HIGH-EFFICIENCY GEOCHEMICAL CHARACTERIZATION OF MUDROCKS

Quantitative mineralogy determined by powder X-ray diffraction (XRD) analysis of mudrocks from core or outcrops requires extensive sample preparation and lengthy acquisition time. In this study, we demonstrate that fine-grained mudrocks, with relatively coarse sample preparation techniques (samples powdered to < 250 µm) and shorter, standardless XRD measurements (2 to 70° 2θ at 0.02°/step 30 minute total scan vs. 2 to 80° 2θ 0.02°/step 80 minute total scan), can generate semi-quantitative mineralogy that is comparable to more refined powder diffraction approaches using internal standards and longer count times. Coarse and refined XRD methods were applied to core samples from the Eocene Green River Formation (Piceance Basin, CO) and Cenomanian-Turonian Eagle Ford Shale (Maverick Basin, TX). The results compared favorable to data from portable X-ray fluorescence (pXRF), infrared spectroscopy, and ICP-MS/AES chemistry. We show that sample micronization, addition of internal standards, and procedures to ensure random orientation are often unnecessary for mudrock XRD analysis due to the small particle sizes that typically make up these samples as viewed when studing during scanning electron microscopy investigation. The data suggest that in many mudrocks, where mean grain size is in the 10¹ µm range, large numbers of samples can be rapidly prepared and analyzed at faster scan rates than is often applied in refined methods, and still yield accurate and reliable mineralogy and approximate chemistry from XRD alone.

Coarse XRD analysis combined with pXRF can be used to employ provenance techniques for mudstones such as Chemical Index of Alteration (CIA), elemental ratios (Si/Al, Ti/Al, Th/U, etc.), Chemical Index of Weathering (CIW), and Index of Compositional Variability (ICV). By using fast, reliable, and non-destructive analytical techniques such as coarse XRD and pXRF, large sections of core can be sampled and analyzed at a higher resolution scale for more detailed mineral and chemical constituent analysis. Detailed mineralogy and chemistry information can reveal details about changes in depositional environment and sediment diagenesis during basin evolution versus with well log interpretation or optical and petrographic investigations typically employed with core material.