Organic geochemical characteristics of the Mesoproterozoic Hongshuizhuang Formation from northern China: implications for thermal maturity and biological sources

The Mesoproterozoic Hongshuizhuang Formation (∼1.4 Ga) from northern China consists of organic-rich sediments which record important environmental and biological information from that time. Organic and element geochemistry techniques were used to assess the syngeneity and indigeneity of hydrocarbons, and the thermal maturity and biological sources of the organic matter in the Hongshuizhuang Formation. Results from a slice-extraction experiment indicate that the exterior of the core was subjected to contamination from drilling, sampling and storage, whereas the organic matter in the interior shale core slices was indigenous. The organic matter in the interior argillaceous dolomite core slices might have been affected by the migration of oil. It is important to remove the outside of the core prior to solvent extraction, as this increases the possibility of analysis of indigenous hydrocarbons. The source rocks of the oil are the black shales of the Hongshuizhuang Formation. Aliphatic and aromatic hydrocarbon distributions indicate that the Hongshuizhuang Formation samples have a low thermal maturity in the early to middle oil window (0.6–0.8 %Ro equivalent), based on comparison with middle Velkerri Formation rocks of about the same age in the McArthur Basin, northern Australia. These data are consistent with results based on the values of T_max and protobitumen reflectance, and suggest that the samples contain well preserved organic matter that has not been thermally destroyed. The redox conditions indicated by the pristane/phytane (Pr/Ph) ratios are not consistent with other evidence. The correlations between n-C₂₀₋/n-C₂₁₊, major elements and Pr/Ph indicate that the Pr/Ph ratios are influenced by biological sources and mineralogy, rather than by redox conditions. Pr and Ph may have been derived not only from chlorophyll-a, but also from archaea. The short and long chain n-alkanes in the Hongshuizhuang Formation were derived from anoxic clay-rich environments and oxic carbonate-rich environments, respectively. The biomarkers observed in the Hongshuizhuang Formation indicate that the organic matter was primarily derived from prokaryotes.