GEOCHEMISTRY AND SEDIMENTOLOGY OF THE LOWER SILURIAN LONGMAXI SHALE IN SOUTHWESTERN CHINA: IMPLICATIONS FOR DEPOSITIONAL CONTROLS ON ORGANIC MATTER ACCUMULATION

Spatial and temporal changes of lithofacies and abundance of organic matter in mudrocks control the distribution of prospective unconventional petroleum reservoirs. Although lithofacies characterization is essential to the exploration of unconventional reservoir, the relationships among organic matter accumulation, depositional processes and water column chemistry are little studied. By combining geochemical analyses and detailed sedimentologic observations, we investigate the depositional controls on organic matter abundance in the prolific black shale of the lower Silurian Longmaxi Formation in southwestern China.

Six primary lithofacies deposited in variable marine water depths and water column chemistry are recognized from a 83 m long core based on organic carbon content (TOC%), mineralogy, trace element compositions, and spectroscopic observations of mineralogy composition, sediment texture and structure, and biota. We found TOC% is well correlated to quartz content and the non-detrital components of V, U, Mo, Ni and Cu, suggesting the enrichment of organic matter in the studied shale was controlled by paleoproductivity and water redox condition. The low Mo/TOC ratio of the lithofacies formed in deep, anoxic environments suggest moderate basin restriction during the deposition. We suggest that radiolarian thrive and the symbiotic relationship between radiolarians and algae may have caused high productivity of organic matter, and organic matter preservation was favored in anoxic water during marine highstands. Three lithofacies assemblages change upward from deep water to shallow water depositional environments, reflecting nearly one third-order sea-level cycle after the early Silurian global glacial ablation. Based on the changes of depositional processes and water column conditions, we developed a depositional model to interpret the evolution of depositional environments of the lower Longmaxi Formation. This study provides an example to better characterize unconventional reservoirs by coupling rigorous geochemical analyses and sedimentologic observations, and has implication to early Silurian marine hydrography.