ABSTRACT

The Natural Gamma Ray Spectrometry Tool (NGS) detects naturally occurring gamma rays of various energies emitted from a formation. Amounts and types of elements present are determined by how the formation was deposited and what has happened to it since deposition. Thorium, uranium, and potassium (Th, U, K) are primarily responsible for the energy spectrum observed by the tool. The individual contributions of the three elements in relation to the total formation weight are calculated from the energy spectra. Elemental concentrations thus calculated have showed correlation to depositional environment, neomorphic and diagenetic processes, clay type, and clay volume. A sophisticated data-processing approach, based upon the principles of natural gamma ray spectrometry and statistical estimation theory, has been implemented to improve the precision of the measured variables. A further reduction of the Th, U, and K statistical error is obtained at the wellsite by utilization of a real-time filtering algorithm implemented with the CSU system. Thus, most of the statistical error associated with the natural radioactivity measurements is removed prior to performing the interpretation. An initial interpretation can be made at the wellsite after logging and filtering. The NGS log is analyzed to define the nature and quantity of the most probable radioactive materials present in the formation. In particular, this is shown to yield an improved shale indicator, and a better estimation of the clay percentage at the wellsite. In complex lithologies, the exact nature of the mixture of radioactive minerals may not be immediately discernible. Hypotheses concerning the types and volumes of the radioactive minerals may be advanced and investigated at the Computing Center by combining the NGS log with other logs for quantitative interpretation and better lithologic description. Additional applications of the NGS measurements include analysis of log data for correlations, facies identification, reconstruction of the environment of deposition, reservoir delineation, and as supportive information for the identification of igneous rocks as well as the determination of permeability and producibility. In summary, the potential of extracting more information about the nature of the formation and reservoir, through the principles of natural gamma ray spectrometry, has been realized with the implementation of the NGS log.

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