Analysis on the occurrence characteristics and resource potential of tar-rich coal in Santanghu and Turpan-Hami Basins

Oil-rich coal is a special coal resource with a tar yield ≥ 7% and a combination of coal, oil, and gas attributes. It is an important resource and development direction for the clean utilization of coal, and has important scientific value in alleviating the shortage of national oil and gas resources and promoting the breakthrough development of coal chemical industry. There are a large number of tar-rich coal resources in the eastern basin of Xinjiang Uygur Autonomous Region. In order to find out the occurrence and distribution of tar-rich coal, the coal accumulation and occurrence characteristics of tar-rich coal were studied by means of sedimentary environment analysis, proximate analysis of coal, ultimate analysis of coal, and Gray-King assay for coal. The geological block method was used to estimate the resources of tar-rich coal. The results show that the tar-rich coal seams in the Santanghu Basin are mainly distributed in the upper section of the Badaowan Formation (J1b2), the Sangonghe Formation (J1s) and the lower section of the Xishanyao Formation (J2x1), the tar yield is about 13.67%, which is high tar-rich coal. The tar-rich coal seams in the Turpan-Hami Basin are developed in the middle section of the Xishanyao Formation (J2x2), and the tar yield is about 7.6%, which are tar-bearing coal and tar-rich coal. The tar-rich coal in the area is generally characterized by ultra-low to medium-low water content, low to medium-low ash content, medium-high to high volatile content, high tar yield, rich in oil generating components such as vitrinite and lipitnite, low degree of coalification, and formed in the lacustrine-deltaic sedimentary environment. The potential of rich oil coal resources in the study area is large. The resources of tar-rich coal in the Santanghu Basin is estimated to be 67.083 billion tonnes above a depth of 2 000 m, and the resources of tar-rich coal in the Turpan-Hami Basin is estimated to be 41.755 billion tonnes above a depth of 2 000 m, and the average thickness of the coal seams is more than 9 m. In order to effectively utilize the development of oil rich coal resources, the technical assumption of Underground Coal Pyrolysis (UCP) is proposed. The UCP refers to the establishment of heat introduction and product production channels in underground coal seams through petroleum engineering technology. Through artificial heating, coal undergoes in-situ carbonization reactions. The pyrolysis products include tar, gas (CH4, H2, light hydrocarbon C2+, etc.), and water. The carbon element in coal is mainly left underground in the form of semicoke, and the generated carbon dioxide can be used to drive oil in the Santanghu and Turpan-Hami Basins. The remaining carbon dioxide can be buried in-situ using the semicoke layer. The UCP achieves a clean conversion of coal resources through “hydrogen extraction and carbon retention”, which is expected to become a technical direction for the low-carbon green development of tar-rich coal.