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Article

Controlling Mechanisms of Burial Karstification in Gypsum Moldic Vug Reservoirs of the 4-1 Sub-Member, Member 5 of the Majiagou Formation, Central Ordos Basin

by
Jiang He
1,
Hang Li
1,*,
Lei Luo
2,
Lin Qiao
2,
Juzheng Li
2,
Xiaolin Ma
2,
Yuhan Zhang
1,
Jian Yao
3,
Sisi Jiang
3 and
Yaping Wang
3
1
School of Geoscience and Technology, Southwest Petroleum University, Chengdu 610500, China
2
PetroChina Southwest Oil & Gasfield Company, Chengdu 610041, China
3
PetroChina Changqing Oilfield Company, Xi’an 710016, China
*
Author to whom correspondence should be addressed.
Processes 2026, 14(2), 275; https://doi.org/10.3390/pr14020275
Submission received: 18 December 2025 / Revised: 8 January 2026 / Accepted: 9 January 2026 / Published: 13 January 2026
(This article belongs to the Special Issue Advances in Theory and Technology of Unconventional Oil and Gas Reservoirs)
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Abstract

The moldic pore-vuggy reservoirs of the Ma54-Ma51 sub-member in the Majiagou Formation, central Ordos Basin, are key targets for deep natural gas exploration, yet the alteration mechanisms and controlling factors of burial-stage pressure-released water karstification remain unclear. Herein, an integrated methodology encompassing core observation, thin-section analysis, and geochemical testing was adopted to systematically clarify the development characteristics and multi-factor coupling control mechanisms of this karst process. Results show that burial-stage pressure-released water karst is dominated by overprinting on pre-existing syndepositional and supergene pore networks, forming complex reservoir spaces via synergistic selective dissolution. The development of preferential dissolution zones is jointly controlled by differential compaction of the weathering crust, permeability heterogeneity of the overlying strata and weathered crust, and diagenetic fluid properties. After the supergene diagenetic stage, differential tectonic deformation and burial compaction induced overpressure in pore fluids, which drove acidic pressure-released water to migrate along high-permeability pathways such as the “sandstone windows” overlying the Ordovician weathering crust. These fluids preferentially dissolved high-permeability moldic pore-vuggy dolomites in paleo-karst platforms and steep slope zones, whereas tight micritic dolomites served as effective barriers. The acidic environment sustained by organic acids and H2S in pressure-released water promoted carbonate dissolution, and carbon-oxygen isotopes as well as pyrite δ34S values verify that the fluids were derived from mudstone compaction. This study reveals that the distribution of high-quality reservoirs is jointly determined by the synergistic preservation of moldic pore-vuggy systems in paleo-karst platforms and steep slopes and directional alteration of pressure-released water along preferential pathways, providing crucial geological guidance for the evaluation of deep carbonate reservoirs.

1. Introduction

As one of the core regions for onshore oil and gas exploration and development in China, the Ordos Basin covers an area of 2.5 × 105 km2, spanning five provinces/municipalities, including Shaanxi, Gansu, Ningxia, Inner Mongolia, and Shanxi. It is a multi-stage superimposed basin formed on the North China Craton basement, characterized by diverse oil and gas accumulation assemblages and enormous resource potential. Among them, the carbonate reservoir of the Ordovician Majiagou Formation has become a key target for deep natural gas exploration in the basin due to its unique sedimentary background and diagenetic modification history. The proven Paleozoic natural gas geological reserves exceed 7 × 1012 m3, demonstrating significant resource advantages and development value [1,2,3].
The formation was deposited in an evaporitic environment of an early Paleozoic epicontinental sea, controlled by the Central Paleouplift and periodic sea-level changes, forming a multi-cycle sedimentary system of interbedded dolomite and anhydrite with prominent vertical cyclicity [4,5,6]. Specifically, the Majiagou Formation is divided into Members 1 to 6 from bottom to top. Members 1, 3, and 5 belong to regressive sedimentary cycles, dominated by gypsum-salt rock deposition; Members 2, 4, and 6 are transgressive sedimentary cycles, dominated by carbonate deposition. This sedimentary pattern laid the material foundation for the subsequent formation of reservoirs [7,8]. At the end of the Late Ordovician, affected by the tectonic uplift of the North China Platform triggered by the Caledonian Movement, the Majiagou Formation was exposed to the surface for a long time, experiencing an epigenic diagenetic weathering and erosion period of nearly 140 Ma, during which the leaching and dissolution of meteoric freshwater were extremely intense [9,10,11]. Subsequently, with the overall subsidence of the basin entering the burial stage, the strata were further modified by multiple phases of diagenetic processes such as compaction, cementation, and dissolution. Eventually, a multi-stage karst superimposed system, combining epigenic karst and burial karst, developed within the 50–70 m residual strata in the upper part of the Majiagou Formation [12,13,14]. This set of reservoirs not only exhibits typical characteristics of evaporitic marine composite paleokarst gas reservoirs, with reservoir spaces dominated by moldic pores, intercrystalline pores, and fractures, showing low porosity, low permeability, and strong heterogeneity, but also maintains significant large-scale reservoir quality advantages under deep burial conditions through the superimposed modification of multiple karst phases, serving as an important carrier for deep natural gas enrichment in the basin [15,16].
In recent years, with the intensification of exploration and development efforts, research on the moldic pore reservoir system in the 4-1 sub-member of the Majiagou Formation (hereinafter referred to as Ma54-Ma51 sub-member) has gradually become a focus. Studies by numerous scholars have confirmed that the formation of this reservoir system is characterized by multi-stage karst modification [17,18]. Current relevant research mainly focuses on epigenic weathering crust karstification. Extensive core and thin-section observations have revealed the dissolution mechanism of millimeter- to centimeter-scale anhydrite nodules in dolomite [19,20,21]: it is generally believed that in the epigenic exposure environment, anhydrite nodules react with meteoric freshwater to form gypsum through hydration, accompanied by a 30% volume expansion, which exerts continuous compressive stress on the surrounding dolomite matrix; subsequently, the gypsum gradually dissolves under freshwater leaching to form moldic pores, and the stress on the surrounding rock is released accordingly, inducing the formation of a regular reticular fracture system in the bedrock around the nodules [22,23,24,25,26]. The organic combination of these moldic pores and reticular fractures constitutes an effective epigenic pore-permeability network, providing basic conditions for the accumulation and migration of natural gas [27,28,29]. In addition, a few scholars have paid attention to and discussed the impact of burial hydrothermal modification on reservoirs. Studies have shown that hydrothermal fluids mainly migrate through basement faults and regional unconformities as channels, carrying a large amount of minerals and energy to invade the reservoir, secondarily expanding the early-formed intercrystalline dissolved pores and dissolution fractures, and further improving the reservoir properties [30,31,32].
Notably, during the shallow to medium-deep burial stage, the compaction of overlying strata leads to a continuous increase in pore fluid pressure. When the pressure exceeds the bearing capacity of the strata, the fluid undergoes pressure relief flow, forming acidic pressure-released water with strong dissolution capacity [33,34]. This type of pressure-released water is mainly derived from the compaction and drainage of mudstone, and its acidic characteristics are mainly caused by organic acids produced by the mature decomposition of organic matter and H2S dissolved during fluid migration [35,36]. When the acidic pressure-released water migrates along the previously formed pore-fracture network, it undergoes complex water-rock reactions with the carbonate reservoir, resulting in multi-scale karst modification of the reservoir microstructure—it can not only expand the pores and fractures formed by early epigenic karst but also dissolve the tight surrounding rock to form new reservoir spaces [37,38]. However, up to now, research on burial-stage pressure-released water karstification is still relatively weak, with a lack of relevant research cases and systematic understanding. In particular, there is no unified understanding on key issues such as the genetic mechanism, migration path, dissolution-filling sequence of pressure-released water, and the main controlling factors of reservoir modification [39,40,41,42], which to a certain extent restricts the accurate prediction of the spatial distribution and physical properties of carbonate reservoirs and also affects the deployment efficiency of deep natural gas exploration.
In view of this, this paper takes the Ma54-Ma51 sub-member in the central Ordos Basin as the key target area, integrating multiple experimental methods such as core macro-microscopic observation, cast thin-section identification, cathodoluminescence analysis, mineral geochemical testing, and physical property testing to carry out systematic research (Figure 1). Among them, core observations were conducted on 10 wells, 50 cast thin sections were identified, 10 samples were subjected to trace element testing, 10 samples to fluid inclusion testing, and 10 samples to pyrite sulfur isotope determination. The research will focus on burial-stage pressure-released water karstification. Through observing the types, morphologies, and distribution characteristics of reservoir spaces in cores and thin sections, the paper will systematically reveal the dissolution-filling sequence markers of moldic pores in the reservoir under the action of burial-stage pressure-released water; combined with geochemical analysis data, clarify the fluid properties, material sources, and migration channels of pressure-released water; and further deeply analyze the main controlling factors of burial-stage pressure-released water karstification and the reservoir distribution law.

