The Genetic Mechanism and Geological Significance of Calcite in Buried-Hill Karstic Reservoirs: A Case Study of the Lower Paleozoic Carbonate Reservoirs in the Bohai Sea

Abstract

Calcite in hydrocarbon reservoirs records abundant information about diagenetic fluids and environments. Understanding the formation mechanisms of calcite is crucial for predicting reservoir characteristics and hydrocarbon migration. This study identifies the types of authigenic calcite present in the Lower Paleozoic carbonate reservoirs of the Bohai Bay Basin through petrographic analysis, cathodoluminescence, and other experimental methods. By integrating electron probe microanalysis, in situ isotopic analysis, and fluid inclusion studies, we further constrain the source of the diagenetic fluids responsible for the authigenic calcite. The results show that there are at least three types of authigenic calcite in the Lower Paleozoic carbonate reservoirs of the Bohai Sea. Calcite cemented in the syn-depositional-to-early-diagenetic stage displays very weak cathodoluminescence, with δ¹³C and δ¹⁸O and paleo-salinity distributions similar to those of micritic calcite. These features suggest that the calcite was formed during burial heating by sedimentary fluids. Calcite filling fractures shows heterogeneous cathodoluminescence intensity, ranging from weak to strong, indicating multiple stages of cementation. The broad elemental variation and multiple cementation events suggest that the diagenetic fluid sources were diverse. Isotopic data show that samples with carbon isotope values greater than −2.9‰ likely formed through water–rock interaction with fluids retained within the strata, whereas samples exhibiting more negative δ¹³C were formed from a mixed-source supply of strata and mantle-derived fluids. Calcite that fills karst collapse pores exhibits alternating bright and dark cathodoluminescence, strong negative δ¹⁸O shifts, and variability in trace elements such as Mn, Fe, and Co. These characteristics indicate a mixed origin of diagenetic fluids derived from both meteoric freshwater and carbonate-dissolving fluids.

1. Introduction

The Lower Paleozoic of the Bohai Bay Basin constitutes a key target for hydrocarbon exploration in China [1,2]. Karst reservoirs are widely developed in this interval, and authigenic calcite is one of the most representative diagenetic minerals within these reservoirs [3]. As the most common mineral in the Earth’s crust, calcite plays a crucial role in reservoir prediction due to its properties. For instance, its high solubility promotes the development of reservoirs with enhanced porosity and permeability [4], while the temperature, salinity, and other parameters of diagenetic fluid recorded in calcite provide valuable insights into the timing, nature, and evolution of fluid activity [5,6]. Previous studies have shown that calcite in the Lower Paleozoic karst reservoirs of the Bohai Bay Basin formed in at least three distinct phases [7,8]. However, research on the origins of these calcites remains limited. Most existing studies have focused primarily on hydrothermal calcite, with systematic investigations of other genesis types still at an early stage [9,10,11]. The Lower Paleozoic in this region has undergone multiple phases of complex uplift-subsidence tectonics and hydrocarbon charging events [12,13]. As a result, calcite preserved in pores and fractures records abundant information on diagenetic fluid activity, hydrocarbon migration, and tectonic evolution. Therefore, advancing our understanding of the genesis of authigenic calcite is of significant scientific value for analyzing reservoir characteristics, interpreting tectonic evolution, and reconstructing hydrocarbon migration patterns in the Lower Paleozoic of the Bohai Bay Basin.

In reservoir studies, carbon and oxygen isotope (δ¹³C and δ¹⁸O) analyses are essential for elucidating the formation mechanisms of carbonate cements. This approach not only effectively distinguishes calcite of different genesis but also provides key insights into diagenetic temperature, fluid salinity, fluid sources, and diagenetic processes [14,15,16,17,18]. As such, δ¹³C and δ¹⁸O analyses hold significant scientific and practical value in deciphering the formation mechanisms of calcite [19,20]. However, in practice applications, isotopic analyses can be influenced by multiple factors, such as the mixing of fluids from different sources and temperature-related isotope fractionation, resulting in data complexity and interpretative uncertainty [21,22]. Therefore, to improve the reliability of isotopic interpretations, it is often necessary to integrate multiple geochemical approaches, particularly elemental analyses and fluid inclusion studies [23]. The combined application of these methods allows for a more comprehensive understanding of calcite formation mechanisms and the diagenetic environmental conditions that they reflect.

