Controlling Factors of Meteoric Diagenesis in Karst Reservoirs: An Example from the Majiagou Formation, Ordos Basin, China

: Karst reservoirs have always been a key ﬁeld of oil and gas exploration. However, quantifying the process of meteoric transformation remains a persistent challenge that limits the accuracy of reservoir quality prediction. To explore the controlling factors of meteoric cementation on karst reservoirs, the Majiagou Formation of the Ordos Basin in China was selected as an example. The petrology; carbon, oxygen, and strontium isotopes; and in situ major, trace, and rare earth elements were used, types and origins of calcite cements were analyzed in detail. The results revealed ﬁve types of calcite cements (Cal-1~Cal-5), four types of cathodoluminescence (CL) intensities (dull, dull red, deep red, and bright red luminescence), and six types of rare earth element patterns (Pattern-1~Pattern-6). These ﬁve types of calcite cements developed in three periods. Cal-1 (transition CL) and Cal-2 (dull CL) were precipitated during the Early Pennsylvanian period, the meteoric freshwater was clean; Cal-3 (transition CL) and Cal-4 (bright red CL) were precipitated at the end of the Late Carboniferous period, the ﬂuids had strong dissolution ability and were polluted by terrigenous debris; Cal-5 (transition CL) was deposited during the burial period, the ﬂuid was pure pore water or groundwater. The control of the cement on the reservoir during the burial period was much weaker than that of meteoric cements. Therefore, explorations of karst reservoirs should be focused on weak cementation during the epigenetic period.


Introduction
Karst reservoirs are a significant focus for oil and gas exploration all over the world, and different degrees of karst transformation in carbonate reservoirs have been discovered [1]. The formation mechanism and transformation process of high-quality karst reservoirs are critical scientific problems of scholarly concern [2,3]. Exploring variables that control meteoric dissolution and cementation difference in karst reservoirs will be conducive to attaining a more comprehensive and profound understanding of these scientific problems.
Statistics indicate that the reservoirs of 20% to 30% of large gas fields are carbonate karst reservoirs related to regional unconformity [4]. Interest has increased in studying karst patterns, such as those in the layered carbonates in the Salitre Formation, São Francisco Craton, Brazil [5][6][7], and their association with Brazilian presalt carbonate reservoirs [8], as well as those from the Paleogene in the southwest of the Zagros Basin [9]. Moreover, karst reservoirs are a key reservoir type in the marine petroliferous basins of China. There are large gas fields related to karst reservoirs that developed in the Ordos Basin during the Middle Ordovician, in the Tarim Basin during the middle-late Cambrian and late Ordovician, and in the Sichuan Basin during the Precambrian and the Carboniferous [10][11][12][13]. Key characteristics of these carbonate karst reservoirs are long development time, multiple karst periods, and complex reservoir evolution, all of which make them difficult to explore [14]. South China blocks/terranes, respectively, modified from ref. [34]; (b) map showing the location of Fuxian area in the Ordos Basin, and the specific core wells locations; (c) stratigraphic, lithology and paleotemperature characteristics from Ordovician to Carboniferous in the study area, sea surface temperature modified from ref. [35].
Affected by the Caledonian movement, the basin was uplifted in the Late Ordovician, and experienced weathering up to 130 Ma; Late Ordovician to Early Carboniferous strata are missing (Figure 1c) [36]. Until the end of the Hercynian movement, under the compression of the North and South troughs, seawater gradually withdrew from the basin area, forming a craton basin that was steep in the south and gentle in the north [37,38].
Carbonate platform with low terrain and uplifted periphery were in the basin from Ma1 to Ma3, but the paleogeomorphology of Ma4 to Ma6 varied greatly because it was affected by the high-frequency variation of the sea level [39,40]. The present study focused on Ma5, which corresponds to the Dapingian-Darriwilian and was mostly deposited in a limited tidal flat environment. Under the influence of high-salinity seawater, the dolomite strata often contain gypsum-salt rocks and/or layers [41]. South China blocks/terranes, respectively, modified from ref. [34]; (b) map showing the location of Fuxian area in the Ordos Basin, and the specific core wells locations; (c) stratigraphic, lithology and paleotemperature characteristics from Ordovician to Carboniferous in the study area, sea surface temperature modified from ref. [35].
Affected by the Caledonian movement, the basin was uplifted in the Late Ordovician, and experienced weathering up to 130 Ma; Late Ordovician to Early Carboniferous strata are missing (Figure 1c) [36]. Until the end of the Hercynian movement, under the compression of the North and South troughs, seawater gradually withdrew from the basin area, forming a craton basin that was steep in the south and gentle in the north [37,38].
Carbonate platform with low terrain and uplifted periphery were in the basin from Ma1 to Ma3, but the paleogeomorphology of Ma4 to Ma6 varied greatly because it was affected by the high-frequency variation of the sea level [39,40]. The present study focused on Ma5, which corresponds to the Dapingian-Darriwilian and was mostly deposited in a limited tidal flat environment. Under the influence of high-salinity seawater, the dolomite strata often contain gypsum-salt rocks and/or layers [41].
The Ordovician was an important geological period for the tectonic climate and biological evolution of the early Paleozoic. In addition, the Late Paleozoic Ice Age (LPIA), which lasted approximately 100 Ma (ca. 360-255 Ma), is a critical turning point in global climate change [42,43]. However, the LPIA was not a long-term continuous glaciation but rather an overall cold glacial event separated by multiple warm events [44]. The precipitation of calcite cement may be the result of multiple warm events in the study area during the Late Carboniferous [45][46][47].

