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Article

Paragenesis and Formation Mechanism of the Dolomite-Mottled Limestone Reservoir of Ordovician Ma4 Member, Ordos Basin

by
Zeguang Yang
1,
Aiguo Wang
1,*,
Liyong Fan
2,3,
Zhanrong Ma
4,
Xiaorong Luo
1,*,
Xinghui Ning
1 and
Kun Meng
1
1
State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China
2
National Engineering Laboratory of Low Permeability Oil and Gas Field Exploration and Development, Xi’an 710018, China
3
Research Institute of Exploration and Development, PetroChina Changqing Oilfield Company, Xi’an 710018, China
4
Exploration Department, PetroChina Changqing Oilfield Company, Xi’an 710018, China
*
Authors to whom correspondence should be addressed.
Minerals 2023, 13(9), 1172; https://doi.org/10.3390/min13091172
Submission received: 29 June 2023 / Revised: 3 September 2023 / Accepted: 5 September 2023 / Published: 6 September 2023
(This article belongs to the Special Issue Mineralogical, Geological and Geochemical Heterogeneities of Carbonate Reservoirs)
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Abstract

:
Despite the discovery of high-producing natural gas reservoirs in the low-permeability dolomite-mottled limestone (DML) reservoir of the fourth Member (Ma4) of the Majiagou Formation in the Ordos Basin, the current understanding of the processes responsible for reservoir formation are still superficial, which extremely restricts the effectiveness of deep petroleum exploration and development in the basin. Therefore, this study analyzed the paragenesis process of the DML reservoir through systematic petrographic and geochemical measurements. The DML consists of burrows and matrix. The burrows are mainly filled with dolomite with a small amount of micrite, calcite cement, and solid bitumen. The matrix mainly consists of wakestone or mudstone. The DML has experienced multiple diagenetic events, including seepage-reflux dolomitization, compaction, calcite cement CaI cementation, micrite recrystallization, dissolution, hydrocarbon charging, calcite cement CaII cementation, and dolomite progressive recrystallization. Dolomitization is critical to the DML reservoir formation. The pore created by dolomitization is the hydrocarbon-migrated pathway and storage space. Due to the difference in Mg2+-rich fluid supply, the degree of dolomitization decreases from west to east, which causes the difference in diagenetic evolution of the western and eastern parts of the study area. The high dolomitization degree led to strong anti-compaction ability in the west, contrary to the east. Thus, the reservoir quality of the west is better than the east.

1. Introduction

Deep carbonate strata have become an important field of study in petroleum exploration [1,2]. With the discovery of a large number of commercial reservoirs, the debate about whether there are effective reservoirs in deeply buried carbonate rock has gradually diminished. The formation mechanism of deep ancient carbonate rock reservoirs has become a center of interest in petroleum geology research. There are two main perspectives about the origin of deeply buried carbonate reservoirs. Some researchers think that because of the overprinting of multiple diagenetic processes, the porosity of carbonate rock decreases with increasing burial depth [3,4], and only dissolution can form a large number of pores in the middle–deeply buried carbonate reservoirs [5,6,7,8]. It has been proposed that thermal sulfate reduction (TSR), organic acids, and deep hydrothermal fluids may be critical causes of dissolution [9,10,11,12,13]. Others have proposed that the development of carbonate rock reservoirs is mainly controlled by sedimentary facies [14,15,16,17], suggesting that porosity in deeply buried carbonate rock reservoirs can be maintained during the burial process. However, the impact of late diagenetic processes, such as dissolution or recrystallization, depends on the depositional processes and early diagenetic events.
Dolomite is commonly porous and, hence, is a major target in petroleum exploration in the carbonate rock field [18]. However, the origin of it remains the subject of considerable debate [19]. There are many thermodynamic, kinetic, and hydrological models (i.e., dolomitization model) that have been offered to explain the formation of dolomite, such as evaporation dolomitization, seepage-reflux dolomitization, mixing-zone dolomitization, burial dolomitization, seawater dolomitization, etc., [18]. One of the most commonly invoked models is seepage-reflux dolomitization [20,21,22,23]. Conceptually, dolomitization by hypersaline reflux proceeds when the evaporation of seawater produces a dense brine, which promotes fluid flow by a density difference [19].
The lithology of the fourth Member of the Majiagou Formation (Ma4 Member) in the Ordos Basin is mainly limestone with a dolomite-mottled texture (dolomite-mottled limestone, DML). It has been proven to be a major regional reservoir [24,25,26,27]. Previous petrology and ichnological analyses have indicated that the mottle depositional texture is a result of biotic burrow fabric dolomitization [28,29]. Reservoirs of bioturbation origin have been reported around the world [30,31,32]. The transformation of original sediments by bioturbation often leads to obvious differences between the burrows and the matrix, resulting in a complex diagenetic process that eventually leads to the heterogeneity of diagenetic features [33]. However, the diagenetic research of the DML of the Ma4 Member is still weak.
Therefore, this study takes the DML reservoir of the Ma4 Member as a case, using petrographic and geochemical measurements to evaluate the paragenetic process that mainly occurred in the burrows. On this basis, the way in which the early dolomitization process related to the burrows controls the subsequent diagenetic processes can be clarified, as well as why it maintains porosity under deep burial conditions. This study is expected to provide a reference for the study of the paragenetic process and formation mechanism of bioturbation carbonate rock reservoirs, as well as regional petroleum exploration.

2. Geological Setting

Ordos Basin is a multicycle superimposed basin in the western portion of the North China Craton. Based on the differences in the basement structure, the basin was divided into six tectonic units, including the Yimeng Uplift, the Yishan Slope, the Weibei Uplift, the Western Thrust Belt, the Tianhuan Depression, and the Jinxi Folded Belt. The study area is mainly located in the middle eastern portion of the Yishan Slope (Figure 1a, [34]). During the paleozoic, the basin experienced the Huaiyuan and Caledonian orogeny, leading to the Ordovician strata being unconformably positioned between the underlying Cambrian strata and the upper Carboniferous strata (Figure 1b, [34]).
The Ordovician Majiagou Formation is a carbonate–evaporite depositional system about 600~1200 m thick. Six members in the Majiagou Formation have been recognized based on the depositional sequences (Ma1, Ma2, Ma3, Ma4, Ma5, Ma6, Figure 1b, [35]). The Ma1, Ma3, and Ma5 Members are composed of evaporite and dolostone, representing regressive cycles. The Ma2, Ma4, and Ma6 Members are composed of limestone and dolostone, representing transgression cycles ([36], Figure 1b). A thick evaporite (about 60~160 m) developed in the sixth sub-member of the Ma5 Member, which divides the Majiagou Formation into “post-salt” and “pre-salt” petroleum systems [37]. At present, there is a high level of exploration activities within the “post-salt” petroleum system. The Jingbian gas field, which holds more than a trillion cubic meters of natural gas, has been discovered in it. However, the exploration progress of the “pre-salt” petroleum system is relatively slow [37]. High-producing commercial natural gas was first discovered in the MT1 well in 2021 [24]. Since then, the Ma4 Member has become the most practical and attractive new exploration field in the region.
Minerals 13 01172 g001
Figure 1. (a) Lithofacies and paleogeography map of the Ma4 period and (b) stratigraphic histogram of the Majiagou Formation of the Ordos Basin (modified from [35,37]).
Figure 1. (a) Lithofacies and paleogeography map of the Ma4 period and (b) stratigraphic histogram of the Majiagou Formation of the Ordos Basin (modified from [35,37]).
Minerals 13 01172 g001
Wushengqi uplift and Mizhi depression were, respectively, developed in the western and eastern parts of the study area during the Ma4 period [34]. Based on this structure pattern, the study area was divided into two parts. The western part is mainly located in the Wushengqi uplift, and the eastern part is located in the Mizhi depression (Figure 1a). There was a secondary uplift that developed in the Mizhi depression. The depositional environment of the study was a restricted platform [36]. The Ma4 Member gradually thins from west to east (Figure 2), and the dolomite content also gradually decreases from west to east.