Based on petrographic observations combined with isotopic and various chemical analytical techniques, this study systematically investigates the characteristics of authigenic calcite in the Lower Paleozoic reservoirs of the Bozhong Sag in the Bohai Sea. The objectives are as follows: (1) to summarize the geological significance of δ¹³C and δ¹⁸O in diagenesis and to explore the material sources of the authigenic calcite and the conditions of the diagenetic fluids and (2) to preliminarily establish the genesis types of authigenic calcite in the Lower Paleozoic karst reservoirs of the Bohai Sea region, which will provide a scientific foundation for further research on the genesis and geological significance of calcite in the Lower Paleozoic of the Bohai Bay Basin.

2. Geological Background

The Bozhong Sag, located at the core of the Bohai Sea structural unit, is the deepest depression in the Bohai Bay Basin [24], covering an exploration area of 8634 km² (Figure 1). Tectonic evolution studies reveal that since the Late Ordovician Caledonian orogeny, the region has successively experienced several tectonic superposition, including the Hercynian, Indosinian, Yanshanian, and Himalayan [13]. Repeated tectonic uplifts have kept the North China Craton in a prolonged state of continental erosion, with the most intense erosion occurring during the Mesozoic Yanshanian movement.

This coupled tectonic–sedimentary regime strongly influenced the Lower Paleozoic carbonate strata, which underwent intense surficial karstification, forming a regionally developed paleoweathering crust-type karst reservoir system [3,11,25]. Today, only the Cambrian and Lower Ordovician strata are preserved in the study area. The Cambrian unconformably overlies the pre-Cambrian metamorphic basement, while the Paleogene directly unconformably overlies the Ordovician (Figure 1).

As an important hydrocarbon-rich sag in the Bohai Bay Basin, the Bozhong sag represents a typical epicontinental carbonate platform depositional system in the Lower Paleozoic [26,27]. This unit comprises several sedimentary subfacies, including micritic limestone from tidal flat and lagoon, dolomicrite from restricted platform, oolitic/bioclastic grainstone from open platform, and sparry grainstone from platform margins [3].

Diagenesis studies reveal that the study area experienced a complex sequence of diagenetic events. During the syndiagenetic stage, meteoric freshwater dissolution occurred. During the early diagenetic stage, dolomitization improved the matrix porosity. During the mesodiagenetic stage, tectonic activity generated extensive fractures. During the epidiagenetic stage, meteoric water leaching led to the development of karstic reservoirs. In the late diagenetic stage, silicification and calcite cementation significantly reduced porosity. Notably, multiple episodes of calcite cementation—including isopachous rim cementation, syntaxial cementation, and vein-like filling—are primary contributors to reservoir quality degradation. Importantly, tectonic reactivation during the Himalayan phase generated micro-fractures that served as pathways for late-stage acidic fluids, which played a significant constructive role in the development of secondary dissolution porosity [3,25,28].

3. Methods

In this study, over 150 samples from eight wells were selected and prepared as thin sections (one-third stained, polished, and uncovered). Petrographic observation under a microscope was carried out to analyze the various types and diagenetic stages of calcite cementation. Based on these observations, more than 20 samples were chosen for geochemical analysis, these samples include micritic calcite as well as authigenic calcite of different cementation types. The experimental work included isotopic analysis, fluid inclusion studies, cathodoluminescence (CL), and electron probe microanalysis (EPMA).

Specifically, the selected samples were ground into 0.05 mm-thick slices and observed using a ZEISS Axio Imager, M2m polarizing microscope. Target areas were marked and pre-screened under CL before isotopic and EPMA testing. All experiments were conducted at the CNOOC Experimental Center (Tianjin, China).

