Origin of bedding-parallel fibrous calcite veins in the Permian carbonate, central Thailand: Constraint on hydrocarbon generation

Bedding-parallel fibrous calcite veins (BPCVs) are found extensively in sedimentary basins and usually developed in petroleum source rock; however, their origin remains debated. This study investigates the formation of BPCVs in the Permian carbonate rocks of the Khao Khwang Formation, central Thailand, through petrographic, geochemical, and isotopic analyses, along with total organic carbon (TOC) content assessment. Five samples, including 9 veins and 5 host rocks, were analyzed. The petrographic observations show that the BPCVs exhibit cone-in-cone structure and beef vein and are classified as unitaxial veins. The geochemical analyses and stable isotope compositions suggest local fluid sources originated from inorganic carbonates and diagenetic formation fluids within the microbial methanogenic zone. The BPCVs have an average δ¹³C_VPDB value of 3.04‰ ± 0.30‰ and an average δ¹⁸O_VPDB value of −10.75‰ ± 0.62‰, while the host rocks have an average δ¹³C_VPDB value of 1.88‰ ± 1.10‰ and an average δ¹⁸O_VPDB value of −10.77‰ ± 0.39‰. The mineral compositions and element contents of the host rock and fibrous calcite veins are similar, with a strong negative relationship between calcium and other elements. The rare earth element (REE) distribution patterns from the calcite veins are similar to the host rocks, with positive Ce anomalies and small positive Eu anomalies. Oxygen isotope data suggest vein formation at temperatures between 62 °C and 75 °C, corresponding to the onset of the oil window. The in-situ U-Pb dating indicates that fibrous calcite veins formed between the Early to Middle Permian period [(260.1 ± 4.5) to (288.9 ± 4.8) Ma)]. Microstructural and geochemical evidence suggests that vein initiation resulted from fluid overpressure of a supersaturation of pore fluids within semi-consolidated sediments, with continuous vein dilatation driven by crystallization forces. This study illustrates that the BPCVs of the Khao Khwang Formation highlight significant potential in accurately assessing the evolution of hydrocarbon generation.