Characteristics and geological significance of escaping gas rich in natural hydrogen from pilot well BYP5 cores of lower sub-member of third member of Shahejie Formation in Zhanhua Sag, Bohai Bay Basin

The pilot well BYP5 is a cored well drilled to explore the oil and gas bearing properties of the highly thermally evolved lower sub-member of the third member of the Paleogene Shahejie Formation in the Bonan deep sag of the Zhanhua Sag, Bohai Bay Basin. The coring interval depth ranges from 4 267.0 to 4 338.1 m. To reveal the oil and gas bearing properties of the cored interval, pyrolysis of frozen, sealed fragments was conducted on typical samples and the escaping gas from the core was collected and quantified for composition analysis. The results show that the cored interval is a high-quality hydrocarbon source rock rich in organic matter and carbonates, with a maturity (Ro) of about 1.2%. Efficient hydrocarbon generation and expulsion likely occurred during thermal evolution, causing the current low free hydrocarbon (S1) and hydrogen index (IH) values. The content of the hydrocarbon gas from the core was generally low, ranging from 0.001 to 0.01 cm3/g, with an average of 0.005 cm3/g. Segments with relatively high levels of escaping hydrocarbon gas corresponded to those with relatively high pyrolysis S1 values. The escaping gas was mainly composed of CH4, CO2, H2, and C2H6, with mole percentages of H2 ranging from 1.08% to 19.23%, with an average of 7.09%, indicating hydrogen-rich characteristics. H2 showed a significant positive correlation with CO2 and a negative correlation with CH4. The escaping gas from the core was likely trapped in-situ, and the formation of H2 might be related to the cleavage of hetero-bonds and demethylation during the pyrolysis of organic matter. Further research is suggested on the formation mechanism, geological exploration, and evaluation of natural hydrogen released during organic matter pyrolysis, so as to provide a basis for the decision-making in the exploration and development of this type of natural hydrogen resource.