Mineralogy from Geochemical Well Logging

-Multivariate statistical analyses of geochemical, mineralogical, and cation-exchange capacity (CEC) data from a Venezuelan oil well were used to construct a model which relates elemental concentrations to mineral abundances. An r-mode factor analysis showed that most of the variance could be accounted for by four independent factors and that these factors were related to individual mineral components: kaolinite, illite, K-feldspar, and heavy minerals. Concentrations of AI, Fe, and K in core samptes were used to estimate the abundances of kaolinite, illite, K-feldspar, and, by subtraction from unity, quartz. Concentrations of these elements were also measured remotely in the well by geochemical logging tools and were used to estimate these mineral abundances on a continuous basis as a function of depth. The CEC was estimated from a linear combination of the derived kaolinite and illite abundances. The formation's thermal neutron capture cross section estimated from the log-derived mineralogy and a porosity log agreed well with the measured data. Concentrations of V, among other trace elements, were modeled as linear combinations of the clay mineral abundances. The measured core V agreed with the derived values in shales and water-bearing sands, but exceeded the day-derived values in samples containing heavy oil. The excess V was used to estimate the V content and API Gravity of the oil. The logderived clay mineralogy was used to help distinguish nonmarine from transitional depositional environments. Kaolinite was the dominant clay in nonmarine deposits, whereas transitional sediments contained more illite. Key Words--Elemental analysis, Illite, Kaolinite, Mineralogical analysis, Multivariate analysis, Neutron capture cross section, Well logging. I N T R O D U C T I O N Formation evaluation in the petroleum industry would be significantly enhanced if it were possible to determine the mineralogy in a given well or field without the time and expense of collecting and analyzing core samples. Such mineralogical information could help in reservoir quality evaluation, production risk analysis, depositional environment interpretation, and well-to-well correlation. Currently, geophysical logs are usually interpreted in terms of general lithology, such as sandstone, shale, or limestone, but only rarely in terms of specific mineral phases. Geochemical logs, i.e., wireline measurements of total chemical concentrations on a continuous basis with depth, offer one possibility for obtaining more accurate mineral information. The use of total chemical composition to estimate mineral abundances is a common practice in igneous petrology, but has met with only limited success in sedimentary rocks (Imbrie and Poldervaart, 1959; Miesch, 1962; Pearson, 1978; Gold et al., 1983). The primary causes of disagreement between calculated and measured mineral abundances are the presence of additional minerals beyond those included in the models and the variability in chemical composition of clay minerals due to ionic substitution and ionic adsorption. At least some of this chemical variability, e.g., Fe in kaolinites, has been linked to the degree of disorder or the "crystallinity" of the clay structure (Mestdagh et al., 1980; Brindley et al., 1986). Such variability in Copyright 9 1986, The Clay Minerals Society composition may be less significant when considering samples from a single geologic setting. This report examines the correlations between chemical composition and mineral abundances in the MFM7S well in the Faja Petrolifera del Orinoco in eastern Venezuela. The well penetrates a deltaic--alluvial to brackish water--sequence of the Oficina Formation composed of unconsolidated sands and muds containing abundant heavy oil (Everett et aL, 1983; Pirie and Everett, 1984). This study focuses on the 460-580-m depth interval from which 114 sidewall core samples were obtained.