Bedding-parallel fibrous calcite veins in Permian carbonate, central Thailand

Bedding-parallel fibrous calcite veins (BPCVs) are prevalent in sedimentary basins and typically formed in petroleum source rock. This study examines the development of BPCVs in the Permian carbonate of the Khao Khwang Formation in central Thailand using petrographic, geochemical, and isotopic investigations, as well as total organic carbon (TOC) evaluation. Five rock specimens, comprising ten veins and five host rocks, were examined. The findings indicate that BPCVs display cone-in-cone and beef structures, categorizing them as unitaxial veins. The geochemical analyses and stable isotope compositions indicate that local fluid sources derived from inorganic carbonates and diagenetic formation fluids in the microbial methanogenic zone. Oxygen isotope analysis indicates that vein development occurred at late diagenetic stage. In situ U-Pb dating reveals that fibrous calcite veins originated from the Early to Middle Permian. The development of BPCVs linked to petroleum source rock provides critical insights into the history of fluids and petroleum generation within sedimentary basins.