Characteristics and the formation mechanism of the dolomite reservoirs for Lower Ordovician Majiagou Formation, central Ordos Basin, China

The distribution of anhydrite contributes to the formation of high-quality dolomite reservoirs. However, the lower part of the Lower Ordovician Majiagou Formation in the central Ordos Basin shows significant natural gas potential despite in anhydrite-depleted settings. Therefore, its formation mechanism is crucial for carbonate hydrocarbon exploration in evaporite-depleted regions globally. Through integrated multidisciplinary analysis (petrography, cathodoluminescence (CL), X-ray diffraction (XRD) and carbon (C)-oxygen (O)-strontium (Sr) isotopes), the sedimentary-diagenetic differentiation mechanism of dolomite reservoirs in the study area was revealed. Two types of dolomite mainly develop in the study area. (Very) finely dolomite (Md1) is composed by micritic or microcrystalline (<20 μm), euhedral to subhedral dolomite crystals with laminated structure observed, the CL shows a very dull or dark red in color, low order degree (0.65 on average), a positive δ¹³C shift (+0.83‰ on average), high Sr isotope ratio (⁸⁷Sr/⁸⁶Sr = 0.70967 on average) and paleosalinity Z value (125.9747 on average). Fine-medium crystalline dolomite (Md2) is composed by finely or medium crystalline (20-60 μm), euhedral to subhedral dolomite crystals, the dark red or orange in color by CL analysis, high order degree (0.85 on average), a negative δ¹⁸O shift (−7.12‰ on average), low Sr isotope ratio (⁸⁷Sr/⁸⁶Sr = 0.70946 on average) and paleosalinity Z value (122.8781 on average). It is indicated that Md1 formed in a restricted platform tidal flat environment with weak hydrodynamic conditions, during the syngenetic-quasi syngenetic stages through the seepage reflux of high-salinity seawater in an open, low-oxidation, low-temperature environment. Md2 formed through the superimposed transformation of the original tidal flat shoal during the shallow burial stage, driven by the reflux of reducing seawater in a closed, low-reduction, higher-temperature environment. In addition, the atmospheric freshwater dissolution and the enhanced euhedral growth of dolomite crystals during the burial stage promote the development of high-quality reservoirs. This study provides novel geochemical and sedimentary insights for predicting dolomite reservoirs in anhydrite-depleted settings, aiding global hydrocarbon exploration in similar basins.

1 Introduction

The genesis of dolomite reservoirs has consistently been a topical issue. The formation of large-scale dolomite reservoirs is controlled by dolomitization models, which is closely related to sedimentary environment, the origin of Mg²⁺, hydrodynamic mechanism, etc (Jones and Xiao, 2005; Warren, 2000). In recent years, various models for dolomitization have been proposed to account for the ubiquitous occurrences of dolomite, such as evaporative, seepage–reflux, mixing zone, burial, hydrothermal, and microbial mechanisms (Hsü and Schneider, 1973; Garven et al., 1999; Davies and Smith, 2006; Bontognali et al., 2012; Özyurt et al., 2023; Özyurt and Kırmacı, 2024; Rahim et al., 2022; Kırmacı et al., 2018), making the development of new basis for the prediction of effective reservoir distribution.

Ordos Basin is the second largest gas producing basin in China, and the middle part of the basin develops the carbonate gas reservoir of the Middle Ordovician Majiagou Formation, covering an area of about 4.8 × 10⁴km² (Shi et al., 2009). Majiagou Formation is a set of carbonate-dominated strata containing evaporite, which can be divided into six sections from bottom to top. Among them, the fifth member of Majiagou Formation can be divided into 10 sub-members from top to bottom (Yu et al., 2012; Zhu et al., 2014). Since the end of the 20th century, the Jingbian gas field with high production has been discovered in sub-member M5¹ to M5⁴ of Majiagou Formation. Recently, more than one million cubic meters of gas production per day are discovered in the M5⁵ to M5¹⁰ sub-members of Majiagou Formation, which proves the high gas production capacity of this formation (Shi et al., 2013; Wu et al., 2014; Huang et al., 2011; Yang et al., 2014; Huang and Bao, 2012).