In situ carbon and oxygen isotope analyses were performed using a Teledyne CETAC Fusions CO₂ laser sampling system (Teledyne CETAC, Omaha, NE, USA) coupled with a Thermo Scientific DELTA V isotope ratio mass spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, USA). The results, calibrated to the PDB standard, had a precision better than 0.1‰ and a spatial resolution of 100–150 µm. Fluid inclusion analyses employed a Linkam-TH600 heating/cooling stage and a ZEISS Axio Scope, A1 microscope (Carl Zeiss AG, Oberkochen, Baden-Württemberg, Germany), with a temperature accuracy of ±1 °C. CL observations were conducted using a RELION CL VI system (RELION INDUSTRIES, Bedford, MA, USA) with a ZEISS Axio Imager A2 microscope (Carl Zeiss AG, Oberkochen, Baden-Württemberg, Germany). EPMA was carried out using a Shimadzu EPMA-1720 (Shimadzu Corporation, Kyoto Prefecture, Japan), offering 1 µm spatial resolution, ~0.01% detection limits, and relative errors of 0.1%–1% for major elements. Laboratory conditions were maintained at ~26 °C and 40%–50% humidity.

δ¹³C and δ¹⁸O provide key insights into diagenetic processes. The Z value (Equation (1)), proposed by Keith and Weber [29], and was used to infer the relative salinity of diagenetic fluid. The paleotemperature of carbonate formation was estimated using Equation (2), developed by Friedman and O’Neil [30], based on oxygen isotope geothermometer. These parameters were used to reconstruct the temperature and salinity conditions during calcite cementation.

Z = 2.048 × (δ¹³C_PDB + 50) + 0.498 × (δ¹⁸O_PDB + 50)

(1)

1000 × lnα_{(calcite–water)} = 2.78 × 10⁶ × T⁻² − 2.89

(2)

• α_{(calcite–water)} = (1 + δ¹⁸O_calcite/1000)/(1 + δ¹⁸O_water/1000): isotope fractionation factor;
• T: Temperature (°C) of the water during carbonate precipitation;
• δ¹⁸O_calcite: in situ laser isotope, PDB standard, ‰;
• δ¹⁸O_water: oxygen isotope of diagenetic fluid, PDB standard, ‰.

4. Results

4.1. Calcite Occurrence Characteristics

The study area has undergone both karstification and burial diagenesis, resulting in a multi-phase, multi-genetic calcite cementation system. Based on petrographic and CL analysis and considering cement occurrence, crystal morphology, and CL response, three genesis types of calcite cement are identified.

4.1.1. Calcite (C1) in Primary or Early Dissolution Pores

C1 occurs primarily as sparry calcite filling primary intergranular and early dissolution pores. It commonly exhibits fibrous, platy, and coating-like crystal habits (Figure 2a–c). The calcite is often cross-cut by later fractures, indicating early diagenetic timing. CL reveals non-luminescence to faint luminescence, consistent with high Fe²⁺ and low Mn²⁺ content, suggesting precipitation in a phreatic environment under near-surface conditions.

4.1.2. Calcite (C2) Characterized by Fracture Fillings

C2 is occurred as a fracture-filling mineral, with cross-cutting relationships indicating at least three distinct cementation phases (Figure 2f). Early-stage fracture fills display little to no CL (Figure 2d,e), with the fracture walls showing evidence of dissolution and being commonly cut by subsequent fractures. Later-stage fractures are straighter and show clear tectonic signatures. Due to repeated tectonic activity, fractures intersect each other, and the calcite exhibits a wide range of CL intensities—from weak to strong (Figure 2d–g)—reflecting multiple fracturing and cementation events.

4.1.3. Karst Collapse Cemented Calcite (C3)

C3 associated with karst collapse is found in reticulate fractures or interclast pores among angular clasts, often showing striated fracture textures (Figure 2h). This features indicates formation in karst environments. C3 exhibits moderate to strong alternating bright and dark or banded CL intensities (Figure 2i), which correspond to fluctuations in oxidation–reduction conditions caused by periodic exposure.

4.2. δ¹³C and δ¹⁸O Distribution Characteristics

The δ¹³C of calcite range from −6.809‰ to 0.646‰, while δ¹⁸O vary from −28.787‰ to −7.071‰ (Figure 3). In situ isotopic analyses were performed on micritic calcite and the three identified calcite types (C1–C3). As an original sedimentary component, micritic calcite shows δ¹³C values between −1.99‰ and 0.02‰ and δ¹⁸O from −9.82‰ to −7.07‰, closely approximating the marine carbonate background (δ¹³C_PDB ~0‰). These values indicate deposition under normal marine conditions. C1 exhibits δ¹³C values from −1.4‰ to 0.187‰ and δ¹⁸O from −11.32‰ to −8.37‰. C2 ranges more widely, with δ¹³C between −6.809‰ and 0.646‰ and δ¹⁸O from −15.62‰ to −9.84‰. The broader δ¹³C range and localized negative excursions suggest varied fluid sources and multi-phase cementation events. C3 shows δ¹³C from −4.129‰ to −0.167‰ and δ¹⁸O from −28.787‰ to −14.348‰, with the pronounced oxygen isotope depletion indicative of meteoric water influence during karst-related diagenesis.

