APPLICATIONS OF SPECTROLITH MINERALOGY FROM NEUTRON CAPTURE SPECTROSCOPY TOOLS FOR FORMATION EVALUATION

Abstract

When performing a petrophysical analysis, an accurate understanding of the mineralogy of the formations of interest is important for two main reasons. Firstly, the mineralogy of the formations controls the matrix density, which is fundamental in the evaluation of porosity using conventional methods. Secondly, an accurate estimate of the fraction of clay minerals present in the formation is essential in accurately correcting resistivity-based saturation estimates for the effects of excess clay conductivity.

Accurately evaluating the mineralogy in formations containing both gas and radioactive minerals such as feldspars can be a challenge. Traditional clay indicators such as gamma ray estimate clay volumes which are too high due to the radioactivity coming from matrix grains such as potassium feldspars, feldspar rich volcanolithic grains and muscovite. In formations having light hydrocarbon such as gas or condensate, density-neutron logs under-estimate the volume of clays due to the light hydrocarbon effects on the density and neutron logs. Moreover, if the logs are to be acquired in a borehole with gas, air or foam, only a limited number of tools sensitive to mineralogy may be run. Using the SpectroLith technique, data from neutron capture spectroscopy tools such as the Reservoir Saturation Tool (RST) or Elemental Capture Spectroscopy Sonde (ECS) can be used to provide answers in these challenging situations.

The RST is normally logged in cased hole for the sigma or carbon/oxygen ratio logs useful for determining formation saturation. The SpectroLith processing extracts lithology information, and in turn the weight proportion of clay, quartz-feldspar-mica, carbonate, pyrite, anhydrite and coal. The ECS tool is optimised for providing SpectroLith results at faster logging speeds in larger boreholes.

This paper demonstrates, through examples from Australia, the applications of SpectroLith results derived from RST and ECS measurements. Two examples with the RST and one with the ECS are presented. In one example, the only available source of mineralogical information was from the results of SpectroLith processing applied to RST data. The second example shows how SpectroLith results can complement a through casing petrophysical evaluation. The example using ECS compares the mineralogy from a traditional analysis with that from SpectroLith processing and demonstrates the improvements to the petrophysical evaluation through a more accurate mineralogical description. Although no complete examples are available in gas wells in Australia at present, SpectroLith provides a promising way for accurately estimating mineralogy in gas wells.