Abstract
Coal samples analyzed from Junggar Basin are characterized by high Total Organic Carbon (TOC) contents (63.7-82.2%) in all samples, except the highly over-mature samples Ha-01 and Ha-02, and high S2 (60.5-83.0 mg HC/g rock) and Hydrogen Index (HI) values (77-118 mg HC/g TOC), indicating the coal rocks have excellent source rock potential. The Tmax values (418-531°C), C29 sterane 20S/(20S+20R) ratios (0.20-0.54) and C31 homohopane 22S/(22S+22R) ratios (0.26-0.61) indicate an immature to highly over-mature stage of organic matter, which is supported by the organic geochemical maturation parameters. In addition, n-alkane carbon numbers range from C11 to C33 with maxima at n-C19 or n-C23 , exhibiting unimodal distribution patterns except Ai-13 and Ha-01, and low Pr/Ph ratios (0.29-0.80) in Ai-01, Ai-13, Ha-01 and Ha-02, indicating a suboxic reductive environment. In Wu-8, Wei-09 and Shinan-20, the Pr/Ph ratios are clearly greater than 1.0 (2.43-3.23), indicating oxic depositional conditions.Furthermore, pristane isomers were identified in extracts from these coal samples. Using coals of different maturity, which correspond to vitrinite reflectances (%Ro ) of 0.36-2.99%, the Pristane Isomerization Ratio (PIR) (PIR = [6(R)10(R)+6(S)10(S)]/6(R)10(S)) ranges from 0.42 to 0.97 for the coal extracts. The value of the PIR, which is a molecular maturity parameter, is evaluated by analyzing a series of samples using known values of the molecular maturity parameter based on the sterane and hopane isomerization indices, and the Methyl Phenanthrene Index (MPI). Changes to the PIR in highly mature Ai-13 (Ro > 1.5%) and over-mature Ha-01 and Ha-02 (Ro > 2.0%) are clear; thus, a linear correlation is evident between the PIR and Ro . The results suggest that the PIR is an appropriate indicator of maturity for the highly mature and over-mature coal samples in the Junggar Basin.
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