4.3. EPMA Elemental Distribution Characteristics

EPMA was conducted on micritic and authigenic calcites with distinct CL and occurrence features (Figure 4). While C1, C2, and C3 share overall elemental trends, notable differences exist (Figure 5). Compared to micritic calcite, authigenic calcite exhibits lower SiO₂ concentrations. C1 shows reduced BaO concentrations, while C2 and C3 display broader elemental variability, particularly elevated MnO and FeO levels. These differences suggest that during formation, the authigenic calcite may have undergone different diagenetic fluid interactions, reflecting a complex diagenetic environment and fluid evolution process.

5. Source of Fluids Forming Calcite Veins

5.1. Applicability of Geochemical Data

The δ¹³C and δ¹⁸O are influenced by variations in both the depositional environment and diagenetic processes. Micritic calcite, serving as the original sedimentary component, is theoretically the most strongly affected by external diagenetic conditions. To assess the extent of post-depositional alteration and ensure the reliability of the isotopic measurements, a comparative isotopic analysis of micritic calcite was performed.

The isotopic composition of micritic calcite in the study area generally aligns with that of global Cambrian–early Ordovician bioclasts, with δ¹⁸O primarily between −7‰ and −10‰ and δ¹³C mainly between −2‰ and 0‰. These values are also consistent with the carbon and oxygen isotopic characteristics of Cambrian–Ordovician limestone samples from the Bohai Bay Basin [31,32]. Additionally, the isotopic Z-value of the micritic calcite range from 119.08 to 122.76, with an average of 121.21, further supporting deposition in a marine environment. Importantly, δ¹³C and δ¹⁸O show no significant covariation (Figure 3), and δ¹⁸O are all greater than −10‰, which further confirms the stability of the diagenetic environment. Overall, the isotopic data from the selected samples effectively represent the diagenetic conditions, providing a reliable basis for investigating diagenetic processes and fluid evolution.

5.2. Diagenetic Fluid Characteristics of C1

The EPMA element distribution pattern of C1 is similar to that of micritic calcite but with a significant decrease of BaO. The comparative analysis of ionic radii indicates that Ba cannot be easily substituted for Ca or Mg in carbonate minerals due to its relatively large radius. In micritic calcite, Ba primarily exists in an adsorbed state within the sediment; thus, during the carbonate cementation—when external fluid sources are absent—the BaO content inevitably decreases.

The isotopic analysis reveals that the δ¹³C and δ¹⁸O of C1 clearly overlap with the distribution range of calcite formed in the syngenetic diagenetic environment (Figure 3). Its Z value (ranging from 119.33 to 123.00, with an average of 121.19) is comparable to that of micritic calcite. This further confirms that C1 formed in a hypersaline environment and is highly correlated with the syngenetic diagenetic setting. Using an estimated δ¹⁸O_SMOW of −1.4‰ for Ordovician seawater as the diagenetic fluid [33], the calculated formation temperature of C1 ranges from 52 to 72 °C, consistent with cementation during the syngenetic-to-early-burial-diagenetic stage (typically <85 °C).

5.3. Diagenetic Fluid Characteristics of C2

Compared to the elemental distribution of C1, C2 exhibits a broader SrO range and elevated BaO content. The increased Ba concentration and high ω(Mn)/ω(Sr) ratios (average 3.797; range: 0.092–26.560) suggest involvement of non-marine or hydrothermal fluids [11,34]. While, the elemental distribution pattern of C2 shows some similarity to that of micritic calcite and C1, reflecting the important role of sedimentary fluids during its formation, C2 also exhibits a relatively wide variation in δ¹³C and δ¹⁸O, a feature that further supports the multi-source origins of the diagenetic fluids.

