Fluid Sources of Bedding-Parallel Fibrous Veins in Lacustrine Shales and Their Implications for Hydrocarbon Accumulation

Bedding-parallel fibrous calcite veins in organic-rich shale are commonly regarded as petrographic archives of abnormal pore-fluid overpressure and associated hydrocarbon expulsion and migration. Their formation reflects a dynamic cycle of fracture opening, fluid ingress and mineral precipitation, and subsequent re-opening. Constraining the evolutionary model of BPFVs and evaluating their influence on hydrocarbon accumulation are therefore of clear significance. In lacustrine shale systems, the sources of vein-forming fluids and the extent to which BPFV development couples with organic-matter maturation, overpressure generation, and hydrocarbon accumulation remain poorly constrained. This study investigates lacustrine shale of the second member of the Paleogene Funing Formation (E₁f₂) in the Qintong Sag, Subei Basin. Core observations, petrographic thin sections, and cathodoluminescence (CL) imaging were used to characterize vein petrography and constrain vein growth stages. Fluid inclusion petrography and microthermometry were conducted to define inclusion assemblages and homogenization-temperature (Th). Carbon–oxygen–strontium (C–O–Sr) isotopes and PAAS-normalized rare earth element (REE) patterns were integrated to diagnose vein forming fluid sources. These datasets were further evaluated against BasinMod-1D burial–thermal–hydrocarbon generation modeling to link Th stages with source-rock maturity and to assess the coupling between BPFVs and hydrocarbon accumulation. The results show that the BPFVs contain a well-defined median zone and symmetric antitaxial fibrous fabrics. Multiple internal growth records indicate repeated fracture opening and sealing. Oil inclusions commonly associated with aqueous inclusions, suggesting that hydrocarbons and formation water entered the bedding-parallel fractures during opening and were co-trapped during vein precipitation. Aqueous-inclusion Th values cluster into two populations (90–100°C and 120–130°C), matching the initial oil window and the main oil generation stage inferred from burial–thermal–hydrocarbon generation histories, and implying at least two vein filling episodes synchronous with source-rock thermal evolution. Geochemical data further show that vein calcite and host-rock carbonates share similar carbon sources and PAAS-normalized REE patterns, with no evidence for high temperature hydrothermal input. These observations indicate that vein forming fluids were dominated by basin-internal diagenetic pore waters, modified by fluids released during hydrocarbon generation and by sustained water–rock interaction. Based on these evidences, we propose a conceptual model for the development of BPFVs. Organic-matter thermal evolution elevates pore-fluid pressure and drives episodic opening of fractures along mechanically weak bedding planes. During opening, these fractures act as short-range pathways for hydrocarbon migration within the shale. Subsequent calcite precipitation partially to completely seals the fractures, preserving time-transgressive fluid properties and migration episodes in veins and fluid-inclusion assemblages. This framework provides key evidence for the dynamic coupling between the formation–evolution of bedding-parallel fractures and hydrocarbon accumulation in lacustrine shale, and offers a reference for reconstructing charging histories and timing in analogous lacustrine shale systems.