Samples Selected
We investigated the Ma5 member dolomite from some drilling wells (R1, R2, FG3, XF2, XF14, and N3) in remnants, Fuxian area, Ordos Basin (Figure 1b). Cores of these drilling wells included Majiagou Fm. karst reservoirs. A total of 23 samples were polished as thin sections for petrological, mineralogical and morphological observations. Types and morphology of carbonate minerals were observed by an OLYMPUS BX51 microscope. The CL imaging were performed on polished and carbon-coated samples to characterize the morphology of minerals. The imaging was conducted with a Quanta 450 FEG SEM equipped with a SDD Inca X-Max 50 and a MonoCL 4 + detector (20 kV accelerating voltage and 20 nA beam current) at the Beijing Research Institute of Uranium Geology, China.
Four representative thin sections were selected for in situ major, trace, and rare earth element analysis. Powdered samples were selected for geochemical analyses (e.g., δ 13 C, δ 18 O, and 87 Sr/ 86 Sr), using a micro-mill with a drilling bit diameter of 0.25 mm. To distinguish between matrix and cement, a total of 20 samples were collected for C and O isotope analysis, and 16 samples were collected for Sr isotope analysis.

In Situ Elemental Analysis
Concentrations of 57 major, trace, and rare earth elements were determined by performing in situ elemental analysis under room temperature in the Research Branch of Southwest Petroleum University, Key Laboratory of Carbonate Reservoirs, China National Petroleum Corporation (CNPC). The test sample was a special sheet prepared for the laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) experiment (thickness 0.04 mm, size 17 mm × 25 mm). Before the test, ultrasonic waves were applied to the sample for 90 min to remove surface impurities; then, the sample was air-dried in a fume hood. The test instrument was a Newwave 193 nm UC laser connected to an Agilent 7800 ICP-MS mass spectrometer; the laser beam spot was 80 microns in diameter, and the frequency was 10 Hz. The standard sample was USGS MACS-3 (calcium carbonate; United States Geological Survey), the monitoring standard sample is JCp-1. To calibrate and calculate the measurement results, a series of reference materials were analyzed before and after the analytical batch and were also inserted within the sequence every five sample positions to ensure the error was less than 10%.
We normalized the REE data with Post-Archean Australian Shale (PAAS) standard samples. Because the Eu of PAAS has a negative anomaly, a chondrite standard sample was selected to normalize the data to avoid incorrectly evaluating the δEu, and the specific calculation formula can be found in ref. [48]. The concavity index (CI) refers to the concavity of the PAAS-normalized REE patterns, and a CI value of~1 indicates a quasi-linear shalenormalized partitioning pattern. The following equation was used to calculate the CI:

Bulk C, O, and Sr Isotopes
Approximately 10 mg of the rock powder samples was reacted with pure phosphoric acid at 72 • C for 1 h. The released CO 2 was collected and tested for C and O isotope composition in a Mat 253 Mass Spectrometer. The results were calibrated and expressed as Vienna Pee Dee Belemnite VPDB (‰); the accuracy was better than ±0.2‰. For strontium isotopes analysis,~100 mg powder was reacted in a Finnigan Mat Triton Tl with 6 M HCl for 24 h at 100 • C. The tested 87 Sr/ 86 Sr ratios were calibrated with a standard from the National Institute of Standards and technology (NBS-987), which has an 87 Sr/ 86 Sr ratio of 0.710273 ± 0.000012.

