COMPARISON OF PORTABLE FIELD XRF (PXRF) AND LABORATORY XRF MEASUREMENTS ALONG A FAULTED NONCONFORMITY INTERFACE

A dramatic increase in seismicity has occurred in the midcontinent region since 2009. Previous work has directly linked this “induced seismicity” to the injection of high-volumes of waste water, associated with oil and gas production, to the reactivation of buried fault systems that occur near the Paleozoic sedimentary bedrock-Precambrian crystalline basement contact (or nonconformity interface).

The subsurface geologic setting impacts the migration of fluids, pore fluid pressures, and fluid-rock interactions which in turn may lead to fault zone weakening and reactivation. However, due to the presence of thick overburden or the lack of accessible data within the midcontinent region, the subsurface geologic setting of this nonconformity interface is not well-constrained. To better understand the chemical alteration and fine-scale textural heterogeneities that may occur along the nonconformity interface, we examine an exposed outcrop site near Gunnison, Colorado that serves as a representative geologic analog for the midcontinent region.

We use a portable handheld X-Ray Fluorescence Unit (pXRF) as a rapid and inexpensive tool to measure mineralogical alteration or variations in whole-rock major and minor trace elements in outcrop. While the pXRF measures what elements are present in the field, it cannot directly measure variations in weight-percent concentrations unless the data is compared and calibrated to laboratory standards. Since these elemental concentrations more accurately reflect alteration and allow for comparison of variations related to structural architecture, we will directly compare the field and laboratory data by using USGS Concentration Standards and a laboratory X-Ray Fluorescence (XRF) instrument to obtain weight percent concentrations. By developing these calibration standards optimized for this specific suite of sedimentary and igneous rocks using the XRF, we can then calculate concentrations obtained from the pXRF in the field. This comparison between the calibrated data sets informs the accuracy of in-situ field data measurements against laboratory results and tests the accuracy of pXRF measurements at the nonconformity interface outcrop analog site and more broadly within outcrops that contain faulted rock sequences comprised of heterogeneous rock properties.