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A worldwide dataset of organic material from 553 samples belonging to coal and carbonaceous materials was used to analyze the evolution of hydrogen index (HI) and bitumen index (BI) with increasing thermal maturity. Basic statistical analyses were applied to detect the boundary lines of HI max and BI max in delineating the upper and lower limits of the HI and BI bands for the majority of samples. In addition, cross-plots of HI or BI versus maturity (Ro% and T max ) also provide criteria for defining the HI max and BI max boundary lines. The constructed HI and BI bands are broad at low maturities and become narrower with increasing thermal maturities. The petroleum generation potential is completely exhausted at the vitrinite reflectance of 2.0-2.2% or T max of 510-520°C. An increase in HI implies extra petroleum generation which was related to changes in structure of organic materials. A declining BI means that the oil expulsion window starts to occur at the vitrinite reflectance range of 0.75-1.05%. The petroleum potential can be divided into four different areas based on the cross-plot of HI versus Ro%. The highest petroleum potential area is located in section II with Ro = 0.6-1.0% and HI [ 100. The oil generation potential is rapidly exhausted at section III with Ro [ 1.0%. This result is also in accordance with the result of curve regression of HI versus Ro% based on 80 samples with Ro = 1.02-3.43% (R 2 = 0.72). Overall, the total oil window can be extended up to Ro = *1.25-1.95%. Finally, in the cross-plots of S1 versus S2, shale or C-shale exhibits a higher and slowly decreased slope, compared with a lower and then sharply increased slope of coal samples, which is attributable to their compositional difference in organic material.

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In this study, 26 samples from northwest Taiwan, 12 from Mainland China, 13 from Australia and 39 from literature were jointly examined to explore relationships among pyrolysis parameters, Vitrinite Reflectance Ro%, and Atomic H/C ratio. Samples of mixed high and low maturity coal were combined in proportions determined by the total quantity in the furnace prior to the Rock-Eval analysis and used to explore the correlation between the pyrolysis parameter, Tmax, and the vitrinite reflectance. These average values were then plotted against the corresponding Tmax results. The experimental results revealed that:(1) For low maturity coal samples that were mixed with different proportions of high maturity coal samples, the Tmax values fell within a range of low maturities. Alternativly, for samples containing the reworked sedimentary materials in the rock formation, the Tmax values were similar to the maturity of young material. (2) For sampling or Rock-Eval analysis of the high maturity materials, contamination with low maturity material should be avoided, even in very small amounts. (3) Afterproportional mixing, there was no evidence of a general linear relationship between the average of vitrinite reflectance, Ro%, and the corresponding Tmax value recorded. The atomic H/C ratio, as well as the BI, HI, QI, S1, and S2, generally decreases while the maturity (Tmax (o C); vitrinite reflectance Ro%) increases. The atomic H/C ratio decreases slightly from 1.1 to 0.7 while maturity increased from Ro 0.55% to 0.85%. Samples with atomic H/C ratio within this range show significant change in certain other geochemical parameters (eg. BI, HI, QI, PI, S1, S2, S1+S2, Tmax). Organic matter in the samples studied is of type II/III kerogen based on the relationship between HI and Tmax. The hydrocarbon potential per unit organic carbon (S1+S2/TOC) of the organic matter in this study to be approximately 100~380, similar to the potential of humic coal used in general gas and oil production. This shows that organic matter in an oil window of Ro%=0.55 and atomic H/C=1.1 have reached a certain maturity and hydrocarbon potential. Overall, when the atomic H/C ratio increases, the BI, HI, QI, S1, and S2 also show an increasing trend; therefore, these parameters and atomic H/C ratio show a certain correlation.