Petrological Characteristics
The Majiagou Fm. mainly developed microcrystalline dolomite reservoirs, the matrix was dolomite with a particle size of~5 µm, and the cements were mostly developed in the gypsum mold pores and associated fractures ( Figure 2). The cement mainly consisted of coarse-crystalline calcite, usually larger than 500 µm. Dolomite silt (Dol-1) was observed in pores and fractures (Figure 2a). Calcite-and dolomite-embayed contacts formed by dedolomitization which were common in the matrix (Figure 2b). Various types of calcite cements were observed under CL; the CL intensities were dull, dull red, deep red, and bright red luminescent (Figure 2c-h). Five types of calcite cements were present in the study area; dull red ring-shaped calcite cements with complete crystal structure, mainly developed at the edges of the gypsum mold pores (Cal-1) (Figure 2c (Figure 2g-i). The CL intensity of Cal-5 was between dull red and deep red luminescent, and the homogeneous temperature was >100 • C (Table 1).

Petrological Characteristics
The Majiagou Fm. mainly developed microcrystalline dolomite reservoirs, the matrix was dolomite with a particle size of ~5 μm, and the cements were mostly developed in the gypsum mold pores and associated fractures ( Figure 2). The cement mainly consisted of coarse-crystalline calcite, usually larger than 500 μm. Dolomite silt (Dol-1) was observed in pores and fractures (Figure 2a). Calcite-and dolomite-embayed contacts formed by dedolomitization which were common in the matrix (Figure 2b). Various types of calcite cements were observed under CL; the CL intensities were dull, dull red, deep red, and bright red luminescent (Figure 2c-h). Five types of calcite cements were present in the study area; dull red ring-shaped calcite cements with complete crystal structure, mainly developed at the edges of the gypsum mold pores (Cal-1) (Figure 2c The CL intensity of Cal-5 was between dull red and deep red luminescent, and the homogeneous temperature was >100 °C (Table 1).
The  (Figure 3f). The patterns of the matrix were Pattern-1 and Pattern-2, those of the calcite cements were Pattern-1~Pattern-5, and that of the dolomite silt was Pattern-6. heavy-REE (HREE) depleted and monotonically decreasing ( Figure 3e); Pattern-6 was positive La and Gd anomalies (Figure 3f). The patterns of the matrix were Pattern-1 and Pattern-2, those of the calcite cements were Pattern-1~Pattern-5, and that of the dolomite silt was Pattern-6.

Evidence of Meteoric Diagenesis Transformation
Dedolomitization, the process of converting dolomite to calcite, o netic environment of gypsum-bearing carbonate formations, providing dissolving gypsum minerals from atmospheric freshwater to replace [53,54]. The concave-convex contact between calcite and dolomite in the feature of dedolomitization (Figure 2b), and calcite cement is a key by C and O isotopes of the calcite cements also indicated a significant p pheric freshwater [13]; in addition, there was a positive correlation betw 3), and the δ 18 O showed an obvious negative bias. Moreover, the Sr iso were high (Figure 4), which suggests meteoric cementation or dissolu with similar varying Sr ratios [56]. However, the cores in deeper layers d obvious dissolution phenomenon. The 87 Sr/ 86 Sr ratio of global runoff i similar to our obtained values [57]; therefore, the high Sr isotopic ratio the presence of meteoric cementation.
The homogenization temperature of fluid inclusions is a good me time of cementation [58]. A type of calcite cement filled the fractures, a zation temperature of the fluid inclusions was >100 °C which much hi mation temperature of meteoric calcite, indicated the remaining presenc during the burial period (Cal-5). The CL intensity of Cal-5 was a trans red and deep red luminescent (Figure 2h), and it was difficult to disti solely by lithological characteristics. The elemental compositions of characteristics that were significantly different from those of other typ the elemental concentrations were lower than those in other types of ce ditionally, the REE pattern was Pattern-5 ( Figure 3e). Compared with th

