RELATIONSHIP OF INORGANIC AND ORGANIC GEOCHEMISTRY IN PETROLEUM SOURCE ROCKS

Advances in the development of petroleum resources in shale formations have spurred new interest in the geochemistry of source rocks and their associated fluids. In this study, we evaluated the inorganic chemistry of high-temperature and pressure leachates of shale source rocks and compared these data to organic geochemical information derived from whole rock pyrolysis. Samples were collected from a drill core in the Wolfberry Formation, a Permian shale source rock in West Texas. Rock samples were powdered in a ball mill and 0.1g of sample was placed in a Parr Bomb™ with 15mL of 0.01M NaCl electrolyte. The bomb was placed in a heating oven for 24 hours at temperatures of 150°C or 195°C. After each experiment, the leachate was filtered through a 0.45µm nylon syringe filter, preserved with acid, and analyzed using an ICP-OES. Powdered rock samples were additionally analyzed for organic geochemical data via whole rock pyrolysis and oxidation using a Vinci Technologies Rock Eval 6™. One set of samples was subjected to leaching experiments after pyrolysis and oxidation had been performed.

The results show that major elements like Ca, Mg, and Si, in the leachates from the Parr Bomb experiments vary as a function of the thermal maturity of the kerogen. Low Ca and higher Mg and Si correlated with areas of higher Tmax (thermal maturity). Trace elements like Ni (6.0 mg/kg), Zn (130 mg/kg), and Fe (260 mg/kg) reached maximum concentrations in the leachates with the highest Tmax. Several samples subjected to leaching after pyrolysis showed large increases (relative to the other leaching experiments) in the concentrations of redox-sensitive trace metals, including Cr, Mo, and V. For example, Cr concentrations in all the samples were below detection in the 150°C and 195°C leachates, but reached a maximum of 250 mg/kg in the leachate from a sample where the organic matter had first been destroyed via pyrolysis. Our study suggests that leaching techniques, in combination with traditional whole rock pyrolysis, can be used to identify and better understand subtle changes in paleodepositional environments. These data may additionally provide useful information regarding the geochemistry of water-rock interaction during petroleum development.