INORGANIC AND ORGANIC GEOCHEMISTRY OF PETROLEUM SOURCE ROCKS

Interest in petroleum production from hydrocarbon-generating source rocks has triggered renewed interest in the geochemistry of these systems. In this study we compare the inorganic geochemistry of fluids from high-temperature and pressure (T&P) leachate experiments with data from whole rock pyrolysis of the same shale source rocks. Leachate experiments were conducted by loading 0.1g of powdered rock sample and 15 mL of a 0.01 M NaCl electrolyte in a Parr Bomb™. The mixtures were reacted for 24 hours in a drying oven at T&Ps of 150°C/69 psi, 200°C/225 psi, or 250°C/577 psi. After cooling to room temperature, the leachate was filtered and analyzed for major and trace element chemistries using a Perkin Elmer™ 8300 DV ICP-OES. Splits from the powdered rock samples were run on a Rock Eval™ pyrolysis/oxidation instrument for determination of organic geochemical parameters, including T_max and TOC. A final set of leaching experiments was conducted using rock powder that had first been subjected to pyrolysis.

Leaching results indicated that the concentrations of some trace elements such as Ba, Cu, and S, increased with increasing experimental T&P, while major elements like Ca, Mg, and K did not change appreciably with experimental T&P. We suspected that the concentrations of elements such as Fe, Al, Si and Mn were primarily controlled by their limited solubilities at room T during sampling. We tested this hypothesis on samples leached at high T&P by acidifying the fluid-rock powder mixtures after they cooled to room T and letting them react for 30 minutes prior to sampling. The acidification process dramatically increased the yields of Fe, Al, Si, Mn, and many other trace elements. Finally, leaching of the samples in which the organic matter had first been destroyed via pyrolysis resulted in the greatest releases of several trace elements (presumably those associated largely with kerogen), including Cr, Mo, and V. The distributions of Cr and V correlated with the thermal maturity (Tmax) of the kerogen, but not with TOC. These relationships could reflect subtle changes in the paleodepositional environments of the kerogen and/or could be a result of re-partitioning of the trace elements during increasing thermal maturity.