Evidence of Meteoric Diagenesis Transformation
Dedolomitization, the process of converting dolomite to calcite, occurs in the epigenetic environment of gypsum-bearing carbonate formations, providing additional Ca 2+ by dissolving gypsum minerals from atmospheric freshwater to replace Mg 2+ in dolomite [53,54]. The concave-convex contact between calcite and dolomite in the matrix is a critical feature of dedolomitization (Figure 2b), and calcite cement is a key by-product [55]. The C and O isotopes of the calcite cements also indicated a significant presence of atmospheric freshwater [13]; in addition, there was a positive correlation between them (Figure 3), and the δ 18 O showed an obvious negative bias. Moreover, the Sr isotopic ratio values were high (Figure 4), which suggests meteoric cementation or dissolution of older rock with similar varying Sr ratios [56]. However, the cores in deeper layers did not exhibit any obvious dissolution phenomenon. The 87 Sr/ 86 Sr ratio of global runoff is 0.7119, which is similar to our obtained values [57]; therefore, the high Sr isotopic ratios strongly suggest the presence of meteoric cementation.
The homogenization temperature of fluid inclusions is a good method to detect the time of cementation [58]. A type of calcite cement filled the fractures, and the homogenization temperature of the fluid inclusions was >100 • C which much higher than the formation temperature of meteoric calcite, indicated the remaining presence of some cements during the burial period (Cal-5). The CL intensity of Cal-5 was a transition between dull red and deep red luminescent (Figure 2h), and it was difficult to distinguish from Cal-3 solely by lithological characteristics. The elemental compositions of Cal-5 had distinct characteristics that were significantly different from those of other types of cement, and the elemental concentrations were lower than those in other types of cement as well. Additionally, the REE pattern was Pattern-5 ( Figure 3e). Compared with the other calcite cements, Cal-5 had unique characteristics, the lowest CI value, low Fe and Mn contents (Figure 6a-c), and monotonically decreasing REE distribution. In addition, there was no positive Eu anomaly, indicating no effects from potential abnormally high temperatures or hydrothermal fluids [59,60]. Combined the ∑REE content and CI, the Cal-5 should precipitate during the burial period was unaffected by terrigenous debris and/or late diagenesis and that the fluid source was residual pore water or pure groundwater (Figure 6d

Characteristics of Multi-Period Meteoric Calcite Cements
The petrographic and isotopic characteristics described above indicate that four types of calcite cements were of meteoric origin. However, during a long-term weathering process, the properties of meteoric freshwater in an open system can easily be changed by factors such as weathering intensity and sea level. To further explore the origins of the calcite cements, in situ REE and major and trace elemental analysis were used to study the samples in detail [30,31].
According to the development positions and the CL intensity, four types of meteoric calcite cements were present in the study area ( Figure 2). The CL intensity in the weathering crust had regular changes. The O2 contents gradually decrease from aerobic respiration to manganese reduction in a suboxic environment, and the CL intensity changes from dull to bright red and continues to transition to dull again [61]. The CL results for Cal-1 were dull red ring-shaped calcite that was more euhedral and had a more complete crystalline form than the others (Figure 2c,d); therefore, it would have precipitated slowly during the early stage, when the pore space was the greatest. The characteristic REE pattern was Pattern-1, the ∑REE content was low, the δCeSN values were generally >1.0, the CIs were ~3.0, and the δEuCN values were all <1.0. REEs are an effective indicator for recovering paleoenvironments (Figure 6d-f), but they are highly susceptible to terrigenous debris and late diagenesis [62]. In general, the mixing of terrigenous detrital materials leads to a significant increase in the ∑REE content [63,64]. The lower ∑REE of Cal-1 could represent a relatively pure meteoric freshwater environment. In addition, δCeSN and δEuCN can be used to assess the redox status of fluid during precipitation as well as the influence

