Field study and rock sampling are performed in two study areas in Phetchabun Province (Figure 1B). In total, 12 rock samples were selected for detailed study from a range of samples from the Khao Kwang and Nam Duk Formations. The 12 samples contained fibrous calcite veins suitable for petrography and geochemical analyses. Generally, bedding-parallel fibrous calcite veins documented in this study occur within calcareous mudstones and argillaceous limestones. Samples referred to as “outcrop samples” in this paper are not taken directly from the surface. Instead, samples were collected by digging some 0.10 and 0.30 m into the outcrop to ensure that visibly unweathered material was obtained. Any remaining weathered parts of the samples were removed before proceeding with detailed analysis. Samples were then cleaned with distilled water, acetone, and dichloromethane to eliminating any remaining surface contamination. All washed samples for organic geochemical analysis were crushed into small chips in a hammer and jaw-crusher and then into powder by dish mill grinder. Powder samples were then put into pre-washed glassware for further extractable organic matter and gas chromatography–mass spectrometric determinations. Petrographic samples were cut into rock slabs, and thin sections were made to observe fibrous calcite characteristics. Samples for isotope analysis were extracted from vein regions in rock slabs by using a dental technician’s drill in traverses that extracted a sample volume every 5 mm along traverses than ran from the host rock across a vein, or stacked veins, and back into host rock on the other side of the vein.

Gas chromatography-mass spectrometry (GC-MS) is an instrumental technique for identifying the molecular mass of organic compounds by their ionized compositions. In this study, GC-MS was performed at the Scientific and Technological Research Equipment Centre Chulalongkorn University to analyze biomarker and non-biomarker compounds from bitumen extracts. The saturated hydrocarbon fraction from the rock samples was dissolved in n-pentane and analyzed with an Agilent 7000C GC/MS (triple quadrupole) machine, with an ion source temperature of 250°C and ionizing energy of 70 eV. The column used in this study is a 30 m long DB-5 (5% Phenyl Methyl Siloxane) with a 250 µm inner diameter and 0.25 µm film thickness. Starting temperature is 80°C and is held for 3 min. The column temperature is heated from 80°C to 310°C at 4°C/minutes and held for 30 min at 310°C. The total analysis time is 90 min. Chromatograms were acquired in scanning: 35–700 molecular weight and selected-ion-monitored (SIM) for compound identification and integration. The distribution of organic compounds in the bitumen extracts was monitored by fragmentograms of n-alkane (m/z 85) and steranes (m/z 217). Details of the analytical procedure used are summarized in Moustafa and Morsi (2012).

Stable isotopes of δ¹³C and δ¹⁸O of the veins and host rock samples are measured by utilizing techniques outlined in Allègre (2008) after extracting powder samples using a dental technician’s drill. The stable isotopes were carried out at Monash University, Australia. The purpose of this study is to better understand fluid carbon sources for the fibrous calcite veins and fluid evolution during different burial stages, tied to calcite precipitation. The carbon sources for the fibrous calcite vein in organic-rich rocks come from inorganic carbon in the host sediment and organic carbon from organic matter (both depositional and catagenic). The inorganic carbon source is expected to have δ¹³C values near the seawater δ¹³C value through time, whereas the organic carbon source is significantly depleted in the host rock by biological processes (Luan et al., 2019). Therefore, if no external fluids contributed to the host rock, especially within low permeability host rocks, the fibrous calcite vein is expected to have bulk isotopic compositions close to the host rock. All analyses are reported in per mill (‰) relative to the Vienna-PeeDee Belemnite (V-PDB) standard.

The dominant lithology of samples from the Khao Khwang Formation is argillaceous limestone. In general, the succession in the study area comprises limestone (approximately 1–50 cm) interbedded with calcareous mudstone to argillaceous limestone (approximately 1–10 cm) (Figures 2A,B). In contrast, the rock samples from the Nam Duk Formation are calcareous mudstones interbedded with sandstones (Figures 2C,D). Bedding-parallel 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 quality of TOC and pyrolysis data as published in previous studies, using standard petroleum industry methodologies, is compromised, likely due to strong maturation (e.g., Chenrai and Fuengfu, 2019). Accordingly, in this study, organic matter source and the depositional conditions are determined using alternative tools such as non-biomarker and biomarker parameters. Molecular geochemical results and biomarker ratios from samples measured in this study are summarized in Table 1.

