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 (δC) ranged from - 24.6 to - 23.7‰. The calorific value (Q), maximum vitrinite reflectance (Ro), 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 and C 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.

Organic geochemistry characteristics of the Permo-Carboniferous coals from the Huainan coalfield, East China: implications for paleoenvironmental changes

“…Biomarkers identi ed in the extracts of coal originate from various biological precursors and are characterized by carbon skeletons that are indicative of their link to speci c bio-precursors (Alexander et al, 1995;Radke et al, 1980aRadke et al, , b, 1986Radke et al, , 1990Radke et al, , 1998Radke et al, , 2000. (Alexander et al, 1995;Armstroff et al, 2006;Fleck et al, 2001;Körmöasab et al, 2020;Li et al, 2017;Wang et al, 2017;Wang et al, 2018). They are often used as chemostratigraphic tools for tracing both paleo oral changes based on the established relationships between oral assemblages and molecular skeletons.…”