The role of deep fluid in the formation of organic-rich source rocks☆

The inner basin has been the focus of organic-rich source rocks -related studies for a long time. In contrast, the effect brought by deep fluids on organic-rich source rocks has not received its much-deserved attention. As a belt connecting the inner and external parts of basins, deep fluids promote the formation of organic-rich source rocks by increasing the marine primary productivity and creating favorable conservation environment. In the productivity facet, deep fluids carry a significant amount of nutrients like NO3−, PO43−, NH4+, hydrothermal gases (i.e., CH4, CO2, H2, and NH3), minor metal elements (i.e., Fe, Mn, Zn, Co, and Cu), as well as microbes from the deep earth like Archaebacteria and thermophilic bacteria. The injection of deep fluids assists organisms to thrive in water and helps facilitate the enhancement of marine primary productivity. The results then create favorable conditions for the formation and enrichment of organic matter. In regards to the organic matter conservation facet, the eruption of deep fluids sends a great deal of CO2 into atmosphere and ocean. By connecting with the Ca2+ and Mg2+ in the water, the CO2 above transforms into carbonate and contribute to the ever increasing water salinity. As a consequence, it will cause the stratification and circulation suspension of the ocean as well as the formation of favorable hydrodynamic conditions and preservation environments. In addition, magma and hydrothermal fluids can also create a reductive environment by sending reductive gases like H2S and CO into the water. In summary, the eruption of deep fluids guarantees the production of essential substances and a favorable environment for the formation and preservation of organic-rich source rocks. Keywords: Deep fluids, Nutrient elements, Reductive environment, The thriving of creature, Organic enrichmen