Molecular structure and origin of solid bitumen from northern Sichuan Basin

Abstract

Four natural solid bitumen samples were collected from the Guangyuan area (northwest Sichuan Basin, southwest China) to characterize their structure. Samples were ground to power (120 mesh), purified by ZnBr2 solution to remove mineral and soluble impurities, and analyzed using Rock-Eval pyrolysis, elemental analysis, Raman spectroscopy, solid bitumen reflectance, and solid-state 13C nuclear magnetic resonance to determine the maturity degree of samples and obtain structural information. Results showed that Guangyuan solid bitumen samples are in low maturity, and three solid bitumen maturity detection methods are compared in this research. With respect to the solid bitumen molecular structure, less than 10 carbon atoms were determined in aliphatic chains and the solid bitumen aromatic clusters ranged from two to four rings, which indicates that the solid bitumen comprises complex geological organic matter, such as kerogen. Four molecular models were established in this research to reflect the basic molecular configuration of natural solid bitumen. Samples were classified into two types based on the structural information and models, and the results imply that the solid bitumen samples may have been derived via different evolutionary processes. This phenomenon indicates that structural studies could play an important role in the research of oxidation and carbon isotope changes in solid bitumen. In summary, this investigation provides an understanding of the evolutionary stages of Guangyuan bitumen, establishes associated molecular structure models, and researches the role of structural information in bitumen source studies. As the focus of this study, the four molecular models were used to restore the structural characteristics of natural solid bitumen as possible. And this study tried to find out the relationship between the solid bitumen structure information and it geology evolution. As the carbon isotope may not accurately determine the source of solid bitumen because of oxidation, molecular information could be used to research the solid bitumen source and associated evolution.