Summary A geochemical logging tool (GLT SM) string, measuring natural, activation, and prompt neutron-capture gamma rays, produces logs of the most abundant and a few trace inorganic element concentrations. Direct measurements of Al concentrations are provided. A geochemically based closure model is used to derive Si, Ca, Fe, S, Gd, and Ti concentrations. The only significant spectroscopically undetermined element, Mg, is inferred by comparing measured with derived photoelectric factor. Analysis algorithms, demonstrations of accuracy and precision, and applications of geochemically derived formation properties are discussed. Introduction Elemental Analysis by Spectroscopy. This paper describes a nuclear geochemical tool string designed to determine a sufficient number of elemental concentrations through logging measurements to permit a satisfactory geochemical description of the formation. The tool string combines measurements of natural radioactivity with the natural gamma ray tool (NGT SM), delayed activation (for aluminum) with a new tool called the aluminum activation clay tool (AACT TM) and tau-gated thermal neutron-capture spectroscopy with the gamma ray spectrometer tool (GST TM). The components of the tool string are described in the next section. Three new features of nuclear spectroscopic logging measurements are introduced in this paper. First, an algorithm is presented to determine Al concentrations, in weight percent, from delayed-activation count-rate measurements. The count rates are corrected for the complex effects of the subsurface environment on the neutron and gamma ray physics. Natural activity measurements of Th, U, and K are directly calibrated in weight percent. Second, a new method is presented to calculate elemental concentrations from pulsed-neutron-capture measurements. Thus, all elemental concentrations are provided in weight percent. In most gamma ray spectroscopy measurements, the fraction of the detected spectrum that can be attributed to a particular element is linearly proportional to the concentration of that element in the volume of the measurement. However, determining the proportionality constant can be essentially impossible under neutron-capture logging conditions when pulsed neutron generators with variable and uncalibrated neutron yields are used. Thus, we have taken a new approach that renormalizes the relative yields from each clement measured through thermal neutron-capture reactions in a self-consistent manner to obtain the elemental concentrations. The key to this approach is focusing only on elements that are contained in the rock and not present in the fluids. The renormalization procedure is based on the geochemical fact that in all core analyses, the rock elemental oxides, measured in weight percent, sum to 100 %. The elements measured by capture, activation, and natural spectroscopy compose, with few exceptions, most of the significant elements seen as oxides, or carbonates, in the formations. Therefore, one can use the absolute elemental concentrations measured by natural K activity and delayed All activity, induced with a calibrated neutron source, to renormalize the prompt capture elemental contributions from the formation rock in a closure relationship. Third, the most significant element not measured spectroscopically, magnesium, can be determined when it is present in a significant concentration by comparing the measured photoelectric cross section for the formation with the theoretical cross section calculated with the assumption that no Mg is present. After the means for determining the elemental concentrations are described, examples are provided to demonstrate the viability of the elemental concentrations determined by neutron-induced gamma ray spectroscopy through comparisons of logging results with laboratory analysis of cores from the logged wells. Examples of elemental concentration repeatability are provided. Finally, the last part of this paper discusses several applications in which elemental concentrations can be used to enhance the formation description. Spectroscopic measurements of gamma rays were originally introduced to separate the contribution of particular elements to the total activity seen in a detector. Natural activity can be separated into the Th, U, and K components. Other logging measurements use a neutron source to initiate reactions that produced gamma rays. However, there are many difficulties associated with relating the fraction of a gamma ray spectrum to the absolute concentration of an element in the formation, and most of the original work concentrated on the use of relative spectral contributions, such as carbon/oxygen, to indicate gross formation properties like oil saturation. Recent geochemical research has shown that when a sufficient number of elemental concentrations is determined, a detailed mineralogy of the formation can be estimated. From this mineralogical description, many formation properties can be better characterized and other properties can be derived that could not otherwise be obtained except, perhaps, by detailed analyses of core samples. These properties include better porosity determination from logs, sandstone classification, cation exchange capacity, grain-size distribution, and permeability. Tool String Description Nuclear geochemical logging involves three separate modes of gamma ray spectroscopy to make a comprehensive elemental analysis of the formation. Fig. 1 shows the recommended configuration of this new openhole tool string. The first measurement is performed by the NGS tool, which passes by the formation before any neutron source can induce radioactivity. The concentrations of K, Th, and U in the formation are derived from the gamma ray spectrum recorded from these naturally radioactive elements and their daughter products. The second and newest measurement in the nuclear geochemical logging toot is performed by the AACT. The AACT is a modified NGS tool. The modification consists of three more windows added to the spectrometer to remove potentially interfering Mn activation. The AACT, the NGS tool above it, and the 252Cf neutron source [carried in the compensated neutron tool with epithermal measurements (CNT-G)] between them allow a measurement of activation gamma rays to derive formation aluminum concentration. The GST at the bottom of the tool string, in Fig. 1, measures the spectrum of capture gamma rays from elements in the formation. The GST uses a pulsed 14-MeV neutron generator to induce the capture reactions. The spectrum from the GST, in conjunction with elemental concentrations from the NGS tool and AACT, allows derivation of the concentration of elements in the formation rock, such as Si, Ca, Fe, S, Ti, K, and Gd. The GST is also sensitive to H and Cl, but these elements are not used in determining the rock geochemistry. Secondary measurements are made by the auxiliary measurement sonde (AMS TM) for determining borehole salinity and mud temperature, the CNT-G for apparent neutron slowing length of the formation, Ls, and as a carrier for the 252Cf source, and the formation neutron capture cross section, sigma form, obtained from the GST. These measurements are used in the environmental correction of the derived Al concentration. Data from the tools in the string are sent by telemetry to the surface by the telemetry communication cartridge or cable communication cartridge (cable communication electronics), also shown in Fig. 1.

