Geochemical well logs: calibration and lithostratigraphy in basaltic, granitic and metamorphic rocks

Abstract

Elemental analyses of rock compositions have been measured in situ with a geochemical logging tool (GLT) in drillholes through igncous and metamorphic terrains. Comparisons of dry weight percent oxide analyses from laboratory measurements on core samples and the log-derived results demonstrate that accuracics are fair in Ocean Drilling Program Hole 504B basalts, good in Cajon Pass, California granites, and very good in Moodus, Connecticut metamorphic rocks and Palisades, New York diabases. Improved accuracies of GLT measurements are attainable through the use of a boron sleeve to isolate the borehole fluid from wellbore effects, and in smaller diameter, land-based boreholes. Precisions within each well are very good in all cases. That is, only a linear calibration shift was required to bring core and laboratory analyses into general agreement in all cases. Variations in mineralogy, derived from inversion models of the log-derived elemental abundances at each well, produce a lithostratigraphic interpretation that is useful for determining the structure of the oceanic crust (Hole 504B), the fractionation sequence of basaltic intrusions in a new rift basin (Palisades Sill), the deformation history near the San Andreas Fault (Cajon Pass), and the variations in degree of metamorphism across a shallow decollcment surface caused by collision tectonics (Moodus).

Natural fracturing and petrophysical properties of the Palisades dolerite sill

“…1). The hole was geophysically surveyed using geophysical and geochemical logging tools (Anderson et al 1990). Goldberg (1997) described the measurements made by these tools in detail.…”

Borehole geophysical techniques to define stratigraphy, alteration and aquifers in basalt

“…The sedimentary-rock-based calibrations and oxide-closure model originally developed for Schlumberger's geochemical method are precise though not accurate for several major elements when tested in basalt by the ODP and closely related studies (Brewer et al, 1989(Brewer et al, , 1990Anderson et al, 1990a). Subsequently, successful calibrations and oxide-closure models were developed for geochemical logging in oceanic basalts as well as in other crystalline rocks (Anderson et al, 1990a,b;Brewer et al, 1990;Draxler, 1990).…”

Evaluation of Quality of Geochemical Log Data in ODP Hole 798B

“…The huge database of information collected has enormous potential for a wide range of geological applications. Relative elemental variations can be used in lithological classification of both igneous and sedimentary environments (Anderson et al, 1990), as well as when determining sediment sources and paleoclimate information (deMenocal et al, this volume). Normative mineralogy determinations from the geochemistry (Herron, 1986;Harvey et al, 1990) have a variety of uses, including petrologic classification and the production of thermal conductivity profiles (Dove and Williams, 1989).…”