Due to the high gas production in the Majiagou Formation of Ordos Basin, numerous models for dolomitization has been proposed including mixed-water dolomitization (Zhao et al., 2005), quasi syngenetic dolomitization (Han and Xin, 1995; He et al., 2014), burial dolomitization (Su et al., 2011; Chen et al., 2018; Fu et al., 2019), and localized hydrothermal dolomitization (Wang et al., 2009; Huang et al., 2011), which believe anhydrite contributes to forming high-quality reservoirs. The study area located in the central Ordos Basin, was initially considered to have low exploration potential due to poorly developed anhydrite and decreasing stratum thickness. However, high gas production from exploratory wells has spurred research into dolomite reservoir research in this region. Given the underdeveloped anhydrite sedimentation, the origin of dolomite reservoirs warrants further discussion. This study investigates the petrological characteristics and formation mechanism of dolomite for the Lower Ordovician Majiagou Formation in central Ordos Basin. Petrographic observations, X-ray diffraction (XRD) analyses and C–O–Sr isotopic data are used to characterize dolomite types and their petrological and geochemical features, clarifying the origin of dolomite reservoir, aiming to provide the geochemical and sedimentary dynamic evidence for predicting dolomite reservoirs in anhydrite-depleted settings and for guiding hydrocarbon exploration in similar basins globally.

2 Geological setting

Ordos Basin is a typical multi-cycle intracratonic basin located in the western North China Block (Figure 1a). It has six tectonic units, namely, the Weibei Uplift in the south, Yimeng Uplift in the north, Jinxi Fold Belt in the east, Yishan Slope in the central, Tianhuan Depression and the Western Margin Thrust Belt in the west (Fu et al., 2019; Cao et al., 2021; Tian et al., 2024). The study area is located in the central Ordos Basin, and in the middle of the Yishan Slope (Figure 1b).

Figure 1

Figure 1. The structural, sedimentary, petrological column characteristics and the location of research area in the Ordos Basin, the red rectangle represents the study area. (a) The location of Ordos Basin in China; (b) the location of core samples in Ordos Basin; (c) geological structures evolution of regional geological structure; (d) sedimentary environment distribution during M₅¹ to M₅¹⁰ Formation; (e) the characteristics of lithology and sedimentary cycle column [reference from Zhou et al. (2023), Shi et al. (2021)].

Ten sub-members comprise the fifth member of Majiagou Formation, namely, Ma5¹ to Ma5¹⁰. During the depositional period of the Ordovician Majiagou Formation, the basin experienced a protracted burial history and tectonic evolution. The “L” type paleo-uplift formed by subduction of Qingling-Qilian paleo-ocean crust controlled the sedimentary environment of the Ordos Basin, especially the central Ordos Basin (Figure 1c) (Zhou et al., 2023; Cao et al., 2018; Xi et al., 2017; Wang et al., 2009; Chen et al., 2020; Feng and Bao, 1999). Therefore, karstic reservoirs are deposited during Ma5¹ to Ma5⁴ period, while dolomite and evaporite are characterized in Ma5⁵ to Ma5¹⁰ sub-member (Xiong et al., 2020; Mou et al., 2023 2020). Among them, the Ma5⁵ and Ma5⁹ were dominated by dolomite, while the Ma5⁶, Ma5⁸ and Ma5¹⁰ formed thick gypsum-salt rock deposits (Zhou et al., 2023). However, affected by tectonic uplift, compared with the eastern Ordos Basin, the central Ordos Basin developed restricted platform sedimentary environment (Figure 1d), the lithology dominated by finely crystalline dolomite or finely to medium crystalline dolomite (Cao et al., 2021; Liu et al., 2020), rather than evaporites.

Specifically, when the rapid transgression cycles developed in the sub-members Ma5⁵, Ma5⁷ and Ma5⁹, the sedimentary system of the restricted platform subfacies was developed. From west to east, dolomite flat, gypsum-bearing dolomite flat were developed successively. In addition, a gypsum-dolomite flat facies could be developed in a limited-range distribution. The lithology of the sediments mainly consists of dolomite, gypsum-bearing dolomite and gypsum-dolomite. While the sub-members Ma5⁶, Ma5⁸ and Ma5¹⁰ are in the period of rapid marine regression cycles, the overall sedimentary facies characteristics and lithologies are similar to those of the sub-members Ma5⁵, Ma5⁷ and Ma5⁹ in the period of rapid marine transgression cycles. The differences of sedimentary subfacies distribution are mainly related to the amplitude of sea oscillations (Yang et al., 2014; Yu et al., 2012; Shi et al., 2013; Wu et al., 2014) (Figure 1e).

