1. Field of the Invention
The present invention is in the field of gamma ray testing of geological formations. In particular, the invention determines the mineralogy of a formation from recorded spectra.
2. Description of the Related Art
Well logging systems have been utilized in hydrocarbon exploration for many years. Such systems provide data for use by geologists and petroleum engineers in making many determinations pertinent to hydrocarbon exploration. In particular, these systems provide data for subsurface structural mapping, defining the lithology of subsurface formations, identifying hydrocarbon-productive zones, and interpreting reservoir characteristics and contents. Many types of well logging systems exist which measure different formation parameters such as conductivity, travel time of acoustic waves within the formation and the like.
One class of systems seeks to measure incidence of nuclear particles on the well logging tool from the formation for purposes well known in the art. These systems take various forms, including those measuring natural gamma rays from the formation. Still other systems measure gamma rays in the formation caused by bursts of neutrons into the formation by a neutron source carried by the tool and pulsed at a preselected interval.
In these nuclear well logging systems, reliance is made upon the physical phenomenon that the energies of gamma rays given off by nuclei resulting from natural radioactive decay or induced nuclear radiation are indicative of the presence of certain elements within the formation. In other words, formation elements will react in predictable ways, for example, when high-energy neutrons on the order of 14.2 MeV collide with the nuclei of the formation elements. Different elements in the formation may thus be identified from characteristic gamma ray energy levels released as a result of this neutron bombardment. Thus, the number of gamma rays at each energy level will be functionally related to the quantity of each element present in the formation, such as the element carbon, which is present in hydrocarbons. The presence of gamma rays at a 2.2 MeV energy level may for example, indicate the presence of hydrogen, whereas predominance of gamma rays having energy levels of 4.43 and 6.13 MeV, for example, may indicate the presence of carbon and oxygen respectively.
In these nuclear well logging systems, it is frequently useful to obtain data regarding the time spectral distributions of the occurrence of the gamma rays. Such data can yield extremely valuable information about the formation, such as identification of lithologies that are potentially-hydrocarbon producing. Moreover, these desired spectral data may not only be limited to that of natural gamma rays, for example, but also may be desired for the gamma ray spectra caused by bombardment of the formation with the aforementioned pulsed neutron sources.
Well logging systems for measuring neutron absorption in a formation use a pulsed neutron source providing bursts of very fast, high-energy neutrons. Pulsing the neutron source permits the measurement of the macroscopic thermal neutron absorption capture cross-section Σ of a formation. The capture cross-section of a reservoir rock is indicative of the porosity, formation water salinity, and the quantity and type of hydrocarbons contained in the pore spaces.
The measurement of neutron population decay rate is made cyclically. The neutron source is pulsed for 20–40 microseconds to create a neutron population. Neutrons leaving the pulsed source interact with the surrounding environment and are slowed down. In a well logging environment, collisions between the neutrons and the surrounding fluid and formation atoms act to slow these neutrons. Such collisions may impart sufficient energy to these atoms to leave them in an excited state, from which after a short time gamma rays are emitted as the atom returns to a stable state. Such emitted gamma rays are labeled inelastic gamma rays. As the neutrons are slowed to the thermal state, they may be captured by atoms in the surrounding matter. Atoms capturing such neutrons are also caused to be in an excited state, and after a short time gamma rays are emitted as the atom returns to a stable state. Gamma rays emitted due to this neutron capture reaction are labeled capture gamma rays. In wireline well logging operations, as the neutron source is pulsed and the measurements made, the subsurface well logging instrument is continuously pulled up through the borehole. This makes it possible to evaluate formation characteristics over a range of depths.
Depending on the material composition of the earth formations proximal to the instrument, the thermal neutrons can be absorbed, or “captured”, at various rates by certain types of atomic nuclei in the earth formations. When one of these atomic nuclei captures a thermal neutron, it emits a gamma ray, which is referred to as a “capture gamma ray”.