3. Materials and Methods

In this study, cores from 13 wells and the Aojiaping outcrop of the Ma4 Member are described (Figure 1a). A total of 33 samples from wells and 1 sample from outcrop were collected and impregnated with blue epoxy to facilitate porosity recognition and then cut to prepare 0.03 mm thickness thin sections. Alizarin Red S and potassium ferricyanide staining were further applied to the thin sections for calcite and dolomite identification. Diagenetic features were examined using a ZEISS Scope A1 polarizing microscope. The selected samples were plated with gold and placed in an FEI Quanta 450 FEG scanning electron microscope (SEM) to observe the microscopic texture of the dolomite mottle and matrix. The sections containing calcite cements were selected for hot cathodoluminescence (CL) characterization on a BII CLF-1 CL stage. The operating voltages were 5–8 kV and the gun current levels were 300–500 μA.
After the above tests, 13 thin sections (0.08 mm thickness) were prepared for in situ geochemical testing to obtain the trace elements, rare earth elements, and strontium isotopes of the different components (including matrix, dolomite, residual micrite, and calcite cement CaI and CaII) of the samples. Trace elements (including rare earth element, REE) and strontium isotopes were analyzed in an Agilent 7900 Inductively Coupled plasma mass spectrometer equipped with ASI Resolution M-50 Excimer laser ablation system in accordance with the methods described in [38,39]. Since the dolomite is mixed with residual micrite and/or calcite cement, it is difficult to separate pure dolomite powder to conduct X-ray diffraction experiments; thus, the stoichiometry of dolomites was calculated using the Mg and Ca contents from trace elements data.
A micro drill was used to prepare 10–50 μg powders from the matrix and mottle of the core samples, respectively. Then, these powders were placed into the Thermo Finnigan 253 plus IRMS isotope ratio mass spectrometer to test their carbon and oxygen isotopes. The testing method used is the same as in reference [40]. All δ13C data are reported relative to the Vienna Pee Dee Belemnite (VPDB). The analyses were conducted in the State Key Laboratory of Continental Dynamics, Northwest University, China.
The REEs are normalized using the North American shale composite (NASC). The ratio of light rare earth elements to heavy rare earth elements (LREE/HREE) was calculated as follows:
LREE/HREE = (LaN + CeN + PrN + NdN)/(ErN + TmN + YbN + LuN)
MREE anomaly, Eu anomaly, and Ce anomaly were calculated as follows:
MREE/MREE* = 2(SmN + EuN + GdN + TbN + DyN + HoN)/(LaN + CeN + PrN +
NdN + ErN + TmN+ YbN + LuN)
Eu/Eu* = EuN/(0.5SmN +0.5GdN)
Ce/Ce* = EuN/(0.5LaN + 0.5PrN)
In Equations (1)–(4), the subscripted N means PAAS-normalized. MREE*, Ce*, and Eu* represent the expected concentrations of these fractions or elements based on a flat or uniformly inclined REE distribution.
In addition, physical property test data of 105 related rock samples were obtained from PetroChina Changqing Oilfield Company. Kendall’s W test was used to test the distribution relationship between the porosity, permeability, and content of dolomite. The result of this test was quantified by the parameter Cohen’s d value (△d). If the △d ranges from 0.90 to 1.00, then the measured variables have a strong correlation.

4. Results

4.1. Petrographic Features of Dolomite-Mottled Limestone (DML) from Core Observation

Core observation indicated that the DML consists of two components: a gray matrix and a grey-black burrow (Figure 3). The width of the burrow is generally about 1 cm, and the extension length is generally several centimeters (Figure 3). Sharp contacts exist between the matrix and burrow (Figure 3).
Based on the ratio of the burrow area to the cross-sectional area of the core, the DML in the study area is divided into three types.
Type I: The burrow area of the core accounts for less than 30%, and the burrows generally appear as isolated layers (Figure 4a,d). Some burrows are gradually disappearing in a certain direction (Figure 4a), and the boundary of the burrow is indistinct (Figure 4a). This type is commonly found in the eastern Mizhi depression, such as in the M105 well and MT1 well.
Type II: The burrow area of the core accounts for 30%~60%, and the burrows appear as layers or mottle. Different burrows usually have point-like contact with each other, with a certain degree of connectivity (Figure 4b,e). This type can be seen throughout the entire study area.
Type III: The burrow area of the core accounts for more than 60%. The burrows overlap with each other, resulting in high connectivity (Figure 4c,f). This type often occurs in the western part of the study area, such as in the T100 well and T95 well.

4.2. Petrographic Features of Dolomite-Mottled Limestone (DML) from Microscopic Observation

4.2.1. Matrix

The matrix is made up of wackestone or mudstone without pores (Figure 5). Abundant bioclasts and gypsum are embedded within the matrix. Bioclast types include Trilobite, Bivalve, Crinoid, Gastropod, and other unidentifiable bioclasts (Figure 5a,c,d). In the western portion of the study area, the particles in the matrix are relatively round (Figure 5c). However, the particles are often in an ellipsoidal shape in the east (Figure 5d).

4.2.2. Burrows

The burrows are mainly filled by the dolomite, with residual micrite and calcite cement (Figure 6).
Dolomite mainly shows euhedral–subhedral crystals with fine size (Figure 6a). Some dolomites have cloudy centers and clear rims (Figure 6b). In the western part of the study area, the dolomite rhomb generally presents as tangential contact (Figure 6b). However, they usually exhibit concave-convex contact in the east (Figure 6c).
Residual micrite and calcite cement exist in both the western and eastern parts of the study area. However, the developed degree is different. Residual micrite usually occurs in the eastern samples. On the opposite, the calcite cements are more developed in the west.
Residual micrites in the intercrystalline pores are rhomb-shaped with varying diameters (Figure 6c). The calcite cements developed in the intercrystalline pores (CaI, Figure 6d) appear blocky, with crystal sizes ranging from 50 μm to 200 μm. The luminescence color of CaI was dim (Figure 6d). The calcite cements developed in the dissolution pores (CaII, Figure 6e) usually have a coarser crystal size, and their luminescence color is bright yellow (Figure 6e). There are still fewer unfilled residual pores, which are generally smaller, with pore sizes mostly less than 50 μm and poor connectivity (CaII, Figure 6f).
The phenomenon of burrows disappearing in cores also can be observed with the microscope. Especially in the eastern part of the study area, lots of burrows suddenly narrow along a certain direction and pinch out (Figure 6g,h).

