Characterization of Mudrock by Integrating Core Description, XRF Chemostratigraphy, Organic Matter Pyrolysis, and Isotope Analysis
Nance, Hardie S.1; Ruppel, Stephen C.; and Rowe, Harry
Mudrocks are difficult to characterize by routine core study. Descriptions provide mostly sedimentary structure information. Mineralogy cannot be defined macroscopically or microscopically. X-ray diffraction (XRD) analysis is expensive with adequate sampling. By contrast, x-ray fluorescence (XRF) can be obtained quickly using hand held scanners and when integrated with selected XRD analysis, provides high resolution interpretation of mineralogical profiles. Stratigraphic variations in mineralogy defined by XRF combined with and trace element, TOC, and in stable organic isotope data comprise a crucial dataset for interpreting basin history from mudrocks.
XRF-elemental profiles define basic facies variations for further investigation. Pyrolysis data provide insights into hydrocarbon-source richness and thermal maturity. Trace element data allow insights into redox conditions and, when combined with major element data, aid correlation of core to geophysical well logs. Organic carbon and nitrogen isotopes can help define organic matter sources, sea-level change, and ocean circulation patterns. Our studies of the Permian Bone Spring Formation, Permian Basin, Texas, illustrate the power of these methods.
Basinal Bone Spring mudrock successions comprise cyclic siliciclastic and carbonate facies. Most of the TOC in these rocks (up to 5.6% by wt.) was preserved during sea-level lowstand characterized by dominantly quartz-rich turbidity currents displaying depleted d13CTOC (down to -28.9‰), and reducing conditions (elevated Mo up to 150 ppm) caused by reduced oceanic circulation over shallow basin sills and recycling of nutrients (elevated d15N up to 17.5‰). During highstands, which were characterized by abundant calcareous turbidity currents, elevated d13CTOC (up to -25.5‰) and more oxidizing conditions (reduced Mo down to ~2 ppm) and slightly less recycling of nutrients(reduced d15N down to 12.2‰), TOC preservation generally was less than 1%. XRF-defined variations in mineralogy form a basis for improved calibration of geophysical well logs. Calibration of well logs with core lends confidence to interpretation of facies from other well logs.
AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013