3 Samples and experimental methods

The core samples of this study were collected from six coring wells in the Wu Qi areas of the Ordos Basin. Petrographic observations were conducted on all core samples, and 16 samples representing different dolomite types were selected for thin section, cathodoluminescence, XRD and C-O-Sr isotope analyses. In order to reflect the original sedimentary characteristics, and guarantee the accuracy of experiments results, we avoid dissolution holes and calcite veins to the largest extent during sampling.

3.1 Petrographic observation

There are 9 of 16 samples are made into thin section, which are doubly polished and half-stained with a mixture of Alizarin-red S and potassium ferricyanide to distinguish calcite and dolomite (Dickson, 1965; Fu et al., 2019; Cantrell et al., 2004). Besides, Cathodoluminescence (CL) analysis of five representative polished thin-sections was undertaken at Xi’an Petroleum University, China, using a CITL 8220 MK3 instrument (operating conditions: 20 kV beam voltage and 200 μA beam current).

3.2 XRD analysis

The Mg/Ca order degree and dolomite content determination can be obtained by XRD analysis. The Mg/Ca order degree was calculated based on the ratio of the reflection intensities of the diffraction peaks (015) (2θ = 35.3°) and (110) (2θ = 37.3°), as proposed by Füshtbauer and Goldschmidt in 1965, which can be used to indicate the degree of crystallization (Zhang et al., 2010; Manche and Kaczmarek, 2021). We put powdered samples into D2 Phaser diffractometer to analysis, with the detection standard SY/T-5163–1995. The results were obtained as weight percentages of CaO and MgO, and final results are converted, calculated and present in Table 1.

Table 1

Table 1. The petrological and mineralogical characteristics of the Ma5⁶ to M5⁸ Formation in the study area.

3.3 Isotope analysis

16 Samples are powered for isotope analysis. For the carbon and oxygen isotope analysis, performed at the Geochemical Testing Department of Chinese Academy of Sciences using a Finnigan MAT252 (Ger many) mass spectrometer, samples are heated to remove organic matter and then release the CO₂ by reacting with anhydrous phosphoric acid under a vacuum at 25°C for 24 h. The testing basis was GB/T6041-2020 (Standard, 2020), and the isotope values were calculated according to the Peedee Belemnitella (PDB) standard (Sauer et al., 2001). Here, we also use the result of C-O isotope analysis to calculate Z value, which is a criteria to indicate the sedimentary environment by the equation:

$\mathrm{Z}=\mathrm{a}\left({\mathrm{\delta }}^{13}\mathrm{C}+50\right)+\mathrm{b}\left({\mathrm{\delta }}^{18}\mathrm{O}+50\right)$

in which a and b are 2.048 and 0.498 respectively. The results of Z value are shown in Table 1, dolomite with a Z value above 120 would be classified as marine, those with Z value below 120 as continental (Li et al., 2014; Keith and Weber, 1964).

The Sr isotope analysis involved the following steps: (1) dissolving the powdered samples in ultrapure acid at 60°C for a duration of 24 h; (2) further separating the Sr using ion exchange resin; and (3) completing the analysis with a thermal ionization mass spectrometer (GV IsoProbe-T), achieving a high measurement accuracy.

4 Results

4.1 Dolomite petrography

There are various types of dolomites during Ma5⁶ to Ma5⁸ period in study area, the lithology distributed by micritic dolomite, fine-crystalline dolomite, and medium-crystalline dolomite according to the crystal size classification scheme (Huang, 2010; Sibley and Gregg, 1987). Here we recognized and focus on two types dolomite below by thin section observation and CL analysis as they widely distribution in the study area.