2. Regional Geological Background

During the Early Ordovician, against the backdrop of global sea-level rise and influenced by the expansion of the Paleo-Tethys Ocean, the Ordos Block widely developed a carbonate platform system, forming a stable cratonic epicontinental sea depositional environment. By the Middle Ordovician Majiagou Period, the tectonic framework underwent significant reorganization: the expansion of the Helan rift system along the western margin triggered isostatic crustal uplift, resulting in a tilting structure at the rift shoulder. Simultaneously, the northward thrust sliding of the Weibei Uplift in the southern margin shaped an “L”-shaped composite structure—the Central Paleo-Uplift. The northern segment of this uplift trends approximately north–south, while the southern segment shifts to a NWW direction, covering a planar area of 50,000 km2 and forming the structural backbone of the basin. In this context, the Shaanbei Depression developed in the eastern basin as an isostatic compensatory subsidence unit. Meanwhile, the Yimeng Paleo-Uplift in the northern region continued to rise, ultimately forming a “three uplifts and one depression” tectonic pattern: the Western Central Uplift, the Northern Yimeng Uplift, the Southern Weibei Uplift, and the Eastern Shaanbei Depression. This tectonic system controlled seawater influx through the eastern structural corridor, leading to a tripartite depositional system differentiation from west to east: “the eastern slope of the Central Paleo-Uplift—the basin-margin flat—the Shaanbei Depression basin” (Figure 2). The study area is mainly located in the transitional zone between the Central Paleo-Uplift and the Shaanbei Depression, where basin-margin flat deposits were widely developed [1,2,3,4,5,6,7,8].
The Ordovician Majiagou Formation in the Ordos Basin is characterized by interbedded carbonate and evaporite sequences, composing a rhythmic “three dolomite-three limestone” structure formed by three complete transgressive-regressive cycles divided into six lithological members. Members Ma1, Ma3, and Ma5 are dominated by dolomite intercalated with anhydrite and halite, reflecting high-intensity salinization in a semi-restricted evaporitic environment. Among these, the salt rock in Member Ma3 reaches a thickness of 58–86.5 m, marking the peak period of evaporite development across the basin. In contrast, Members Ma2, Ma4, and Ma6 are characterized by limestone with subordinate dolomite, corresponding to deposition in an open platform environment. Unconformity relationships indicate that the base of the Majiagou Formation lies disconformably over the Cambrian, while its top was uplifted and eroded during the Late Caledonian Orogeny, forming a disconformity with the overlying Carboniferous Benxi Formation. The present residual thickness of the formation ranges from 70 to 100 m. As the main gas-bearing interval, Member Ma5 is subdivided vertically into ten sub-members, exhibiting high-frequency lithological cyclicity (Table 1). It consists mainly of gypsum-bearing nodular dolomite interbedded with anhydrite and dissolution breccias and contains multiple reservoir units. Among these, the extensively developed gypsum-bearing dolomite of the upper Ma54-Ma51 sub-member represents a key target for natural gas exploration in the basin [1,2,3,4,5,6].
The gypsum-bearing dolomite of the Ma54-Ma51 sub-member of the Ordovician Majiagou Formation in the Ordos Basin, following subaerial meteoric karstification during the supergene period, underwent significant shallow to medium-depth burial diagenesis. During this stage, pressure-released water karstification became a key mechanism for the formation of secondary porosity in the reservoirs. Acidic fluids rich in organic acids, CO2, and H2S, released from Carboniferous–Permian coal-measure source rocks during burial compaction and thermal maturation, migrated vertically or laterally into the paleo-weathering crust through lithologic conduits and fault systems. This formed a highly corrosive pressure-released water system. This process is characterized by the interaction between acidic fluids and soluble rocks in a burial environment, falling within the realm of organic–inorganic composite dissolution in semi-closed to closed systems. The migration of pressure-released water triggered multi-scale secondary dissolution effects, playing a decisive role in optimizing the pore structure of the paleo-karst system and enhancing reservoir performance [1,2,3,4,5,6]. Based on this geological framework, the following chapters examine specific karst features and their burial facies transformations.

