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Article

Karstification and Its Role in the Formation of Carbonate Reservoirs: A Case Study from the Ordovician Majiagou Formation in Jingbian, Ordos Basin, North China

by
Xiaoxia Peng
1,
Guobin Li
2,
Xin Cheng
1 and
Ling Guo
1,*
1
State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China
2
Northwest Branch, PetroChina Research Institute of Petroleum Exploration & Development, Lanzhou 730020, China
*
Author to whom correspondence should be addressed.
Minerals 2023, 13(11), 1425; https://doi.org/10.3390/min13111425
Submission received: 24 September 2023 / Revised: 31 October 2023 / Accepted: 3 November 2023 / Published: 9 November 2023
(This article belongs to the Section Mineral Exploration Methods and Applications)
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Abstract

:
The discovery of natural gas reserves in Lower Paleozoic carbonate formations has generated significant enthusiasm regarding the potential for oil and gas exploration in the deeper carbonate reservoirs of the Ordos Basin. Significant progress has been made in the field of natural gas exploration, especially within the carbonate reservoirs of the Ordovician Majiagou Formation. In this study, we conducted a comprehensive analysis, including microscopic examination of thin sections and the inspection of 60-m cores from five wells, to classify pore types and investigate various forms of karstification in the fifth member of the Majiagou Formation. Our findings have identified distinct pore types, including interparticle pores, separate-vuggy pores, and touching-vuggy pores, in this formation. The dominant forms of karstification in this area were identified as syngenetic karstification, weathering crust karstification, and burial karstification. Importantly, our investigation emphasizes the significant influence of factors such as sea level fluctuations, exposure duration, and the presence of acidic formation water in shaping the observed patterns of karstification. Syngenetic karstification and shallow-buried karstification have emerged as key stages in the formation of natural gas reservoirs. Moreover, our research emphasizes the importance of structural and composite fractures observed in the fifth member of the Majiagou Formation. These unfilled fractures serve as crucial microfracture networks, facilitating oil and gas migration and contributing to the overall development of reservoirs. The research results are of great significance for understanding the formation process of carbonate reservoirs in the Majiagou Formation, Ordos Basin.

1. Introduction

The process of karstification in carbonate rock formations has been extensively documented and bears substantial relevance to the field of oil and gas exploration. In recent decades, there has been a surge in interest regarding fractured and karstified carbonate oil and gas reservoirs, as well as aquifers [1,2,3,4,5].
In China, marine carbonate sequences are notably abundant in oil and gas resources, and paleokarst carbonate reservoirs are a pivotal type of reservoir. Remarkable breakthroughs have been achieved in the study of paleokarst reservoirs, such as the identification of a dolomite gas reservoir in the Ordovician Majiagou Formation in the central Ordos Basin [6,7,8,9], as well as the exploration of an Ordovician limestone reservoir in the Tahe oilfield, Tarim Basin [10,11,12,13]. Additionally, the Carboniferous dolomite gas reservoir in the Huanglong Formation within the Sichuan Basin serves as an exemplary case study [14,15,16].
In the Middle Ordovician, marine carbonate strata was deposited within the Majiagou Formation in the eastern region of the Ordos Basin. Towards the conclusion of the Middle Ordovician, the North China platform underwent uplift and remained exposed until sedimentation recommenced during the Late Carboniferous. Throughout this uplift phase, a sedimentary discontinuity layer developed, reaching thicknesses of up to 150 m [17]. In particular, the Ma54–Ma51 sub-members situated in the upper section of the Majiagou Formation experienced pronounced weathering and leaching processes. The resultant reservoir underwent substantial superposition transformation, leading to the development of weathering crust karst and buried karst [9,18].
The development of carbonate reservoirs is intricately controlled by three primary geological factors: sedimentation, diagenesis, and tectonic stress [19,20,21,22,23]. Microphases of deposition serve as the foundational material for reservoir formation and development. Structural fractures play a pivotal role in reservoir development, with paleokarst acting as a critical factor in fracture formation [24,25]. Carbonate rock karstification can be categorized into three distinct types based on the timing and environmental conditions: syngenetic karstification, weathering crust karstification, and buried karstification [24,26]. An examination of previous research indicates that the three karstification types have had different impacts on the development of the reservoir in the Majiagou Formation in the Jingbian region of the Ordos Basin. However, detailed investigations into the extent of their respective impacts have been lacking, and such insights are crucial for guiding natural gas exploration efforts.
In the Ma54–Ma51 sub-members of the Jingbian area, the gas-producing reservoir primarily consists of gypsum mold pore mud silty dolomite. Several hypotheses have been proposed to elucidate the genesis of this reservoir. The first perspective suggests that the reservoir formed during the initial sedimentary phase, influenced by sea-level fluctuations and the dissolution of atmospheric freshwater [17,27]. An alternative viewpoint posits that the reservoir developed during a period of exposure to supergene rocks. In this scenario, the dissolution of anhydrous nodules through supergene karstification is considered pivotal [9,17,28]. Another hypothesis suggests that the reservoir took shape during the exposed karst period of the Majiagou Formation, spanning from the Caledonian to the Early Hercynian era. According to this theory, lithofacies and paleogeographic units played a decisive role in determining the distribution of high-quality reservoirs [29]. Finally, there is the theory that the reservoir’s formation occurred during the burial diagenetic stage, with the dissolution of anhydrous nodules driven by organic acid activity in a medium-deep burial environment [17,30].
Extensive weathering, denudation, and karst superposition transformation have significantly altered both macro and micro characteristics of the Majiagou Formation in the Jingbian area. These transformations have posed challenges to gaining a comprehensive understanding of the reservoir’s characteristics. Consequently, our study is centered on newly acquired drilling cores from the Ma54–Ma51 sub-members, which host a recently discovered gas field within the Ordos Basin. Through systematic core observation, detailed description, analysis of logging data, examination of conventional thin sections, and the study of injected thin sections, our aim is to provide insights into the type and developmental characteristics of reservoir karstification, the underlying karstification mechanisms, and the pore development patterns within the sub-members. Furthermore, we explore the influence of dolomite karstification on the reservoir. These findings hold significant promise for advancing reservoir prediction models and will serve as valuable guidance for the exploration of similar natural gas fields in our study area.

