Mechanism of organic matter accumulation in a marine-terrestrial transitional residual bay environment: A case of Early Cretaceous organic-rich shales in the Qiangtang Basin

This paper is the result of oil and gas exploration engineering.

Objective Residual bay basin was developed during the transition from marine to continental deposition in the Qiangtang Basin. This transition environment enables Qiangtang Basin to generate organic-rich shales. The mechanism of organic matter accumulation is still controversial due to its complicated organism origin.

Methods In this study, we choose residual bay organic-rich shales from the Changliangshan section to study the mechanism of organic material accumulation. Redox-sensitive indices including EF values, Mn contents and U/Th ratios reveal that the water volume was dyoxic to anoxic condition during the shale deposition. The relatively low primary productivity during deposition is supported by the relatively low biogenic barium concentrations.

Results Climate change indices including oxygen-isotope, Mg/Ca and Fe/Mn ratio show that a significant climatic change has taken place during the shale deposition. Climate has changed into warm and humid during the shale deposition compared with dry and cold during underlying marl deposition.

Conclusions Combined with sedimentary rate and type Ⅱ kerogen, this paper proposed that warm and humid climate conditions, relatively high sedimentation rate, and high input from terrigenous higher plants are the main factors controlling organic matter accumulation in the Changliangshan shales.