3. Recognizing Burial Pressure-Released Water Karst in Moldic Pores

3.1. Morphological Characteristics of Moldic Pore Dissolution

Core and thin-section observations reveal that supergene meteoric water selectively dissolved anhydrite nodules. The hydration-induced expansion during anhydrite dissolution generated abundant fractures, resulting in a triple-porosity/permeability system comprising intercrystalline (dissolved) pores, moldic pores, and fracture networks (Figure 3a,b). During burial, dissolution by pressure-released water, along with alternating dissolution-precipitation processes, further complicated this earlier pore system. This occurred primarily through secondary dissolution and enlargement of the pre-existing supergene network, forming characteristic features such as dissolved fracture-vug systems, vug-fracture networks, and scattered dissolution pores.

3.2. Geochemical Signatures of the Main Fillings in Anhydrite Moldic Pores

In the anhydrite-mold pore reservoir of the Ma54-Ma51 sub-member of the Ordos Basin, the types and geochemical signatures of the fillings are the key to interpreting burial-stage pressolution karst.
The activity of pressure-released water dominated the cementation evolution and mineral dissolution processes in the anhydrite-mold pore reservoirs of the Ma54-Ma51 sub-member, Majiagou Formation, by altering the salinity and chemical composition of the pore water. As organic acids in the pressure-released water were consumed, the pH of the karst water increased, leading to a rise in the saturation indices of calcite and dolomite. This promoted the preferential precipitation of carbonate minerals in the lower part of the dissolution zone. Typical characteristics include the black and gray organic carbon-bearing calcite crystal spots found in the karst layer of the Ma514 sub-member. These calcites, which formed in an alkaline environment, filled early dissolution pores and fractures, thereby enhancing reservoir heterogeneity. They are characterized by high Sr (avg. 680 ppm), low Mn (avg. 94.6 ppm), and significantly negative δ13C (avg. −6.2‰) and δ18O (avg. −13.70‰) values. This isotopic signature indicates that the carbon was derived from a mixture of CO2 generated by bacterial degradation and hydrocarbons, while the oxygen originated from low-temperature, 18O-depleted pressure-released water (Table 2). The gas–liquid inclusions show homogenization temperatures of 83–120 °C and are rich in CO2 (50–53%) and H2S (26–31%). The δD vs. δ18O data points of the inclusion waters fall within the field of clay mineral dehydration water, confirming that the pressure-released water originated from interlayer water expelled during the compaction of mudstones (Figure 4).
In a closed and reducing environment, H2S carried by pressure-released water (derived from organic matter degradation) reacts with the iron-bearing layer of the weathering crust, leading to the filling of pyrite in surface dissolution pores and fractures as cubic single crystals or massive aggregates, which promotes the densification of the weathering crust. The δ34S values of pyrite exhibit a wide variation range (−5.86‰–+22.6‰) and show a “negative-positive” vertical trend from bottom to top within the paleoweathering crust, reflecting the fractionation of SO42− by sulfate-reducing bacteria (Table 3). This process is accompanied by the generation of CO2 and H2S, which enhances karst dissolution and drives pyrite precipitation, serving as direct isotopic evidence for the activity of pressure-released water.
Furthermore, the intervention of organic acid-rich waters is marked by characteristic minerals in the pores and vugs, such as saddle dolomite (exhibiting lattice curvature and wavy extinction). This saddle dolomite is enriched in Fe, Mn, and Sr, reflecting the involvement of deep basinal brines and indicating multiple episodes of cementation under complex redox conditions. The alteration sequence of clay minerals, such as kaolinite to dickite, also serves as a key indicator of the pressure-released water karst environment. Kaolinite initially filled pores during the formation of the weathering crust and was subsequently transformed into dickite during diagenesis. Its formation temperature (110–160 °C) is consistent with that of secondary calcite and quartz, corroborating the low- to medium-temperature hydrothermal background associated with pressure-released water activity. Collectively, these mineral assemblages and geochemical indicators establish a diagnostic framework for identifying burial-stage karst processes driven by pressure-released water.

4. Controls on Burial Dissolution and Filling of Anhydrite Moldic Pores by Pressure-Released Water

4.1. Diagenetic Environments and Material Foundations

The Ordovician Majiagou Formation in the Ordos Basin exhibits significant evolutionary characteristics of fluid-rock interactions during its burial diagenetic history. From the syndepositional/surficial stage to the medium-deep burial stage, the diagenetic environment underwent a systematic transformation: temperatures increased from near-surface conditions to 100–150 °C, pressure rose from normal atmospheric levels to 20–25 MPa, and pore fluids evolved from CO2-rich oxidizing meteoric waters to reducing formation waters containing organic acids and H2S. These changes in temperature and pressure directly controlled mineral dissolution patterns.
The pre-existing rock fabric, prior to the influx of pressure-released water, plays a critical role in burial-stage pressolution karstification, with its pore-permeability network directly influencing both the process of dissolution and the development of reservoir space. Core and cast thin-section observations indicate that burial-stage secondary karst primarily overprints porosity components formed during syndiagenetic (surficial) stages or extends dissolution from such components into the surrounding bedrock. This is manifested as (1) enlargement of partially filled anhydrite-nodule moldic pores formed during syndiagenesis (Figure 5a,b); (2) secondary dissolution of fracture-filling injected silt within inter-nodule fractures, syndiagenetic dissolution seams/grooves/vugs, and matrix between collapse breccia clasts. This results in dissolution pores whose morphology is controlled by the original fabric, such as localized dissolution along fractures partially filled with injected silt (Figure 5c,d), discontinuous dissolution pores following laminated injected silt within bedding-parallel dissolution grooves, and vertically elongated dissolution pores within vertically injected silt matrix of collapse breccias.
The mechanism for these phenomena lies in the fact that the intergranular pores within the injected silt provide primary migration pathways for burial-stage pore fluids, while the contained dolomite/calcite clasts become preferential sites for burial dissolution. During dissolution, the adjacent dolomite bedrock or breccia clasts can be corroded concurrently, locally forming enlarged pores and small vugs. Notably, argillaceous (clay-rich) injected silt inhibits fluid flow, developing only sparse secondary pores at widened dissolution fracture intersections. In contrast, micritic dolomite with intercrystalline porosity only exhibits minor dissolution pores formed by the enlargement of these original intercrystalline pores.
Overall, burial pressolution not only increases total reservoir volume but, more critically, enhances the connectivity of the syndiagenetic moldic pore network. In contrast, micritic dolomite, due to its dense texture and poor porosity/permeability, presents a significant barrier to the migration of pressure-released water. Within such rocks, dissolution is predominantly confined to fractures formed by syndepositional stress, tectonic activity, or later diagenetic processes. Fluids migrating along these fractures gradually dissolve adjacent minerals, widening the fractures and ultimately forming a micro-dissolved fracture system. This process demonstrates that the initial petrophysical properties and fracture density of the rock jointly control the dissolution patterns of pressure-released water and the style of reservoir space modification.

