BATCH LEACHING OF HYDROCARBON SOURCE ROCKS AT 150C UNDER VARIABLE CONCENTRATIONS OF CHLORIDE AND ORGANIC ACIDS

Several studies have suggested a link between the generation of petroleum and the formation of base metal, primarily Pb, Cu, and Zn, deposits in sedimentary basins. The metals incorporated into basinal brines could be derived from a number of sources, including host rock units. In this study, we evaluated hydrocarbon source rocks, specifically organic-rich shales, as a source of trace metals for basinal brines under conditions of varying NaCl and dissolved organic acids. In addition to these experiments, we evaluated the relationship between chloride molality and trace metal concentration using existing data from the U.S. Geological Survey’s produced waters database. Batch leaching experiments were performed with powdered source rocks from the Eagle Ford, Marcellus, and Wolfcamp Shales. Approximately 1 g of rock was combined with 10 mL of liquid with NaCl molarities of 0.01, 0.1, 0.5, and 1. Experiments were conducted at 150°C for 24 hours in Savillex™ digestion vessels. Some of the experiments were additionally subjected to 1M of sodium acetate. After the leaching period, the liquid was filtered (0.2 µm) and analyzed using an ICP-OES instrument. The experimental results showed that very little Pb, Cu, or Zn were leached from the source rocks under the tested conditions. This could be attributed in part to the oxidizing conditions of the experiments and the formation of iron oxides that sequestered trace metals within the rock material. We did, however, see substantial leaching of some elements (Ca, S, Sr, K, Mg, V, etc.). The produced waters database included over 400, 1100, and 1200 measurements of Pb, Cu, and Zn, respectively. The median Pb, Cu, and Zn concentrations for these samples, was 0.03, 0.04, and 0.13 mg/L, respectively. Trace metal concentrations in samples from the Appalachian, Gulf Coast, Green River, and Black Warrior basins were positively correlated with chloride molarity. Experimental data collected thus far does not approximate the metal concentrations seen in produced waters, hinting at additional complexities associated with metal sources and loading. Additional work will be necessary to better simulate subsurface conditions to evaluate the role of hydrocarbon source rocks as a source of metals in basinal brines.