4.1.1 (Very) finely crystalline dolomite (Md1)

Md1 dolomite is well-developed in M5⁶ and M5⁹ Formation. In thin section, Md1 dolomite is gray or dark gray in colors (Figure 2), dolomite crystals are 5–20 μm in size, euhedral to subhedral of dolomite crystalline, featured by thin clay laminations (Figure 2a), micrite texture with organic matter filled (Figure 2b), dominated by microcrystalline or fine-crystalline structure and the crystal morphology is rather difficult to be identified. CL analysis shows a dull (Figure 2d) or dark red color (Figure 2e), and dissolution pores or filled by calcite can be observed occasionally (Figures 2c, f). It has the order degree of 0.53–0.79 (Table 1), with an average value is 0.65. The low value of order degree indicate that Md1 dolomite grows rapidly, lacking the conditions to form a crystalline structure. It likely that this type of dolomite was formed under weak to moderate hydrodynamic conditions, precisely in the tidal flat environment at the top of the upward shallowing cycle, being a product of the quasi syngenetic stage (Zhang et al., 2019; Warren, 2000; Sanz-montero et al., 2008).

Figure 2

Figure 2. Petrographic features of Md1 dolomite during M5⁶ to M5⁸ formation in Ordos Basin. (a) Very finely crystalline dolomite, euhedral to subhedral, thin clay laminations, Well A3, 4089m; (b) very finely crystalline dolomite, microfracture (blue in color) or microfractures with organic matter filled (black in color), Well A2, 3,934.9m; (c) finely crystalline dolomite, dissolution pores filled by calcite, Well A3, 3,962.5m; (d) coupled CL photomicrograph of showing dull red luminescence of (d), bright red luminescence of calcite, Well A3, 3,962.5m; (e) finely crystalline dolomite, coupled CL photomicrograph show dull red luminescence, Well A4 4,010.2m; (f) finely crystalline dolomite, crystalline size ranges relatively widely, dissolution pores generated, Well A1, 3,958.6 m.

4.1.2 Finely to medium crystalline dolomite (Md2)

Md2 dolomite is typically light brown, gray or light gray in color under thin section, with crystals size ranging from 20 to 60 μm, which is widely developed in M₅⁸ sub-member formation. It is observed to have fine to medium crystalline texture, with euhedral to subhedral crystals (Figure 3a). The dissolution or intergranular pores and fracture generated, some of them are filled with calcite can be occasionally seen (Figures 3b-e). Under CL, the Md2 dolomite displays dull or relatively bright red luminescence color (Figure 3f), and has the order degree of 0.78–0.98 (with an average value 0.85) (Table 1). The relatively high value of order degree indicates a sufficient conditions to form a crystalline structure to generate a higher euhedral degree. This type of dolomite is frequently observed in the tidal flat environment under weak hydrodynamic conditions and related to the reflux and percolation dolomitization during the shallow burial stage (Zhang et al., 2019; Warren, 2000; Su et al., 2011).

Figure 3

Figure 3. Petrographic features of Md2 dolomite. (a) Medium dolomite, euhedral to subhedral crystals, Well A4, 4,044.1m; (b) finely to medium dolomite, intercrystalline and dissolution pores developed, Well A2, 3,957.5m; (c) finely to medium dolomite, dissolution pores filled with calcite, Well A2, 3,957.5m; (d) medium dolomite, euhedral to subhedral crystals, inter-crystalline pores developed (blue color), Well A5 3,930.1m; (e) medium dolomite, micro-fracture can be observed, Well A5, 3,934.9m; (f) coupled CL photomicrograph of showing bright red luminescence of (e), Well A3, 3,962.5 m.

4.2 C-O isotopes

The stable isotope analysis results for carbon and oxygen are presented in Table 1 and Figure 4, which are commonly used for diagenetic fluid tracing (Major et al., 1992; Kaufman et al., 1993; Berra et al., 2020). In the study area, dolomite samples have δ¹³C values of −1.32‰-1.82‰, with the average value is 0.11‰ (Md1 dolomite = −0.68‰ to +1.82‰, and Md2 dolomite = −1.32‰–0.66‰), and δ¹⁸O value of −8.1‰ to −5.26‰, with the average value is −6.54‰ (Md1 dolomite = −6.92‰ to −5.26‰, Md2 dolomite = −8.1‰ to −6.03‰). Previously measured C and O isotopes in the middle Ordovician seawater between −2.0‰ and +0.5‰ and −7‰ and −5‰, respectively (Allan and Wiggins, 1993; Bai et al., 2016). Therefore, samples of study area from M₅⁶ to M₅⁸ Formation basically fall within the range of carbon isotope, and display the slight negative oxygen isotope characteristics compared to the Ordovician seawater. In detail, the average δ¹³C and δ¹⁸O value of Md1 dolomite (mean value of δ¹³C = +0.83‰, δ¹⁸O = −5.79‰) is higher than that of Md2 dolomite (mean value of δ¹³C = −0.48‰, δ¹⁸O = −7.12‰).

