Differential origins of pyrolysis tar yield and hydrocarbon generation characteristics in the Yan’an Formation coals, Ordos Basin, China

The tar yield plays a crucial role in determining the development and utilization direction of coal, while hydrocarbon generation kinetics are key factors influencing the in-situ pyrolysis potential and temperature conditions of tar-rich coal. However, the varying origins of tar yield and hydrocarbon generation characteristics across different coal samples remain unclear. This study investigated the differences in hydrocarbon generation characteristics and geochemical properties between tar-rich coal and tar-containing coal from the Yan’an Formation of the Huanglong Jurassic coalfield in the Ordos Basin, elucidating the effects of organic macerals, parent material composition, sedimentary environment, and inorganic components on tar yield and hydrocarbon generation characteristics. The findings indicate that the hydrocarbon generation process of tar-rich coal is characterized by a low hydrocarbon generation threshold and a short conversion cycle. Conversely, the hydrocarbon generation process of tar-containing coal has the characteristics of a relatively high hydrocarbon generation threshold and an extended conversion cycle. The parent material composition of tar-rich coal contains a higher proportion of aquatic organisms and prokaryotic bacteria, as well as angiosperms and Cupressaceae and Taxodiaceae plants, compared to tar-containing coal. Moreover, the tar-rich coal is deposited in a more reducing environment and exhibits lower moisture and ash contents, along with reduced concentrations of radioactive elements, relative to tar-containing coal. The tar yield of the Yan’an Formation coal is positively correlated with the content of vitrinite and liptinite in organic macerals, the proportion of aquatic organisms and prokaryotic bacteria in the parent material composition, the proportion of angiosperms and the Cupressaceae and Taxodiaceae plants among terrestrial higher plants, as well as the reducibility of the sedimentary environment. Conversely, tar yield diminishes with increased moisture and ash contents, as well as higher concentrations of radioactive elements. Additionally, the activation energy value and dispersion of the Yan’an Formation coal decrease with changes in the aforementioned parameters of organic macerals and parent material composition, while they increase with variations in the aforementioned parameters related to inorganic components. The research results offer valuable insights for predicting and facilitating the in-situ pyrolysis conversion of tar-rich coal.