Estimation of Mineralogy and Lithology from Geochemical Log Measurements (1)

Abstract

Mineralogy, chemistry, and physical properties of core samples from a research well in Grand County, Utah, have been determined to assess the applications of geochemical logging tool measurements to reservoir characterization. The cored interval (over 827 ft or 252 m of continuous core) samples a wide range of lithologies, including subarkose, sublitharenite, litharenite, feldspathic litharenite, quartz wacke, lithic wacke, siltstone, mudstone, and coal. Total clay mineral contents range from 2 to almost 60 wt.% and consist primarily of illite and kaolinite with subordinate ordered, mixed-layer illite/smectite. Dolomite cement is pervasive; locally, large amounts (up to 25 wt.%) of siderite or ankerite are present. Total feldspar minerals comprise 5-18 wt.% with sodium-ri h plagioclase slightly more abundant than potassium feldspar. Continuous geochemical well logs of 12 elements (K, U, Th, Al, Si, Ca, Fe, S, Gd, Ti, Cl, and H) were obtained using logging tools that measure neutron capture, neutron activation, and natural gamma-ray activities. Applications of these geochemical log measurements to assess mineralogy and lithology for the research well are complicated by (1) the near compositional colinearity of the assemblage illite + kaolinite + K-feldspar, (2) the presence of dolomite and iron-bearing carbonates, (3) the predominance of Na-plagioclase over K-feldspar, and (4) the abundance of sedimentary and metamorphic rock fragments in the litharenite group of lithologies. Because of these mineral and textural features, successful application of the log measurements to reservoir characterization requires calibration to core analyses. Mineral logs calculated using least-squares inversion programs accurately describe changes in mineralogy and mineral proportions once the mineral assemblage is identified. Lithology is best estimated based on these mineral logs. Alternatively, up to seven general lithology types can be discriminated using the lithology ratio (Al[2]O[3] + Fe[2]O[3])/(SiO[2] + Al[2]O[3] + Fe[2]O[3]), when chemical data are calibrated with petrographic data.