ABSTRACT

Formation matrix properties, such as matrix density, can be estimated from the elemental concentrations available from modern, openhole, nuclear spectroscopy logging techniques. Although this estimation is similar to that of mineral-based interpretation frequently practiced today, it can preempt the a priori selection of minerals by solving for matrix properties directly from the elements. This simple approach greatly enhances the ability to perform wellsite interpretations in both simple and complex formations. The interpretation for the matrix density is derived from a comprehensive database containing hundreds of core samples analyzed for both mineralogy and chemistry. The chemical analysis includes not only the major elements, but also the minor and trace elements that significantly influence wireline log responses. These data are used to forward model the matrix which is then solved as a linear combination of four elements (silicon, calcium, iron, sulfur) that are measured by prompt neutron capture spectroscopy. Comparisons are shown between measured and derived matrix density along with statistical measures of goodness of fit. Although in many cases the errors could be reduced by local optimization, the overall agreement is quite good. Although matrix density is empirically derived, the rationale is straightforward. For example, in sandstone, matrix density is approximately equal to that of quartz and feldspar, and it increases as the concentration of calcium- and iron-bearing minerals increases. Therefore calcium and iron heavily influence matrix density. The feldspar minerals are less dense than quartz and are not well sensed by the elements Si, Ca, Fe, and S. Therefore, separate algorithms are presented for non-arkosic, sub-arkosic, and arkosic environments.

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