What does this research mean for the field?

Organic matter enrichment and kerogen type in the Upper Triassic Xujiahe Formation are determined by a ternary coupling of depositional environment, biological assemblage, and preservation conditions, resulting in distinct facies-dependent distributions across the northeastern Sichuan Basin. Novelty: ClaimNovelty.INCREMENTAL. Consensus alignment: ConsensusAlignment.NEUTRAL.

May 31, 2026Scientific Reports0 citationsOpen Access

Multiple source enrichment model of organic matter in fifth member of Xujiahe Formation of Upper Triassic, northeastern Sichuan Basin

JLJ Li YWY Wang XWXiandong Wang

Key Points

This study aims to clarify the heterogeneity of organic sources in the Upper Triassic fifth member of the Xujiahe Formation.
Selected three typical wells (DS2, LQ1, PA3) for analysis.
Used petrology, organic geochemistry, biomarkers, kerogen maceral analysis, and rock pyrolysis.
Developed a multi-source organic matter enrichment model.
Identified three sedimentary facies with distinct organic matter assemblages.
Well DS2 showed type III kerogen dominated by vitrinite (93%) with low HI.
PA3 displayed mixed type II-III kerogen with sapropelinite (35–49%) and vitrinite (45–53%), average HI 111 mg/g.

Abstract

In order to clarify the heterogeneity of organic sources in the Upper Triassic fifth member of the Xujiahe Formation (T3x5) on the northeastern margin of the Sichuan Basin, three typical wells (DS2, LQ1, PA3) were selected. Using petrology, organic geochemistry, biomarkers, kerogen maceral analysis, and rock pyrolysis, the sedimentary environment and organic matter (OM) sources were investigated, leading to a multi-source OM enrichment model. Results distinguish three sedimentary facies and corresponding OM assemblages. Well DS2 (east) deposited in delta plain swamp microfacies; well PA3 (south-central) in delta front and littoral-shallow lake transitional facies; well LQ1 (mid-west) in littoral-shallow lake facies. These facies control multiple OM sources (higher plants, cyanobacteria, algae). OM type is ultimately determined by a ternary coupling of depositional environment, biological assemblage, and preservation conditions. DS2 is dominated by vitrinite (93%), no sapropelinite, low HI—typical type III kerogen from higher plants. PA3 shows coexistence of sapropelinite (35–49%) and vitrinite (45–53%), average HI 111 mg/g (range 0.9–279.7), indicating mixed type II and III kerogen with local algae-rich intervals. LQ1 is enriched in sapropelinite (41–45%) and liptinite (8–11%), hydrogen-rich components > 50%, average HI 126.7 mg/g, representing stable type II kerogen. The enrichment model thus controlled by multiple organic sources and facies: in the eastern delta plain swamp, oxidation and degradation formed hydrogen-poor type III OM; in the central transitional environment, fluctuations produced mixed type II-III; in the mid-western lacustrine facies, a stable reducing environment promoted hydrogen-rich type II source rocks. The humid tropical-subtropical Late Triassic climate provided abundant biological sources, and the sedimentary facies—through proximity to provenance, redox conditions, and organisms—ultimately determined OM composition.

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