Prior art methods exist for determining attributes of a formation from logging results. For example, U.S. Pat. No. 4,712,424, to Herron, performs an elemental analysis of core data as well as a mineralogical analysis. Based on a regression analysis of the core data, an element-mineral transformation matrix is determined. This predetermined transformation matrix is then applied to elemental analysis made from nuclear logs in an earth formation.
There are several problems with the Herron method. First is the problem of sampling: there is no such thing as a universal transformation matrix that will convert any elemental analysis, regardless of geologic setting, into a mineralogical analysis. The transformation matrix therefore has to be derived on a sample that is truly representative of the earth formation to which the matrix is being applied. This is not an easy task, and there still remains the problem of knowing when a particular transformation matrix is being applied to data for which it is not suitable. Detailed mineralogical and elemental analysis of a wide variety of core samples is not an easy task. A second issue has to do with the actual inversion, and the fact that physically unrealistic mineralogical analyses may result, specifically in the form of negative values of a mineral. Such negative values are indications that either the matrix is being applied beyond its range of applicability or the elemental analysis may be incorrect, or both may be occurring.
U.S. Pat. No. 4,394,574, to Grau et al., discusses investigating the composition of a geological formation traversed by a borehole by measuring an energy spectrum of the radiation within the borehole. The measured spectrum is thereafter analyzed by comparing it with a composite spectrum, made up of standard spectra of constituents postulated to comprise the formation-borehole system. As a result of such analysis, the proportions of the postulated constituents in the formation are determined.
U.S. Pat. No. 4,390,783, to Grau, discusses an iterative technique in which the offset of the background spectrum is varied until a goodness of fit parameter is optimized. The magnitude of the background spectrum is normalized by the ratio of the number of background counts in the gross energy spectrum to the number of counts in the background spectrum. Subsequently, the background spectrum is normalized by the ratio and then subtracted from the gross inelastic spectrum in order to determine a net inelastic spectrum.
Methods of decomposing obtained spectra into constituent spectra have been discussed, for instance, in SPE 7430 “Laboratory and Field Evaluation of an Inelastic-Neutron-Scattering and Capture Gamma Ray Spectroscopy Tool”, 1978., by Hertzog, SPE9461, 1980; SPE “Prompt Gamma-Ray Spectral Analysis of Well Data Obtained with NaI(T1) and 14 MeV Neutrons,” 1986, by Grau and Schweitzer; and Neutron-Induced Gamma Ray Spectroscopy for Reservoir Analysis, June 1983, by Westaway et al. The methods discussed in these papers correct an obtained inelastic spectrum by subtracting a background spectrum. Statistical analysis of obtained spectra is discussed in “Statistical Precision of Neutron-Induced Gamma Ray Spectroscopy Measurements” by Roscoe et al., November-December, 1987, The Log Analyst.
U.S. Pat. No. 5,471,057, to Herron, discusses a method for determining the elemental concentrations in an underground formation by irradiating the formation with neutrons, detecting the γ ray spectrum arising from neutron capture by the formation and analyzing the spectrum to determine relative elemental yields which are converted to elemental concentrations. This method avoids the need for activation measurements or natural radiation measurements and is made possible by applying a factor to modify the determined yield of iron (Fe) from the spectrum to compensate for the absence of measurement of aluminum (Al) and for the absence of potassium (K) when not measured directly. The apparatus of Herron '057 can comprise a neutron source, such as a broad energy chemical source, e.g. AmBe, or a pulsed accelerator source, a γ ray detector for detecting capture γ rays and means for analyzing the spectra detected by the determining the elemental concentrations in the formation.
There is a need for a more complete analysis of the obtained gamma ray spectra. A separation of inelastic and capture gamma ray spectra yields a more complete understanding of the elemental composition of a geological structure. Consequently, an advantage can be obtained through a combined analysis of both inelastic and capture spectra in terms of their formation constituents. Such a method should not give physically unrealistic analyses. The present invention fulfills this need.