PRE- AND POST-PYROLYSIS LEACHING OF EAGLE FORD SHALE SAMPLES TO BETTER EVALUATE THE INORGANIC GEOCHEMISTRY OF KEROGEN

Measurements of the inorganic geochemistry of petroleum source rocks have become increasingly important for the exploration of unconventional resources. Elemental concentration data have been used to assess mineralogy and stratigraphy and have served as proxies for TOC and paleodepositional environments. One of the challenges in interpreting elemental data from the analysis of bulk rocks, however, is in establishing which trace elements are associated primarily with the kerogen. In this study, we test a new method for evaluating the inorganic geochemistry of hydrocarbon source rocks. The method uses high temperature and pressure leaching of powdered rock samples pre- and post-pyrolysis. Chip samples were used for testing. Batch leaching was conducted by adding 0.05 to 0.1 g of powdered rock sample (from a drill core in the Eagle Ford Shale) to approximately 15 mL of electrolyte solution inside a Polytetrafluoroethylene (PTFE) container within a Parr™ bomb acid digestion vessel. The sample was heated to 250°C (and 580 psi) for 4 hours. After the sample cooled, 1mL of ultra-pure 10M HCl was added to the leachate-rock powder mixture. The same procedure was performed on sample splits that were first pyrolyzed using a Vinci Technologies™ Rock-Eval 6 instrument to destroy kerogen. Leachate solutions were analyzed using a Perkin Elmer™ 8300DV ICP-OES. Results demonstrate that there was little difference in pre- and post-pyrolysis elemental concentrations in leachate for Zn, Mn, V, Ca, and S, while leaching after pyrolysis samples increased the concentrations of Sr, Cu, Ni, Co, Fe, Cr, K, Al, Si, and Ti at specific depth intervals within the core. The elements that increased in concentration post-pyrolysis are most likely those that are intimately associated with the kerogen, including clay minerals. The elements least influenced by pyrolysis are more likely associated with carbonates and sulfides. Further evaluation of the trace element data that was most likely associated with kerogen helps to identify clear differences in marine versus terrestrial input sources for organic carbon in the Eagle Ford.