Depositional Environment and Petroleum System Significance of Shale- And Siltstone-Dominated Source Rocks: Evidence from Well A-1X, Nam Con Son Basin, Offshore Vietnam
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The Nam Con Son Basin (NCSB), offshore southeastern Vietnam, is among the largest and most hydrocarbon-prolific basins in Southeast Asia. While coal-bearing intervals are widely recognized as the primary source rocks, the role of shale- and siltstone-dominated successions has not been fully evaluated. This study integrates bulk geochemistry, organic petrology, and biomarker data from Well A-1X to assess the petroleum potential and depositional environment of shale and siltstone intervals.
A total of 94 fine-grained clastic samples (90 shales and 4 siltstones) were analyzed. TOC values for shales range from 0.50 to 2.60 wt.% (mean 1.10 wt.%), with the majority classified as “Good” (1–2 wt.%) according to petroleum geochemical standards. Siltstones yielded TOC between 0.90–1.16 wt.% (mean 0.99 wt.%), confirming fair-to-good organic enrichment despite coarser grain size. Stratigraphically, Oligocene shales are richer (mean 1.30 wt.%) compared to Miocene equivalents (mean 1.05 wt.%), indicating that the Oligocene succession forms the main shale source rock interval in Well A-1X.
Thermal maturity, based on vitrinite reflectance, increases with depth. Miocene shales are immature to marginally mature (0.31–0.52%Ro, mean 0.44%Ro), whereas Oligocene shales reach the early oil window (0.46–0.62%Ro, mean 0.57%Ro). Biomarker signatures support these results: shallow Oligocene shales exhibit n-alkane dominance at n-C17–C18 (algal contribution), while deeper intervals show enrichment in long-chain n-C25–C31, reflecting significant terrestrial higher-plant input. Pristane/Phytane ratios of 4–9 suggest deposition under suboxic to oxic deltaic conditions. CPI values averaging 1.03 indicate thermal maturity consistent with early oil generation.
Collectively, these findings demonstrate that shale and siltstone intervals in Well A-1X, though less organic-rich than coals, are effective secondary oil-prone sources. Their widespread stratigraphic distribution and confirmed maturity enhance the robustness of the Nam Con Son petroleum system and highlight new opportunities for exploration, particularly in Oligocene reservoirs charged by shale-derived hydrocarbons.
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