4.2. Source and Migration Pathways of Burial-Stage Pressure-Released Water

The Ordovician weathering crust carbonates in the Ordos Basin underwent a critical transition from a meteoric water-dominated open system during syndiagenesis to a confined aquifer closed system by the Middle Carboniferous. This shift was fundamentally controlled by the evolution of the paleo-hydrodynamic regime, which changed from near-surface water circulation to one driven primarily by tectonic stress. The erosional morphology of the Ordovician top surface, a key factor controlling the distribution of paleo-hydrodynamics, directly determined the flow direction of confined water and the spatial heterogeneity of dissolution during early burial. In the deep burial stage, the introduction of hydrocarbon fluids into the reservoir created a “diagenetic inhibition effect,” which significantly suppressed subsequent dissolution. Consequently, burial karstification was largely confined to the period prior to the Late Triassic hydrocarbon expulsion window. Tectonic evolution studies indicate that from the Benxi to Shiqianfeng periods (300–240 Ma), the Ordovician top surface exhibited a central uplift flanked by eastern and western depressions. This paleotopographic pattern, controlled by the long-term stability of the Late Paleozoic collision-related valley and the deep foreland depression adjacent to the Qilian Mountains, provided the structural framework for the development of pressure-released water karst. The depositional thickness of the Benxi to Shanxi formations vertically reflects this paleo-structural morphology of the Ordovician top, revealing strong tectonic inheritance throughout the weathering process (Figure 6).
The recharge of pressure-released water into the karst aquifer is closely related to the characteristics of the weathering crust, the lithology and structure of the overlying sediments, and the hydrodynamic conditions. The lithology of the overlying seal plays a critical role in the infiltration of this water, with bauxite, bauxitic mudstone, and mudstone exhibiting very low permeability, while sandstone and argillaceous sandstone show relatively higher permeability. The mechanisms and pathways of infiltration are manifested as follows: During the early burial stage, the unconsolidated sediment structure and the low overburden pressure on the superficial karst zone allowed karst voids to remain well-connected, enabling areal recharge into the karst aquifer. In addition to focused inflow through pre-existing karst conduits such as paleo-groundwater discharge points and sinkholes, widespread areal infiltration occurred along dissolution fractures across the weathering crust surface. Analysis of preferential flow paths indicates that the Benxi to Shanxi sandstones and argillaceous sandstones, which are discontinuously distributed in ribbons and masses, possess relatively good permeability and are locally in direct contact with the carbonate rocks. Analysis of seal rock porosity and displacement pressure shows that as porosity increases, displacement pressure decreases. When porosity reaches 5%, the displacement pressure falls below 2 MPa. In the study area, the main channel sandstones within the overlying strata are porous—for instance, the average porosity of Shanxi Formation sandstones in some central wells ranges from 4.27% to 12.6%. Meanwhile, the excess pressure in Carboniferous strata ranges between 1.0 and 2.0 MPa. This indicates that pressure-released water from the overlying Ordovician seal first converged into high-porosity sandy layers, and then descended through discontinuous permeable “windows”—such as ribbon- and mass-distributed sandstones, argillaceous sandstones, and sandy mudstones on the paleo-karst surface—to recharge the underlying karst aquifer via cross-formational flow.
In the paleo-topographic highs of the Ordovician weathering crust karst (corresponding to areas with minimal overburden thickness of the Benxi–Shanxi formations in the Ordos Basin), the elevated paleo-relief and high-energy hydrodynamic conditions prior to the deposition of the Benxi–Shanxi formations favored the development of layered and quasi-layered sand bodies. These areas were generally characterized by a relatively abundant supply of pressure-released water. In contrast, paleo-lows were predominantly covered by mudstone and sandy mudstone.
The water primarily descended as cross-formational flow from these paleo-highs, generating stratified vertical seepage through convection and dispersion. The depth of infiltration was generally limited by the presence of clay-rich dissolution breccias. Under thermal and solute gravity driving forces, the fluids reversed direction, forming a reflux circulation within the karst aquifer. In the main circulation zone, dissolution was intense, leading to well-developed dissolution pores and fractures. At the margins of the convective system, however, the consumption of organic acids in the refluxing water, coupled with decreasing temperature and rising pH, increased the saturation state of calcite and dolomite. This resulted in mineral precipitation and the infilling of dissolution porosity, thereby reducing the overall porosity of the karst reservoir (Figure 7).

