Organic geochemistry of the Jurassic tar-rich coal in Northern Shaanxi Province

The organic geochemical characteristics of tar-rich coal are of great significance to study the paleosedimentary environment and evolution of coal seams. Taking coal samples from 6 main coal seams of Yan'an Formation in Northern Shaanxi Province as the research object, this paper analyses the organic geochemical characteristics of coal seams by coal quality analysis and gas chromatography-mass spectrometry. The results showed that coal samples had a high abundance of chloroform asphalt "A" and a low saturated hydrocarbon/aromatic hydrocarbon ratios, characterized by the unimodal pattern of n-alkanes and dominated by medium chainsaturated hydrocarbons. The ratios of Pr/C₁₇, Ph/C₁₈and Pr/Ph were relatively high, regular sterane and diasteranes of C₂₈ and C₂₉ were dominant in steranes. The organic matter was in the low-mature-mature stage, which was a favorable stage for perhydrousvitrinite and lipinite to generate oil. The source of organic matter were from terrestrial organic matte and aquatic organisms and which formed in the anoxic environment and undergoed strong biodegradation. All of these provided favorable conditions for the formation of perhydrousvitrinite.