Figure 4

Figure 4. Cross-plot of δ¹³C and δ¹⁸O values in different dolomite types. The gray box represent Middle Ordovician marine calcite signature according to Veizer et al. (1999).

4.3 Sr isotopes

Sixteen dolomite samples analyzed were also for Sr isotope test (Figure 5; Table 1). The ⁸⁷Sr/⁸⁶Sr ratios for the dolomite ranges from 0.70921 to 0.71062 (the average value is 0.70955). Besides, the ⁸⁷Sr/⁸⁶Sr value of Md1 dolomite is 0.70943–0.70995, with an average of 0.70967, Md2 dolomite has the ⁸⁷Sr/⁸⁶Sr ratios from 0.70921 to 0.71062 (average value is 0.70946, n = 9), which is lower than Md1 dolomite. Most of ⁸⁷Sr/⁸⁶Sr ratios from Md2 samples are higher than of the Middle Ordovician marine carbonate (between 0.7087 and 0.7092, from Veizer er al., 1999, Huang et al., 2011; Edwards et al., 2015), which may be associated with the presence of anhydrite (Su et al., 2011).

Figure 5

Figure 5. Cross-plot of δ¹⁸O and ⁸⁷Sr/⁸⁶Sr values in different dolomite types. The gray box represent Middle Ordovician marine calcite signature according to Veizer et al. (1999).

5 Discussion

5.1 Petrographic implications

Md1 dolomite is predominantly characterized by microcrystalline to subhedral dolomite crystals associated with anhydrite that has been replaced by calcite precipitation (Figure 2F). Previous studies have suggested that microcrystalline dolomite can form in evaporitic settings at relatively low temperatures (Gregg and Shelton, 1990; Fu et al., 2011; Loyd and Corsetti, 2010). The original limestone structure has been preserved within the microcrystalline dolomite, indicating its likely origin from near-surface dolomitization in a restricted, shallow environment during early diagenesis. Md1 dolomite pervasively replaces the matrix across all facies, especially in laminates where complete replacement occurs. The textures of Md1 dolomite and its close association with restricted-marine deposits suggest formation in a near-surface, low-temperature, saline environment with a high density of nucleation sites (Gregg and Shelton, 1990). Through petrographic observations, M1 dolomite is characterized by rich in terrigenous clay minerals, widely distributed in thin layers, and the bedding structure (Figure 2a) during the quasi-syngenetic dolomitization stage.

The Md2 dolomite is mainly composed of fine to medium-crystalline (30–100 μm), euhedral to subhedral dolomite crystals (Figure 3a), which preferentially replace the limestone matrix. The Md2 dolomite is characterized by partial to complete replacement of micritic limestone/silty limestone. This phenomenon indicates that there are a large number of replacement residues of micritic calcite or uncompletely dissolved micritic dolomite residues within the large crystals. The Md2 dolomite typically exhibits structural disruption and indistinct original sedimentary features. The high order degree (average value is 0.85) suggests that the concentration of magnesium ions in the fluid during dolomitization has decreased, and the replacement rate is relatively slow. Therefore, fine to medium dolomite crystals have formed, suggesting that Md2 dolomite formed during a period of shallow burial.

5.2 Petrogenesis of dolomite

C and O isotopes are commonly used for diagenetic fluid tracing as their compositions in the dolomite and the dolomitized rocks are closely related to the salinity and temperature of diagenetic fluids (Major et al., 1992; Kaufman et al., 1993; Berra et al., 2020). In addition, we use Keith and Weber’s salinity index calculation formula Z to analyze diagenetic fluids. The ratio of ⁸⁷Sr/⁸⁶Sr can also reflect the information of diagenetic fluids, and thus is also widely used in the dolomite petrogenesis discussion (Wang et al., 2009).