The results of stable carbon and oxygen isotope analyses of the samples from both Formations are summarized in Table 2 and plotted in Figure 8. The δ¹⁸O values of the fibrous calcite veins and host rock matrices of the Nam Duk samples are more depleted than the Khao Khwang samples. The fibrous calcite veins from the Nam Duk samples also show higher values of δ¹³C compared to the Khao Khwang samples, indicating overall heavier carbon isotope compositions. The carbon and oxygen isotope compositions of the Khao Khwang and Nam Duk samples are plotted along with the sample profile in Figure 9 and Figure 10, respectively. The carbon and oxygen isotope compositions of the fibrous calcite veins are generally quite similar to those in the host rock matrix, especially within the Khao Khwang samples (Figures 9, 10). The carbon isotope compositions of the fibrous calcite vein are slightly more elevated than in the host rock matrix, presumably reflecting incorporation of organic carbon into vein cement.

Biomarker analysis of soluble organic matter (bitumen) from the rock samples hosting the fibrous calcite veins has contributed to a better understanding of the characteristics of the depositional environments (e.g., Waples and Machihara, 1991; Sivan et al., 2008). In this study, organic facies and depositional environment are interpreted using Pr/Ph ratio, Pr/n-C₁₇ and Ph/n-C₁₈ cross plots and sterane distributions (e.g., Shanmugam, 1985). Samples from both Formations have similar isoprenoid, n-alkane and sterane distributions, and thus are similar in depositional environment, organic matter type and maturity (Figures 5–7; Table 1). Molecular geochemical data from both Formations suggest that the rock samples are deposited in a marine environment with a mixed source of organic matter from the marine and terrestrial. However, there are slight differences in likely paleo-geography and thermal maturation between the two Formations.

Based on these data, rock samples from the Khao Khwang Formation are believed to have been deposited in a lagoonal/estuarine or shallow marine environment. In contrast, based on n-alkane and sterane distributions, the Nam Duk Formation was deposited in a somewhat deeper water slope environment. That is, a higher proportion of land plant material was deposited within the Khao Khwang Formation. These interpretations coincide with results from previous studies on depositional environments for both Formations (e.g., Chitnarin et al., 2008; Chenrai and Fuengfu, 2019). The same settings of the relevant organic facies are confirmed by the bivariate plot trends of the Pr/Ph ratio, the Pr/n-C₁₇ and Ph/n-C₁₈ cross plot and the tertiary diagram of regular steranes (Figure 7).

Total organic carbon (TOC) contents in the Khao Khwang Formation imply fair to excellent source rock potential (Racey, 2011; Chenrai and Fuengfu, 2019). Unfortunately, there is no similar data measure for the Nam Duk Formation. However, the Pr/n-C₁₇ and Ph/n-C₁₈ cross plot reveals that the samples from both Formations contain a mixed kerogen, between type III and type II, deposited under sub-oxic to oxic conditions. It should be noted that these analyzed data are from outcrop samples that may be somewhat oxidized, tied to exposure to the more oxic conditions near the present-day landsurface. If organic matter was first deposited in anoxic to sub-oxic conditions, the rock samples would be considered potential oil and gas sources for both Formations. Abundant framboidal pyrites having a various size from 5.7 µm to 0.5 mm within the rock samples, especially from the Khao Khwang Formation, also suggest that the samples were deposited under relatively anoxic conditions (Wilkin et al., 1996; Liu et al., 2019). The Nam Duk Formation was likely deposited further from a terrestrial organic matter source than the Khao Khwang Formation, based on n-alkane and sterane distributions. This implies fine-grained rocks in the Khao Khwang Formation have a higher petroleum potential than the Nam Duk Formation. Thermal maturation of the Nam Duk Formation is interpreted to be higher than the Khao Khwang Formation, based on the Pr/n-C₁₇ and Ph/n-C₁₈ cross plot (Figure 6). This higher thermal maturation interpretation is supported by the δ¹⁸O values of the Khao Khwang samples being more negative than the Nam Duk samples (Figure 8), indicating that the calcites within the Khao Khwang samples were precipitated at somewhat higher temperatures (Al-Aasm et al., 1993).

