Analysis and characterization of samples from sedimentary strata with correlations to indicate the potential for hydrocarbons

Abstract

The Rock-Eval pyrolysis and TOC analysis have been widely used to evaluate the source rock quality. The atomic H/C ratio of kerogen, however, has been overlooked in source rock evaluation. In this study, coal and carbonaceous samples, including 26 from northwestern Taiwan, 12 from China, and 4 from the United States were analyzed, and integrated with 157 published data, to explore the significance of atomic H/C ratio as a parameter of source rock evaluation. Two different linear trends were observed in the cross-plot of S 1 versus S 2. Field outcropped shale or C-shale exhibits a steeper slope compared to that of coal samples which can be attributed to the compositional difference in their organic material. A rather strong positive correlation for H% versus S 2 illustrates the contribution of H-containing macerals, especially exinite. Organic matters in the samples studied are of type II/III kerogen based on the relationship between HI and T max. The H/C ratio, as well as the HI, S 1, and S 2, generally decreases with the maturity increasing. The H/C ratio decreases slightly from 1.1 to 0.7 with the maturity increasing from R o 0.55 to 0.85%. Samples with H/C ratio in this range show significant change in certain other geochemical parameters (e.g. HI, S 1, S 2, S 1 + S 2, S 1/(S 1 + S 2), S 1/TOC, (S 1 + S 2)/TOC, T max). The (S 1 + S 2)/TOC ratio (defined as QI) was used as an indicator of the hydrocarbon potential. The QI, HI, and H/C ratio show a certain correlation, all increasing accordingly. The QI of the samples analyzed in this study is approximately 100–380 (mgHC/gTOC), similar to that of most humic coals for oil and gas generation. Samples with R o value lower than 0.55% always show significant variation in their HI, ranging from 80 to 520 mgHC/gTOC. It is inferred that hydrocarbon potential started from R o 0.55% and atomic H/C ratio 1.1 in this study.