Research on tar-rich coals: Progress and prospects

Tar-rich coals are a coal-based oil and gas resource that integrates the properties of coal, oil, and gas. Given China's energy endowment of abundant coal resources but limited oil and gas reserves, exploiting tar-rich coals plays a significant role in alleviating China's tight supply of oil and gas resources and achieving green development and low-carbon utilization of coals. Using tar-rich coals as the keyword, we searched the CNKI and Web of Science databases for academic articles and patents published from 1985 to the end of 2023. Based on the search results, we statistically analyzed the process and main aspects of research on tar-rich coals, sorting out the academic interests and frontier fields and forecasting future trends. Findings reveal that the current academic interests in research on tar-rich coals focus on their pyrolysis, occurrence characteristics, sedimentary environments, pore and molecular structures, tar yield forecast, microbial degradation, resource potential, and production and utilization. Tar-rich coals contain hydrogen-rich structures that can produce oil and gas through pyrolysis, such as the side chains and bridge bonds of aliphatic structures and weakly bonded structures on the periphery of condensed aromatic nucleus. Tar-rich coals were predominantly formed in sedimentary environments with a stable terrestrial sediment supply, a warm and humid climate, and strong reducing conditions. Pore structures influence the pyrolysis efficiency, tar precipitation, and hydrocarbon migration of tar-rich coals, whereas molecular structures (primarily influenced by aliphatic hydrogen content and weak hydrogenated bonds) determine the oil-generating potential of tar-rich coals. Tar-rich coals convert into oil and gas through four stages under biodegradation: microbial hydrolysis, fermentation, hydrogen/acetic acid generation, and methane generation. With the achievement of technical breakthroughs in geological area selection and the integration of multiple disciplines, future trends in research on tar-rich coals will center on the indicators and methods for forecasting and evaluating their oil-yield property, the sources and quantitative identification of hydrogen-rich components, the sealing performance of surrounding rocks and its evaluation methods for in-situ exploitation, and technologies for microbial degradation combined with pyrolysis. The results of this study will lay a foundation for ascertaining both current directions and future trends of research on tar-rich coals.