2. Geological Setting

The Ordos Basin holds significant importance as one of China’s primary petroliferous basins, encompassing an extensive area of approximately 371,000 km2 along the western margin of the North China platform (Figure 1a) [9]. Structurally, the basin’s Mesozoic characteristics allow for its division into six secondary structural units: the Weibei uplift, the Yimeng uplift, the Western Shanxi flexure fold belt, the Western edge thrust belt, the Tianhuan depression, and the Yishan slope. Specifically, the Jingbian gas field is strategically situated within the central expanse of the Yishan slope, covering an area of roughly 9000 km2 [29] (Figure 1b).
The Ordovician Majiagou Formation in the Jingbian area is stratified into six members, denoted as Ma1 through Ma6 from bottom to top. The Ma1, Ma3, and Ma5 members primarily consist of argillaceous silty dolomite, gypsum, and salt rock, while the Ma2, Ma4, and Ma6 members are predominantly characterized by argillaceous limestone or granular argillaceous limestone [31,32] (Figure 1c). During the Middle Ordovician Majiagou deposition period, the Jingbian area underwent the development of a marine restricted or semi-restricted platform environment within a hot, arid climatic setting. This period saw the deposition of various carbonate rocks, including mud powder crystal dolomite, mud powder crystal dolomite containing anhydrite nodules, and columnar crystals [17,33,34].
Towards the conclusion of the Middle Ordovician, the entire North China platform experienced a significant uplift, enduring approximately 150 million years of weathering and denudation. This extended period of exposure subjected the Majiagou Formation to multi-stage and long-term weathering, denudation, and karst transformation. As a result, the Ma6 member at the top of the Majiagou Formation is nearly absent in the study area [6,9,35,36]. The Ma54–Ma51 sub-members within the study area have also been subject to varying degrees of denudation. The remaining reservoir rocks consist primarily of anhydrite nodule mud silty dolomite, anhydrite columnar crystal mud silty dolomite, and karst breccia dolomite of the Majiagou Formation. Notably, paleokarst features are predominantly found within anhydrite nodules and mud powder dolomite containing anhydrite columnar crystals. Because anhydrite exhibits considerably higher solubility in comparison to dolomite, dolomite compositions with a greater concentration of anhydrite, particularly nodule anhydrite, have led to the formation of a larger number of pores [6].

3. Methodolgy

3.1. Core Analysis

We performed core investigation in wells Y904, Y917, Y918, Y969, and S301 in Jingbian, Ordos Basin. Preliminary qualitative characterization of carbonated lithologies, laminations, and karst dissolution-related features was performed. A total of 120 plugs (2.5 cm diameter) were collected and cut to a height of 3 cm for petrographic and petrophysical analysis.

3.2. Laboratory Analysis

We performed the petrographic analysis of 120 thin sections obtained from the rock plugs to document and analyze the porosity types, diagenetic processes, and percentages at the micrometer scale. A Zeiss Axio Scope.A1 polarized light microscope with a digital camera was used for the petrographic study. We followed Lucia (2007) to describe the porosity [11]. Colored epoxy impregnation was used to define the pore space.

4. Results

In the carbonates, karstification ranged from slight to intensely karstified beds in drilling cores and thin sections (Figure 2). Dissolution is weakly localized along bedding surfaces (Figure 2a), bed-parallel stylolites (Figure 2b,c), and fractures, which are partially or completely filled with calcite cement (Figure 2d). The Majiagou Formation in Jingbian, Ordos Basin, is made up of four main facies: carbonate packstones, carbonate grainstones, crystalline carbonates, and calcirudyte (Figure 2g–i). The carbonates include karst dissolution cavities such as vugs, channels, and circlular-to-elongated caves (Figure 2e,f).

4.1. Pore Types

Pore types were identified following Lucia’s (2007) classification [11]. The pore types commonly found in the carbonate rocks are as follows: (I) interparticle pores, consisting of both intercrystal pores (Figure 3a) and intergrain pores (Figure 3b); (II) separate-vuggy pores, including moldic pores (Figure 3c,d) and intraparticle pores. Intraparticle pores are further subdivided into intragrain (Figure 3e), intracrystal (Figure 3f); (III) touching-vuggy pores, including cavernous pores (Figure 4a), fractures partially dissolved (Figure 4b), partially filled solution-enlarged fractures (Figure 4c), fractures connecting moldic pores (Figure 4d,e), and microfractures in calcirudyte (Figure 4f).

4.2. Syngenetic Karstification

Syngenetic karstification is widely developed in the carbonate rocks of the Majiagou Formation in the Jingbian area of the Ordos Basin, driven by sea-level rise and fall. Some typical signs and characteristics of syngenetic karstification are as follows.

