Impact of Hydrothermal Fluids on Hydrocarbon Generation and Solid Bitumen Formation in the Kongdian Formation, Huanghua Depression, China.

Hydrothermal fluid plays a crucial role in the generation and migration of hydrocarbons within sedimentary basins. Herein, we employed bulk analysis and high-resolution mass spectrometry techniques to investigate the transformation dynamics from source rock to hydrocarbons under conditions influenced by magmatic activities in the Kongdian Formation, Huanghua Depression, China. The results revealed that hydrocarbon generation in the Ek₂ shale of the study area was significantly influenced by abnormal heating from hydrothermal fluids. High temperatures associated with these fluids accelerated the conversion of organic matter within source rocks, enhancing hydrocarbon generation rates. Subsequently, the hydrocarbons migrated into fracture networks, where they solidified as low-reflectance solid bitumen, forming trapped fractures of pyrobitumen and authigenic mineral aggregates leached from thermal fluid. High aliphatic fractions were noted in the source rock extracts, while extracts from low-reflectance solid bitumen exhibited higher aromatic fraction. Aliphatic and aromatic compounds in extracts from both the low-reflectance solid bitumen and the source rock exhibited similar maturities and origins. Regarding polar compounds, the compound classes O₁, O₂, N₁O₁, and N₁O₂ showed higher abundances in source rock extracts compared to those in low-reflectance solid bitumen, while the S₁ and N₁ classes showed the opposite trend. Thus, fractionation clearly occurs when hydrocarbons expelled from source rocks by thermal fluids solidify into low-reflectance solid bitumen. This unique study provides valuable insights into understanding the fate of hydrocarbons originating from source rocks heated by thermal fluids, and explores the potential for unconventional oil in regions with intense hydrothermal alteration.