covered with a muscovite mica detector in the form of a mask and placed adjacent to the core of a nuclear reactor along with a uranium-doped glass covered with a second muscovite mica detector where both are irradiated with thermal neutrons. The epoxy wafer, uranium-doped glass, and their attached muscovite mica detectors are then removed from the reactor and the muscovite mica detectors are immersed in hydrofluoric acid to etch the induced fission tracks caused by the induced fission of uranium in the apatite grains and the uranium-doped glass. The concentration of .sup.238 U and the fission track density per unit volume are determined for a volume of an apatite grain beneath a selected area of the apatite grain and the fission track age for each grain is determined. In the methods currently practiced within the industry, the chemical composition of the apatite grains is then determined by a process known as microprobe analysis. This process consists of placing the apatite grains under an electron beam thereby inducing each affected apatite grain to produce X-rays. Through the careful monitoring and detection of these emitted X-rays, it is possible to determine whether the apatite grain being subjected to the electron beam is fluorine-rich, chlorine-rich, or water-rich, or some combination of these types. The currently practiced process of determining the chemical composition of the apatite grains is an extremely costly, time consuming, and labor intensive process, and, as such, it is rarely done with sufficient completeness. Additionally, the level of expertise required of the person performing the actual steps of the analyses currently used is greater than in that of the present invention. The method of the present invention demands a high level of expertise only during the interpretation phase rather than during the actual performance of the analyses, consumes less time for a given number of samples, and results in greatly increased capital savings. Read, U.S. Pat. No. 1,799,604 discloses a method and apparatus for identifying precious gems and crystals which operates upon the principle of an initial ray or beam of light striking a diamond and being reflected or refracted into secondary rays of light whose intensity and direction are dependent upon the angles, faces and imperfections in the diamond. This apparatus allows the recording of the secondary rays so that the diamond or crystal can be identified thereby under identical conditions. Grayson, U.S. Pat. No. 4,093,420 discloses a method of prospecting for accumulations of minerals based upon organic material present in the rock samples taken at differing locations and depths. This method is based upon the amount of light emitted or absorbed by the specific organic particles within the rocks and the gradients between samples taken at the same location but at different depths are plotted on a map. By repeating this procedure for numerous locations, the contours which will appear on the map will encircle the mineral deposit. Trossarelli, U.S. Pat. No. 4,906,093 discloses an illuminator device for the spectroscopic observation of samples wherein the substance under examination is illuminated by a source of white light and possesses optical fibers for transmitting the residual illumination light passed through the substance observed to an observation spectroscope. Dobrilla, U.S. Pat. No. 4,925,298 discloses a method for measuring and plotting the etch pit density on the surface of an etched test wafer. In this method, a beam of light is focused onto an etched wafer and the intensity of the light reflected is compared with the intensity of a reference wafer to calculate the etch pit density of the etched wafer. This procedure is repeated for different areas of the etched wafer so that any variances within each wafer may be detected. In the method of the present invention, the chemical composition of the apatite grains is determined by taking measurements of etch figures formed by the intersection of etched naturally occurring fission tracks or other crystallographic imperfections, such as other charged-particle tracks, defects, dislocations, fluid inclusions, mineral inclusions, polishing scratches, and fractures, with the planar surface of the apatite grain being observed. The purpose of the measurements is to determine if the apatite is of a fluorine-rich, chlorine-rich, or water-rich nature. Apatite grains which are fluorine-rich are identified by the characteristic dimensions of the etch figures within their etched planar surfaces. The dimensions of the etch figures in fluorine-rich apatite and relatively non-fluorine-rich apatite are taken and, together with other information gathered by methods of analysis already used within the geological sector of the scientific community, the pooled fission track ages and the pooled distributions of perceived track lengths pertaining to the fluorine-rich apatite grains and the relatively non-fluorine-rich apatite grains, respectively, are determined. While the prior art discloses methods and apparatus with which to observe mineral or rock samples and even calculate the density of etch pits on the surface of a wafer containing crystals which has been etched, the actual use of the dimensions of the resulting etch pits and the etch figures they form has not been practiced to determine chemical composition of the crystalline structure which has been etched. SUMMARY OF THE INVENTION It is an objective of this invention to provide a method for utilizing measurements of two-dimensional etch figures formed by the intersection of etched, naturally occurring fission tracks and other crystallographic imperfections with the surface of the apatite grain in which they exist to determine the chemical characteristics and composition of the apatite. Apatite grains are obtained from rock samples from the bore of a well or from the surface of the Earth for the purpose of determining the geological evolution of the rock samples. By observing and measuring the etch pits and the etch figures they form after exposing naturally occurring fission tracks and other crystallographic imperfections to acid, it may be determined whether the apatite grains being analyzed are predominantly fluorine-rich (or fluorapatite), chlorine-rich (or chlorapatite), or water-rich (or hydroxyapatite), and, using already existing methods, it will allow the pooled fission track ages and the pooled distributions of perceived track lengths to be determined for the predominantly fluorine-rich apatite grains and for the predominantly non-fluorine-rich apatite grains. The present method utilizes the shape and dimensions of two-dimensional etch figures on etched planar surfaces of apatite grains in order to determine the chemical composition of apatite grains obtained from rock samples. Over time, fission tracks accumulate in the apatite grains due to the self-destruction of the nuclei of the trace element .sup.238 U. The fission that occurs when one of these nuclei spontaneously destructs causes damage to the surrounding crystalline structure of the host apatite grain. The resulting damage is an elongated path known as a fission track. While these fission tracks cannot be seen optically, they may be enlarged or etched, sufficiently to be viewed using either an optical microscope or a scanning-electron