4.2.3. Solid Bitumen

In addition to the dolomite, micrite, and calcite cement, there is solid bitumen filled in partial burrows, and they present four occurrences. (1) Enriched at the interface between burrow and matrix (Figure 7a). (2) Existing in the residual intercrystalline pores of the dolomite (Figure 7b). (3) Existing in the dissolution pores (Figure 7c). (4) Covering the calcite cement CaII (Figure 7d).

4.3. Porosity and Permeability Features of Dolomite-Mottled Limestone (DML)

A total of 105 measurements of porosity and permeability DML data were collected, including 2 measurement matrix data and 103 measurement burrows data (Appendix A). The porosity of the matrix ranges from 0.13% to 0.24%, with an average of 0.19%, and permeability ranges from 0.0021 mD to 0.0027 mD, with an average of 0.0024 mD. The porosity of burrows ranges from 0.29% to 3.33%, with an average of 1.05% ± 0.50%, and its permeability ranges from 0.0038 mD to 0.2075 mD, with an average of 0.0231 mD ± 0.0288 mD (Appendix A, Figure 8). The porosity of burrows is about five times greater than that of the matrix, and the permeability is about ten times greater than that of the matrix. According to Kendall’s W test, the △d of both porosity and permeability and dolomite content are 1.00.
There were also differences in the porosity and permeability of DML between the eastern and western portions of the study area (Appendix A, Figure 8). The porosity of the eastern DML ranges from 0.30% to 2.43%, with an average of 0.97%; the permeability ranges from 0.0038 mD to 0.0490 mD, with an average of 0.0173 mD. The porosity of the western DML ranges from 0.29% to 3.33%, with an average of 1.14%; the permeability ranges from 0.0039 mD to 0.2075 mD, with an average of 0.0319 mD. The porosity and permeability of the western samples are greater than those of the eastern samples.

4.4. Geochemical Features of Dolomite-Mottled Limestone (DML)

4.4.1. Trace Elements

The average contents of Mn, Fe, and Ni elements of the matrix are 10.35 ppm, 999.56 ppm, and 0.83 ppm, respectively. The average contents of Mn, Fe, and Ni elements of dolomite are 26.22 ppm, 2358.77 ppm, and 0.97 ppm, respectively. The average contents of Mn, Fe, and Ni elements of residual micrite are 25.10 ppm, 1990.11 ppm, and 2.55 ppm, respectively. The average contents of Mn, Fe, and Ni of calcite cement CaI are 9.09 ppm, 457.19 ppm, and 0.45 ppm, respectively. The average contents of Mn, Fe, and Ni elements of calcite cement CaII are 23.40 ppm, 374.73 ppm, and 0.42 ppm, respectively (Table 1 and Figure 9).

4.4.2. Rare Earth Elements (REE)

The REE distribution of the matrix, dolomite, residual micrite, and calcite cement CaI and CaII are illustrated in Figure 10 and Appendix B. The REE distribution of the matrix is characterized by LREE enrichment (LREE/HREE range 1.12~2.72, average 1.90), with no Ce anomaly (Ce/Ce* range 0.89~1.02, average 0.95), and negative Eu anomaly (Eu/Eu* range 0.37~1.78, average 0.90). The REE distribution of dolomite shows a nearly “linear” pattern (LREE/HREE range 0.46–1.43, average 0.94), with no Ce anomaly (Ce/Ce* range 0.90–1.07, average 1.01) and negative Eu anomaly (Eu/Eu* range 0.54–1.32, average 0.83). The REE distribution of residual micrite shows LREE enrichment (LREE/HREE range 0.32–3.57, average 1.80), with no Ce anomaly (Ce/Ce* range 0.97–1.04, average 0.99), and no Eu anomaly (Eu/Eu* range 0.58–1.83, average 1.01). CaI presented the “MREE–bulge” structure (MREE/MREE* range 1.28~2.15, average 1.62), with no Ce anomaly (Ce/Ce* range 0.90~1.02, average 0.98), and slightly negative Eu anomaly (Eu/Eu* range 0.19~1.44, average 0.85). The REE distribution of CaII was different from that of CaI, with significant LREE losses (LREE/HREE range 0.45~0.57, average 0.50), MREE enrichment (MREE/MREE* range 1.68~1.86, average 1.77), with no Ce anomaly (Ce/Ce* range 0.96~1.02, average 0.99), and slightly negative Eu anomaly (Eu/Eu* range 0.55~1.14, average 0.85).

4.4.3. Isotopic Geochemistry

As shown in Figure 11 and Table 2, the δ13C and δ18O values of the matrix range from −1.2‰ to 0.4‰ (average −0.32‰) and −9.0‰ to −5.5‰ (average −7.01‰), respectively. Those of burrows range from −0.6‰ to 0.9 ‰ (average 0.13‰) and −8.5‰ to −4.5‰ (average −6.44‰), respectively.
The 87Sr/86Sr values of the matrix, dolomite, residual micrite, and calcite cement CaI and CaII range from 0.70870 to 0.70942 (average 0.70905), 0.70883 to 0.71005 (average 0.70942), 0.70877 to 0.70920 (average 0.70901), 0.70890 to 0.71111 (average 0.70977), and 0.71016 to 0.71068 (average 0.71042), respectively (Figure 11 and Table 1).

4.4.4. Stoichiometry of Dolomites

The stoichiometry of dolomites is listed in Table 1. The values are range from 44.14% to 48.80%. Moreover, these values present a decreasing trend from the west to the east in the study area (Figure 12).

5. Discussion

5.1. Modification of Sediments by Bioturbation

The Ni (average 2.55 ppm) and REE (average ΣREE = 0.55) contents of residual micrite are greater than the matrix, indicating their enrichment in marine organic matters [31], such as the remains of planktonic animals and plants, as well as mucus and feces secreted by organisms [31]. It can be reasonably inferred that the residual micrite was initially deposited on the seafloor. The process whereby sediment deposited at the seafloor was backfilled into the borrows is easy to associate with the “refuse dump model” of infauna behavior [42]. In this model, infauna feed deep in the sediments, excrete feces on the sediment surface (Figure 13a), and then introduce surface material into the burrow to compensate for the space created during the feeding process (Figure 13b).
Many burrows (i.e., the precursor of the mottled depositional texture) are created during the feeding process, as observed in the cores (Figure 3 and Figure 4). Because of the higher porosity and permeability (Figure 8, Appendix A), burrows are more efficient than matrices for diagenetic fluid migration. Thus, they are the major migration path for diagenetic flows.