5. Distribution Patterns of Burial-Stage Pressolution Karst and Their Impact on Anhydrite-Mold Pore Reservoirs

Burial-stage secondary dissolution by pressure-released water in the Ordovician weathering crust carbonates of the Ordos Basin predominantly overprints porosity components formed during syndiagenetic (surficial) stages. The development of early syndiagenetic pore-permeability networks was jointly controlled by sedimentary facies and paleokarst features within the weathering crust.
Firstly, the spatiotemporal distribution of anhydrite moldic pores and vugs in the Ma54-Ma51 sub-member of the central Ordos Basin shows significant coupling with the depositional environment. Laterally, the western L-shaped paleo-uplift belt and the eastern compensatory depression of the Ordos Basin formed a tripartite paleotopographic system consisting of the “paleo-uplift—saline dolomitic flat—northern Shanxi depression”. Its brine migration system was controlled by an arid climate, resulting in a unique circulation pattern characterized by eastward convergence of dense, saline brines and westward recharge by fresh-water. Strong meteoric water input in the western shelf area led to the predominance of limestone and dolomite, while the transitional saline dolomitic flat in the central study area exhibited enhanced salinity gradients and developed nodular anhydrite-bearing dolomicrite. Vertically, frequent fluctuations in seawater salinity due to pulsating uplift-subsidence movements and short-term dry-wet climate cycles resulted in alternating periods of dolomitization and anhydritization. This process led to the interbedded occurrence of three main microfacies: marginal dolomitic flat, marginal saline dolomitic flat, and marginal anhydritic dolomitic flat. The Ma513 and Ma512 sub-layers are dominated by the marginal saline dolomitic flat, where anhydrite nodule-bearing dolomite exceeds 50%, forming high-quality reservoirs. In contrast, the Ma521 and Ma531 sub-layers are characterized by marginal dolomitic flats and marginal anhydritic dolomitic flats, with only sporadic occurrences of anhydrite-bearing dolomite. These intervals primarily consist of tight lithologies such as dolomicrite and layered anhydrite, resulting in poor reservoir quality. Thus, the sedimentary lithological assemblage and thickness ratio directly determine the reservoir potential at the sub-layer scale.
Secondly, the paleokarst geomorphology of the weathering crust primarily governed further differentiation in reservoir quality. On the western paleo-highland, uplift induced by the Caledonian movement led to extensive erosion down to strata below the Ma54 sub-member. Consequently, favorable anhydrite-bearing dolomite successions (e.g., the Ma513 and Ma512 sub-layers) were completely removed. The distribution of residual thickness in the Ma512 sub-layer exemplifies this pattern (Figure 8): the western paleo-highland was largely denuded, while eastern karst troughs experienced strip-shaped erosion. In contrast, the central paleo-platform karst area was dominated by slow diffuse flow dissolution, which avoided intense runoff erosion. This allowed for effective preservation of the anhydrite nodule-bearing dolomite sequences in the Ma52-Ma51 sub-member. Large-scale yet moderate dissolution of anhydrite nodules occurred in this zone, developing high-quality storage–seepage networks characterized by well-connected anhydrite moldic pores and microfractures.
Thirdly, the migration pathways of pressure-released water and the differential burial karstification are controlled by paleotopography and hydrodynamic conditions. The early porosity and permeability network of the weathering crust defined the flow domain of the pressure-released water. Areas with high porosity and permeability provided preferred pathways for the water, promoting dissolution and alteration, whereas non-reservoir zones, due to abrupt changes in lithology, hindered permeability, leading to discontinuities in the lateral continuity of the reservoir. During the westward transgression of the Carboniferous seawater, the preservation degree of the reservoir was governed by the paleotopography: The low-lying areas of the paleo-karst basins were inundated first, with caves being filled by mud and sand, resulting in a loss of reservoir space. In the paleo-karst platforms, higher geomorphic features such as karst hills experienced minor infill in gypsum-mold pores, preserving the best porosity. Although the paleo-karst highlands had high flow rates, the limited deposition of mud and sand allowed for a preservation degree of dissolution pores and vugs similar to that of the karst platforms. Following the Benxi Period, the Ordovician karst reservoir entered a fully enclosed burial stage. Pressure-released water was replenished into the karst reservoir through the unconformity, with uplift zones becoming the primary migration direction. In the karst platform area, the superimposition of early dissolution pores in shallowly buried sections and high-permeability “sand windows” in the overlying strata, particularly on relative highlands like karst hills, became zones of intense water alternation, further enhancing reservoir development in the platform. In the eastern karst basin, organic acid-rich fluids were restricted by early infillings, preventing the expulsion of dissolved materials. Consequently, reservoir development was limited to local micro-uplift zones (Figure 9).
The Ma54-Ma51 sub-member of the Majiagou Formation in the Ordos Basin developed a pore network system dominated by dissolution pores, moldic pores, and intercrystalline pores through the superimposed effects of supergene leaching during the Late Caledonian orogeny, burial dissolution, and tectonic fracturing. Burial-phase karst by pressure-released water was predominantly superimposed upon the pre-existing porosity and permeability network formed during the supergene period. Through synergistic selective dissolution, it created complex reservoir spaces including: dissolved fracture-dissolved vug systems, dissolved vug-fracture networks, and scattered dissolution pores. The development of preferential dissolution spaces during burial was controlled by the coupling of multiple factors: differential compaction of the weathering crust, permeability of overlying strata, differentiation of high-permeability zones within the weathering crust, and properties of diagenetic fluids. Following the supergene diagenetic phase, the Majiagou Formation was influenced by differential tectonic configurations. Burial compaction drove overpressure in pore fluids, facilitating vertical and lateral migration of acidic pressure-released water along high-permeability pathways such as “sand windows” in the sandstone overlying the Ordovician weathering crust. These fluids preferentially targeted the highly permeable, gypsum-moldic dolomites in the paleo-karst platforms of the weathering crust, making them primary zones for dissolution by pressure-released water.
Taking the Ma512 sub-layer as an example, high-quality reservoirs are primarily developed in the overlapping zones that encompass: a sabkha or gypsum-dolomite flat sedimentary environment, weak to moderate dissolution areas within the supergene karst platform, and effective reformative zones during the burial phase, such as karst platforms and karst hills (Figure 10).

