Variation in maceral composition and gas content with vitrinite reflectance in bituminous coal of the eastern Ordos basin, China

Abstract

Development of coal petrology is of great importance in both understanding the coal formation and its controlling effect on the coalbed methane content. We report microscopic analyses and field desorption test results of 113 coal samples from epicontinental (lower) and continental (upper) strata in the eastern Ordos basin, China. The maceral group composition shows great variation, and the vitrinite increases from 50% in the north to approximately 75% in the south of the study area, corresponding to the increase of maximum vitrinite reflectance (Ro,max) from 0.5% to 2.5%. Liptinite shows a power function decrease with Ro,max from 0.5% to 1.5%, and the mineral composition is relatively high in low Ro,max samples. The photomicrographs of the same rank coals also exhibit heterogeneity, and the higher Ro,max coals show more compact microstructures. Gas content varies between 2 and 10m3/t and exhibits a sharp increase in areas where Ro,max is about 1.0% demonstrating that the gas content in the middle and south areas is primarily controlled by the preservation conditions. The gas content shows an “increase–decrease” trend with depth increment, with the transition point at 1000m for Ro,max between 0.4% and 1.2%, and 1400m for Ro,max between 1.2% and 2.5%.The relationships between maceral group composition and gas content have also been discussed. The gas content generally shows a positive correlation with the vitrinite percentage, Ro,max and the Langmuir volume. The Langmuir volume increases linearly as with the Ro,max values, while the Langmuir pressure is relatively high in low rank coals. The epicontinental and continental coals do not show a significant difference in the organic compositions, except that the sulfur content of 1.9% in lower coals and 1.0% in the upper coals. Furthermore, the epicontinental coals exhibit relatively better gas preservation and production conditions with higher gas content and Langmuir volumes. Results from the same source coal samples show strong heterogeneity in maceral composition and gas content variations and illustrate the controlling effect of Ro,max on coal petrology and the importance of preservation conditions on gas content.