Veizer et al. reported that the carbon isotopes of Cambrian–Ordovician carbonate sediments are greater than −2.9‰ [31]. In this study, the samples with δ¹³C greater than −2.9‰ have Z values ranging from 115.60 to 123.63, with an average of 118.11, while those with δ¹³C_PDB less than −2.9‰ have Z values ranging from 106.98 to 113.24, with an average of 110.49. This significant difference in Z values indicates that the diagenetic process of C2 involved fluids of varying salinities, further revealing the multi-source origins of the diagenetic fluids.

For samples with δ¹³C greater than −2.9‰, a significant positive correlation between δ¹³C and δ¹⁸O is observed (Figure 6), with a correlation coefficient of approximately 0.55. This manifests as a covariance line with synchronous negative shifts. Such a pattern reflects the diagenetic changes occurring during the burial-heating process, largely because oxygen isotope exchange reactions are highly sensitive to temperature—resulting in a typically wider range of oxygen isotope variation compared to carbon isotopes. The carbon isotope values range from −2.55‰ to 0.646‰, with an average of −1.474‰, falling within the expected range for calcite under a Lower Paleozoic sedimentary background. Based on fluid inclusion microthermometry (Figure 7), the diagenetic temperatures associated with burial heating are estimated to range from 70 °C to 150 °C. Using the corresponding δ¹⁸O (−15.62‰ to −9.84‰), the calculated δ¹³O_SMOW of the diagenetic fluid ranges from −0.21‰ to 2.02‰. This range aligns well with δ¹³O_SMOW (approximately −5‰ to 4‰) observed in formations from China’s major oil and gas basins that are effectively sealed and remain unaffected by external fluids [35]. In summary, the carbon isotopic characteristics of calcite with δ¹³C greater than −2.9‰ indicate that the carbon source is derived from sedimentary fluids entrapped within the reservoir. This further demonstrates the fluid isolation during burial diagenesis and its controlling effect on diagenetic evolution.

For samples exhibiting a negative carbon isotope shift (δ¹³C_PDB < −2.9‰), the carbon isotopic signature serves as a key indicator of the diagenetic fluids. In this study, we trace the fluid isotopic trends resulting from the mixing of carbon sources with depositional-burial fluids. In nature systems, the isotopic range of carbon in carbon reservoirs is relatively fixed, representative carbon sources for negatively shifted carbon isotopes mainly include atmospheric freshwater, organically derived CO₂ (e.g., from organic matter or thermally induced decarboxylation), and inorganically derived CO₂ (e.g., mantle-derived CO₂) [36,37].

In deeply buried hydrocarbon reservoirs, CO₂ produced by the thermal decarboxylation of organic matter and mantle-derived CO₂ are considered the primary contributors for calcite formation [14]. In the Bohai Bay Basin, the δ¹³C of mantle-derived CO₂ are generally between −5.9‰ and −3.2‰ [38,39], closely matching the δ¹³C observed in these calcite samples (ranging from −6.809‰ to −3.297‰, with an average of −4.746‰). When integrated with temperature data from fluid inclusions (Figure 7), this isotopic signature suggests that calcite affected by mantle-derived fluids formed under deep burial conditions at temperatures ranging from approximately 150 °C to 212 °C. Based on the δ¹⁸O distribution of these calcites (−15.387‰ to −12.807‰), the deduced diagenetic fluid δ¹⁸O_SMOW is estimated to range from 4.884‰ to 5.997‰. These values are consistent with those of carbonates formed in magmatic environments (5.5‰ to 8.5‰) [40]. Collectively, these findings indicate that the formation of calcite with strongly negative δ¹³C values in the study area was significantly influenced by mantle-derived fluids. This highlights the critical role of deep-sourced materials in driving diagenetic processes during advanced burial stages.

5.4. Diagenetic Fluid Characteristics of C3

C3 displays a marked negative oxygen isotopic shift, which provides critical constraints on the nature of the diagenetic fluids. Typically, the oxygen isotopic compositions of atmospheric freshwater and magma sources show lighter δ¹⁸O_SMOW (with atmospheric precipitation ranging from −55 to +10‰ and magma sources from +5.5 to +8.5‰). However, the minimum δ¹⁸O_PDB of C3 is −28.787, which is notably lower than that of magma sources. In domestic karst reservoirs, carbonates formed by freshwater leaching generally exhibit lower δ¹⁸O_PDB [15,41]. Furthermore, the Z values for C3 range from 110.42 to 116.03, with an average of 113.50, indicating that the diagenetic fluid during C3 formation had a relatively low mineralization. It is therefore inferred that the ion source for C3 formation was supplied by atmospheric freshwater.