5.2.1 Petrogenesis of Md1 dolomite

The results from C and O isotope tests of Md1 dolomite in the study area indicate the value of δ¹³C is −0.68‰–1.82‰ (average value is 0.83‰), and δ¹⁸O is −6.92‰ to −5.26‰ (average value is −5.79‰). Compared to C and O isotopes measurement value in the Ordovician seawater (Allan and Wiggins, 1993), Md1 dolomite samples have high δ¹³C values and high anhydrite contents, which are likely to be related to evaporated seawater (Figure 4) (Wang et al., 2009; Shi et al., 2013; He et al., 2014). According to the O isotope value decreases gradually as the temperature rises (Zhao et al., 2005; Shields et al., 2003; Yu et al., 2012), it is deduced that strong evaporation, relatively shallow seawater, and the relatively high temperature depositional environment when the Md1 dolomite was formed, leading to the characteristics of slight negative oxygen isotope excursion (Figure 5). Meanwhile, the range of Z value from 124.84696 to 126.9936 with an average of 125.9747, indicating the dolomite should be formed in the high-salinity environment. The value of ⁸⁷Sr/⁸⁶Sr ratio is 0.70943–0.70995, and mean value is 0.70967 (Table 1), which is higher than those of contemporaneous seawater, indicating that the Md1 dolomite may be associated with the presence of anhydrite or the product of reflux and infiltration metasomatism by magnesium-rich concentrated brine from the tidal flat or overlying layers (Xiong et al., 2018; Su et al., 2011).

Moreover, according to the previous research results (Yu, 2019), the content of major and trace elements are relatively low, specifically, the average contents of major elements Fe, Na, and K are 0.24%, 0.03% and 0.07%, and the mean value of trace elements Mn, Ba, Sr, Pb and Zn are 74.14 × 10⁻⁶, 8.3 × 10⁻⁶, 83.97 × 10⁻⁶, 0.8 × 10⁻⁶, and 3.31 × 10⁻⁶ (Table 2). As shown in Figures 6a, b, the distribution characteristics of high value of Fe and Mn, and low value of Na and Sr indicating the Md1 dolomite was formed in a sedimentary environment with high salinity and restricted environment, and was influenced by atmospheric freshwater (Jiang et al., 2015; Bai et al., 2016; Yu, 2019). In addition, the low value of order degree and black-brown color in CL analysis (Figures 2d, e) also indicates the high-temperature environment. Therefore, the Md1 dolomite is formed from evaporated seawater after quasi syngenetic.

Table 2

Table 2. Analysis of major and trace elements for various type of dolomite during the Ma₅⁶ to Ma₅⁹ period in the study area [Referenced from Yu (2019)].

Figure 6

Figure 6. The characteristics of Major and Trace Elements in (a, b) Md1 and (c, d) Md2 dolomite of the Ordovician Majiagou Formation in the Study Area [Referenced from Yu (2019)].

5.2.2 Petrogenesis of Md2 dolomite

The value of C isotope ranges from −1.32‰ to 0.66‰ with mean value is −0.48‰, which is equivalent to the δ¹³C value of global seawater at the same time (Figure 4) (Veizer et al., 1999), indicating that the dolomitizing fluid was seawater-derived fluid. The δ¹⁸O values of Md2 dolomite is −6.03‰ to −8.16‰, the average value is −7.12‰, which is lower than that of Md1 dolomite. Previous research indicates that the δ¹⁸O values of dolomites formed in hydrothermal development environments are mostly less than −10.0‰ (Huang et al., 2011). The δ¹⁸O values of the dolomite samples in the study area are all greater than −10.0‰ (Figure 5), eliminating the possibility of external hydrothermal fluid influence. The Z value is 121.3482–124.4332 with an average of 122.8781, being lower than that of Md1 dolomite, suggesting a reduction in dolomitization fluids salinity. In addition, the value of order degree is relatively high (mean value is 0.85) (Table 1), and the CL analysis Figure 3f shows a dark red or orange in color (Figure 1d) with the fine-crystalline grains can be observed. According to the relationship between C and O isotopes of dolomite and diagenesis proposed by James and Choquette (1986), it is held that Md2 dolomite should originally be the product of the deep burial stage. However, microscopic observations show that there is no saddle-shaped or coarse-grained dolomite in the study area.

