Geochemical characteristics of n-alkanes and isoprenoids in coal seams from Zhuji coal mine, Huainan coalfield, China, and their relationship with coal-forming environment

摘要

Ten coal seams in Upper Shihezi Formation, Lower Shihezi Formation, and Shanxi Formation from the Zhuji mine, Huainan coalfield, China, were analyzed for n -alkanes and isoprenoids (pristine and phytane) using gas chromatography-mass spectrometry (GC-MS), with an aim of reconstructing the coal-forming plants and depositional environments along with organic carbon isotope analyses. The total n -alkane concentrations ranged from 34.1 to 481?mg/kg. Values of organic carbon isotope (δ_(13)C~(org)) ranged from ??24.6 to ??23.7‰. The calorific value (Q~(b,d)), maximum vitrinite reflectance (Ro~(max)), proximate, and ultimate analysis were also determined but showed no correlation with n -alkane concentrations. Carbon Preference Index (CPI) values ranged from 0.945 to 1.30, suggesting no obvious odd/even predominance of n -alkane. The predominance of C~(11)and C~(17) n -alkanes implied that the coal may be deposited in the fresh and mildly brackish environment. According to the contrary changing trend of pristine/phytane (Pr/Ph) ratio and boron concentrations, Pr/Ph can be used as an indicator to reconstruct the marine transgression-regression in sedimentary environment of coal formation. The influence of marine transgression may lead to the enrichment of pyrite sulfur in the coal seam 4-2. C3 plants (??32 to ??21‰) and marine algae (??23 to ??16‰) were probably the main coal-forming plants in the studied coal seams. No correlation of the n -alkane concentration and redox condition of the depositional environment with organic carbon isotope composition were found.