Geochemical data reveal that relative to the elemental partitioning pattern of micritic calcite (Figure 8), C3 exhibits a pronounced enrichment in Mn and Fe, accompanied by a depletion in Mg and Si. In natural waters, silicon is primarily present as undissociated siloxane molecules, while Mg²⁺ is prone to isomorphic substitution with Fe²⁺ and Mn²⁺. Moreover, compared with modern seawater, continental freshwater contains relatively lower concentrations of Mg and Sr but higher levels of Mn and Fe [42,43]. In the study area, the MnO content in C3 ranges from 0.001% to 0.558% (averaging 0.107%), and the FeO ranges from 0.007% to 0.358% (averaging 0.131%); both are higher than the values found in the sedimentary background micritic calcite (0.013% and 0.026% for MnO and FeO, respectively). The CL characteristics of the minerals provide insight into the geochemical nature of the diagenetic fluids. C3 displays intergrown, intercalated, or ring-banded cementation with bright red and very bright orange luminescent phases. This multiphase luminescence directly reflects the spatial segregation of trace elements—particularly Mn²⁺ (a luminescence activator) and Fe²⁺ (a quencher)—in the diagenetic fluids, indicating the mixing of multiple fluid sources.

The concentrations of Mn and Co vary in response to redox conditions. Under reducing conditions, the solubility of Mn increases, whereas Co is more readily transported into sediments. Therefore, low Co/Mn ratios indicate an oxidizing environment and can be used as an indirect proxy for the influence of meteoric water. In normal marine settings, the dissolved Co/Mn ratio is approximately 0.187 [44]. If diagenesis is affected by oxidative conditions, this ratio will decrease. In the C3 samples, the Co/Mn ratio ranges from 0.029 to 9.343. The lower values clearly reflect oxidation of the cement, which indirectly indicates the influence of meteoric karst water.

Compared to seawater, river water—representing meteoric karst water—contains significantly higher concentrations of dissolved Mn. For example, the Mn concentration in seawater is about 0.182 nmol/kg, whereas in river water it reaches approximately 149 nmol/kg [44]. As such, Mn concentrations can serve as an indicator of meteoric water input. With increasing proportions of freshwater in mixed-source fluids, the mass fraction of MnO increases, while the Co/Mn ratio decreases accordingly. These geochemical characteristics suggest that the diagenetic fluids in the C3 samples were derived from surface karst water—a mixture of meteoric water and calcium carbonate dissolution water.

6. Origin and Geological Significance of Calcite

Based on the above analysis, the diagenetic characteristics of different types of calcite in the study area have been summarized (

Table 1. Diagenetic indicators of different types of calcite.

Diagenetic Stage	Syngenetic to Early Diagenesis	Epidiagenetic	Mesogenetic to Telogenetic
Fluid Sources	Seawater, Pore Fluids	Karst Water	Pore Fluids	Hydrothermal Fluids
Geochemical Parameters	The mass fraction of BaO exhibiting a pronounced decrease (compared to micritic calcite); δ13C: −1.40~0.19; δ18O: −11.32~−8.37.	The concentrations of Na, Mn, and Fe ions increase, while those of Mg and Si ions decrease (compared to micritic calcite); δ13C: −4.13~−0.17; δ18O: −28.79~−14.35.	The EPMA elemental distribution pattern is similar to that of micritic calcite; δ13C: −2.55~0.646; δ18O: −15.62~−9.84.	Higher Ba ion concentrations (compared to C1); ω(Mn)/ω(Sr) > 2; δ13C: −6.81~−3.30; δ18O: −15.39~−12.81.
Diagenetic phenomena	In primary intergranular pores, C1 occurs as fibrous, platy, or coating, and under cathodoluminescence it either shows no luminescence or only extremely weak luminescence.	C3 that fills reticulate fractures or polygonal (turtle-cracked) fractures formed by karst collapse; and displays alternating bright and dark or banded medium-to-strong cathodoluminescence.	Since the Cenozoic, under the influence of multiple phases of tectonic activity, intersecting fractures and intense water–rock interactions have led to several episodes of calcite cementation. In particular, since the Pliocene, deep major fractures have connected mantle-derived inorganic CO2 with diagenetic fluids, resulting in the formation of hydrothermal calcite.