microscope, by exposing the planar surface of the apatite grain to an acidic solution and preferentially dissolving the crystallographic damage that constitutes the naturally occurring fission tracks. The etching process transforms fission tracks that intersect the etched planar surface of an apatite grain into etch pits. An etch pit is a polyhedral (or three-dimensional) recess issuing from the etched planar surface of the apatite grain; prior to becoming an etch pit, the space within the polyhedral recess was initially composed of apatite material that ultimately was preferentially dissolved by the acidic solution. An etch figure is the polygonal (or two-dimensional) cross-section of the etch pit where it intersects the etched planar surface of an apatite grain. Other crystallographic imperfections are etched in a similar manner to that of naturally occurring fission tracks, and these include other charged-particle tracks, defects, dislocations, fluid inclusions, mineral inclusions, polishing scratches, and fractures. Additionally, naturally occurring fission tracks which do not intersect the etched planar surface of an apatite grain may still be etched if they intersect another fission track or a crack in the crystal structure of the apatite grain that does intersect the etched planar surface. In this manner, some subsurface fission tracks can be etched and viewed. The intersection of the etch pits with the etched planar surface of an apatite grain causes polygonal apertures or etch figures to be formed on that surface. If the crystallographic c-axis of an apatite grain is parallel to the etched planar surface, one of the two orthogonal axes of each etch figure will be parallel to the c-axis of the apatite grain. One of the measurements taken is the maximum diameter of the etch figure taken along a line segment which is parallel to the crystallographic c-axis of the apatite grain. Another measurement taken is the maximum diameter of the etch figure taken along a line segment which is perpendicular to the crystallographic c-axis of the apatite grain. Arithmetic mean maximum etch figure diameters parallel and perpendicular to the crystallographic c-axis are calculated for each apatite grain studied by taking the average of the respective diameters for a series of etch figures measured for each apatite grain. The mineral apatite is categorized as being fluorine-rich, chlorine-rich, or water-rich apatite, or some combination of these types. By measuring the arithmetic mean maximum diameter parallel to the crystallographic c-axis and the arithmetic mean maximum diameter perpendicular to the c-axis of the etch figures, it is possible to identify the fluorine-rich apatite grains. Etch figures in fluorine-rich apatite tend to be relatively small in size and exhibit a shape characterized as short and narrow. Chlorine-rich apatite tends to exhibit etch figures that are longer and proportionately wider while water-rich apatite etch figures exhibit dimensions which are approximately equal in length to those of chlorine-rich apatite while having a proportionately narrower diameter perpendicular to the crystallographic c-axis. The crystallographic defects within the apatite crystal or grain that make up a naturally occurring fission track exhibit a natural tendency to convert back to pristine (undamaged) crystalline material. This process is referred to as fission track annealing or annealing and occurs at all temperatures near the Earth's surface. Annealing is the process by which fission tracks are eliminated wholly or partially from their host apatite grain. The annealing process is most rapid at temperatures between 70.degree. C. and 130.degree. C. over geologic time. Annealing occurs more rapidly in fluorine-rich apatite in comparison to the annealing rate in chlorine-rich apatite. Fluorine-rich apatite is most useful for the study of oil formation whereas chlorine-rich apatite is useful for the study of natural gas formation. Because fluorine-rich apatite occurs most commonly in nature and also because the dimensions and characteristic shape of etch figures formed in fluorine-rich apatite are most easily identified and measured it is desirable to perform analyses on apatite that is predominantly fluorine-rich. However, analyses are also performed on relatively non-fluorine-rich apatite, when such apatite is present, as the present invention provides a method to distinguish between these apatite types. Non-fluorine-rich apatite is apatite that is not categorized as fluorine-rich. In addition to the measurements of the arithmetic mean maximum etch figure diameters parallel and perpendicular to the crystallographic c-axis for each apatite grain studied, the fission track ages and the perceived track length distributions of fluorine-rich and non-fluorine-rich apatite grains are also measured using presently practiced methods. The etch figure measurements provide a means to group the apatite grains into fluorine-rich apatite and relatively non-fluorine-rich apatite and enable the calculation of a pooled fission track age and a pooled distribution of perceived track lengths corresponding to each apatite type. Furthermore, the etch figure measurements eliminate the need to perform expensive microprobe analyses to determine the chemical composition of the apatite grains. Fission track age measurements for apatite grains require that the etched apatite grains be masked by a thin sheet of muscovite mica detector and then be placed in close proximity to the core of a nuclear reactor along with a uranium-doped glass covered by a second muscovite mica detector to be irradiated with thermal neutrons in order to determine the amount of .sup.238 U in the apatite grains. It is presently common practice to irradiate the apatite grains and the muscovite mica detectors prior to measuring the chemical composition of the apatite grains by microprobe analysis. This approach necessarily exposes the analyst to radioactive material but it is practiced because of the high cost of the microprobe analyses and the requirement that all other measurements be completed for the apatite grains prior to microprobe analysis in order to most efficiently employ this method of chemical composition measurement. The method of the present invention eliminates the requirement of prolonged exposure and handling of the radioactive apatite grains after irradiation by permitting a thorough analysis of the chemical composition of the apatite grains to be performed prior to irradiation of the apatite grains in the nuclear reactor. Following irradiation of the apatite grains, it is only necessary to examine the relatively less radioactive muscovite mica detectors that were irradiated in contact with the apatite grains and the uranium-doped glass. The method of the present invention for performing fission track analysis utilizing the dimensions of etch figures in apatite eliminates the requirement of performing costly microprobe analyses to determine the chemical composition of the apatite grains and in doing so allows more rapid analysis, eliminates the requirement for a high level of expertise at all levels of analyses, and eliminates the need to be exposed to radioactive material for extended periods of time while performing the analyses. Further features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.