4.2.1. Selective Dissolution

Selective dissolution of the fifth member of the Majiagou Formation in the Jingbian area primarily affected the silty dolomite in the member. The selective dissolution is most obviously manifested in intermittent exposure of dolomite containing anhydrite, formed in a flat tidal environment during sea level decline. Extensive atmospheric rainfall means that the freshwater body was not saturated with CaSO4, and anhydrite agglomerates or nodules were selectively dissolved to form mold spores and intragranular dissolution pores. Moldic holes formed by the dissolution of plate- or strip-shaped anhydrite are found in Ma5 (Figure 5a). Some plate-shaped anhydrite has only partially dissolved in the interiors and at the edges of particles, forming intragranular dissolution holes (Figure 5b). Anhydrite nodules are distributed in silty dolomite containing gypsum mud in the form of spots (Figure 5c,d), and the pores formed in ‘honeycombs’ after dissolution (Figure 5e). The middle and lower parts of moldic pores formed after dissolution of anhydrite nodules are filled with silt, while the upper parts are not filled, forming a distinct top–bottom structure. At the same time, microfractures developed around some of the nodules (Figure 5f), which play a positive role in improving the reservoir.

4.2.2. Solution Fracture, Lithology Mutation, and Brecciation

Solution fractures and caves are found in the dolomite reservoir in Ma5 in the Jingbian area. The solution fractures are often distributed along the stratum, with irregular boundaries (Figure 5g). The extension lengths of the fractures are often short, 1–3 cm, but they are densely distributed, and the interiors are not filled (Figure 5h). Small, near-circular, karst caves are also relatively common in Ma5 and are not filled (Figure 5i).
In periods of high-frequency sea-level change, especially regression–transgression transition periods, transgressive sediment filling, gravity faulting, and slump deposition are common [9] and provide the most direct evidence of syngenetic karstification. Sediment filling during transgression occurred often in Ma5 in the Jingbian area, and the lithology changes from gray silty fine-grained dolomite to dark gray dolomite mudstone. Collapsed deformation structures also occur in the upper transgression filling species (Figure 5j), and some unconsolidated gravels are also found (Figure 5k). Brecciation is not widely developed in Ma5 in the study area. The breccia is light gray in color, with particle sizes of 0.2–1 cm. Most of the breccias are sub-angular and are filled with silt or fragments of the surrounding rock (Figure 5l).

4.3. Weathering Crust Karst

The development of weathering crust karst is related to long-term exposure of strata over large areas and is often related to the development of unconformities [24,37]. The identifying features of weathering crust karst include the development of erosion unconformities, underground dissolution, associated hole systems, and internal filling [24].

4.3.1. Erosion Unconformity

At the end of the early Ordovician, the entire Ordos Basin was uplifted, so the basin has no Silurian, Devonian, or lower Carboniferous strata. Stratigraphic correlation of a bottom-leveled well in the Ma541 sub-member shows that the residual thicknesses of the Ma53–Ma51 sub-members in the Majiagou Formation are different, with the Ma511 stratum retained in well J2. By contrast, the Ma51 and Ma52 sub-members are denuded in well Y902-3, indicating an unconformity surface at the top of the Majiagou Formation (Figure 6), which is stratigraphic evidence of paleokarst at the top of the Ordovician system in the formation. The occurrence of an unconformity surface at the top of the Ordovician has also been confirmed in a number of previous studies (Figure 7) [38,39], as has the development of ancient weathering crust and paleokarst in the formation [7,8,9,36,40].

4.3.2. Karst Characteristics of Weathered Crust

Karst breccias are observed in the Ma55–Ma51 sub-members in the Jingbian area. The karst breccia gravel is of large diameter, up to 5 cm. The gravels are generally disorderly and poorly sorted. The larger gravel particles are well rounded, with the small gravels filled between the larger particles being poorly rounded and mostly of sub-angular shape (Figure 8a). In some karst breccias, the gravel presents a high-angle vertical shape (Figure 8b) or is embedded in a fine-grained matrix. The matrix shows plastic deformation, characteristic of in situ collapse and sedimentation (Figure 8c). Some karst breccia gravels are weakly oriented, reflecting short-distance transportation, and the matrix is primarily gray and dark gray (Figure 8d,e). In breccias affected by water flow, the gravel is well-sorted and well-rounded. Fine-grained matrixes represent a flowing state (Figure 8f), reflecting transportation and deposition over greater distances.

4.4. Buried Karstification

Buried karstification refers to the phenomenon and process of dissolution in the middle and deep burial stage, which is mainly related to burial diagenesis. Buried karstification is manifested in further expansion and dissolution of pre-existing fracture networks and pores [24,41,42]. Organic acids associated with the thermal evolution of organic matter and hydrocarbon generation play an essential role in this dissolution process [43].
After weathering and denudation, with the unconformity developed, the Majiagou Formation entered a burial period. Under pressure from overlying strata, acid pore water generated during hydrocarbon generation formed dissolution pores and fractures in the dolomite reservoir. In addition, under the action of overlying pressure or tectonic extrusion pressure, the consolidated rock dissolved, forming suture lines (Figure 8i). Secondary calcite (Figure 8g) and quartz (Figure 8h) developed in solution moldic pores in the study area, perhaps following reactions between pore water and rock. When the calcite reached equilibrium and achieved supersaturation, it precipitated, filling the moldic pores. At the same time, K+, SiO2, and Al2O3 formed minerals such as quartz that filled the solution mold pores. Recent research has shown that solution moldic pores in the Ordovician Majiagou Formation in the Ordos Basin have experienced multi-stage filling. The first stage is filling with silt formed at the same time as the moldic pores. The second stage is filling with anhydrite, automorphic dolomite, or continuous calcite. The third stage is filling with iron-free calcite and quartz, and the fourth stage is filling with iron dolomite or iron calcite [5]. Buried karstification of the Majiagou Formation in the study area produced sutures, calcite, and dolomite precipitated in moldic pores, and also structural fractures, solution expansion structural fractures (Figure 8j), and intergranular solution pores (Figure 8l). In addition, some solution mold pores are filled with minerals such as fluorite and quartz (Figure 8k), which may be related to deep hydrothermal activity.

