Influence of volcanic and hydrothermal activity on organic matter enrichment in the Upper Triassic Yanchang Formation, southern Ordos Basin, Central China

Based on the field investigation of oil shales in the southern Ordos Basin, detailed observations, and systematic sample analyses of core samples from a well that penetrated an oil shale, the influence of volcanic and hydrothermal activity on the Chang 8 Chang 7 members of the Yanchang Formation on organic matter (OM) enrichment was geochemically and petrologically analyzed. According to maceral analyses and rock pyrolysis, the kerogen types of the samples in the study area are widely distributed. Those of the oil shale with the highest OM abundance are mostly type II-I, and are immature or in a stage of low-maturity. Six layers of tuff and tuffaceous mudstone were identified by core observation and element characteristics; they are characterized by higher total rare earth element (Sigma REE) and delta Ce values and Th contents, and lower (La/Yb)(N) and delta Eu values. Mineral assemblages of anhydrite pyrite marcasite and relevant diagrams of the elemental geochemistry confirmed the existence of hydrothermal deposits. The high Y-n/Ho-n and U/Th ratios and total sulfur (TS) contents indicate that the Chang 7 Member experienced four episodes of intense hydrothermal activity. Early in the Chang 7(3) Member, volcanic ash and hydrothermal fluids brought in a large amount of nutrients, which not only increased primary productivity, but also increased the salinity of the water column, enhanced anoxia in the sedimentary environment of the lake bottom, promoted the preservation of OM, and finally formed the oil shale layer with the richest OM. However, the strong volcanic eruption in the late Chang 7(3) Member led to changes in the regional climate, causing it to dry and become colder and resulting in a decrease of primary productivity. Earthquakes triggered by volcanism resulted in turbidite deposits, which destroyed the preservation and diluted the enrichment of OM. This study reveals in detail the influence of volcanic and hydrothermal activity on OM enrichment during the evolution of the basin, and thereby deepens our theoretical understanding of OM enrichment.