5.2. Diagenetic Process

5.2.1. Diagenetic Sequence from Petrography

Multiple phases of diagenetic events and minerals were identified based on petrographic works. The major diagenetic events include dolomitization, micrite recrystallization, compaction, cementation, dissolution, hydrocarbon charging, and dolomite recrystallization. Based on the description in Section 4.1 and Section 4.2, as well as the discussion following, the diagenetic sequence and model are illustrated in Figure 14 and Figure 15, respectively.
Dolomitization is the earliest diagenetic event and only occurs in the burrows, resulting in the dolomite replacing the backfilled material and forming the dolomitic mottle texture in the limestone. However, the degree of the replacement decreased from west to east, as evidenced by the micrite being more residual in the eastern part of the study area (Figure 6c).
Due to the spatial difference of the initial dolomitization, subsequent diagenesis also exists between the west and east. In the western part of the study area, the calcite cement CaI filled into the intercrystalline pores of the dolomites (Figure 6b,d). In the eastern part of the study area, due to the relatively incomplete dolomitization, lots of micrites remained and recrystallized during the subsequent burial period and, therefore, present varying diameters (Figure 6c).
The combination of dolomite and calcite cement has a stronger anti-compaction ability [43,44], so the compaction was relatively minor in the west, as evidenced by the mostly rounded particles (Figure 5c) and tangential contact of dolomite rhombs (Figure 6b). Thus, most burrows have been preserved and formed the DML (Figure 4c,f). Due to the weak anti-compaction ability caused by the low degree of dolomitization, most particles were compacted to an ellipse shape (Figure 5d), dolomite rhombs exhibited concave–convex contact (Figure 6c), and the incompletely or slightly dolomitized burrows were destroyed by compaction in the eastern part of the study area (Figure 4a and Figure 6g,h).
The textural relationship between the solid bitumen and other minerals provides a perspective to understand the diagenetic sequence during the middle-deep burial period. Solid bitumen filled in the dissolution pores (Figure 7c,d), indicating that they formed after dissolution. In addition, the solid bitumen covering the calcite cement CaII (Figure 7d) reveals that the CaII formed coeval with or after hydrocarbon charging. Otherwise, there would be no space for hydrocarbon charging.
In addition, the cloudy centers and clear rim texture of dolomites (Figure 6b) indicate there is dolomite recrystallization [45]. It commonly occurs during the burial process and is induced by increasing temperature and pressure [46].

5.2.2. Constraints from Geochemistry

The inferred diagenetic sequence from the petrographic observations is also supported by geochemical data.
Mixing terrigenous detrital materials and the dissemination of Fe- or Mn- oxides will disturb the REEs distribution of marine carbonate rocks [47]. Zr and Th are commonly used to screen for terrestrial debris pollutants [48,49]. In the studied samples, the Zr and Th contents range from 0 to 42.79 ppm (average 3.33 ppm) and 0 to 4.79 ppm (average 0.59 ppm), respectively, both of which were much lower than those in terrestrial debris (the average contents of Zr and Th were 210 ppm and 15 ppm, respectively [50]), indicating that the samples had not been contaminated by terrestrial debris. The lack of correlation between Fe and Mn contents with ΣREE (Figure 16) indicates that the samples had not been contaminated by Fe- or Mn- oxides.
The δ13C and 87Sr/86Sr values of dolomites largely overlap those of Ordovician seawater (Figure 11, [41]), indicating the original dolomitization fluid was seawater-derived. Although some δ18O values of dolomite negative drift from the range of Ordovician seawater, they are still greater than −10.00‰, indicating the formation of dolomite is not affected by the hot fluids [51], which is consistent with the analysis from the negative Eu anomaly of dolomite (Figure 10, [52]). The Mn and Fe contents of dolomite were greater than those of the matrix (Figure 9), as well as the “linear”-type pattern of REE distribution of dolomite (Figure 10), indicating that dolomites were formed in a shallow burial environment [53]. Moreover, the stoichiometry of the dolomites demonstrates a decreasing trend from west to east (Figure 12), indicating that there was a difference in Mg2+-rich fluid supply. These occur when seepage-reflux dolomitization happens (Figure 17). Strong evaporation caused the salinity of seawater to increase, as evidenced by the Znacl value calculated from δ13C and δ18O values, and the hypersaline seawater flowed seawards (i.e., from west to east) along the pathway consisting of burrows and was gradually depleted (Figure 17), which caused the above geochemical features of dolomite.
The majority of the 87Sr/86Sr values of calcite cement CaI overlap those of Ordovician seawater (Figure 9b, [41]), and their REE distribution of CaI appeared to have an “MREE-bulge” structure (Figure 8), with no Ce anomaly (Figure 10), revealing that they were precipitated from marine pore water in the shallow burial period [52]. At the shallow burial period, Fe-oxides adsorbed with MREE were reduced in this zone, releasing rich MREE and forming the MREE bulge [53]. Three samples have very high 87Sr/86Sr values and do not fit with the Ordovician seawater strontium isotope range, which could be attributed to the diagenetic alteration [54,55].
The bright yellow luminescence color and REE distribution of calcite cement CaII are obviously different from those of CaI (Figure 6 and Figure 10), indicating its origin differs from CaI. However, limited by the spatial resolution of carbon and oxygen isotopes analysis, it is difficult to determine the diagenetic fluid source of the CaII; thus, we can only infer its origin is related to hydrocarbon charging based on the textural relationship between solid bitumen and CaII [56].
In addition, geochemical evidence reveals dolomite recrystallization. The δ18O and 87Sr/86Sr values of some dolomite drifted from the range of Ordovician seawater, indicating the progressive recrystallization of dolomite. Due to its temperature sensitivity, the 18O of dolomite will be depleted as the burial depth increases [57]. The 87Sr/86Sr value will be enriched during the burial process [57].

5.3. Reservoir Formation Mechanism Dominated by Dolomitization

Based on the above analysis, the diagenetic events dolomite-mottled limestone (DML) experienced include seepage-reflux dolomitization, compaction, calcite cement CaI cementation, micrite recrystallization, dissolution, hydrocarbon charging, calcite cement CaII cementation, and dolomite progressive recrystallization (Figure 14 and Figure 15). The critical diagenetic events modified the porosity and permeability, including dolomitization, CaI cementation, compaction, dissolution, and CaII cementation (Figure 15).
Dolomitization is the earliest and the most important diagenetic event. The higher the dolomite content, the greater the porosity and permeability of the DML (Figure 8), revealing the control of dolomitization over the reservoir’s quality. On the one hand, dolomitization tends to create pores in the precursor limestone [58]. On the other hand, dolomites, a product of dolomitization, have a strong anti-compaction ability [43,44], which makes the pore space well-preserved in the burial condition. Therefore, the spatial difference of dolomitization caused by the difference in Mg2+-rich fluid supply led to the pores of the DML reservoir in the west being both more initially developed and well preserved than those of the east. This finally caused the porosity and permeability of the western DML reservoir to be greater than the east.
The porosity and permeability of the burrows were about 5 and 10 times that of the matrix (Appendix A), respectively. Solid bitumen, the trace of hydrocarbon charging [59], developed in the intergranular pores (Figure 6b), dissolution pores (Figure 6c), and interfaces of the burrow and matrix (Figure 6a), indicating that the pore created by the dolomitization was major migration path and storage space of hydrocarbon.
Both cementation and dissolution are conducted on the basis of dolomitization. The object of CaI cementation is the intercrystalline pores of dolomite (Figure 6b,d). The objects of dissolution were mainly calcite cement CaI and residual micrite in the intercrystalline pores of dolomite. The object of CaII cementation is dissolution pores. The compaction is also controlled by the dolomitization. Controlled by the spatial difference of dolomitization, the compaction degree presents an increasing trend from west to east (Figure 5c,d), which is contrary to the degree of dolomitization.
Thus, the early dolomitization controlled the DML reservoir formation. It controlled not only the reservoir quality and hydrocarbon migration and storage but also the spatial difference of subsequent diagenetic events.

