Geochemistry and sedimentology of the Lower Silurian Longmaxi mudstone in southwestern China: Implications for depositional controls on organic matter accumulation

Abstract

Spatial and temporal changes of lithofacies and abundance of organic matter in mudstones control distribution of prospective unconventional petroleum reservoirs in a sedimentary basin. Lithofacies characterization is an essential prerequisite to understanding of organic matter accumulation, depositional processes and water column chemistry. This research combines geochemical analyses and detailed sedimentologic observations in order to investigate the depositional controls on organic matter abundance in the prolific black mudstone of the Long-1 Member of the Lower Silurian Longmaxi Formation in southwestern China.Six primary lithofacies deposited at variable marine water depths and under differing water column chemistry were identified from an 83 m long core based on microscopic observations of sediment texture and structure, and biota, and analyses of total organic carbon (TOC) content, mineralogy, and trace element abundances. Our results show that TOC content is well correlated to biogenic quartz content and the non-detrital components of V, U, Mo, Ni and Cu, suggesting the accumulation of organic matter in the studied mudstone was controlled by high paleoproductivity and anoxic water conditions. The good correlations also suggest that the abundant organic matter was produced by algal blooms, which are typically associated with radiolarian thrive because of the symbiotic relationship between algal and radiolarian. The low Mo/TOC ratios of the three lithofacies formed in deep, anoxic environments were similar to the ratios of modern sediments deposited in anoxic-euxinic environments, suggesting moderate basin restriction during deposition. The mudstone in the lower Longmaxi Formation may be part of the globally abundant organic-rich mudstones of Early Silurian age, whose depositional mechanism have yet to be fully explored.The distribution of lithofacies in the studied interval shows an overall trend from deep water to shallow water depositional environments. We developed a detailed depositional model to interpret the evolution of depositional environments of the lower Longmaxi Formation. This study provides an example to how better characterize unconventional hydrocarbon systems through coupling of rigorous geochemical and sedimentological analysis.