Using MicroBeam XRF to Complement Downhole Elemental Spectroscopy Data
Jennifer L. Kharrazi, Tina R. Hill
Downhole elemental spectroscopy tools have been used in the oil industry for several decades to obtain elemental concentrations of the subsurface. These elemental concentrations can be used to determine mineralogy, grain density and a variable matrix porosity, which in turn provide an advanced formation evaluation, enabling more accurate lithologies, improved saturations and geomechanical properties of the subsurface formations. Laboratory analyses such as XRD and, less often, conventional XRF are used to validate and improve mineralogical models from downhole spectroscopy tools. These analyses provide laboratory-quality data to compare to downhole data, with the caveat that the same volume of sample is not represented in the subsurface as in the lab analysis. A less standard method of laboratory analysis that can add an extra dimension to downhole spectroscopy data is that of MicroBeam XRF (XRF). With µ-XRF it is possible to study complex inhomogeneous samples (such as core or cuttings) and obtain spatially resolved elemental information from major elements to low-ppm trace element level which can be used to enhance commonly obtained elements of downhole tools. The advantage of this method is that it provides X-ray maps of elemental distribution to pinpoint element location, and thus the minerals which make up the rock fabric. The additional dimension of concentration overlaid with location offers key information regarding formation properties. For example, spectroscopy tools, XRD and XRF can quantify calcite, but XRF can establish whether calcite is present as discreet shells/fossils or as carbonate cement, which have different impacts on the depositional environment interpretation as well as the flow and mechanical properties of the rocks. This study will examine downhole spectroscopy data alongside XRF mapping to determine the additional benefits of spatially resolved rock fabric information. The results will be compared with OH data and XRF and XRD data from a nearby well.