Characteristics of Multi-Period Meteoric Calcite Cements
The petrographic and isotopic characteristics described above indicate that four types of calcite cements were of meteoric origin. However, during a long-term weathering process, the properties of meteoric freshwater in an open system can easily be changed by factors such as weathering intensity and sea level. To further explore the origins of the calcite cements, in situ REE and major and trace elemental analysis were used to study the samples in detail [30,31].
According to the development positions and the CL intensity, four types of meteoric calcite cements were present in the study area ( Figure 2). The CL intensity in the weathering crust had regular changes. The O 2 contents gradually decrease from aerobic respiration to manganese reduction in a suboxic environment, and the CL intensity changes from dull to bright red and continues to transition to dull again [61]. The CL results for Cal-1 were dull red ring-shaped calcite that was more euhedral and had a more complete crystalline form than the others (Figure 2c,d); therefore, it would have precipitated slowly during the early stage, when the pore space was the greatest. The characteristic REE pattern was Pattern-1, the ∑REE content was low, the δCe SN values were generally >1.0, the CIs werẽ 3.0, and the δEu CN values were all <1.0. REEs are an effective indicator for recovering paleoenvironments (Figure 6d-f), but they are highly susceptible to terrigenous debris and late diagenesis [62]. In general, the mixing of terrigenous detrital materials leads to a significant increase in the ∑REE content [63,64]. The lower ∑REE of Cal-1 could represent a relatively pure meteoric freshwater environment. In addition, δCe SN and δEu CN can be used to assess the redox status of fluid during precipitation as well as the influence of temperature [65]. Cal-1 was formed under oxidizing conditions and were not affected by abnormal thermal events. Elemental features can effectively identify the origin of authigenic minerals: in particular, the composition and distribution pattern of REE can precisely identify the fluid source [62,66]. The characteristics of Cal-1 are a complete euhedral form; a unique ring-shaped CL luminescent; Pattern-1; moderate CI, Fe, Mn, and Ba content; and low ∑REE all suggest that the meteoric fluid may have been polluted by few terrigenous (Figure 6; Table 2) [40,64]. Moreover, the fluid content was low; therefore, precipitation occurred only at the pore edges and not on a large scale.
The characteristics of Cal-2 were similar to those of Cal-1, and the CL was dull luminescent. Cal-2 first developed at the pore edges or as the central fulcrum for subsequent precipitation (Figure 2d,e). The characteristic pattern of Cal-2 was Pattern-3, and most of the δCe SN values were far less than 1.0 ( Figure 6f). Therefore, the diagenetic fluid of Cal-2 would have been similar to that of Cal-1 except that the diagenetic fluid of Cal-2 might have been more biased toward oxidizing conditions. In addition, the REE distribution features were Pattern-3 and partial Pattern-1, and the significant negative Ce anomaly is typical characteristics of meteoric cementation. The low Fe, Mn, Ba, and Al contents, low ∑REE, and low CI value further demonstrate that the source was pure meteoric fluid during this period and that the pollution by terrigenous debris was low (Table 2, Figure 6).
The CL of Cal-3 was a transition luminescence that exhibited changes between dull red and deep red luminescence, and Cal-3 was more extensive than the other types of cement ( Figure 2e). The highest concentrations of Fe, Sr, and Ba were present in Cal-3 and indicated a strong influence of terrigenous debris. Fe, Mn, Ba, Al, and Sr are key elements for identifying the origin of calcite [64]. Fe and Mn in carbonate mostly come from early precipitation or late metasomatism and generally enter the carbonate minerals in the form of divalent ions to replace Ca 2+ at high temperatures and under reducing conditions [13]. However, during the epigenetic and shallow burial period, the environmental conditions are mostly low temperature and oxidizing and are not suitable for Fe 2+ and Mn 2+ to enter the crystal. Therefore, the Fe and Mn contents in the meteoric calcite show a wide range, indicating precipitation form a variety of fluid sources (Figure 6a,b). Owing to long-term weathering, there were clear differences in the contents of terrigenous debris, which would have been carried by different stages of fluids. Notably, Al and Ba mainly from terrigenous sources could be used to characterize the differences in the contents of terrigenous debris (Figure 6c). Cal-3 was widely distributed in the study area; its petrological characteristics were not obvious, but the characteristic REE pattern was Pattern-4 ( Figure 3d), it had high ∑REE contents and the highest CI, and most of the δCe SN values were distributed between 0.9 and 1.0 except for individual positions (Figure 6f). CI can effectively identify fluid sources. Compared with the traditional HREE/LREE versus MREE/MREE* plot [67], CI is better suited for distinguishing potential effects of seawater, pore water, and detrital signals in authigenic minerals [66]. Therefore, Cal-3 was most affected by terrigenous debris, consistent with the trends shown by ∑REE and other elements [66], and the environment may have been suboxic. The reason for the enrichment of MREE in Cal-3 was the enhanced solubility of meteoric fluids caused by weathering [68]. Oxidative weathering of pyrite (common near regolith), other sulfides, and/or siderite often leads to the formation of secondary iron oxides [69], whose modification of REE is focused on the depletion of LREE and on the enrichment of MREE [70].
The CL of Cal-4 was bright red luminescent, its elemental characteristics were similar to those of Cal-3, and the REE pattern was Pattern-1 ( Figure 3a). Therefore, Cal-4 developed in an environment similar to that of Cal-3, which was strongly affected by terrigenous debris. Cal-4 was the last cementation to occur during the epigenetic stage. The high contents of Mn, Fe, and ∑REE indicate that the source was not pure, given that the REE distribution was Pattern-1 (Figure 3a), which is completely consistent with that of the matrix. Therefore, the meteoric fluid would have had strong dissolving capability. During this period, the fluid would have flowed through the matrix and strongly transformed the elemental composition; this is consistent with the observation of similar 87 Sr/ 86 Sr ratios for the matrix and cement ( Figure 5). Weathering caused an increase in the 87 Sr/ 86 Sr ratios [71], and the meteoric fluid that contained weathering information precipitated at the residual positions of the pores and/or cracks to form Cal-4 after modifying the matrix.
The origins of Cal-1 and Cal-2 were similar, and those of Cal-3 and Cal-4 were similar, which suggests that the meteoric calcites were mainly precipitated in two periods in the study area. The initial fluid was clean and was not strongly affected by terrigenous debris. However, the subsequent fluids were mixed with terrigenous debris and had strong dissolution capability. As the matrix was transformed, the REE distribution characteristics were inherited by the freshwater calcite.