5. Discussion

5.1. Syngenetic Karst and Its Control on Formation of Pores

Syngenetic karst refers to early exposed karst caused by freshwater leaching due to sea level fluctuations or thinning of deposition during sedimentation [5,9,24]. In regressive sedimentary sequences, shallow-water carbonate sedimentary bodies are sometimes exposed to the sea surface or atmospheric freshwater leaching environments, forming pores of varied sizes and shapes in carbonate rocks. The results of this study show that it is generally difficult for ancient weathering crusts to form from syngenetic karstification. However, this karst type displays some typical identifying features, including seepage silt, a top–bottom structure, mold pores, and intragranular dissolution pores formed by selective dissolution [44], near original brecciation, a lithologic catastrophe surface, dissolution fractures, and small karst caves [45].
Syngenetic karstification occurs in a syngenetic atmospheric diagenetic environment, controlled by sea-level rise and fall. Dissolution pores formed by selective dissolution, such as mold pores and intragranular dissolution pores in carbonate rocks, are generally regarded as the primary identifying characteristics of syngenetic karstification in carbonate rocks [24].
During syngenetic karstification of the Ma54–Ma51 sub-members of the Majiagou Formation in the study area, the formation of dolomite intergranular pores was accompanied by the flow of atmospheric fresh water. Firstly, aragonite, anhydrite, and other minerals that migrate easily dissolved to form moldic solution pores (Figure 5a–c). Anhydrite nodules expanded in volume during the dissolution process and microfractures formed along their peripheries. During the later period, further dissolution occurred, forming annular dissolution fractures along the nodules (Figure 5f). This enhancement of earlier dissolution formed channels that connected adjacent mold dissolution holes and intergranular solution holes to form a karst area with a ‘honeycomb’ structure (Figure 5e). As dissolution continued, lath and nodule anhydrite gradually migrated away due to chemical dissolution and mechanical transportation, leaving solution cracks and caves (Figure 5g,i). Subsequently, increased numbers of solution moldic pores, intergranular matrix pores, and microfractures formed during burial compaction, with the isolated solution mold pores, intergranular pores, and karst caves becoming connected, which significantly improved the permeability of the reservoir. The reservoir space at this time mainly consisted of solution moldic pores and intergranular solution pores, providing optimal reservoir physical properties with an average porosity of 3.54% and an average permeability of 0.53 × 10−3 μm2. This kind of mud silty dolomite with solution moldic pores is the most important reservoir rock type in the Ma54–Ma51 sub-members.

5.2. Weathering Crust Karst and Its Control on the Formation of Pores

Weathering crust karstification forms various types of holes and karst landforms [29], modifying the initial stratigraphic structure into karst rocks. Common karst rocks include karst formation and karst transformation rocks [6]. Karst breccia is the basic type of karst formation rock. The development of karst breccia is one of the most easily identifiable features of weathering crust karstification [29].
Based on previous research results, and according to karst hydrodynamics, the aquifer is divided into a surface karst zone (deep green in Figure 9), a horizontal undercurrent zone (orange, blue, and yellow-green in Figure 9), and a deep slow flow zone (green in Figure 9). By analyzing the rock and pore characteristics of the weathering crust karst, we assembled a model of weathering crust karst, layer distribution, and karst zoning (Figure 9).
Some karst breccia and other surface residues with collapse origins are found in the surface karst zone (Figure 2i). Intergranular pores, supported by debris, are developed in the breccia dolomite (Figure 10b). Some microcracks (Figure 10a,d,f,g) or residual holes (Figure 10b) also occur at the edges of or inside the particles.
The vertical seepage zone is in the underground seepage zone, above the surface and on the underground phreatic water surface. Atmospheric freshwater containing CO2 in the upper part infiltrates the structural fractures, physically and chemically dissolving the flowing carbonate rocks, forming solution expansion joints and small karst caves vertically (Figure 10e,f). These spaces are mostly filled with mud, sand, and materials seeping from various sources. The vertical seepage zone in the Ma5 member is mostly in the upper part of the Ma51 sub-member and has medium reservoir performance.
A horizontal subsurface flow zone lies generally 20–300 m below the erosion surface and forms the central part of the aquifer [24]. The horizontal subsurface flow zone of Ma5 is mostly in the Ma54 sub-member and the lower part of the Ma51 sub-member (Figure 9), encompassing a medium dissolution sub-zone with substantial dissolution and good reservoir performance. However, the abundant formation of cloud breccia and a high concentration of complex matrices within the intense dissolution sub-zone result in inferior reservoir physical properties. In the horizontal subsurface flow zone, groundwater flows with strong dissolution ability formed nearly horizontal solution fractures and caves, as well as underground rivers and karst breccia formed by collapse. For example, in well Y917, a solution fracture in the silty dolomite of the Ma5 member is 10 cm wide with an irregular boundary (Figure 5g). The edges of solution fractures were further corroded and expanded by dissolution in the burial stage (Figure 5g). Vertical solution fractures and nearly horizontal bedded dissolution joints also occur in the silty dolomite of Ma5 (Figure 5h), indicating that the dissolution of this layer may be affected by both the vertical seepage of groundwater and horizontal subsurface groundwater flow. In addition, some anhydrite nodules distributed along the layer dissolved in the horizontal seepage zone to form solution pores (Figure 5i). The horizontal subsurface flow zone, particularly the medium dissolution sub-zone, has an excellent reservoir space and seepage channel due to its many solution fractures and caves. Argillaceous silty dolomite with solution moldic pores provides the most favorable reservoir-forming zones in Ma5 in the study area.
The deep slow flow zone is located beneath the horizontal subsurface flow zone, with prolonged movement and alternation of groundwater and weak karstification. Scattered dissolved pores, fractures, and caves occur widely, filling when they reach supersaturation due to the continuous supply of dissolved substances in the water, which has a destructive effect on the reservoir.

