Molybdenum speciation tracking hydrocarbon migration in fine-grained sedimentary rocks

Molybdenum (Mo) systematics (i.e., total concentration and isotope ratios) are widely used for the reconstruction of paleo- redox conditions in sedimentary records. However, the geochemical processes affecting the distribution of Mo in carbona- ceous fine-grained sedimentary rocks remain unclear. This study investigates how the organic matter type may control Mo concentration and speciation in siltstone/shale strata of the Lower Triassic Montney Formation in the Western Canadian Sed- imentary Basin (WCSB) to identify the effects of hydrocarbon expulsion and migration, fluid/rock interactions, and thermal maturity on Mo geochemistry when used as a paleo-redox proxy. The bulk Mo concentration in the studied samples varies from 0.2 to 86 ppm (mean = 13 +/- 16.7 ppm), with Mo enrich- ment factors of 22.7 +/- 24.4 suggesting an anoxic paleo-depositional setting. The X-ray Absorption Near Edge Spectroscopy (XANES) results show the presence of two separate Mo species within distinct geographic locations: (1) a group with a higher average Mo oxidation state where Mo is mostly surrounded by oxygen (O) atoms in an octahedral configuration, and (2) a group showing a lower average Mo oxidation state where Mo is mostly surrounded by sulfur (S) atoms in a tetrahedral configuration. The reduced Mo species are distributed in the vicinity of fault-related hydrothermal diagenesis/dolomitization zones. In contrast, the oxidized Mo species are found associated with samples enriched with solid bitumen/pyrobitumen. The results of our study show Mo speciation can significantly help to elucidate complex paleo-redox histories. (c) 2020 Elsevier Ltd. All rights reserved.