Impact of hydrothermal activity on organic matter quantity and quality during deposition in the Permian Dalong Formation, Southern China

Abstract

Abstract The very high temperature at the center of a hydrothermal system is detrimental for the deposition of organic-rich sediments. However, it's not known how the petroleum source rock develops at the boundary of a hydrothermal system where the seawater temperature was only slightly elevated. The petroleum generation potential, using pyrolysis-GC, GC-MS and FT-ICR MS techniques, of eight organic-rich Permian shale samples from Shangsi Section China were systematically investigated. Principal component analysis reveals that the quantity and quality of the organic matter in the shale is dominated by biomass productivity, the redox condition, and terrestrial organic matter input. Rare earth element distribution pattern reveals that the two samples with highest TOC contents show a slight Eu anomaly which indicates the sample was influenced by a hydrothermal system to some degree. A depositional model showing how the influences from hydrothermal event promote the development of petroleum source rock is summarized: (1) At the edge of a hydrothermal system, the seawater-temperature can be elevated by 5–10 °C which further promotes the development of phytoplankton and zooplankton. (2) An upwelling can be intrigued and chemical weathering will be accelerated which both introduce excessive nutrients and increase the biota productivity. (3) Flux with the terrestrial advanced plant can also be accelerated to the shelf area by the volcano-induced climate change. (4) The consumption of the extensive biomass induced by the hydrothermal activity would expand the oxygen minimum zone, and further enable more organic matter to be preserved in shale.