Bedding-parallel calcite veins are widely documented, as are interpretations of the formative mechanisms driving aligned fibrous crystal growth within sedimentary strata during burial diagenesis and compaction (Franks, 1969; Cobbold et al., 2013). Nam Duk samples are dominated by beef veins with a smooth continuous boundary between the fibrous calcites and host rock bedding surface. This probably indicates that the beef veins grew at a consistent rate. On the other hand, the Khao Khwang samples mostly fibrous calcite cone-in-cone veins, with abundant inclusions of host-mud. These samples likely grew under more fluctuating conditions, with times of rapid growth indicated by intervals enclosing more mud inclusions. The ability of the calcite to encase mud inclusions likely indicates that the host rock was also semi to unconsolidated at the time of calcite crystal growth. Beef veins without mud inclusions suggest a more brittle or consolidated host rock at the time of fibrous calcite precipitation (e.g., Meng et al., 2017). Hence, beef veins in the Nam Duk Formation likely formed during deeper burial diagenesis and in more compacted situations than cone-in-cone veins in the Khao Khwang Formation. Both the cone-in-cone and beef vein fabrics are a fibrous growth response to saturated conditions in a hydrologic regime derived from local carbonaceous fluids within the host rock. Individual crystal fibers grew vertically and inward during the periodic opening of the vein space.

Stable isotope compositions can be used to refine interpretations of growth style and relative timing of fibrous calcite development. Negative signatures in organic matter can provide evidence of an organic bicarbonate (HCO₃⁻) sourced in zones of bacterial sulphate reduction in shallow-burial anaerobic environments (Irwin et al., 1977). The bicarbonate fixed in fibrous calcites precipitated in this zone usually inherit δ¹³C signatures from this organic matter; hence a much-depleted carbon isotope composition is expected to occur in calcites formed in this zone (δ¹³C < −25‰ PDB; Irwin et al., 1977). After sulphate is totally consumed in the bacterial sulphate reduction zone, the microbial methanogenic zone is entered; there Archaeal methanogens can continue to be active until temperatures reach ∼75°C (the petroleum generation window; Morad, 2009).

Samples from the two Formations show positive δ¹³C values, suggesting that the heavy carbon in the formative bicarbonate was influenced by microbial activity in a methanogenic zone that underlay the bacterial sulphate reduction zone. There the carbon dioxide (CO₂) generated within the microbial methanogenic zone tends to have heavy carbon isotope compositions (δ¹³C ≈ 0–15‰ PDB) via microbial fermentation (Raiswell, 1987; Wolff et al., 1992). Thus, if the buried organic-rich rocks still have organic matter remaining after passing through the bacterial sulphate reduction zone, they can generate carbon dioxide with δ¹³C enriched carbon. This leads to progressive ¹³C enrichment in bicarbonate fixed as calcite forming within the rocks. Fibrous calcite veins in this zone are expected to be similar to the veins in the two Formations. Overall δ¹³C carbon composition will be positive if there is no subsequent external fluid intrusion, as is likely due to the ongoing low permeability of the host rocks (Luan, et al., 2019). Thus, moderate positive δ¹³C values from the Khao Khwang and Nam Duk Formations possibly suggest a carbon source from carbon dioxide influenced by microbial methanogenesis.

Oxygen isotope compositions of the fibrous calcite veins from both Formations are more depleted when compared to the range of Permian carbonate rock precipitation in equilibrium with Permian seawater as −4.8‰ PDB (Veizer et al., 1999). The measured negative oxygen isotope signatures (≈−10 to −13‰) are too negative and likely reflect ongoing local rock-buffered rock-fluid interactions, carbonate precipitation and re-equilibration is occurring in modified marine pore waters within adiagenetic systems at somewhat deeper burial depths where the temperature are progressively increasing (Hodgson, 1966; Dickson and Coleman, 1980; Marshall, 1982; Nuriel, et al., 2011; Uysal, et al., 2011; Warren et al., 2014).