). During the syngenetic-to-early-diagenetic stage, the pore water expelled by mechanical compaction, similar to sedimentary fluids, is a high-salinity solution that provides the material foundation for calcium carbonate precipitation. The solubility of calcium carbonate decreases with increasing temperature, which leads to the cementation of C1 during burial and temperature increase.

Following the deposition of Lower Paleozoic carbonates, the Bohai Bay Basin underwent multiple stages of tectonic uplift, particularly during the Yanshanian orogeny, which led to intense karstification and the development of karst-related pores and fractures. Surface karst water, composed of meteoric water and calcium carbonate dissolution water, infiltrated deep into the ancient karst layers, reaching the phreatic zones, and even deeper. These phreatic zones are located in the groundwater’s saturation zone, where CO₂ saturation led to the cementation of C3 [45].

During the Himalayan tectonic period, the basin underwent multiple stages of rifting, subsidence, and strike-slip movements, which were accompanied by prolific fracture development. The Shahejie and Dongying formations in the BoZhong sag developed high-quality hydrocarbon source rocks, with the hydrocarbon generation threshold around 3100 m [46]. Under the conditions of moderate to deep burial, the source rocks underwent intense water–rock reactions due to acid generation, and diagenetic fluids migrated along dominant pathways, forming C2 cement filling fractures.

Since the Pliocene, deep large faults have connected mantle-derived inorganic CO₂ with diagenetic fluids, providing the ionic source for the formation of hydrothermal calcite.

In the Bohai Sea area, the primary fractures and pores of the Lower Paleozoic carbonate reservoirs are commonly filled by early calcite due to the influence of pore water and burial heating and have generally lost their porosity. The formation of C3 confirms the development of karst reservoirs in buried hills. The intense karstification in the study area led to the formation of developed karst-type reservoirs, which are often characterized by accelerated drilling rates, enlargement of the wellbore, and mud loss during geological drilling near unconformities. Additionally, the physical properties show a decreasing trend in porosity with increasing depth (Figure 9). Under the microscope, the development of networked fractures confirms that karstification plays a significant role in the formation of buried-hill karst-type reservoirs (Figure 9). The development of C2 indicates that tectonic activity since the Cenozoic has created multiple generations of intersecting fractures. Data statistics reveal anomalously high permeability and porosity values in deep strata (Figure 9). Reservoir profiles in the BZ28-1 area show that faults cut through deep strata, creating developed fractured reservoir spaces [47]. In the case of the ancient buried-hill oil and gas reservoir in Bohai Zhong 21–22 [39], hydrocarbons and mantle-derived gas have been injected, suggesting that after fractures were filled by C2, effective pore spaces still developed, providing favorable pathways for the migration and accumulation of oil, gas, and mantle-derived fluids.

7. Conclusions

C1 formed during the syn-depositional-to-early-diagenetic stage, under the influence of pore fluid and burial-induced temperature increase, resulting in the formation of sparry calcite that filled both primary intergranular pores and early diagenetic dissolution pores.

The cementation of C2 is controlled by multiple fluid sources. For calcite with carbon isotope values greater than −2.9‰, its formation mechanism is similar to that of C1, where diagenetic ions sequestered in the strata are released into pores and fractures under the influence of burial heating and related environmental changes, precipitating as calcite cement. In contrast, for calcite with carbon isotope values lower than −2.9‰, the supply of mantle-derived fluids, together with the mixing of formation-trapped fluids, provides the necessary material for calcite cementation.

During its geological history, the study area underwent intense surficial karstification, during which the mixing of meteoric freshwater with ions released from karst processes supplied the material for C3 cementation.

The development of C2 and C3 in the study area demonstrates that the formation of reservoirs is influenced by tectonic fracturing and karstification. Karstification has created significant karstic reservoir spaces near unconformity surfaces, which serve as the primary reservoirs. Tectonic activity has formed fractures that provide essential storage space for the formation of inner reservoirs.