5.3. Buried Karst and Its Control on the Formation of Pores

Buried karst refers to karstification occurring within deeply buried carbonate reservoirs, primarily associated with deep fluid activities, tectonic fractures, and suture lines from pressure dissolution [46]. The buried karst stage can be divided into three sub-stages according to burial depth; namely, shallowly buried karst stage (buried depth < 1000 m), medium buried karst stage (buried depth 1000–2600 m), and deeply buried karst stage (buried depth > 2600 m) [32]. The buried karst stage of the Ma51–Ma54 sub-members in the study area has experienced compaction, dissolution, fracturing, and filling. Compaction has had little effect on reservoir formation in the shallow burial stage, confirmed by the fact that there is no deformation of solution pores with lath anhydrite (Figure 5a) and nodule anhydrite (Figure 5c). In the shallow-buried karst stage, the acidic water in the overlying Carboniferous coal-bearing stratum migrated to the dolomite reservoir in the Ma5 member through the unconformity surface. Under these conditions, with weak acidic formation water, small amounts of authigenic quartz precipitated in the pores and filled them (Figure 8h).
In the middle-buried karst stage, the formation temperature was generally 50–100 °C, and the karstification process in the Ma51–Ma54 sub-members was mainly dissolution. At this stage, hydrocarbons reached maturity, and the expulsion of hydrocarbons from organic matter resulted in the release of a broad spectrum of organic acids. The resulting weakly acidic water dissolved the cloud seepage silt (Figure 10e,i), dolomite, and calcite cement that had earlier filled the solution moldic pores, forming secondary dissolution pores.
In the deeply buried karst stage, the formation temperature was generally 100–160 °C or even higher. Hydrocarbons, generated from source rocks, permeated the reservoir pores. The elevated temperature led to further cracking of the hydrocarbons, forming asphaltene or carbonaceous substances that filled the sutures (Figure 10h). In the deeply buried karst stage, the solution pore and fracture system sometimes filled with minerals related to underground hydrothermal fluids such as fluorite and pyrite. Pyrite generally invaded along the unconformity surface at the top of the Majiagou Formation and its connected fracture system. The Ma51–Ma54 sub-members in the Jingbian area experienced complex buried karstification, with the relative degrees of karstification and mineral filling determining whether the pores are effectively preserved. In the middle- and shallow-buried karst stages, the silty sand and bright calcite in the early solution moldic pore dissolved. The weakly acidic formation water inhibited the precipitation of calcite so that the solution moldic pores are well preserved (Figure 10i), finally forming a silty dolomite reservoir containing solution moldic pores. Dissolution did not occur in the middle- and shallow-buried karst stages, so the silt in the solution moldic pores formed during the syngenetic karst stage remains. The formation water was alkalescent, and large amounts of calcite and dolomite precipitated, filling the solution moldic pores, which is unfavorable for the preservation of reservoir space.

6. Conclusions

(1)
The pore types in the fifth member of the Majiagou Formation in the Jingbian area of the Ordos Basin include interparticle pores, separate-vuggy pores, and touching-vuggy pores. Interparticle pores consist of both intercrystal and intergrain. Separate-vuggy pores consist of moldic and intraparticle pores. Touching-vuggy pores consist of cavernous pores and fractures.
(2)
The dolomite reservoir in the fifth member of the Majiagou Formation in the Jingbian area of the Ordos Basin has experienced three stages of karstification: syngenetic karst, weathered crust karst, and buried karst. These stages were influenced by fluctuations in sea level, periods of exposure, and the release of organic acids during hydrocarbon generation.
(3)
Syngenetic karst and medium, shallowly buried karst are the critical stages for the formation and preservation of reservoir space. Mold pores and intragranular dissolution pores formed in syngenetic karst by selective dissolution are especially important in the formation of carbonate gas reservoirs. Pores and fractures formed in the weathering crust provide some reservoir space in carbonate. Buried karstification in the study area produced sutures, calcite, and dolomite precipitated in moldic pores, and also structural fractures, solution expansion structural fractures, and intergranular solution pores.
(4)
The Majiagou Formation exhibits well-developed structural and diagenetic fractures, which play a crucial role in facilitating the migration of natural gas reservoirs.

Author Contributions

Conceptualization, X.P. and L.G.; methodology, X.P.; software, X.P. and G.L.; validation, G.L., X.C. and L.G.; resources, X.P.; writing—original draft preparation, X.P.; writing—review and editing, X.C. and L.G.; visualization, X.P.; supervision, X.C. and L.G.; funding acquisition, L.G. All authors have read and agreed to the published version of the manuscript.

Funding

The author(s) receipt of the following financial support for the research, authorship, and publication of this article: This research was supported by the NSFC (Nos. 42130206, 41302076), Major Scientific and Technological Projects of the Changqing Oilfield (ZDZX-2021-03) and MOST Special Fund, awarded by the State Key Laboratory of Continental Dynamics, Northwest University (201210128).

Data Availability Statement

The authors declare that all analytical data supporting the findings of this study are available within the paper or cited in peer-review references.

