Organic Matter Composition and Hydrocarbon Generation Potential of Coal Seams in Well A-1X, Nam Con Son Basin, Offshore Vietnam
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The Nam Con Son Basin (NCSB) is one of the most hydrocarbon-prolific basins in offshore Vietnam. Although shales and siltstones have recently received greater attention, coals remain the dominant source rocks in the basin. This study provides a detailed geochemical characterization of coal seams from Well A-1X, focusing on organic matter composition, maturity, and hydrocarbon generation potential.
A total of 12 coal samples were analyzed for TOC, Rock-Eval pyrolysis, vitrinite reflectance (%Ro), and biomarker parameters. TOC values are exceptionally high, ranging from 62.4 to 87.9 wt.%, with a mean of 78.6 wt.%, classifying all samples as Excellent source rocks (Peters & Cassa, 1994). Despite this richness, Rock-Eval hydrogen index (HI) values are relatively low (~150–220 mg HC/g TOC), indicating a gas-prone Type III kerogen typical of higher-plant derived organic matter.
Thermal maturity assessments reveal that coals are at the threshold of petroleum generation. Vitrinite reflectance ranges from 0.55–0.62%Ro (mean 0.59), placing them at the early oil window, while Tmax values (437–443 °C) support this interpretation. However, bitumen extracts yield relatively low hydrocarbon fractions (average 18.3% of total extract) compared to shales, suggesting limited liquid hydrocarbon generation potential.
Biomarker evidence further confirms terrestrial origin and oxic depositional settings. n-Alkane distributions are dominated by long-chain n-C25–C31 hydrocarbons, characteristic of higher-plant waxes. Pristane/Phytane ratios are consistently high (13–16), indicating strongly oxic conditions typical of delta-plain swamp environments. CPI values of 1.14–1.21 reflect early maturity but preservation of odd-carbon dominance in higher plant waxes.
These results demonstrate that coals in Well A-1X are exceptionally organic-rich and regionally important gas-prone source rocks. While they provide limited oil, their vast thickness and continuity make them the primary hydrocarbon source in the Nam Con Son Basin, supplying gas and condensate to regional petroleum accumulations. Their characterization enhances understanding of the basin’s petroleum system and highlights the contrast with shale-dominated intervals.
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