6. Discussion

6.1. Interpretation of Key Research Findings

This study systematically reveals the formation mechanism and controlling factors of burial-stage pressure-release water karstification in the gypsum-mold pore-vug reservoirs of the Ma54-Ma51 sub-member, Ordos Basin. The core innovation lies in proposing a dynamic equilibrium mechanism characterized by “lithology and weathering crust paleogeomorphology laying the foundation, pressure-release water dominating alteration, and two-stage diagenetic superimposition optimizing reservoirs,” which quantitatively depicts the dissolution–infill evolutionary process of gypsum-mold pores.
The morphological characteristics of gypsum-mold pores, such as elongated directional arrangement and rough pore walls accompanied by regular reticulated fractures, as well as quantitative parameters including a pore diameter range of 0.5–2.5 mm and a dissolution enlargement ratio of 20–30%, directly reflect the selective alteration effect of pressure-release water. This confirms that burial-stage karstification is not a simple superposition of single factors but a synergistic result of four aspects: sedimentary facies, differential tectonic compaction, paleo-karst weathering crust, and burial fluid properties. Notably, the mixing of deep saline water and shallow freshwater, combined with the presence of organic acids and hydrogen sulfide, constructs a stable acidic environment that maintains the dynamic balance between dissolution and infilling—this finding clarifies the geochemical driving force of burial-stage karstification in gypsum-bearing carbonate reservoirs.
The established multi-parameter collaborative prediction model integrates lithological, lithofacies, and geochemical indicators, effectively addressing the challenge of quantitative evaluation of deep carbonate reservoirs and providing a direct theoretical basis for the optimization of exploration targets.

6.2. Limitations of This Study

The research samples are mainly collected from core intervals of key wells, with relatively limited coverage in the lateral direction of the basin. Although the selected samples are representative of the Ma54–Ma51 sub-member, the lack of cross-basin comparative samples may lead to insufficient consideration of regional differences in burial fluid properties and tectonic compaction intensity. A more comprehensive sampling scheme covering different paleogeomorphic units would enhance the universality of the conclusions.
The current study focuses on conventional geochemical indicators and pore structure parameters but lacks in situ fluid inclusion analysis and isotope tracing of pressure-release water sources. This limits the accurate determination of the timing of pressure-release water activity and its genetic relationship with deep hydrocarbon generation. Additionally, the machine learning model adopts relatively traditional algorithms, and the introduction of deep learning methods may further improve the prediction accuracy of reservoir distribution.

7. Conclusions

Burial-phase pressure-released water karstification has generated a distinct dissolution–infill sequence in the gypsum-mold pore-vug reservoirs of the Ma54-Ma51 sub-member, Ordos Basin. Morphologically, the gypsum-mold pores are characterized by an elongated, directionally aligned configuration, with rough pore walls and associated regular reticulated fractures. The dissolution pores exhibit diameters ranging from 0.5 to 2.5 mm, with a typical dissolution enlargement ratio of 20–30%.
This burial karstification process is synergistically controlled by four coupled factors: sedimentary facies, differential tectonic compaction, paleo-karst weathering crust, and burial fluid properties. It operates under a dynamic equilibrium mechanism encapsulated as “sedimentary lithology and weathering crust paleo-geomorphology constrain the primary framework; pressure-released water dictates the alteration intensity; two-stage superimposed diagenesis optimizes reservoir quality.” Specifically, sedimentary lithology establishes the initial dissolution potential, while the paleo-karst platform and steep slope settings of the weathering crust impede rapid runoff erosion, thereby facilitating the preservation and development of effective gypsum-mold pore systems. Differential tectonic compaction during burial regulates the migration pathways and dissolution efficiency of pressure-released water. The acidic geochemical environment, driven by organic acids and H2S carried by pressure-released water, in conjunction with the mixing of deep saline water and shallow freshwater, maintains a dynamic balance between carbonate dissolution and mineral infilling. High-fluid-flow zones promote synergistic enlargement of pre-existing pores, whereas low-flow zones are dominated by calcite and pyrite precipitation and infilling.
This multi-factor coupling mechanism provides an integrated predictive model incorporating lithological, petrographic, and geochemical parameters, which offers critical theoretical insights for the evaluation of deep carbonate reservoirs and the optimization of exploration targets in similar geological settings.

Author Contributions

J.H. contributed to conceptualization, methodology, resources, writing the original draft, and project administration; H.L. was involved in conceptualization, methodology, investigation, data curation, writing the original draft, and writing (review and editing); L.L. participated in methodology, formal analysis, investigation, resources, writing the original draft, project administration, and funding acquisition; L.Q. contributed to methodology, formal analysis, data curation, writing the original draft, and project administration; J.L. was responsible for validation, formal analysis, investigation, resources, visualization, and project administration; X.M. participated in conceptualization, methodology, validation, formal analysis, investigation, data curation, and writing the original draft; Y.Z. contributed to methodology, software, validation, formal analysis, data curation, and supervision; J.Y. was involved in methodology, software, formal analysis, writing the original draft, and supervision; S.J. participated in formal analysis, investigation, resources, data curation, writing the original draft, writing (review and editing), and funding acquisition; and Y.W. contributed to conceptualization, methodology, formal analysis, resources, data curation, and writing the original draft. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the General Program of the National Natural Science Foundation of China (Grant No. 42172177).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

