Differential Evolution and Main Controlling Factors of Inner-Platform Carbonate Reservoirs in Restricted–Evaporative Environment: A Case Study of O₂m₅⁶ in the Ordos Basin, North China

Abstract

The potential for oil and gas exploration within inter-salt reservoirs is substantial, primarily due to their significant heterogeneity, which complicates accurate predictions. This study focuses on the inter-salt reservoirs of the sixth sub-member of the fifth member of the Majiagou Formation (hereafter referred to as O₂m₅⁶) in the Ordos Basin, North China. Utilizing core samples, thin sections, and petrophysical data, we investigated the differential evolution and primary controlling factors of the inter-salt carbonate reservoirs. The key findings are as follows: (1) During the sedimentary phase of O₂m₅⁶, high-energy sediments, such as shoals and microbial mounds, were deposited in highlands, while low-energy sediments, including dolomitic lagoons and gypsiferous lagoons, emerged in depressions from west to east. (2) In a restricted–evaporative environment, highlands are prone to karstification, which significantly enhances the development of inter-salt reservoirs and generates a variety of reservoir spaces, including interparticle dissolved pores, growth-framework dissolved pores, and micropores between vadose silts. (3) The presence of alternating highlands and depressions obstructs seawater flow, leading to a progressive increase in salinity from west to east. This process ultimately facilitates the infilling of reservoir spaces with calcite, anhydrite, and halite cements in the same direction. (4) The three components—reservoir rocks, karstification, and infilling features—exert varying effects in the region and collectively govern the north–south distribution of inter-salt reservoirs. Overall, this study examines the characteristics and controlling factors of carbonate reservoirs within a restricted–evaporative platform environment and provides pertinent research cases for the exploration of inter-salt reservoirs.

1. Introduction

The carbonate–evaporite symbiotic systems have been extensively distributed throughout geological history [1] and are known to contain significant oil and gas resources [2]. Previous research has categorized carbonate reservoirs into two main types: pre-salt reservoirs and inter-salt reservoirs, based on the distribution of carbonate and evaporite formations. Pre-salt reservoirs exhibit favorable geological conditions for hydrocarbon accumulation and are associated with numerous significant oil and gas fields worldwide, including the Cretaceous system of the Santos Basin in Brazil [3,4], the Jurassic Ghawar oil and gas field in Saudi Arabia [5], and the Carboniferous–Permian Karachaganak oil field in Kazakhstan [6]. In contrast, inter-salt reservoirs, while possessing similar reservoir formation conditions, have received comparatively less scholarly attention than pre-salt reservoirs [7,8]. Exploration activities have revealed that inter-salt reservoirs possess unique characteristics regarding development patterns, lateral scale, and preservation [9,10]. Firstly, inter-salt reservoirs are interbedded with evaporite due to environmental conditions such as restricted sedimentary seawater and high-frequency sea-level fluctuations [11,12,13]. This results in relatively limited thickness and lateral extension of individual reservoir sets within carbonate rocks [14,15]. Secondly, due to the high solubility and precipitation of evaporite minerals [1], when heavy brine migrates back into the underlying carbonate rocks, the precipitated evaporite minerals will infill and deplete pore spaces [7,16]. Finally, the reservoir spaces infilled with evaporite minerals undergo repeated closure and opening during burial stage, which not only complicates the formation of reservoirs but also influences the migration of hydrocarbon substances [7]. In summary, the presence of evaporite minerals contributes to the heterogeneity of inter-salt reservoirs, thereby complicating the prediction of favorable inter-salt reservoirs. Therefore, there is a pressing need for increased research efforts focused on inter-salt reservoirs.

The Ordovician Majiagou Formation in the Ordos Basin exhibits multiple sets of carbonate–evaporite symbiotic systems, with the largest one occurring in the sixth sub-member of the fifth member, hereafter referred to as O₂m₅⁶ [17]. Extensive drilling core and well logging data indicate that O₂m₅⁶ encompasses inter-salt reservoirs characterized by favorable physical properties, thereby establishing it as a principal pay zone in the Majiagou Formation [15,16,17]. As a result, O₂m₅⁶ is considered an optimal subject for investigating the mechanisms of reservoir formation and the distribution of inter-salt reservoirs in carbonate–evaporite symbiotic systems. Previous studies on this reservoir have predominantly concentrated on regions adjacent to erosion lines, positing that reservoir formation is primarily associated with karstification rather than evaporites. Recently, as exploration efforts have shifted towards the central and eastern parts of the Ordos Basin, it has been discovered that numerous thin reservoirs have developed between evaporites, revealing a more intricate distribution of the O₂m₅⁶ reservoirs [18]. A significant challenge in this context is that high-quality carbonate reservoirs linked to evaporites are frequently infilled with evaporite minerals, leading to ambiguities regarding the formation, preservation, and heterogeneity of inter-salt reservoirs, which seriously restrict the exploration of O₂m₅⁶. Therefore, further research in this domain is warranted.

This paper aims to review the types of reservoir rocks, the characteristics of reservoir spaces, the development environments, and the physical properties of reservoirs across various geomorphic units, utilizing extensive drilling core and thin section data from O₂m₅⁶. Building upon this foundation, the study seeks to elucidate the differential formation mechanisms of carbonate reservoirs within the restricted–evaporative platform by comprehensively considering the influence of geological factors, such as paleogeomorphology, sea-level fluctuations, and seawater differentiation, on inter-salt reservoirs.

