Abstract
Bedding-parallel fibrous calcite veins crop out at two Permian carbonate localities in the Phetchabun area, central Thailand, within the Nam Duk and Khao Khwang Formations. Samples are studied to determine their petrographic, geochemical and isotopic character, depositional and diagenetic associations and controls on the formation of fibrous calcite across the region. Biomarker and non-biomarker parameters are used to interpret organic matter sources in the vein-hosting units, the depositional environment and levels of source rock maturation in order to evaluate source rock potential in the two Formations. Carbon and oxygen isotope values of the veins and the host are determined to discuss the source of carbonates and diagenetic conditions. The petroleum assessment from the Khao Khwang and Nam Duk Formations suggests that both Formations are a petroleum potential source rock with type II/III kerogen deposited in an estuarine environment or a shallow marine environment and a slope-to-basin marine environment or an open marine environment, respectively. The bedding-parallel fibrous calcite veins from the Khao Khwang and Nam Duk Formations are divided into two types: 1) beef and, 2) cone-in-cone veins. The carbon and oxygen isotope compositions from the fibrous calcite veins suggest that the calcite veins could be precipitated from a carbon source generated in the microbial methanogenic zone. The results in this study provide a better understanding of the interrelationship between the bedding-parallel fibrous calcite veins and petroleum source rock potential.
Highlights
Our study focuses on the carbonates and siliciclastic rocks of the Khao Khwang and Nam Duk Formations that crop out in this area (Udchachon et al, 2014)
Beddingparallel fibrous calcite veins are around 1–7 cm from exposed bedding plane surfaces in their host rock outcrops (Figures 2E,F) and oriented approximately parallel to the inferred bedding planes in both Khao Khwang and Nam Duk Formations (Figures 2E,F)
The bedding-parallel fibrous calcite veins from the Khao Khwang and Nam Duk Formations are divided into two types; beef and cone-in-cone veins
Summary
Bedding-parallel fibrous calcite veins are found in sedimentary rocks from Cambrian to Palaeogene strata in many locations worldwide (Cobbold and Rodrigues, 2007; Cobbold et al, 2013; Heindel et al, 2015; Kershaw and Guo, 2016; Ma et al, 2016; Cabral et al, 2019). Such fibrous calcite veins are globally identified across organic-entraining sedimentary basins, and are used in petroleum exploration as monitors of hydrocarbon generation (e.g., Zanella, and Cobbold, 2012; Meng et al, 2017; Luan et al, 2019; Zhang et al, 2021). Times of elevated carbon dioxide concentration are generally associated with mass extinction events, including end-Permian (P-T) boundary and the end-Triassic (T-J) boundary, and are tied to large scale volcanic eruptions (e.g., Greene et al, 2012; Kershaw and Guo, 2016)
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