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Mineralogy and Geochemistry |
Schlumberger-Doll Research, Old Quarry Road, Ridgefield, Connecticut 06877-4108, U.S.A.
Recent advances in geochemical logging and interpretation have made it possible to obtain in situ concentration logs for at least ten of the chemical elements present in sedimentary formations: Al, Si, Ca, Fe, S, Ti, K, Th, U, Gd and possibly Mg. Each of these elements is concentrated in the solid portion of the formation as opposed to the pore fluids, and together these elements provide an array of measurements with a large dynamic range and tremendous diagnostic strength for geological interpretation.
The combination of a few diagnostic elements, such as silicon, aluminum, and calcium, provides sufficient information for a rapid but accurate lithological description. In the case of siliciclastic reservoir rocks, it is possible to discriminate between sand and shales and to determine types of sandstones using the ratios of SiO2/Al2O3 and Fe2O3/K2O. Calcium is used in conjunction with these ratios to differentiate between non-calcareous, calcareous, and carbonate rocks. On a more sophisticated level, a set of chemical abundances can be incorporated into a sedimentary normative analysis to determine quantitatively both the framework and clay mineralogy of siliciclastic formations. Derived mineral assemblages can provide valuable information for the interpretation of depositional environments and diagenesis. In shales, elemental data can be used alone or in conjunction with derived mineralogy to derive total organic carbon and thereby begin to evaluate source rock potential. By using individual elemental concentration logs or any of the interpreted formation units it is possible to enhance the characterization of vertical sequences and the recognition of well-to-well correlations.
