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<p>While high-quality lacustrine source rocks are conventionally associated with deep to semi-deep lake facies, this study investigates the hydrocarbon generation potential of deltaic distributary bay environments to host effective petroleum source rocks. Through integrated geochemical characterization and paleoenvironmental reconstruction of organic-rich mudstones from the Triassic Yanchang Formation’s Chang 9 Member (Longdong region, Ordos Basin)—deposited in a delta-lacustrine transitional system—we demonstrate the viability of such facies as hydrocarbon source units. Analytical results reveal that Chang 9 deltaic distributary bay mudstones exhibit source rock potential, displaying moderate to excellent quality with total organic carbon (TOC) averaging 2.0% and hydrogen index (HI) values approximating 300 mg HC/g TOC. Organic petrography indicates dominance of Type II kerogen, consistent with thermal maturity levels reflected by vitrinite reflectance (Ro) values of ~ 0.90%. Deposition occurred under weakly oxic to suboxic water column conditions within a warm-humid paleoclimate regime, with estimated water depths ranging from 5.9 to 40.0 m. The paleolake maintained brackish salinity and moderate biological productivity during deposition. Semi-deep lacustrine facies contain the most prolific source intervals, characterized by organic matter predominantly derived from lacustrine algae. Secondary but commercially significant source potential occurs in inter-distributary bay facies proximal to the paleo-lake center, where lacustrine algal biomass remains the primary organic contributor, though with non-negligible terrigenous organic input. This type of source rock is widespread distributed due to the low-gradient setting of the Chang 9 deltaic-lacustrine depositional system resulting in extensive lateral continuity of organic-rich shales (thickness: 15–25 m; aerial coverage &gt; 5,000 km<sup>2</sup>). This facies-controlled heterogeneity underscores that deltaic distributary bay deposits—under appropriate hydrodynamic and nutrient conditions—can indeed develop effective source rocks, as exemplified by the Chang 9 system. These findings provide critical insights for hydrocarbon resource assessment and exploration targeting in the Longdong region’s delta-lacustrine transitional systems.</p>

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Potential for effective source rocks in deltaic distributary bays: evidence from the Chang 9 Member, Yanchang Formation, Ordos Basin, China

  • 期刊论文

  • 发表日期：

    2025年10月9日

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  • Feng Ma,
  • Zhongsheng Shi,
  • Ting Wang,
  • Bintao Chen,
  • Luo Xue,
  • Fei Xu,
  • Lun Ma

摘要

While high-quality lacustrine source rocks are conventionally associated with deep to semi-deep lake facies, this study investigates the hydrocarbon generation potential of deltaic distributary bay environments to host effective petroleum source rocks. Through integrated geochemical characterization and paleoenvironmental reconstruction of organic-rich mudstones from the Triassic Yanchang Formation’s Chang 9 Member (Longdong region, Ordos Basin)—deposited in a delta-lacustrine transitional system—we demonstrate the viability of such facies as hydrocarbon source units. Analytical results reveal that Chang 9 deltaic distributary bay mudstones exhibit source rock potential, displaying moderate to excellent quality with total organic carbon (TOC) averaging 2.0% and hydrogen index (HI) values approximating 300 mg HC/g TOC. Organic petrography indicates dominance of Type II kerogen, consistent with thermal maturity levels reflected by vitrinite reflectance (Ro) values of ~ 0.90%. Deposition occurred under weakly oxic to suboxic water column conditions within a warm-humid paleoclimate regime, with estimated water depths ranging from 5.9 to 40.0 m. The paleolake maintained brackish salinity and moderate biological productivity during deposition. Semi-deep lacustrine facies contain the most prolific source intervals, characterized by organic matter predominantly derived from lacustrine algae. Secondary but commercially significant source potential occurs in inter-distributary bay facies proximal to the paleo-lake center, where lacustrine algal biomass remains the primary organic contributor, though with non-negligible terrigenous organic input. This type of source rock is widespread distributed due to the low-gradient setting of the Chang 9 deltaic-lacustrine depositional system resulting in extensive lateral continuity of organic-rich shales (thickness: 15–25 m; aerial coverage > 5,000 km2). This facies-controlled heterogeneity underscores that deltaic distributary bay deposits—under appropriate hydrodynamic and nutrient conditions—can indeed develop effective source rocks, as exemplified by the Chang 9 system. These findings provide critical insights for hydrocarbon resource assessment and exploration targeting in the Longdong region’s delta-lacustrine transitional systems.