6. Conclusions

The dolomite-mottled limestone (DML) in the Ma4 Member of the Ordos Basin consists of burrows and matrix. The burrows are mainly filled by dolomite with a smaller amount of micrite, calcite cement, and solid bitumen and are the main occurrence site of diagenetic events. The main diagenetic events include seepage-reflux dolomitization, compaction, calcite cement CaI cementation, micrite recrystallization, dissolution, hydrocarbon charging, calcite cement CaII cementation, and dolomite progressive recrystallization. The dolomitization is critical to the DML reservoir formed. The pore created by dolomitization was the hydrocarbon-migrated pathway and storage space. Due to the difference in Mg2+-rich fluid supply, the degree of dolomitization decreases from the west to the east, which caused the difference in diagenetic evolution in the western and eastern parts of the study area. The high dolomitization degree led to strong anti-compaction ability in the west, contrary to the east. Thus, the porosity and permeability of the DML reservoir of the western part of the study area are greater than those of the east.

Author Contributions

Conceptualization, Z.Y. and X.L.; methodology, X.L.; software, Z.Y.; validation, Z.Y., X.L. and A.W.; formal analysis, Z.Y.; investigation, Z.Y.; resources, L.F. and Z.M.; data curation, L.F. and Z.M.; writing—original draft preparation, Z.Y.; writing—review and editing, X.L.; visualization, K.M. and X.N.; supervision, L.F.; project administration, X.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by PetroChina Changqing Oilfield Company (ZDZX2021-05).

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Appendix A

Table A1. Porosity and Permeability of the Dolomite-Mottled Limestone.
Table A1. Porosity and Permeability of the Dolomite-Mottled Limestone.
WellSample TypeTop Depth (m)Bottom Depth (m)Permeability
(mD)		Porosity
(%)		Content (%)
CalciteDolomiteAcid-Insolu
GP1burrow2664.792664.840.01681.0747.446.95.7
GP1burrow2672.192672.240.01340.5270.826.32.9
GP1burrow2673.212673.260.01610.7677.915.96.2
GP1burrow2674.382674.430.01340.8486.410.82.8
GP1burrow2677.022677.070.00390.6374.622.33.1
GP1burrow2684.072684.120.00590.7177.511.511
GP1burrow2688.352688.40.01571.0156.630.912.5
GP1burrow2689.032689.080.01310.7350.739.49.9
GP1burrow2690.182690.230.02090.5175.519.45.1
GP1burrow2695.022695.070.01311.3659.3346.7
GP1burrow2696.162696.210.01830.9877.512.510
GP1burrow2698.092698.140.03290.487814.37.7
GP1burrow2699.22699.250.02510.3180.510.59
GP1burrow2700.322700.370.02830.5775.9213.1
GP1burrow2701.172701.220.01830.537315.611.4
GP1burrow2703.042703.090.01830.8579.912.37.8
GP1burrow2704.032704.080.01312.0977.714.18.2
GP1burrow2705.062705.110.01971.6160.52910.5
GP1burrow2707.212707.260.01691.1375.916.47.7
GP1burrow2708.152708.20.01691.0584.512.53
GP1burrow2709.212709.260.01130.9766.3249.7
GP1burrow2710.022710.070.01410.3261.5299.5
GP1burrow2711.282711.330.01130.7276.913.29.9
GP1burrow2712.012712.060.01130.4245.845.38.9
GP1burrow2716.232716.280.01131.4779.713.66.7
MT1burrow2627.172627.220.01671.157015.414.6
MT1burrow2634.092634.140.02191.2455.935.58.6
MT1burrow2636.12636.150.01090.746423.712.3
MT3burrow2935.322935.470.01991.215530.414.6
MT3burrow2938.062938.210.00870.9958.431.610
MT3burrow2953.542953.660.00421.3176.4167.6
MT3burrow2954.012954.130.00420.9868.126.15.8
MT3burrow29572957.150.03931.2641.3499.7
MT3burrow2961.022961.170.02161.2447.5475.5
MT3burrow2962.662962.820.00381.1976.916.96.2
MT3burrow2966.092966.280.0111.2354.932.612.5
MT3burrow2969.092969.290.01291.2967.526.16.4
MT3burrow2970.072970.270.00861.1579.215.75.1
MT3burrow2976.882977.040.0040.9274215
MT3burrow2977.662977.770.01391.3172.415.711.9
MT3burrow2978.052978.240.00420.9872.217.310.5
MT3burrow3026.033026.190.03471.4182.213.74.1
MT3burrow3027.113027.290.01321.364.2296.8
S103burrow2833.152833.20.01582.4348.443.58.1
S114burrow2656.8226570.0191.0372.819.97.3
S114burrow2674.522674.640.0490.6376.915.18
S131burrow2392.382392.490.030.685105
S131burrow2395.32395.480.0350.4264342
S131burrow2396.362396.50.0440.967294
MT2burrow2547.652547.70.00760.377.812.110.1
MT2burrow2548.32548.350.01260.4584.310.84.9
SH473burrow3973.53973.550.01061.5465.823.810.4
SH473burrow4032.264032.310.01671.157016.513.5
SH473burrow4038.084038.130.04711.1260.42712.6
J29burrow4052.494052.540.01632.0875.623.11.3
J29burrow4053.334053.380.02921.884441.414.6
J29burrow4055.484055.530.02251.4361.630.67.8
J29burrow4056.014056.060.0112.1168.222.39.5
J29burrow4078.284078.330.01360.9564.126.79.2
J29burrow4079.444079.490.01080.7871.4253.6
J29burrow4080.114080.160.00620.776.418.94.7
J29burrow4082.014082.060.09430.8261.624.913.5
J29burrow4083.374083.420.02110.544.44312.6
J29burrow4085.164085.210.01880.7974.219.46.4
SH476burrow3451.313451.420.00390.9177.418.24.4
SH476burrow3453.163453.30.01050.6967.831.80.4
SH476burrow3454.273454.430.0090.6972.222.25.6
SH476burrow3455.123455.220.00591.3983.7160.3
SH476burrow3458.033458.20.01043.3356.840.42.8
T83burrow3669.43669.60.0271.2357367
JT1burrow3760.23760.250.02540.8866.621.511.9
JT1burrow3764.023764.070.02670.847.837.714.5
JT1burrow3769.253769.30.02150.6354.432.812.8
JT1burrow3773.13773.150.02351.39612514
JT1burrow3776.113776.160.02351.342.646.414.6
JT1burrow3779.153779.20.01450.7867.628.73.7
JT1burrow3795.423795.470.02721.1563.922.513.6
JT1burrow3797.023797.070.01620.297216.411.6
JT1burrow3798.213798.260.03781.571.414.813.8
JT1burrow3819.083819.130.01640.8958.636.64.8
JT1burrow3822.223822.270.01070.8483.112.44.5
JT1burrow3826.353826.40.01340.7671.723.84.5
JT1burrow3827.073827.120.01340.9955.438.46.2
JT1burrow3830.033830.080.01080.8875.614.410
T95burrow3496.053496.10.01331.946034.75.3
T95burrow3499.043499.090.05991.0655.239.15.7
T95burrow3502.23502.250.0240.7975.816.67.6
T95burrow3526.033526.080.06490.3267.225.57.3
T95burrow3528.113528.160.00440.6181.7135.3
T117burrow3526.783526.920.0431.1955.539.64.9
T117burrow3527.283527.480.01830.938411.84.2
T117burrow35283528.190.20751.5149.147.53.4
T117burrow3530.143530.320.02090.5760.227.911.9
T117burrow3533.073533.240.00390.8378.216.15.7
T117burrow3534.163534.330.01050.772.424.33.3
T117burrow3535.13535.20.05021.3857.930.511.6
T104burrow3853.293853.460.01530.7977.815.46.8
T104burrow3854.243854.40.0451.8446.349.44.3
T104burrow3855.333855.480.18782.1650.339.89.9
T104burrow3856.353856.460.00571.4162.326.910.8
T104burrow3857.973858.120.01021.3779.213.27.6
T116burrow33923392.120.00481.624.323.52.2
T97burrow3259.493259.540.05730.9760.1309.9
MT2matrix2542.152542.20.00270.2491.31.27.5
T100matrix3669.043669.090.00210.1391.13.35.6