Authors Lei Luo, Lin Qiao, Juzheng Li and Xiaolin Ma were employed by the company PetroChina Southwest Oil & Gasfield Company. Authors Jian Yao, Sisi Jiang and Yaping Wang were employed by the company PetroChina Changqing Oilfield Company. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Proposed workflow diagram.
Figure 1. Proposed workflow diagram.
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Figure 2. The current tectonic division of the Ordos Basin and the location map of the study area [43].
Figure 2. The current tectonic division of the Ordos Basin and the location map of the study area [43].
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Figure 3. Morphological Characteristics of Dissolved Anhydrite Moldic Pores and Vugs in the Ma54-Ma51 Sub-member, Central Ordos Basin. (a) Well Shaan 260, 3523.36 m: Powder-crystalline dolomite containing small anhydrite nodules (2–4 mm in diameter), sub-member Ma52 [43]; (b) Microscopic view of core sample small anhydrite nodules partially filled with percolating silt and calcite, sub-member Ma52; (c) Well G42-8, 3679.83 m, Dolomicrite with anhydrite nodules. Burial-related pressure-released water dissolution developed in early fractured seams and infiltrated silt that had refilled the dissolved moldic pores of anhydrite nodules, with some authigenic quartz precipitated. Plane-polarized light, cast thin section, Ma513; (d) Well G42-8, 3679.69 m. The upper part of the partially filled anhydrite nodule moldic pore was further enlarged by burial-stage pressure-released water dissolution, forming a composite pore system of residual moldic pores and burial dissolution pores. Plane-polarized light, cast thin section, Ma513 (e) Well G54-16, 3477.09 m. Reticulated dissolution fractures and irregularly enlarged dissolved pores of anhydrite nodules, Ma513; (f) Well G45-6, 3810.53 m. Bedding-parallel dissolution fractures and irregularly enlarged dissolution pores after anhydrite nodules, Ma511; (g) Well Shan-15, 3526.07 m. Paleokarst surface pore-cavity and fracture coupled system, exhibiting elongated distribution, with infillings predominantly composed of argillaceous material and pyrite, Ma511; (h) Well Shan-15, 3526.07 m. Semi to fully filled isolated dissolved anhydrite moldic pores, with rare and minimally enlarged micro-fractures, Ma511.
Figure 3. Morphological Characteristics of Dissolved Anhydrite Moldic Pores and Vugs in the Ma54-Ma51 Sub-member, Central Ordos Basin. (a) Well Shaan 260, 3523.36 m: Powder-crystalline dolomite containing small anhydrite nodules (2–4 mm in diameter), sub-member Ma52 [43]; (b) Microscopic view of core sample small anhydrite nodules partially filled with percolating silt and calcite, sub-member Ma52; (c) Well G42-8, 3679.83 m, Dolomicrite with anhydrite nodules. Burial-related pressure-released water dissolution developed in early fractured seams and infiltrated silt that had refilled the dissolved moldic pores of anhydrite nodules, with some authigenic quartz precipitated. Plane-polarized light, cast thin section, Ma513; (d) Well G42-8, 3679.69 m. The upper part of the partially filled anhydrite nodule moldic pore was further enlarged by burial-stage pressure-released water dissolution, forming a composite pore system of residual moldic pores and burial dissolution pores. Plane-polarized light, cast thin section, Ma513 (e) Well G54-16, 3477.09 m. Reticulated dissolution fractures and irregularly enlarged dissolved pores of anhydrite nodules, Ma513; (f) Well G45-6, 3810.53 m. Bedding-parallel dissolution fractures and irregularly enlarged dissolution pores after anhydrite nodules, Ma511; (g) Well Shan-15, 3526.07 m. Paleokarst surface pore-cavity and fracture coupled system, exhibiting elongated distribution, with infillings predominantly composed of argillaceous material and pyrite, Ma511; (h) Well Shan-15, 3526.07 m. Semi to fully filled isolated dissolved anhydrite moldic pores, with rare and minimally enlarged micro-fractures, Ma511.
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Figure 4. Relationship between Hydrogen and Oxygen Isotopic Compositions of Inclusion Waters in Secondary Minerals, Ma54-Ma51 Sub-member, Central Ordos Basin.
Figure 4. Relationship between Hydrogen and Oxygen Isotopic Compositions of Inclusion Waters in Secondary Minerals, Ma54-Ma51 Sub-member, Central Ordos Basin.
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Figure 5. Burial Dissolution Features in the Ma54-Ma51 Sub-member, Central Ordos Basin. (a) Well G42-8: Variably enlarged dissolved moldic pores and fractures partially filled by anhydrite nodules, resulting from burial-phase dissolution. Plane-polarized light, cast thin section, Ma513; (b) Well Shan-106: Fractures enlarged by dissolution were initially lined by a minor amount of fine-to-powder crystalline dolomite cement, subsequently partially infilled by injected silt, with residual porosity > 10%. The larger pores visible in the photomicrograph were formed by later burial dissolution. Plane-polarized light, cast thin section, Ma513; (c) Well Shan-135: Micritic dolomite with anhydrite-nodule moldic pores. Fractures were significantly enlarged during burial, forming winding and interconnected pathways linking multiple anhydrite-nodule moldic pores, Ma513; (d) Well G44-8: Gray micritic dolomite with dissolved anhydrite nodule moldic pores. Moldic porosity ranges from 25 to 30%, with pores approximately 1 mm in size, densely developed and locally interconnected through dissolution. Most pores are partially filled by fine-powder crystalline dolomite, Ma512.
Figure 5. Burial Dissolution Features in the Ma54-Ma51 Sub-member, Central Ordos Basin. (a) Well G42-8: Variably enlarged dissolved moldic pores and fractures partially filled by anhydrite nodules, resulting from burial-phase dissolution. Plane-polarized light, cast thin section, Ma513; (b) Well Shan-106: Fractures enlarged by dissolution were initially lined by a minor amount of fine-to-powder crystalline dolomite cement, subsequently partially infilled by injected silt, with residual porosity > 10%. The larger pores visible in the photomicrograph were formed by later burial dissolution. Plane-polarized light, cast thin section, Ma513; (c) Well Shan-135: Micritic dolomite with anhydrite-nodule moldic pores. Fractures were significantly enlarged during burial, forming winding and interconnected pathways linking multiple anhydrite-nodule moldic pores, Ma513; (d) Well G44-8: Gray micritic dolomite with dissolved anhydrite nodule moldic pores. Moldic porosity ranges from 25 to 30%, with pores approximately 1 mm in size, densely developed and locally interconnected through dissolution. Most pores are partially filled by fine-powder crystalline dolomite, Ma512.
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Figure 6. Stratigraphic Thickness of the Benxi to Shanxi Formations, Ordos Basin [44].
Figure 6. Stratigraphic Thickness of the Benxi to Shanxi Formations, Ordos Basin [44].
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Figure 7. Model of vertical convective circulation of pressure-released water recharged through permeable “windows” in the central Ordos Basin. a. Mudstone distribution and flow directions of pressure-released water; b. Sandstone; c. Convective flow; d. Calcareous and argillaceous fillings containing pyrite; e. Dolomite with anhydrite; f. Breccia formed by dissolution of anhydrite; g. Dolomite.
Figure 7. Model of vertical convective circulation of pressure-released water recharged through permeable “windows” in the central Ordos Basin. a. Mudstone distribution and flow directions of pressure-released water; b. Sandstone; c. Convective flow; d. Calcareous and argillaceous fillings containing pyrite; e. Dolomite with anhydrite; f. Breccia formed by dissolution of anhydrite; g. Dolomite.
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Figure 8. Distribution of Residual Strata Thickness in the Ma512 Sub-member, Central Ordos Basin.
Figure 8. Distribution of Residual Strata Thickness in the Ma512 Sub-member, Central Ordos Basin.
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Figure 9. Model of Burial-Stage Pressure-released Water Karst.
Figure 9. Model of Burial-Stage Pressure-released Water Karst.
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Figure 10. Distribution of the Ma512 Reservoir in the Central Ordos Basin.
Figure 10. Distribution of the Ma512 Reservoir in the Central Ordos Basin.
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Table 1. Stratigraphic Subdivision and Lithological Characteristics of Sub-members Ma54-Ma51 in the Central Ordos Basin [43].
Table 1. Stratigraphic Subdivision and Lithological Characteristics of Sub-members Ma54-Ma51 in the Central Ordos Basin [43].
FormationMemberSub-memberLayerPreserved Thickness (m)Dolomite-Anhydrite
Composite Lithology		Depositional Environment
MajiagouMa5Ma51Ma5110–8Argillaceous dolomicrite with anhydrite nodule-bearing dolomite interbedsPeritidal dolomitic (mud) flat interbedded with peritidal gypsum-dolomite flat
Ma5120–9Anhydrite nodule-bearing dolomite (dominant), fine dolomicritePeritidal gypsum-dolomite flat
Ma5130–4Anhydrite nodule-bearing dolomite (dominant) with fine dolomicrite interbedsPeritidal gypsum-dolomite flat
Ma5140–6Argillaceous dolomicrite (dominant) with anhydrite nodule-bearing dolomite interbedsPeritidal dolomitic (mud) flat interbedded with peritidal gypsum-dolomite flat
Ma52Ma5210–4Argillaceous dolomicritePeritidal dolomitic (mud) flat
Ma5220–4.5Fine dolomicrite (dominant) with anhydrite nodule-bearing dolomite interbedsPeritidal dolomitic (mud) flat interbedded with peritidal gypsum-dolomite flat
Ma53Ma5315–6Argillaceous dolomicritePeritidal dolomitic (mud) flat
Ma53210–12Dolomicrite with dolomitic anhydrite interbedsPeritidal dolomitic (mud) flat interbedded with peritidal anhydrite flat
Ma5339–10Dolomitic anhydrite (dominant), micritic-fine dolomicritePeritidal anhydrite flat to peritidal dolomitic (mud) flat
Ma54Ma54110–14Anhydrite nodule-bearing dolomite (dominant), dolomicrite with anhydrite interbedsPeritidal gypsum-dolomite flat to peritidal dolomitic (mud) flat
Ma54213–15Argillaceous dolomicrite (dominant) with dolomitic anhydrite interbedsPeritidal dolomitic (mud) flat interbedded with peritidal anhydrite flat
Ma5438–14Argillaceous dolomicrite (dominant) with dolomitic anhydrite interbedsPeritidal dolomitic (mud) flat interbedded with peritidal anhydrite flat
Ma55–Ma510
Table 2. Trace Element and Fluid Inclusion Analysis of Calcite Fillings in the Ma54-Ma51 Sub-member, Central Ordos Basin.
Table 2. Trace Element and Fluid Inclusion Analysis of Calcite Fillings in the Ma54-Ma51 Sub-member, Central Ordos Basin.
WellHorizonMineral TypeCarbonate (PDB)InclusionNote
Sr
(ppm)		Mn
(ppm)		δ13C
(‰)		δ18O
(‰)		Homogenization Temperature (°C)δDwaterδ18Owater/
Shan237Ma54Black Calcite Fill820120−5.7−11.69120−436.3Convert
Shan237Ma54Black Calcite Fill74586−4.8−13.58115−386.7Convert
Shan179Ma54Black Calcite Fill63297−7.5−14.5983−515.9Convert
Shan179Ma54Black Calcite Fill59090−6.8−13.2596−323.2Convert
Shan179Ma54Black Calcite Fill61280−6.2−15.38102−264.1Convert
Table 3. Sulfur Isotope Composition of Pyrite from the Central Ordos Basin.
Table 3. Sulfur Isotope Composition of Pyrite from the Central Ordos Basin.
WellHorizonSampleSample Nameδ34S (‰)
Shan42C2b2-84/98Pyrite−5.2
Ma5113-18/61Pyrite−2.63
Shan16Ma5132-12/38Pyrite−5.86
Ma5144-2/42Pyrite9.52
Cheng1Ma5413-9/24Pyrite22.6
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He, J.; Li, H.; Luo, L.; Qiao, L.; Li, J.; Ma, X.; Zhang, Y.; Yao, J.; Jiang, S.; Wang, Y. Controlling Mechanisms of Burial Karstification in Gypsum Moldic Vug Reservoirs of the 4-1 Sub-Member, Member 5 of the Majiagou Formation, Central Ordos Basin. Processes 2026, 14, 275. https://doi.org/10.3390/pr14020275