An experimental system for gamma ray spectroscopy logging has been developed which uses prompt and capture gamma radiation induced in formations by 14-MeV neutrons from a pulsed-neutron generator to determine relative concentrations of various elements in the formation. The logging system uses computerprocessing techniques based on spectral modeling that has been developed to analyze the inelastic and capture gamma ray data obtained with a borehole spectrometer.The physics of the production of gamma rays from fast-neutron interactions with elemental nuclei in formations is discussed, leading to a simple but realistic interpretation model for the tool's response. This model is confirmed by laboratory and field tests.The relative spectroscopic contributions from carbon, oxygen, silicon, calcium, iron, chlorine, and hydrogen are used for various cased-hole and open hole logging applications. Particular emphasis is placed on the carbon/oxygen ratio used to obtain oil saturation independent of formation-water salinity. Carbon/oxygen ratio determinations made in the laboratory are compared with values predicted on the basis of known lithologies, porosities, and oilsaturation changes.In addition, the spectral contributions from iron, silicon, and calcium are used to interpret lithology; hydrogen, silicon, and calcium contributions are used to determine the effects of porosity; and chlorine and hydrogen contributions are used to investigate salinity changes.Field-test log examples of these elemental determinations are shown.

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Knowing how environmental properties affect dense nonaqueous phase liquid (DNAPL) solvent flow in the subsurface is essential for developing models of flow and transport in the vadose zone necessary for designing remediation and long-term stewardship strategies. For example, one must know if solvents are flowing in water-wetted or solvent-wetted environments, the pore-size distribution of the region containing DNAPLs, and the impact of contaminated plumes and their transport mechanisms in porous media. Our research investigates the capability and limitations of low-field proton nuclear magnetic resonance (NMR) relaxation decay-rate measurements for determining environmental properties affecting DNAPL solvent flow in the subsurface. The measurements that can be performed with the laboratory low-field system can also be performed in situ in the field with the current generation of commercial borehole logging tools. The oil and gas industry uses NMR measurements in deep subsurface, consolidated formations to determine porosity and hydrocarbon content and to estimate formation permeability. These determinations rely on the ability of NMR to distinguish between water and hydrocarbons in the pore space and to obtain the distribution of pore sizes from relaxation decay-rate distributions. In this paper, we will show how NMR measurement techniques can be used to characterize, monitor, and evaluate the dynamics of mixed fluids (water-DNAPL) in unconsolidated near-surface porous environments, and describe the use of proton NMR [Formula: see text] (spin-spin relaxation time) measurements in unconsolidated sandy-soil samples to identify and characterize the presence of DNAPLs in these environments. The potential of NMR decay-rate distributions for characterizing DNAPL fluids in the subsurface and understanding their flow mechanisms has not previously been exploited; however, near-surface unsaturated vadose zone environments do provide unique challenges for using NMR measurements. These challenges are addressed through systematic laboratory experiments and a program of research to extend and adapt current field NMR measurements to near-surface environmental problems.