Appendix B

Table A2. REEs content of dolomite-mottled limestone.
Table A2. REEs content of dolomite-mottled limestone.
Sample IDWell/
Outcrop		Depth
(m)		TypeContent (ppm)LREE/
HREE		MREE/
MREE*		ΣREE (ppm)Ce/
Ce*		Eu/
Eu*
LaCePrNdSmEuGdTbDyHoErTmYbLu
ZD-122T1003669.94matrix0.030.030.020.030.030.020.020.020.020.020.020.020.010.011.801.560.31.021.07
dolomite0.020.030.020.020.020.030.030.030.030.020.030.010.030.011.181.780.331.070.93
residual micrite0.050.050.050.050.040.040.040.050.040.040.040.040.030.051.211.390.611.000.92
CaII0.02 0.02 0.02 0.02 0.02 0.03 0.03 0.04 0.03 0.04 0.04 0.05 0.04 0.01 0.571.860.410.961.14
ZD-40T1043853.72matrix0.040.030.030.030.020.020.020.030.030.030.010.020.020.021.691.430.350.970.69
dolomite0.010.010.010.010.020.020.010.020.020.020.020.010.030.010.791.830.221.011.18
residual micrite0.070.070.060.060.050.060.040.060.060.060.040.040.050.051.471.500.770.981.30
CaI0.030.030.020.030.020.020.020.020.020.030.020.020.020.011.821.550.311.021.01
ZD-16Q443444.5matrix0.030.020.020.020.010.020.010.010.010.010.010.010.010.012.721.270.20.901.78
dolomite0.010.010.010.010.010.010.020.010.010.020.010.010.010.010.841.910.160.980.56
CaI0.030.030.020.020.030.020.020.010.010.020.020.010.010.021.771.370.271.000.84
YX-9LT22816.4matrix0.030.030.030.030.040.030.040.030.020.020.030.020.020.031.391.660.40.980.79
dolomite0.010.010.010.010.010.010.010.010.010.020.010.010.020.010.921.320.161.051.22
residual micrite0.040.040.030.030.030.030.030.020.030.020.020.020.020.011.721.380.371.001.02
AJP-15*AJP/matrix0.040.040.040.040.050.020.040.030.030.030.020.020.010.031.921.670.441.010.44
dolomite0.010.010.010.010.010.010.010.020.010.010.010.010.010.011.091.440.150.970.79
residual micrite0.050.040.040.040.040.030.030.030.020.020.020.010.020.022.571.430.410.970.76
CaI0.000.010.010.010.020.010.020.030.020.030.020.040.040.030.231.530.290.900.87
ZD-153M1053029.33matrix0.040.030.030.030.020.010.020.020.020.010.010.010.010.022.251.230.280.890.37
dolomite0.010.010.010.010.010.020.020.010.010.010.020.010.010.011.101.830.171.021.32
residual micrite0.040.030.030.030.020.010.020.020.020.010.010.010.010.003.571.200.261.040.58
CaI0.030.030.030.030.020.000.030.020.010.020.010.010.020.012.611.280.270.980.19
ZD-32SH4763453.43matrix0.030.020.020.020.020.020.020.020.020.020.010.010.010.002.411.640.240.941.22
dolomite0.020.020.020.020.020.010.010.020.020.020.010.020.020.011.431.380.240.990.56
residual micrite0.040.030.030.030.020.030.010.020.030.020.020.030.010.011.731.390.330.991.83
ZDX-39MT32946.44matrix0.050.050.050.040.040.040.030.040.050.040.040.040.050.041.121.350.60.981.14
dolomite0.010.010.010.010.010.010.020.010.010.010.010.010.010.010.761.930.151.040.54
residual micrite0.040.040.040.050.060.040.070.050.080.120.100.110.170.150.321.191.120.980.69
CaI0.040.040.040.040.040.030.040.050.040.030.040.040.030.041.071.370.540.940.71
CaII0.02 0.02 0.01 0.02 0.02 0.01 0.02 0.05 0.04 0.04 0.03 0.05 0.05 0.02 0.451.680.41.020.55
ZD-93T953533.96matrix0.020.020.030.020.020.010.020.010.010.010.020.010.010.011.821.120.220.890.61
dolomite0.010.010.020.010.010.010.020.010.020.010.020.020.010.020.851.220.20.980.69
Y-60M762671.3dolomite0.010.010.010.010.020.010.020.010.010.010.020.020.010.030.461.310.21.040.80
CaI0.070.070.080.070.090.110.070.050.040.040.020.010.030.023.662.150.771.021.44
ZDX-58MT12643.48dolomite0.020.020.020.020.030.010.010.020.020.030.020.020.020.010.911.700.270.900.60
CaI0.160.180.190.210.230.210.230.210.170.140.110.110.090.081.902.102.321.020.90
Note that sample ID marked with * represents the sample was obtained in outcrop and MREE*, Ce*, and Eu* represent the expected concentrations of these fractions or elements based on a flat or uniformly inclined REE distribution.

