Abstract

Mudrocks are difficult to characterize by routine core analyses. Descriptions provide mostly sedimentary structure information. Mineralogy is difficult to quantify macroscopically or in thin section. X-ray diffraction (XRD) analysis is expensive with adequate sampling. Transformation of high speed x-ray fluorescence (XRF)-scanning data to mineral data uses relationships between XRF and limited numbers of XRD analyses, thus enabling interpretation of mineralogical profiles with XRF data collected at close intervals. Long-term trends shown by stratigraphic variations in mineralogy and trace elements, TOC, and in stable organic isotopes show the power of integrated data sets in interpreting basin history with mudrocks.

XRF-elemental profiles provide locations for further investigation. RockEval data provides insights into hydrocarbon-source richness and thermal maturity. Trace element data allows insights into redox conditions and, with mineral data, aids correlation of core to geophysical well logs. Organic C and N isotopes may support hypotheses regarding organic sources, sea-level phase, and circulation. An example from the Bone Spring Formation (Lower Permian), Permian Basin, Texas, demonstrates these methods.

Bone Spring mudrock successions from the Delaware silled-basin show siliciclastic- and carbonate facies organized into stratigraphic cycles at multiple scales. For the Bone Spring succession most abundant TOC (up to 5.6% by wt.)was preserved during sea-level lowstand (dominately quartz-rich intervals, depleted d13CTOC down to -28.9±) characterized by siliciclastic turbidity currents and reducing conditions (elevated Mo up to 0.015%) caused by reduced oceanic circulation over shallow basin sills and recycling of nutrients (elevated d15N up to 17.5±). TOC preservation generally was less than 1% during highstand (abundant calcareous intervals, elevated d13CTOC up to -25.5±) characterized by carbonate turbidity currents and more oxidizing conditions (reduced Mo down to 0.0002%) and less recycling of nutrients (reduced d15N down to 12.2±). Variations in mineralogy affect responses of geophysical well logs where carbonates show low gamma ray values and highest resistivity; argillaceous rocks show low resistivity and elevated gamma ray responses. Maturity is in the oil window. Calibration of geophysical well logs with core lends confidence to interpretation of facies from well logs recorded in other wells.