Acknowledgments

We are deeply grateful to the Editor-in-chief and anonymous reviewers for their very constructive comments, which improved the manuscript considerably.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Location of Ordos Basin (a), tectonic division of the Ordos Basin and location of study area (b), and composite stratigraphic column of the 5th member of the Ordovician Majiagou Formation. The 5th member of the Ordovician Majiagou Formation can be divided into ten sub-members (c) (modified from [9]).
Figure 1. Location of Ordos Basin (a), tectonic division of the Ordos Basin and location of study area (b), and composite stratigraphic column of the 5th member of the Ordovician Majiagou Formation. The 5th member of the Ordovician Majiagou Formation can be divided into ten sub-members (c) (modified from [9]).
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Figure 2. Dissolution features at core scale: (a) moldic pores partly filled with carbonate silt, which are weakly localized along bedding surfaces (red arrows), Well Y918, 3276.60 m; (b) bed-parallel stylolites (yellow arrows), Well Y969, 3448.30 m; (c) vertical fracture partly filled with calcite (red arrow) and bed-parallel stylolites (yellow arrows), Well Y 914, 3372.30 m; (d) dissolution cavities and channels (red arrow), Well Y917, 3395.70 m; (e) vertical fracture, completely filled with calcite (red arrow), Well Y918, 3278.70 m; (f) dissolution cavities and circular-to-elongated caves, Well Y918, 3275.70 m; (g) carbonate packstones, Well Y918, 3275.10 m; (h) carbonate grainstones, parallel and cross bedding; Well Y914, 3374.51 m; (i) calcirudyte with fractures, Well Y904, 3438.41 m.
Figure 2. Dissolution features at core scale: (a) moldic pores partly filled with carbonate silt, which are weakly localized along bedding surfaces (red arrows), Well Y918, 3276.60 m; (b) bed-parallel stylolites (yellow arrows), Well Y969, 3448.30 m; (c) vertical fracture partly filled with calcite (red arrow) and bed-parallel stylolites (yellow arrows), Well Y 914, 3372.30 m; (d) dissolution cavities and channels (red arrow), Well Y917, 3395.70 m; (e) vertical fracture, completely filled with calcite (red arrow), Well Y918, 3278.70 m; (f) dissolution cavities and circular-to-elongated caves, Well Y918, 3275.70 m; (g) carbonate packstones, Well Y918, 3275.10 m; (h) carbonate grainstones, parallel and cross bedding; Well Y914, 3374.51 m; (i) calcirudyte with fractures, Well Y904, 3438.41 m.
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Figure 3. Interparticle pore space: (a) interparticle pores, intercrystal, Well Y918, 3277.54 m; (b) interparticle pores, intergrain, Well Y917, 3398.30 m. Separate-vuggy pores: (c) moldic porosity due to selective dissolution of minerals (plate-or strip-shaped anhydrite); (d) moldic porosity due to the selective dissolution of cement rims around grains, Well Y917, 3398.10 m; (e) intragrain development of moldic porosity and early dolomite inside grains (red arrows), Well Y918, 3275.7 m; (f) intracrystal dissolution porosity inside dolomite crystals (red arrows), Well Y918, 3276.55 m. All microscopic photos were taken under one nicol light.
Figure 3. Interparticle pore space: (a) interparticle pores, intercrystal, Well Y918, 3277.54 m; (b) interparticle pores, intergrain, Well Y917, 3398.30 m. Separate-vuggy pores: (c) moldic porosity due to selective dissolution of minerals (plate-or strip-shaped anhydrite); (d) moldic porosity due to the selective dissolution of cement rims around grains, Well Y917, 3398.10 m; (e) intragrain development of moldic porosity and early dolomite inside grains (red arrows), Well Y918, 3275.7 m; (f) intracrystal dissolution porosity inside dolomite crystals (red arrows), Well Y918, 3276.55 m. All microscopic photos were taken under one nicol light.
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Figure 4. Touching-vug pores: (a) cavernous, Well Y918, 3277.45 m; (b) partially dissolved calcite vein (yellow arrow), Well 918, 3276.55 m; (c) solution-enlarged fracture, Well 904, 3439.00 m; (d) microfractures (yellow arrows) connecting moldic pores (red arrows), currently all filled by the same calcite cement, Well Y904, 3439.00 m; (e) microfracture (yellow arrows) connecting moldic pores (red arrows), currently partly filled by calcareous silt and calcite cement, Well Y918, 3277.45 m; (f) boundary presenting porosity more concentrated on calcirudyte, Well Y904, 3438.41 m. All microscopic photos were taken under one nicol light.
Figure 4. Touching-vug pores: (a) cavernous, Well Y918, 3277.45 m; (b) partially dissolved calcite vein (yellow arrow), Well 918, 3276.55 m; (c) solution-enlarged fracture, Well 904, 3439.00 m; (d) microfractures (yellow arrows) connecting moldic pores (red arrows), currently all filled by the same calcite cement, Well Y904, 3439.00 m; (e) microfracture (yellow arrows) connecting moldic pores (red arrows), currently partly filled by calcareous silt and calcite cement, Well Y918, 3277.45 m; (f) boundary presenting porosity more concentrated on calcirudyte, Well Y904, 3438.41 m. All microscopic photos were taken under one nicol light.
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Figure 5. Syngenetic karstification in 5th member of the Majiagou Formation in Jingbian, central Ordos Basin. (a) moldic holes formed by the dissolution of plate- or strip-shaped anhydrite, Well Y918, 3276.55 m; (b) intragranular dissolution holes in plate-shaped anhydrite, Well Y918, 3276.55m; (c) anhydrite nodules in silty dolomite, Well Y918, 3276.55 m; (d) anhydrite nodules in silty dolomite containing gypsum mud, Well Y918, 3276.70 m; (e) cellular cave formed by dissolution in cores, Well Y918, 3276.70 m; (f) microfractures developed around some of the nodules, Well Y918, 3276.55 m; (g) solution fractures distributed along the stratum, with irregular boundaries, Well Y917, 3395.70 m; (h) unfilled short fractures, Well Y917, 3397.50 m; (i) small, near-circular, unfilled karst caves, Well Y917, 3398.10 m; (j) deformation structures, Well Y918, 3280.67 m; (k) unconsolidated gravels and deformation structures, Well 918, 3280.67 m; (l) sub-angular breccias in dolorudite, Well Y917, 3398.20 m. All microscopic photos were taken under one nicol light.
