Using Portable LIBS for Real-Time Elemental Analysis and Chemostratigraphic Correlation While Drilling

Laser-induced breakdown spectroscopy (LIBS) is a versatile analytical technique increasingly used for real-time acquisition of elemental data on drilling rigs. The LIBS instrument is compact, portable, and surprisingly insensitive to vibration. It is well suited to the analysis of cuttings, whereby results can be immediately applied to decision-making for drilling operations. Sample preparation is simple and data is generated within 20-30 minutes. Robust values for 40 elements can be acquired for most shales, greywackes, and lithic arenites (including REE and other low-abundance trace elements). For quartzose arenites and clean carbonates, data for 15-30 elements can be obtained.

We have identified five critical factors for the successful application of LIBS to geological materials: 1) developing a sample pellet binder that has minimum effect on spectral response, 2) identifying compromise laser-pulse parameters to optimize spectral response for the most elements, 3) identifying compromise acquisition parameters, such as delay and gate width, 4) understanding the physical effects of laser coupling with the sample and their potential impact on data quality, 5) applying non-standard mathematical processing to extract the greatest number of elemental values, at good accuracy and precision, from the complete spectra.

Our primary application of LIBS is geosteering wells by elemental chemostratigraphy. A pre-drill study on core and cuttings is conducted on ICP-OES/MS instruments, in order to establish a chemostratigraphic zonation for the interval of interest. That data is then used to calibrate the LIBS instrument for wellsite application. For siliciclastics, the provenance signal dominates the chemostratigraphy, though modifications due to paleoredox conditions, diagenesis, and pedogenesis can be recognized and exploited. For carbonates, signals are usually a mixture of depositional, diagenetic, and pedogenic with some depositional signal surviving even protracted carbonate diagenesis. LIBS data acquired from cuttings while drilling allows accurate “on-the-fly” correlation to the chemostratigraphic zonation, improving knowledge of stratigraphic position.