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Minerals 13 01172 g002
Figure 2. The profile of T100-SH476-M105-MT3-M76 wells (see Figure 1a for the profile location).
Figure 2. The profile of T100-SH476-M105-MT3-M76 wells (see Figure 1a for the profile location).
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Minerals 13 01172 g003
Figure 3. Petrographic feature of dolomite-mottled limestone. (a) Core photo, T100 well, 3668.3 m; (b) Sketch of photo (a).
Figure 3. Petrographic feature of dolomite-mottled limestone. (a) Core photo, T100 well, 3668.3 m; (b) Sketch of photo (a).
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Minerals 13 01172 g004
Figure 4. Type of dolomite-mottled limestone (a) core photo, red arrows in figure indicate the burrows gradually disappear, M105 well, 3030 m. (b) Core photo, Q44 well, 3444.5 m. (c) Core photo, T95 well, 3553.96 m. (d) Sketches of photo (a). (e) Sketches of photo (b). (f) Sketches of photo (c).
Figure 4. Type of dolomite-mottled limestone (a) core photo, red arrows in figure indicate the burrows gradually disappear, M105 well, 3030 m. (b) Core photo, Q44 well, 3444.5 m. (c) Core photo, T95 well, 3553.96 m. (d) Sketches of photo (a). (e) Sketches of photo (b). (f) Sketches of photo (c).
Minerals 13 01172 g004
Minerals 13 01172 g005
Figure 5. Petrographic features of the matrix of DML. (a) Photomicrograph of mudstone matrix, GP1 well, 2677.61 m. (b) SEM Photomicrograph of wackestone matrix, MT1 well, 2639.80 m. (c) Photomicrograph of wackestone matrix, note that the particles have a rounded shape, SH476 well, 3452.74 m. (d) Photomicrograph of wackestone matrix, note that the particles were flattened, LT2 well, 2816.4 m.
Figure 5. Petrographic features of the matrix of DML. (a) Photomicrograph of mudstone matrix, GP1 well, 2677.61 m. (b) SEM Photomicrograph of wackestone matrix, MT1 well, 2639.80 m. (c) Photomicrograph of wackestone matrix, note that the particles have a rounded shape, SH476 well, 3452.74 m. (d) Photomicrograph of wackestone matrix, note that the particles were flattened, LT2 well, 2816.4 m.
Minerals 13 01172 g005
Minerals 13 01172 g006
Figure 6. Petrographic features of the burrows of DML. (a) The burrow is mainly filled with dolomites, T95 well, 3533.96 m. (b) Most of the dolomite rhombs presented as tangential contact, and the intercrystalline pores were cemented by the calcite cement CaI, T95 well, 3533.96 m. (c) Most of the dolomite rhombs exhibit concave–convex contact, and the intercrystalline pores are filled with the residual micrite with varying diameters, M76 well, 2671.3 m. (d) The CL photo of CaI (upper image) and its polarized light photo (lower image)—note that the luminescence color is dim, T95 well, 3533.96 m. (e) The CL photo of CaII—note that the luminescence color is bright yellow, MT3 well, 2946.44 m. (f) The residual intercrystalline pores, T100 well, 3669.42 m. (g) Burrow disappears along the direction indicated by the arrow, LT2 well, 2816.4 m. (h) Burrow disappears along the direction indicated by the arrow, MT3 well, 2957.82 m.
Figure 6. Petrographic features of the burrows of DML. (a) The burrow is mainly filled with dolomites, T95 well, 3533.96 m. (b) Most of the dolomite rhombs presented as tangential contact, and the intercrystalline pores were cemented by the calcite cement CaI, T95 well, 3533.96 m. (c) Most of the dolomite rhombs exhibit concave–convex contact, and the intercrystalline pores are filled with the residual micrite with varying diameters, M76 well, 2671.3 m. (d) The CL photo of CaI (upper image) and its polarized light photo (lower image)—note that the luminescence color is dim, T95 well, 3533.96 m. (e) The CL photo of CaII—note that the luminescence color is bright yellow, MT3 well, 2946.44 m. (f) The residual intercrystalline pores, T100 well, 3669.42 m. (g) Burrow disappears along the direction indicated by the arrow, LT2 well, 2816.4 m. (h) Burrow disappears along the direction indicated by the arrow, MT3 well, 2957.82 m.
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Minerals 13 01172 g007
Figure 7. Petrographic features of the solid bitumen. (a) Solid bitumen enriched at the interface of mottle and matrix, M105 well, 3029.33 m. (b) Solid bitumen filled in the residual intercrystalline pores of dolomite, T95 well, 3496.18 m. (c) Solid bitumen filled in the dissolution pores, T100 well, 3669.94 m. (d) Solid bitumen covered the calcite cement CaII, T104 well, 3853.72 m.
Figure 7. Petrographic features of the solid bitumen. (a) Solid bitumen enriched at the interface of mottle and matrix, M105 well, 3029.33 m. (b) Solid bitumen filled in the residual intercrystalline pores of dolomite, T95 well, 3496.18 m. (c) Solid bitumen filled in the dissolution pores, T100 well, 3669.94 m. (d) Solid bitumen covered the calcite cement CaII, T104 well, 3853.72 m.
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Minerals 13 01172 g008
Figure 8. Relationship between the porosity, permeability, and dolomite content of DML. (a) Cross-plot of porosity against dolomite content. (b) Cross-plot of permeability against dolomite content.
Figure 8. Relationship between the porosity, permeability, and dolomite content of DML. (a) Cross-plot of porosity against dolomite content. (b) Cross-plot of permeability against dolomite content.
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Minerals 13 01172 g009
Figure 9. Histogram of trace element contents of the matrix, dolomite, residual micrite, and calcite cements CaI and Ca II of DML (note that Ni content in the figure is the average value of the sample × 10).
Figure 9. Histogram of trace element contents of the matrix, dolomite, residual micrite, and calcite cements CaI and Ca II of DML (note that Ni content in the figure is the average value of the sample × 10).
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Minerals 13 01172 g010
Figure 10. Average REE distribution of the matrix, dolomite, residual micrite, and calcite cements CaI and CaII of DML.
Figure 10. Average REE distribution of the matrix, dolomite, residual micrite, and calcite cements CaI and CaII of DML.
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Minerals 13 01172 g011
Figure 11. Isotopic features of the DML. (a) Cross-plot of δ13C and δ18O values of the matrix and dolomite. (b) 87Sr/86Sr ratios of matrix, dolomite, CaI and CaII. The small squares of different colors represent different sample points. The δ13C, δ18O and 87Sr/86Sr values range of Ordovician seawater is marked with blue box, data from reference [41].
