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

Abstract

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 (δ¹³C_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 C₁₁ and C₁₇n-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.