The results of the major and trace element experiments indicate that, compared with Md1 dolomite, the contents of Fe (mean value = 4.21%) and Mn (mean value = 283.45 × 10⁻⁶) elements in Md2 dolomite are higher, and the content of Sr element is lower (mean value = 75.21 × 10⁻⁶) (Figures 6c, d). The intensification of burial during diagenesis processes creates a reductive, high-temperature depositional environment that promotes Fe and Mn incorporation into the lattice. Besides, The decrease in Sr content during recrystallization is attributed to dolomite recrystallization (Ni et al., 2010; Kramer et al., 2001; Liu et al., 2020). This suggests that Md2 dolomite has progressed from the quasi syngenetic stage to the burial stage (Bai et al., 2016; Yu, 2019; Xiong et al., 2020), forming dolomite with larger grains and better self-consistency. The characteristics of larger crystalline grain can be observed in thin section (Figure 3a). Therefore, Md2 dolomite is interpreted as the product of shallow burial dolomitization.

5.3 Dolomitization mechanism and dolomites evolution

5.3.1 Dolomitization mechanism

During the lower and middle formation of the Middle Ordovician Majiagou period in the study area, the Ordos Basin near the equator experienced an arid climate with strong seawater evaporation and concentration (Bai et al., 2016; Chen et al., 2018; Fu et al., 2019).

Specifically, shallow-water carbonate platform tidal flat developed in the study area, increasing seawater salinity led to the development of high-frequency transgressive system cycle. It contributes primarily micritic limestone deposition, with only minor, discontinuous dolomite layers near the Yimeng ancient uplift (Figure 7a). In the later regression cycle, the study area featured a high-salinity, water-limited shallow sea environment dominated by evaporation. High-salinity seawater rapidly altered the early marlstone surface, forming thin, laterally unstable layers of quasi syngenetic micritic and fine-crystalline dolomite (Figure 7a). By the end of the sedimentation period of the Middle Ordovician Majiagou Formation, despite the shallow burial near the central paleo-uplift, the overall burial depth increased. This led to enhanced oxidation and higher formation temperatures. High-salinity, Mg²⁺-rich seawater slowly seeped into the underlying unconsolidated limestone, displacing primary pore water and causing metasomatic dolomitization. This process formed thick, laterally stable, coarse-grained medium-crystalline dolomite, providing an ideal environment for natural gas accumulation in the Majiagou Formation (Figure 7b).

Figure 7

Figure 7. (a) The syngenetic-quasi syngenetic stages dolomitization model. (b) The shallow burial stage dolomitization model.

5.3.2 Dolomites evolution

Combining our petrological and geochemical experiment results and their implication with the depositional setting and burial history of the study area from previously research result. The main types of diagenesis include dolomitization, filling, dissolution, cementation, recrystallization and dedolomitization in the study area during the Ma₅⁶ to M₅⁸ Formation (Yu, 2019; Shi et al., 2013). Four diagenetic stages have been experienced: the quasi-syndepositional near-surface to early diagenetic shallow burial diagenetic stage, the supergene atmospheric freshwater diagenetic stage, the middle diagenetic shallow-to-medium burial diagenetic stage, and the late diagenetic medium-to-deep burial diagenetic stage (Figure 8) (Su et al., 2011; Wu et al., 2014).

Figure 8

Figure 8. The diagenetic sequence of the Ma5⁶ to M5¹⁰ Formation in the study area [referenced from Su et al. (2011), Yu (2019)].

During the stage of quasi-syndepositional near-surface to early diagenetic shallow burial diagenetic stage, including compaction, the first stage of dolomitization, weak recrystallization, and cementation (Yu, 2019; Wu et al., 2014). The seawater with high-salinity, magnesium-rich in the dry, hot and restricted depositional environment, limestone or muddy limestone is transformed into micritic dolomite or finely crystalline dolomite through metasomatism. Subsequently, weak recrystallization prompted the grains of micritic dolomite or fine-crystalline dolomite to coarsen, forming fine-to medium-crystalline dolomite with larger intercrystalline pores (Figure 3b). Meanwhile, a part of carbonate grains were cemented by calcite, and evolved into dolomite through dolomitization (Liu et al., 2020; He et al., 2014).

