Improved Methodology For High-Resolution Bed Mineralogy From Wireline Logs

Cores are the most valuable material to characterize sub-surface formations but also the most costly. Wireline or Logging While Drilling (LWD) logs offer an alternative but the resolution is commonly poorer than core information. Recent developments in instrumentation enable to determine the mineralogy of logged formations using the gamma spectrum emitted in response to excitation by fast neutrons. The natural and neutron-induced gamma spectra are converted first into a bulk chemical composition, then into a mineralogical composition. By combining this mineralogical information and the lithofacies from borehole images, the resolution of the mineralogical analysis can be improved. This combination enables to discriminate homogeneous layers with a heterogeneous mineralogy (e.g. an argillaceous sandstone), from successions of thin heterolithic layers (e.g. a succession of thinly layered sandstones and shales). Such analysis was applied to the 12.25-inch hole section in a well from Malaysia. Three curves are presented: the lithology defined by the analysis of the gamma ray spectra, the sedimentological facies inferred from borehole images and the integration of these two independent records. The new curve is then compared to the core description. The borehole image enables us to precisely locate bed boundaries, identify thinly bedded intervals, massive layers and bioturbated layers. The mineralogical information enables us to determine accurately the lithology. Overall, the result is comparable to a core description although it remains dependent on a number of factors including the quality of processing and the quality of the initial records. Additionally, the lithology is defined on bulk-rock chemical composition, not grain-size distribution. The combination of the mineralogy resolved by the analysis of spectral gamma ray with the lithofacies information from the borehole images provides a more complete, more accurate and more precise description of the formation than could be obtained from conventional curves.