ELEMENT PARTITIONING INTO GENERATED PETROLEUM AND HYDROTHERMAL FLUIDS AS DETERMINED BY HYDROUS PYROLYSIS OF RETORT SHALE

Little is known about the redistribution of inorganic constituents of petroleum source rocks among oil, gas, water, and rock during petroleum generation. We have measured the partitioning of 65 elements during experimental hydrous pyrolysis of a thermally immature, organic-rich, metalliferous, black shale (Retort Shale member of the Permian Phosphoria Formation). The sample was crushed to gravel-size pieces, which were immersed in distilled water or a brine solution in a sealed Hastelloy C-276 reactor. The reactor was fitted with ceramic liners in such proportions that the rock pieces remained in contact with the liquid phase throughout the experiment. 72-h runs were made at 240, 300, and 355 ^oC. Rock-free experiments were used to monitor the analytical procedures as well as the contribution of reactors and liners to the elemental budget.

Quantification of element partitioning during the experiments is complicated by physical partitioning of the reactor by ceramic liners that prohibited the movement of the liquid phases but allowed open communication of the vapor phase throughout the multichambered reactor. Comparison of beginning and ending rock composition shows that most elements, particularly C, As, Cd, Se, Zn, Fe, and S, were mobilized during the experiments. Several elements including Se, V, Ni, As, Cr, and Fe were partitioned into the oil. Elevated amounts of several elements including Fe, Mn, Co, Ni, Zn, and Th remained in the aqueous solution after precipitation of multicomponent sulfides, phosphate, and silicates that occurred during cooling of the reactor. Hydrogen, H₂S, methane and a variety of other volatile hydrocarbons were generated in the experiments. Some elements such as F, P, Ca, and, Si were transported in the vapor phase.

These results have significant implications for ore genesis, rock alteration during catagenesis, and environmental concerns related to the movement and concentration of environmentally sensitive elements such as Se, As, and Cd in continental basins and marine basins where oil and associated brine are likely to leak into sea water.