Figure 11. Isotopic features of the DML. (a) Cross-plot of δ13C and δ18O values of the matrix and dolomite. (b) 87Sr/86Sr ratios of matrix, dolomite, CaI and CaII. The small squares of different colors represent different sample points. The δ13C, δ18O and 87Sr/86Sr values range of Ordovician seawater is marked with blue box, data from reference [41].
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Minerals 13 01172 g012
Figure 12. Lateral variability in stoichiometry of the dolomite.
Figure 12. Lateral variability in stoichiometry of the dolomite.
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Minerals 13 01172 g013
Figure 13. The refuse dump model of infauna behavior. (a) Infauna feed deep in the sediments and excrete feces on the sediment surface, (b) followed by introducing surface material into the burrow to compensate for the space created during the feeding process. Modified from [42].
Figure 13. The refuse dump model of infauna behavior. (a) Infauna feed deep in the sediments and excrete feces on the sediment surface, (b) followed by introducing surface material into the burrow to compensate for the space created during the feeding process. Modified from [42].
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Minerals 13 01172 g014
Figure 14. Paragenetic sequence of the dolomite-mottled limestone (note that the thickness of rectangles represents the relative intensity of the diagenesis event).
Figure 14. Paragenetic sequence of the dolomite-mottled limestone (note that the thickness of rectangles represents the relative intensity of the diagenesis event).
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Minerals 13 01172 g015
Figure 15. Sketches of the paragenetic evolution of dolomite-mottled limestone.
Figure 15. Sketches of the paragenetic evolution of dolomite-mottled limestone.
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Minerals 13 01172 g016
Figure 16. (a) Cross-plot of ΣREE and Mn contents of the dolomite-mottled limestone. (b) Cross-plot of ΣREE and Fe content of the dolomite-mottled limestone (the red lines are the fitted trend line).
Figure 16. (a) Cross-plot of ΣREE and Mn contents of the dolomite-mottled limestone. (b) Cross-plot of ΣREE and Fe content of the dolomite-mottled limestone (the red lines are the fitted trend line).
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Minerals 13 01172 g017
Figure 17. The seepage-reflux dolomitization model of the dolomite-mottled limestone.
Figure 17. The seepage-reflux dolomitization model of the dolomite-mottled limestone.
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Table 1. Elemental composition and Sr isotopic composition of the analyzed dolomite-mottled limestone.
Table 1. Elemental composition and Sr isotopic composition of the analyzed dolomite-mottled limestone.
Sample IDWellDepth (m)TypeContent (ppm)Stoichiometry (Mole% MgCO3)87Sr/86Sr
MgMnFeZrThNi
ZD-122T1003669.94matrix/14.09960.111.490.310.93 /0.70926
dolomite117,494.79 27.1915,117.390.390.362.80 47.17 0.70953
residual micrite/21.171324.621.600.671.17 /0.7092
CaII/16.78360.3700.010.45/0.71016
ZD-40T1043853.72matrix/13.83477.580.320.320.35 /0.70875
dolomite125,438.07 38.78620.310.630.350.65 48.80 0.70883
residual micrite/44.651684.169.340.972.74 /0.70883
CaI/14.31477.610.120.780.49 /0.70952
ZD-16Q443444.5matrix/10.29480.060.140.120.82 /0.70878
dolomite114,602.07 28.661695.241.330.322.75 46.55 0.70921
CaI/7.61438.400.300.130.41 /0.70890
YX-9LT22816.4matrix/9.332464.080.970.431.58 /0.7087
dolomite110,832.62 21.98868.210.910.210.46 45.72 0.70904
residual micrite/15.781768.801.310.551.76 /0.70877
AJP-15 *AJP/matrix/7.881861.830.240.141.03 /0.70903
dolomite103,979.94 22.53754.860.300.110.34 44.14 0.70941
residual micrite/7.40552.870.510.220.47 /0.70912
CaI/6.18474.570.000.010.30 /0.71020
ZD-153M1053029.33matrix/7.58591.900.480.230.63 /0.70892
dolomite112,566.29 21.951197.760.830.310.75 46.10 0.70986
residual micrite/10.801015.990.650.291.31 /0.70912
CaI/7.17552.710.190.170.67 /0.70944
ZD-32SH4763453.43matrix/8.07542.980.420.110.36 /0.70922
dolomite116,284.90 22.35934.430.970.240.81 46.91 0.71005
residual micrite/14.30762.240.650.250.81 /0.70914
ZDX-39MT32946.44matrix/11.74710.374.740.740.74 /0.70942
dolomite112,662.96 26.732455.000.770.100.62 46.12 0.71002
residual micrite/61.576822.1142.794.799.61 /0.70890
CaI/11.02617.610.192.150.43 /0.70913
CaII/30.01389.090.110.040.38/0.71068
ZD-93T953533.96matrix/10.32907.130.280.111.02 /0.70938
dolomite117,101.37 21.11464.080.070.130.26 47.09 0.70898
CaI///////0.70932
Y-60M762671.3dolomite108,622.35 30.84881.210.760.160.39 45.22 0.70973
CaI/10.93359.402.600.130.52 /0.71057
ZDX-58MT12643.48dolomite113,202.68 26.35958.030.780.220.81 46.24 0.70892
CaI/6.40280.010.000.420.30 /0.71111
Note that sample ID marked with * represents the sample was obtained in outcrop.
Table 2. The isotopic composition of the dolomite-mottled limestone.
Table 2. The isotopic composition of the dolomite-mottled limestone.
Sample IDWellDepth (m)Typeδ13Cδ18OZNacl
(VPDB, ‰)(VPDB, ‰)
Y-60M762671.3matrix−1.2−6.1121.77
burrow−0.6−6.3122.93
ZDX-58MT12643.48matrix0−5.7124.44
burrow−0.2−6.2123.8
ZD-153M1053029.33matrix−0.2−8.1122.87
burrow0.7−8124.79
ZDX-39MT32946.44matrix−0.3−8.8122.28
burrow−0.6−8.5121.72
YX-9LT22816.4matrix−0.1−7.8123.15
burrow0.6−6.7125.24
ZD-16Q443444.5matrix−0.8−6.3122.58
burrow0.3−5.6125.12
AJP-15 *AJP/matrix−1.1−9120.53
burrow−0.6−6.3122.99
ZD-93T953533.96matrix0−5.7124.45
burrow0.3−5.2125.32
ZD-40T1043853.72matrix0.2−6.3124.54
burrow0.9−6.1126.13
ZD-32SH4763453.43matrix−0.4−7.8122.61
burrow0.1−7.4123.85
ZD-122T1003669.94matrix0.4−5.5125.41
burrow0.5−4.5125.97
Note that sample ID marked with * represents the sample was obtained in outcrop.
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Yang, Z.; Wang, A.; Fan, L.; Ma, Z.; Luo, X.; Ning, X.; Meng, K. Paragenesis and Formation Mechanism of the Dolomite-Mottled Limestone Reservoir of Ordovician Ma4 Member, Ordos Basin. Minerals 2023, 13, 1172. https://doi.org/10.3390/min13091172

AMA Style

Yang Z, Wang A, Fan L, Ma Z, Luo X, Ning X, Meng K. Paragenesis and Formation Mechanism of the Dolomite-Mottled Limestone Reservoir of Ordovician Ma4 Member, Ordos Basin. Minerals. 2023; 13(9):1172. https://doi.org/10.3390/min13091172

Chicago/Turabian Style

Yang, Zeguang, Aiguo Wang, Liyong Fan, Zhanrong Ma, Xiaorong Luo, Xinghui Ning, and Kun Meng. 2023. "Paragenesis and Formation Mechanism of the Dolomite-Mottled Limestone Reservoir of Ordovician Ma4 Member, Ordos Basin" Minerals 13, no. 9: 1172. https://doi.org/10.3390/min13091172

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