The dolomite origins and dolomitizing processes have been extensively studied by petroleum geologists because the dolostones provide approximately 50% of the carbonate hydrocarbon reservoirs (Zenger et al., 1980; Sun, 1995; Ma et al., 2007). This percentage increases to more than 80% at deep-burial formations (i.e., depths greater than 4,000 m) as dolostones are commonly more porous and permeable than limestones because of mineralogical alterations during burying diagenesis (Warren, 2000; Machel, 2004). More importantly, the evolution of reservoir porosities related to dolomitizing process has a great influence on the reservoir quality of carbonates. Hence, deciphering dolomitization is significant for high-quality reservoir prediction in carbonates (e.g., Liu et al., 2016, 2017). In addition, carbonate properties are sensitive to diagenetic alterations (particularly dolomitizing process) because mineralogical and/or chemical modifications occurred easily in ancient carbonates due to their unstable chemical properties (Azmy et al., 2001; Azomani et al., 2013). Previous studies have revealed that dolomitization could exert both direct and indirect controls on the reservoir porosity of carbonates, indicating that the dolomitization-induced porosities have both syn- and post-dolomitization origins (e.g., Machel and Burton, 1994; Machel, 2004; Davies and Smith, 2006; Eren et al., 2007). The dolomitizing process may increase, sustain, or suppress porosity development in ancient carbonates, depending on sedimentary and/or diagenetic controls, such as the textures and fabrics of precursor aragonites/calcites, depositional belts, marine environments, and dolomitizing mechanisms (Warren, 2000; Liu et al., 2016). Particularly, dolomitization has been enigmatic and a plethora of mechanisms have been proposed to describe this process, including seepage-reflux, sabkha-style, meteoric-mixing zone, organic-origin, burial, and hydrothermal fluid dolomitizations (Adams and Rhodes, 1960; Friedman and Sanders, 1967; Badiozamani, 1973; Hsu and Schneider, 1973; Baker and Kastner, 1981; Braithwaite and Rizzi, 1997; Huang et al., 2008). Nonetheless, these dolomitizing patterns are usually subjects of debate when being applied to a certain region. For instance, Land (1985) and Hardie (1987) have cast doubts on Baker and Kastner’s (1981) previous experiments by questioning its extremely high temperatures (greater than 200°C) that might not be applicable to wide dolomitization at seafloor environments featured by deep-water depths and low temperatures. Currently, geochemical methods are widely applied to determine dolomite origins, coupled with traditional depositional, petrographic, and diagenetic elucidation. For instance, the oxygen isotope (δ¹⁸O) is widely used as an effective proxy to reveal the flowing directions and temperatures of dolomitizing fluids because oxygen isotopic composition is sensitive to salinity and temperature. Moreover, other trace elements (Fe, Mn, and Sr) and stable/radioactive isotopes (δ¹³C and ⁸⁷Sr/⁸⁶Sr) provide effective tools to trace dolomitizing fluids (Kaufman et al., 1991; Ronchi et al., 2011; Liu et al., 2016, 2017).

The Pingdingshan section, outcropped at South China, highlighted its significance in the following aspects: 1) The western Pingdingshan interval is recognized as a potential profile of the “Global Stratotype Section and Point” recording the Induan-Olenekian boundary. Insightful evidence revealing the paleo-environment of the Permian Ocean is required to validate this boundary and further understand the Permian–Triassic transition (Tong et al., 2003). 2) The Permian Chihsian biomicrites contribute to regional hydrocarbon source rocks and provide potential reservoirs in the upper part of South China. Deciphering dolomite origins and related transporting fluids has significant implications for hydrocarbon evaluation (Liu et al., 2011).

In this article, we provide newly collected core samples from the Chihsia Formation in the Pingdingshan profile, Chaohu area. Dolomite occurrence is the focus in lithologic and mineralogic studies. Coupled with field study and regular petrological investigation, integrated geochemical proxies were presented and analyzed to reveal the dolomite origins of the carbonate successions from the Chihsia Formation. The current study has the significance of elucidating petroleum systems related to the Permian–Triassic transition in the South China.

The Chaohu area is located on the Lower Yangtze Plate tectonically. The deposition of this area was controlled by the Jiangnan Faulting event, developing marine sedimentation from the Sinian to the Triassic with total strata thickness of ∼14,000 m. The Pingdingshan section is outcropped at a syncline structure which was formed due to an ancient continuous folding movement characterized by a northeastern trend. From northeast to southwest, the Yufudacun syncline, the Ma’anshan monocline, the Fenghuangshan anticline, the Pingdingshan syncline, and the Majiashan monocline are sequentially outcropped (Figure 1A; Liu et al., 2022). The Chihsia Formation, characterized by wide occurrence and stable distribution with a thickness of 100∼200 m, is an important regional marine hydrocarbon source rock in South China and the most representative carbonate source rock in the Lower Yangtze region. The Chihsia Formation is partly developed at the Pingdingshan section (thickness of ∼35 m), Chaohu area, disconformably in contact with the underlying Chuanshan Formation and overlying Gufeng Formation, respectively. At the studied interval, the Chihsia Formation can be divided into several lithology sets from bottom to top, that is, the bottom coal seam, the lower-part bituminous carbonates, the middle-part siliceous limestones, the upper-part bioclastic carbonates, and the top dolomitized limestones (Figure 1B). During this geological period, the sedimentary environment experienced a constant transgression of seawater, thus accommodating sediments’ transition from platform of shoal clastic with coal to slope marine carbonates.