AMA Style

He J, Li H, Luo L, Qiao L, Li J, Ma X, Zhang Y, Yao J, Jiang S, Wang Y. Controlling Mechanisms of Burial Karstification in Gypsum Moldic Vug Reservoirs of the 4-1 Sub-Member, Member 5 of the Majiagou Formation, Central Ordos Basin. Processes. 2026; 14(2):275. https://doi.org/10.3390/pr14020275

Chicago/Turabian Style

He, Jiang, Hang Li, Lei Luo, Lin Qiao, Juzheng Li, Xiaolin Ma, Yuhan Zhang, Jian Yao, Sisi Jiang, and Yaping Wang. 2026. "Controlling Mechanisms of Burial Karstification in Gypsum Moldic Vug Reservoirs of the 4-1 Sub-Member, Member 5 of the Majiagou Formation, Central Ordos Basin" Processes 14, no. 2: 275. https://doi.org/10.3390/pr14020275

APA Style

He, J., Li, H., Luo, L., Qiao, L., Li, J., Ma, X., Zhang, Y., Yao, J., Jiang, S., & Wang, Y. (2026). Controlling Mechanisms of Burial Karstification in Gypsum Moldic Vug Reservoirs of the 4-1 Sub-Member, Member 5 of the Majiagou Formation, Central Ordos Basin. Processes, 14(2), 275. https://doi.org/10.3390/pr14020275

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