Figure 5. Syngenetic karstification in 5th member of the Majiagou Formation in Jingbian, central Ordos Basin. (a) moldic holes formed by the dissolution of plate- or strip-shaped anhydrite, Well Y918, 3276.55 m; (b) intragranular dissolution holes in plate-shaped anhydrite, Well Y918, 3276.55m; (c) anhydrite nodules in silty dolomite, Well Y918, 3276.55 m; (d) anhydrite nodules in silty dolomite containing gypsum mud, Well Y918, 3276.70 m; (e) cellular cave formed by dissolution in cores, Well Y918, 3276.70 m; (f) microfractures developed around some of the nodules, Well Y918, 3276.55 m; (g) solution fractures distributed along the stratum, with irregular boundaries, Well Y917, 3395.70 m; (h) unfilled short fractures, Well Y917, 3397.50 m; (i) small, near-circular, unfilled karst caves, Well Y917, 3398.10 m; (j) deformation structures, Well Y918, 3280.67 m; (k) unconsolidated gravels and deformation structures, Well 918, 3280.67 m; (l) sub-angular breccias in dolorudite, Well Y917, 3398.20 m. All microscopic photos were taken under one nicol light.
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Figure 6. Stratigraphic correlation map of wells J15-1, Y902-3, Y903, Y921-2, J3-1, and J2 in the study area. GR denotes Gamma ray and AC denotes acoustic time difference. The dashed line represents unconformity surface. The solid lines represent the interface of the strata. Different yellow boxes represent different strata.
Figure 6. Stratigraphic correlation map of wells J15-1, Y902-3, Y903, Y921-2, J3-1, and J2 in the study area. GR denotes Gamma ray and AC denotes acoustic time difference. The dashed line represents unconformity surface. The solid lines represent the interface of the strata. Different yellow boxes represent different strata.
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Figure 7. Typical seismic geological profile showing the unconformity of O3m and P1ty in the central Ordos Basin [39]. Pt3—Neoproterozoic, Є2m—Cambrian Mantou Formation, Є2x—Cambrian Xuzhuang Formation, Є2z—Cambrian Zhangxia Formation, Є3—upper Cambrian, O2m—Ordovician Majiagou Formation, P1ty—Permian Taiyuan Formation, P2s—Permian Xiashiahezi Formation, P2sh—Permian Shangshiahezi Formation, P1s—Permian Shiqiangfeng Formation.
Figure 7. Typical seismic geological profile showing the unconformity of O3m and P1ty in the central Ordos Basin [39]. Pt3—Neoproterozoic, Є2m—Cambrian Mantou Formation, Є2x—Cambrian Xuzhuang Formation, Є2z—Cambrian Zhangxia Formation, Є3—upper Cambrian, O2m—Ordovician Majiagou Formation, P1ty—Permian Taiyuan Formation, P2s—Permian Xiashiahezi Formation, P2sh—Permian Shangshiahezi Formation, P1s—Permian Shiqiangfeng Formation.
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Figure 8. Characteristics of weathering crust and buried karstification in the 5th member of the Majiagou Formation in the Jingbian area, central Ordos Basin: (a) karst breccia containing dolomitic breccia; the gravels are disorderly arranged, poorly sorted, and supported by breccia; the gravels with particle sizes greater than 1 cm are relatively well rounded and sub-angular; the gravels with particle sizes of less than 1 cm are poorly rounded, angular–sub-angular and the gravels are filled with sand, silt, and mud, Well Y904, 3440.40 m; (b) karst breccia containing dolomitic breccia, with a maximum diameter of 6 cm; some gravels are nearly vertical, mixed with sand debris and gravel debris, poorly sorted, sub-angular–subcircular, and filled with sand and silt debris, Well Y904, 3440.10 m; (c) karst breccia containing dolomitic breccia, which is disorderly embedded in dark-gray argillaceous rock, with a large amount of pyrite, Well Y969, 3448.50 m; (d) karst breccia, poorly sorted, sub-angular, with the weak qualitative arrangement of gravel, indicating that it is affected by water transportation, Well S301, 3358.57 m [29]; (e) karst breccia, medium gravel sorted, sub-angular, Well Y904, 3438.41 m; (f) karst breccia, medium gravel sorted, and matrix support; the matrix is mainly argillaceous silty dolomite; the matrix part is in plastic shape, reflecting that under the influence of water transportation cracks develop along the edge of gravel debris, and some cracks cut through gravel debris, Well Y904, 3438.41 m; (g) secondary calcite filled in the solution moldic pore, Well Y918, 3278.00 m; (h) moldic pore formed by dissolution of nodular anhydrite; in the later burial stage, the internal hole is filled with a small amount of quartz, Well Y918, 3279.20 m; (i) stylolites in dolorudite, Well Y969, 3448.30 m; (j) fracture expansion due to dissolution, Well Y918, 3277.45 m; (k) massive fluorite fills dissolved pores and coexists with authigenic quartz, Well S23 [5]; (l) intergranular dissolved pores and structural fractures, Well Y918, 3277.45 m. All microscopic photos were taken under one nicol light.
