Origin and geological control of desorbed gas in multi-thin coal seam in the Wujiu depression, Hailar Basin, China

To understand the natural gas characteristics of multi-thin coal seam, this study selected the desorbed gas of coal seams in different layers of Well A in the Wujiu depression, Hailar Basin in northeast Inner Mongolia. The results show that the heavy hydrocarbon content of desorbed gas increases significantly with the increasing depth. Methane carbon (delta(13)C1) and ethane carbon (delta(13)C2) isotope values are vertically become heavier downwards, while the delta(13) values did not change significantly. The kerogen is close to the III-II mixed type with the source rocks mainly deposited in a shore/shallow lake or braided river delta front, and the gas produced has certain characteristics of oil associated gas. However, the characteristics of oil associated gas produced by the organic formed in the shallow-water environment (braided river delta plain) are not obvious. The sandstone pore and fracture systems interbedded with multi-thin coal seam are well developed. And it is conducive to the migration of methanogenic micro-organisms to coal seams via groundwater, making it easier to produce biogenic gas under this geological condition. During the burial evolution of coal-bearing strata in the study area, when the burial depth reaches the maximum, there are significant differences in the paleotemperature experienced by different vertical coal seams, caused by a highpaleogeothermal gradient, increasing the delta C-13(2) of desorbed gas with increasing depth. The above research indicates that there is less biogenic gas in the multi-thin coal seams with relatively developed mudstone, and the multi-thin coal seams with relatively developed sandstones have obvious biogenic gas characteristics. Therefore, for the exploration and development of biogenic gas in low-rank multi-thin coal seams, it is necessary to give priority to the layer with high sandstone content.

Automated summary

This study analyzed the natural gas characteristics of multi-thin coal seams by examining the desorbed gas from different layers of Well A in Hailar Basin. The results showed an increase in heavy hydrocarbon content with depth. Isotope values of methane carbon and ethane carbon became heavier downwards, while the delta13C values did not change significantly. Well-developed sandstone pore and fracture systems facilitated the migration of methanogenic micro-organisms to coal seams, making the production of biogenic gas easier under these geological conditions.