Summary Elemental analysis of inelastic and capture gamma ray spectra from specially constructed laboratory formations is presented. The analysis provides formation oil saturation, water salinity, porosity, and lithology provides formation oil saturation, water salinity, porosity, and lithology after accounting for elements in casing, cement, and borehole fluid. These data permit quantitative formation analysis for many borehole geometries and elemental contents. Introduction The Gamma-Ray Spectroscopy Tool (GST TM) is a down hole NaI(Tl) based nuclear gamma ray spectrometer system, which detects gamma rays produced from reactions induced by 14-MeV neutrons irradiating the formation. Elemental data are obtained in two modes of operation: inelastic and capture-tau. The inelastic mode is based on a pulsed 100- sec timing cycle during which three spectra are accumulated:a gross inelastic gamma ray spectrum obtained during the neutron burst,a background spectrum obtained immediately after the burst, which is scaled and subtracted from the gross inelastic spectrum to obtain a net inelastic spectrum, anda 50- sec capture gamma ray spectrum obtained after the background spectrum. The net inelastic spectrum and the 50- sec capture spectrum are analyzed by a weighted-least-squares (WLS) fitting procedure to obtain information on the atomic concentration of carbon (C), oxygen (O), hydrogen (H), silicon (Si), calcium (Ca), chlorine (Cl), iron (Fe), and sulfur (S) in the formation. The capture-tau mode acquires an optimized capture spectrum (that provides information on H, Si, Ca, Fe, Cl, and S concentrations) and also measures the formation thermal neutron decay time, . The basic design and interpretation of both the inelastic mode and the capture-tau mode 3 of the GST tool have been presented previously. The use of inelastic neutron scattering for carbon and oxygen spectroscopy logging to measure oil saturation has been investigated for many years. More recently, the use of other elemental yields to improve the interpretation of oil saturation and to provide valuable information on formation salinity, porosity, and lithology has been demonstrated. While these investigations have emphasized the value of data on elemental concentrations for defining formation properties, they also have clarified further the advantages of having prior knowledge of tool responses under a wide variety of borehole conditions. The ultimate goal of the GST tool is to provide a measurement of elemental concentrations in the formation. These concentrations then can be used to provide detailed descriptions of formation fluid types and minerals. However, the measurement also is sensitive to the elemental concentrations in the region surrounding the tool, where borehole fluid, casing, and cement contain elements found in the formation. Since a gamma ray from an element in the cement looks the same as the gamma ray from that same element in the formation, the determination of the formation elemental concentration can become ambiguous. However, if the cement thickness is known, then the yield that results from an element in the cement can be obtained from laboratory measurements of cement in combination with formations that are free of the particular element. Similar techniques can be used to investigate the contribution to the spectral yields from other changes in the borehole environment. The potential number of formation and borehole conditions would lead to an unmanageably large set of measurements to determine all GST tool environmental effects. We have, therefore, obtained sufficient data from laboratory measurements to provide the expected response of the tool, in both modes of operation, to the most commonly encountered field conditions. The measured environmental effects data also provide the critical results for defining and testing interpretation models. The parameters that have been varied, with indications of the range of variation are shown in Table 1. Both open- and cased-hole responses have been obtained. For all cased-hole measurements, the casing is cemented. It should be noted that all laboratory measurements were made with a freshwater Type H portland cement. If, for example, other cements had been used, the results portland cement. If, for example, other cements had been used, the results would be altered by the relative elemental concentrations to those in Type H portland cement. Data have been obtained with a laboratory sonde and a GST prototype in specially constructed laboratory formations. The inelastic mode data were obtained with no external boron-loaded fluid excluder while the capture-tau mode data were obtained with a 5 1/2-in. [14-cm] OD boron-loaded fluid excluder. This corresponds to normal field operation. Although the absolute value of GST elemental responses may vary slightly with changes in tool design, the overall systematics of the responses reflect the actual physical changes in formation and borehole elemental concentrations and physical changes in formation and borehole elemental concentrations and will be invariant to small changes in final tool design. JPT p. 1527

Geochemical analyses of oil bearing formations, based on nuclear techniques, are often limited by the current level of uncertainties on neutron cross section data. The problems created for nuclear geochemistry are most acute in the areas of high energy neutron scattering, inelastic neutron scattering gamma-ray production, and production of prompt gamma rays and delayed activations generated by (n,p) and (n,ar) reactions for neutron energies up to 14 MeV. We will present examples illustrating the ways in which spectroscopic measurements are used for geochemical applications and how the uncertainties in the nuclear cross section data affect the usefulness of the results. Particular elements that cause significant problems in the analysis are emphasized. INTRODUCTIONThe discovery of hydrocarbon bearing geological formations and the probability of successful fluid recovery is dependent on the identification of fluids and a knowledge of the mineralogy of the surrounding rock.' An important technique for providing information on these questions is provided by continuous subsurface measurements in a borehole of the elemental concentrations of the' surrounding rock and fluid? With the

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