Evolution Process of the Karst Reservoir Affected by Meteoric Diagenesis
The paleoenvironment is a significant factor affecting the precipitation of carbonate minerals. When the temperature increases from 5 • C to 70 • C, the precipitation rate of calcite can be increased by 3-4 times [72]. In their experiments, Burton and Walter demonstrated that for both aragonite and calcite, the precipitation rate was greatly increased at 35 • C compared with 5 • C and 17 • C [73]. The calcite cements in the study area were mainly precipitated at 319.0~292.7 Ma [22], which was at the low point of the paleotemperature of the LPIA (Figure 1a). Long-term low temperatures are not favorable for the precipitation of authigenic calcite minerals, but recent studies have determined that the LPIA was not a long-term continuous glacial action but rather was composed of glacial events separated by multiple warm events [44,74]. In the NCC, a set of basin-wide stable bauxite deposited developed during the Late Carboniferous as a result of deposition in a warm and humid climate (Figure 1c). Therefore, at 315 Ma, there would have been a long warm event in the NCC [46]; Davydov and Cózar used C and O isotope excursions to identify a period of global warming (c. 319 Ma) [45]; and a short-term climate warming (c. 304 Ma, Kasimovian period) was identified, in which atmospheric CO 2 partial pressure doubled [47].
Terrigenous debris is a major source of strontium in authigenic carbonate minerals [64]. Because the main period of calcite precipitation in the study area was during the Late Carboniferous, the NCC was within a period of uplift and denudation, and seawater had not yet re-entered the craton basin; therefore, it was difficult for seawater Sr to act as a strontium source in the precipitation of calcite cement. The Sr contents also indicated the weathering intensity and the amount of terrigenous debris, which showed similar characteristics to the contents of other elements. This phenomenon might have been caused by the influence of Benxi Formation bauxite.
Geochemical information indicated meteoric calcite cements precipitated in two periods (Figure 7), which matched with the in situ U-Pb dating reported by Zhou et al. (2022) [22]. The iron in the calcite cement mainly came from the overlying bauxite [13], so the period of Cal-1 and Cal-2 precipitation would have been earlier than that of the Moscovian bauxite and would have occurred within the warming event that occurred at c. 319 Ma. Correspondingly, the Fe and Mn contents of Cal-3 and Cal-4 were higher than in other stages. These large amounts of Fe and Mn suggest that the fluid was more contaminated by terrigenous debris than the other calcites [75], and the terrigenous debris might have originated from bauxite that was deposited simultaneously or earlier. The CL intensity showed transitions and bright red luminescence (Figure 2e), indicating that the fluid environment had changed from the strongly oxidizing conditions of the original meteoric fluid. Regarding manganese reduction, Mn 4+ underwent a reduction reaction and became divalent before it could enter the interior of the calcite lattice [61]. A suboxic environment was favorable for a positive δCe SN (Pattern-2) value and for a CL transition and bright red luminescence. The period of Cal-3 and Cal-4 precipitation would have been within the climate warming event during the Kasimovian (Figure 1c). During the Permian-Triassic, the Ordos Basin experienced multiple thermal events and the geothermal temperature of the Majiagou Fm. gradually evolved to 100 • C (Figure 7). The acidity of the formation fluid may have been weakened because of the transformation of clay minerals, resulting in the precipitation of Cal-5 ( Figure 3e). However, owing to formation fluid low content, the late calcite cement (Cal-5) precipitated during the burial period had a weaker effect on the stimulation of reservoirs and on hydrocarbon accumulation than meteoric calcite. precipitation of Cal-5 ( Figure 3e). However, owing to formation fluid low content, the late calcite cement (Cal-5) precipitated during the burial period had a weaker effect on the stimulation of reservoirs and on hydrocarbon accumulation than meteoric calcite.