Affected by the Caledonian movement, the study area experienced intermittent uplift. Although not exposed to the surface for a long time, the upper layers, which are closer to the weathering crust, have also suffered the erosion or dissolution of atmospheric freshwater during the exogenic period (Shi et al., 2013; Su et al., 2011). Therefore, a large number of dissolution pores, caves and fissures with irregular shapes are formed (Figure 3c), which increases the storage capacity of the dolomite reservoir.

During the medium burial diagenesis stage, the diagenetic processes mainly consist of dolomitization, organic acid dissolution, intense recrystallization, and filling (Xiong et al., 2020; Yu, 2019; Huang et al., 2014). The organic acid dissolution process enhance the reservoir capacity, making the dissolution pores, cavities, and fissures enlarge during the epigenetic period. Recrystallization, filling, compaction and pressure solution, cementation and metasomatism, severely damaged reservoir capacity, resulting in a significant reduction in pore volume, during the medium-deep burial diagenesis stage (Wu et al., 2014; Liu et al., 2020). However, exceptionally, the burial dolomitization has enhanced the euhedral degree of dolomite crystalline, providing a large number of intercrystalline pores (Figure 3b) and positive to the high-quality reservoirs develop.

6 Conclusion

(1) According to the classification scheme for the grain size of carbonate rocks, and considering the characteristics of petrography, two types of dolomite are mainly developed in the study area of the Yishan Slope in the Ordos Basin during M₅⁶ to M₅⁸ Formation, namely, (very) finely dolomite (Md1) and fine to medium crystalline dolomite (Md2). The Md1 dolomite characterized by microcrystalline (<20 μm), subhedral dolomite crystals, no or dull red in CL analysis. The Md2 dolomite is characterized by finely to medium-crystalline (20–60 μm), euhedral to subhedral dolomite crystals, with a dark red or orange in color by CL analysis.

(2) Based on systematic analysis of petrology, carbon-oxygen isotopes, major and trace elements from different types of dolomites, two main dolomitization processes were identified: quasi syngenetic and shallow burial reflux infiltration. The dolomitizing fluids were primarily marine-derived. Md1 dolomites formed in relatively open, low-oxidation, low-temperature conditions during the syngenetic-quasi syngenetic stage, while Md2 dolomites formed in the closed, low-reduction, higher-temperature conditions during shallow burial.

(3) A systematic investigation was carried out on the types of diagenesis in the study area, and the diagenetic sequence was established. The M₅⁶ to M₅⁸ Formation primarily exhibit dolomitization, filling, dissolution, cementation, recrystallization, and desulfatization. The Md1 dolomite induced to be associated with the presence of anhydrite or the product of reflux and infiltration metasomatism by magnesium-rich concentrated brine from the tidal flat or overlying layers, which is formed from evaporated seawater after quasi syngenetic. The Md2 dolomite dominated by a larger crystalline grain, a higher content of Fe and Mn, low content of Sr, indicating the stronger dolomitization and reductive sedimentary environment. Therefore, the Md2 dolomite is interpreted as resulting from shallow burial dolomitization.

Data availability statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

Author contributions

KT: Investigation, Writing–original draft, Writing–review and editing, Conceptualization, Methodology. JZ: Data curation, Investigation, Writing–original draft. XY: Data curation, Methodology, Writing–original draft. CX: Data curation, Investigation, Methodology, Writing–review and editing, Writing–original draft. JC: Data curation, Formal Analysis, Writing–review and editing. LM: Investigation, Writing–review and editing. WZ: Investigation, Writing – review and editing

Funding

The author(s) declare that financial support was received for the research and/or publication of this article. The project received funding from the Key R&D Plan of Shaanxi Province: Intelligent Evaluation Technology for Tight Gas Reservoir Driven by Data Mechanism Hybrid (Projects #2023-YBGY-308) and Investigation and Application of Microscopic Gas-Water Seepage Mechanisms in Tight Sandstone Reservoirs (Projects #2024ZC-KJXX-132).

Conflict of interest

Authors KT, JZ, XY, JC, LM, and WZ were employed by Shaanxi Yanchang Petroleum Group Co., Ltd.

Author CX was employed by PetroChina Changqing Oilfield Company.

Publisher’s note

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