Characteristics of different types of soluble organic matter in low- to middle-rank coals: Implications for tar generation and migration during tar-rich coal pyrolysis

Abstract

Soluble organic matter (SOM) in low- to middle-rank coal can be categorized into free (FSOM), mineral-associated (MSOM) and chemical-associated (CSOM) states. However, the characteristics of these different types of SOM remain poorly understood in tar-rich coal research, primarily due to the long-standing perception of coal as a conventional gas source rock. This paper investigates the geochemical characteristics of different types of SOM and the molecular structure characteristics of their residues after successive treatments, discusses the influence of organic matter maturity and tar yield on these characteristics, and clarifies the generation and migration mechanisms of pyrolysis tar. Nine low- to middle-rank coals with different maturity and tar yield collected from the Ordos Basin underwent successive treatments to obtained FSOM, MSOM and CSOM. Comparison of organic characteristics reveals: (1) SOM content follows the order of FSOM> CSOM> MSOM. The FSOM and MSOM are primarily composed of aromatics, while asphaltenes are the main components of the CSOM. (2) SOM polarity follows the order of CSOM> MSOM> FSOM, the polarity of FSOM and MSOM is influenced by both organic matter maturity and tar yield, while the polarity of CSOM is primarily affected by matter origin. (3) SOM maturity shows the order of CSOM> MSOM> FSOM, the geochemical characteristics of SOM are influenced by both organic matter maturity and tar yield. Comparison of molecular structure characteristics reveals: (1) Oxygen functional group content gradually increases across all SOM fractions, while aliphatic functional group content gradually decreases across all SOM fractions. (2) Molecular structure is influenced by both organic matter maturity and tar yield. Comprehensive comparison of the organic geochemistry and molecular structure characteristics reveals: (1) Group composition of CSOM is significantly affected by aromatic and hydroxyl functional groups. (2) FSOM and MSOM primarily contribute to the free hydrocarbons during tar-rich coal pyrolysis. (3) Migration pathway of pyrolysis tar follows the order of silicate minerals, carbonate minerals, pores and expulsion. These differences provide new insights into the understanding of pyrolysis tar generation and migration mechanisms.