Conclusions
(1) The dolomite reservoirs in the Majiagou Formation of the Ordos Basin were formed through a long-term karst process, resulting in high-quality reservoirs. However, the presence of simultaneously precipitated meteoric calcite cement (Cal-1~Cal-4) and burial cementation (Cal-5) has caused reservoir damage.
(2) Cal-1 had a rhombohedral habit, and the REE pattern was flat. Cal-2 had dull luminescence, and had the lowest Fe, Mn, Al, and ∑REE contents; the REE pattern had a negative Ce anomaly. The REE pattern of Cal-3 showed MREE enrichment. Cal-4 had bright red luminescence and the highest contents of Fe, Mn, Al, and ∑REE. In addition, the REE pattern of Cal-5 decreased monotonically.
(4) The cementation by meteoric calcite (Cal-1~Cal-4) occurred prior to the oil and gas charging period, causing significant damage to the karst reservoirs compared to Cal-5. Among them, Cal-3 had the most detrimental impact. Therefore, for oil and gas exploration and development in the study region, areas with higher paleogeographical locations during the critical period (Late Carboniferous) should be prioritized.

Conclusions
(1) The dolomite reservoirs in the Majiagou Formation of the Ordos Basin were formed through a long-term karst process, resulting in high-quality reservoirs. However, the presence of simultaneously precipitated meteoric calcite cement (Cal-1~Cal-4) and burial cementation (Cal-5) has caused reservoir damage. (2) Cal-1 had a rhombohedral habit, and the REE pattern was flat. Cal-2 had dull luminescence, and had the lowest Fe, Mn, Al, and ∑REE contents; the REE pattern had a negative Ce anomaly. The REE pattern of Cal-3 showed MREE enrichment. Cal-4 had bright red luminescence and the highest contents of Fe, Mn, Al, and ∑REE. In addition, the REE pattern of Cal-5 decreased monotonically. (3) Cal-1 and Cal-2 formed during the Bashkirian period (c. 319 Ma), while Cal-3 and Cal-4 precipitated during the Kasimovian period (c. 304 Ma) after bauxite precipitation. These meteoric cementation events were influenced by specific geological conditions. (4) The cementation by meteoric calcite (Cal-1~Cal-4) occurred prior to the oil and gas charging period, causing significant damage to the karst reservoirs compared to Cal-5. Among them, Cal-3 had the most detrimental impact. Therefore, for oil and gas exploration and development in the study region, areas with higher paleogeographical locations during the critical period (Late Carboniferous) should be prioritized.