Hand specimens were collected from the Chihsia Formation outcropped at the Pingdingshan section of the Chaohu City, Anhui Province. The sampled interval is shown in Figure 1B. Micro-area sampling method was applied using a dental drill (diameter of 20 μm) coupled with microscopic observation to obtain pure dolomite or calcite components. Meanwhile, thin sections without coverslips were prepared for conducting the laser ablation process.

The prepared powder and thin-section samples were sent for mineralogic determination, minor/trace and rare-earth element measurement, fluid-inclusion homogenization thermometer analysis, and oxygen and carbon isotopic measurement. Liquid-vapor fluid-inclusions were observed from double-polished, 0.8-mm–thick thin sections. Ice melting and homogenization temperatures (T_h) were measured on inclusions using the Linkam MDS-600 heating-freezing stage linked with microthermometric-analysis software. The temperature accuracy measured is better than ±1°C. Salinities were calculated from T_h values using the equation NaCl (wt%) = 1.78T_m + 0.0442T_m ² + 0.000557T_m ³ proposed by Hall et al. (1988) in the H₂O–NaCl system. Inclusion populations were described by linking UV fluorescent sources, including their morphologies, distribution, sizes, and presence of hydrocarbons. The specific analyzing procedure was performed referring to Shepherd et al. (1985).

Micro-area element analysis was conducted to determine trace and rare-earth element contents for different types of dolomites and calcite matrices. A traditional acidic dissolving method was applied to obtain element concentrations using the ICP-MS method with Finnigan-Mat Element-3 of radio-frequency power of 1,200 kW and vaporized sample flow velocities of 0.660 L/min. The detection limits range from 0.001 to 1 ppm with respect to different elements, and the accuracy is better than ±5%. The specific workflow was conducted referring to Zhang et al. (2019).

For carbon and oxygen isotope determination, the laser ablation method was applied. The LSX-200 UV laser ablation system was equipped with a NdYAG laser emitter. A laser beam with a wave length of 1,046 was focused on the thin sections in a vacuum environment. The agitated carbon dioxide was purified and imported into an isotope mass spectrometer (Type MAT 252) for δ¹⁸O and δ¹³C testing. The detailed parameter settings and workflow were referred from Liu et al. (2017).

All the above experiments were conducted at the State Key Laboratory of Petroleum Resources and Prospecting in China University of Petroleum, Beijing.

The matrix composition of the sampled interval at the Chihsia Formation is mainly composed of micritic calcites, with crystal sizes commonly less than 20 μm (Figure 2A). Stylolites are developed in micritic calcites (Figure 2B). The dolomites developing in the cored interval can be classified into three categories according to crystal sizes, that is, the micritic to fine-sized dolomites, the sucrosic dolomites of medium- to coarse-sized crystals, and the saddle dolomites. The micritic to fine-sized dolomites are characterized by laminar distribution with crystal sizes commonly smaller than 20 μm (Figure 2C). This type of dolomite usually displays tight lithology. Gypsum and/or halite molds are widely developed in micritic dolomites (Figure 2D), reflecting an evaporative environment. The micritic dolomites are closely associated with matrix calcites and show dark red light under CL observation (cathode-luminescence). This type of dolomite mainly occurs in the upper part of the Chihsia Formation as interbeds of matrix calcite. The sucrosic dolomites are featured by their typical sucrosic texture, developing medium- to coarse-sized crystal sizes (ranging from 50 to 300 μm). The crystal textures are planar-s and planar-e, with the latter being dominant (Figure 2E). Meanwhile, fluid-inclusions aligned to the growth direction are widely developed in crystals (Figure 2F). The saddle dolomites are mainly developed in interparticle or vuggy porosities of the matrix carbonates as cements (Figure 2G), particularly in karst breccias as assemblages (Figure 2H). This type of dolomite is featured by large crystal sizes (i.e., coarse than 500 μm) and curved crystal surfaces. The large crystal is dominantly planar-e type.

According to our study, the following conclusions can be addressed.

1) Sample validity is feasibly examined through the Y/Ho ratio, trace element and carbon-oxygen isotopic correlation, and high field-strength metal contents.