Figure 8. Characteristics of weathering crust and buried karstification in the 5th member of the Majiagou Formation in the Jingbian area, central Ordos Basin: (a) karst breccia containing dolomitic breccia; the gravels are disorderly arranged, poorly sorted, and supported by breccia; the gravels with particle sizes greater than 1 cm are relatively well rounded and sub-angular; the gravels with particle sizes of less than 1 cm are poorly rounded, angular–sub-angular and the gravels are filled with sand, silt, and mud, Well Y904, 3440.40 m; (b) karst breccia containing dolomitic breccia, with a maximum diameter of 6 cm; some gravels are nearly vertical, mixed with sand debris and gravel debris, poorly sorted, sub-angular–subcircular, and filled with sand and silt debris, Well Y904, 3440.10 m; (c) karst breccia containing dolomitic breccia, which is disorderly embedded in dark-gray argillaceous rock, with a large amount of pyrite, Well Y969, 3448.50 m; (d) karst breccia, poorly sorted, sub-angular, with the weak qualitative arrangement of gravel, indicating that it is affected by water transportation, Well S301, 3358.57 m [29]; (e) karst breccia, medium gravel sorted, sub-angular, Well Y904, 3438.41 m; (f) karst breccia, medium gravel sorted, and matrix support; the matrix is mainly argillaceous silty dolomite; the matrix part is in plastic shape, reflecting that under the influence of water transportation cracks develop along the edge of gravel debris, and some cracks cut through gravel debris, Well Y904, 3438.41 m; (g) secondary calcite filled in the solution moldic pore, Well Y918, 3278.00 m; (h) moldic pore formed by dissolution of nodular anhydrite; in the later burial stage, the internal hole is filled with a small amount of quartz, Well Y918, 3279.20 m; (i) stylolites in dolorudite, Well Y969, 3448.30 m; (j) fracture expansion due to dissolution, Well Y918, 3277.45 m; (k) massive fluorite fills dissolved pores and coexists with authigenic quartz, Well S23 [5]; (l) intergranular dissolved pores and structural fractures, Well Y918, 3277.45 m. All microscopic photos were taken under one nicol light.
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Figure 9. Karstification model of the Ordovician weathering crust in Jingbian, Ordos Basin.
Figure 9. Karstification model of the Ordovician weathering crust in Jingbian, Ordos Basin.
Minerals 13 01425 g009
Minerals 13 01425 g010
Figure 10. Characteristics of pores formed by karstification in the 5th member in Jingbian, central Ordos Basin: (a) with the enhancement of dissolution, the strip-shaped dissolution holes form minor dissolution fractures, (blue) cast thin sections, Well Y917, 3398.10 m; (b) intergranular pores are developed in karst breccia, which may be due to the action of water flow, leaving pores between gravelly particles, Well Y904, 3445.00 m; (c) intergranular pores, developed in the cloud seepage silty sand matrix in karst breccia, (blue) cast thin section, Well Y904, 3438.41 m; (d) microcracks, developed in karst breccia of collapse origin, developed along the edge of gravel debris in some cloud gravel debris, one nicol light (blue), Well Y904, 3438.41 m; (e) the cloud seepage silty sand in the solution moldic pore is dissolved; (f) the dissolution pore is formed along the edge and the internal cloud seepage silty sand grain solution pore; some pores contain authigenic quartz, Well Y918, 3278.00 m; (g) shear fracture developed in argillaceous silty dolomite, fracture filling–semi filling, Well Y904, 3439.00 m; (h) the suture formed by pressure dissolution in the burial stage is filled with asphaltene.; in addition, a group of microfractures is developed and weak dissolution occurs at the edge of the fracture, Well Y918, 3277.45 m; (i) the cloud seepage silt in the solution moldic pore is dissolved to form dissolution pores, Well Y918, 3279.20 m. All microscopic photos were taken under one nicol light.
Figure 10. Characteristics of pores formed by karstification in the 5th member in Jingbian, central Ordos Basin: (a) with the enhancement of dissolution, the strip-shaped dissolution holes form minor dissolution fractures, (blue) cast thin sections, Well Y917, 3398.10 m; (b) intergranular pores are developed in karst breccia, which may be due to the action of water flow, leaving pores between gravelly particles, Well Y904, 3445.00 m; (c) intergranular pores, developed in the cloud seepage silty sand matrix in karst breccia, (blue) cast thin section, Well Y904, 3438.41 m; (d) microcracks, developed in karst breccia of collapse origin, developed along the edge of gravel debris in some cloud gravel debris, one nicol light (blue), Well Y904, 3438.41 m; (e) the cloud seepage silty sand in the solution moldic pore is dissolved; (f) the dissolution pore is formed along the edge and the internal cloud seepage silty sand grain solution pore; some pores contain authigenic quartz, Well Y918, 3278.00 m; (g) shear fracture developed in argillaceous silty dolomite, fracture filling–semi filling, Well Y904, 3439.00 m; (h) the suture formed by pressure dissolution in the burial stage is filled with asphaltene.; in addition, a group of microfractures is developed and weak dissolution occurs at the edge of the fracture, Well Y918, 3277.45 m; (i) the cloud seepage silt in the solution moldic pore is dissolved to form dissolution pores, Well Y918, 3279.20 m. All microscopic photos were taken under one nicol light.
Minerals 13 01425 g010
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Peng, X.; Li, G.; Cheng, X.; Guo, L. Karstification and Its Role in the Formation of Carbonate Reservoirs: A Case Study from the Ordovician Majiagou Formation in Jingbian, Ordos Basin, North China. Minerals 2023, 13, 1425. https://doi.org/10.3390/min13111425

AMA Style

Peng X, Li G, Cheng X, Guo L. Karstification and Its Role in the Formation of Carbonate Reservoirs: A Case Study from the Ordovician Majiagou Formation in Jingbian, Ordos Basin, North China. Minerals. 2023; 13(11):1425. https://doi.org/10.3390/min13111425

Chicago/Turabian Style

Peng, Xiaoxia, Guobin Li, Xin Cheng, and Ling Guo. 2023. "Karstification and Its Role in the Formation of Carbonate Reservoirs: A Case Study from the Ordovician Majiagou Formation in Jingbian, Ordos Basin, North China" Minerals 13, no. 11: 1425. https://doi.org/10.3390/min13111425

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