2. Geologic Setting

The Ordos Basin is located on the western edge of the North China Platform (Figure 1a), with an area of about 33.7 × 10⁵ km². The basin has evolved in multiple tectonic cycles [19]. During the Caledonian cycle, the Paleo-Asian Ocean and the Central Qilian Massif experienced subduction and subsequent collision with the North China Plate [20]. This geological activity led to the formation of various tectonic units, including the Yimeng paleo-uplift, Qingyang paleo-uplift, and the Lvliang underwater low-uplift surrounding the basin. The study area is located in the central–eastern depression confined by these three uplifts (Figure 1a). In the Ordovician period, the Majiagou Formation developed a sedimentary system characterized by rhythmically interbedded carbonate and evaporite layers, a result of the isolation of seawater in the basin due to the surrounding paleo-uplifts [17] (Figure 1b). This formation has experienced sedimentation through three marine transgression–regression cycles, identified as O₁m₁, O₁m₃, and O₂m₅ within the highstand system tract (HST), and O₁m₂, O₂m₄, and O₂m₆ within the transgression system tract (TST) [21,22]. In the context of high-frequency sea-level fluctuations, the O₂m₅^7-10 developed a multi-layered dolomite–anhydrite symbiotic system [23], while the overlying O₂m₅⁶ represents the most extensive evaporite system within the Majiagou Formation.

During the sedimentary period of O₂m₅⁶, the east–west trending compressive stress shaped a south–north trending concave–convex landform pattern within the study area. From west to east, the geological features include the Taolimiao west depression, Taolimiao highland, Taolimiao east depression, Hengshan highland, and East salty depression (Figure 1c,d) [17]. Over time, the water in these areas became increasingly saline, leading to the sequential development of various sedimentary environments, including dolomitic restricted lagoons, dolomitic–gypsiferous evaporative lagoons, gypsiferous evaporative lagoons, and salty evaporative lagoons [17]. In terms of rock types, micritic dolomite and thinly layered anhydrite are developed in the Taolimiao west depression and the Taolimiao underwater low highland. In contrast, rhythmically interbedded sediments of dolomite and anhydrite dominate the Taolimiao east depression and Hengshan highland, while substantial halite deposits are locally observed on the western side of Hengshan highland. Furthermore, very thick halite layers are present in the East salty depression, often interbedded with thin layers of dolomite and mudstone (Figure 2). The frequency of development of carbonate reservoirs in the study area exhibits a decreasing trend from west to east (Figure 2).

3. Samples and Methodology

The geomorphological analysis conducted during the sedimentary period of during the sedimentary period of O₂m₅⁶ in the Ordos Basin [17] has led to the classification of the study area into five distinct development zones of carbonate reservoirs: Zone A (Taolimiao west depression), Zone B (Taolimiao highland), Zone C (Taolimiao east depression), Zone D (western edge of Hengshan highland), and Zone E (eastern edge of Hengshan highland), as illustrated in Figure 1c,d. This classification facilitated a comprehensive examination of the sedimentary environment, reservoir rock types, and reservoir spaces within the study area, allowing for a comparative analysis of the reservoir properties and characteristics across the five zones.

The cores, 500 m in total, from 12 wells in the study area were observed and interpreted in detail to identify rock types, reservoir rock/space types, and sedimentary structures. Specific coring intervals from five representative wells (one from each of the five zones) were selected based on the chronostratigraphic framework (Figure 2) to ensure lateral comparability. Lithological sequences, texture profiles, karst features, and reservoir properties were documented for these intervals. Thin sections were prepared from 200 core samples, which were subsequently impregnated with blue epoxy resin to enhance pore visibility and stained with Alizarin red S to distinguish between calcite and dolomite. Finally, the Leica DM2500P polarizing microscope was used to describe and photograph the microscopic features of these thin sections. Moreover, 440 sidewall coring samples underwent testing for effective porosity and absolute permeability at the Exploration and Development Research Institute of PetroChina Changqing Oilfield Company, in accordance with the Porosity and Permeability Measurement of Core in Net Confining Stress (SY/T6385-2016) standards [26].

4. Results

4.1. Facies Sequences

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Zone A

The facies sequence predominantly consists of carbonate rocks, characterized by a typical lithological association that includes micritic dolomite, laminar anhydrite-bearing dolomite, grain dolomite, and mosaic anhydrite. The interval from 3727.0 to 3720.1 m in the representative Well T105 comprises three distinct cycles (Figure 3①), with individual cycle thicknesses ranging from 1 to 2.5 m. Cycle S1 is characterized by a lithological association of layered stromatolite, massive micritic dolomite, and laminar anhydrite-bearing dolomite, arranged from the base to the top. Cycle S2 features micritic dolomite with biological burrows at the bottom, transitioning to dolarenite with small irregular cavities upwards, and culminating in mosaic anhydrite at the top. Cycle S3 exhibits micritic dolomite interbedded with grain dolomite, which is 0.8 m thick at the bottom, followed by thinly layered stromatolite with bedded channels above the micritic dolomite and concluding with mosaic anhydrite at the top. In Zone A, the thickness of an individual shoal is considerable, measuring up to 1.2 m. However, the presence of anhydrite and laminar stromatolite is comparatively infrequent.

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Zone B

Zone B is characterized by well-developed carbonate rocks, in contrast to the less developed evaporites. The complete lithological sequence, arranged from the bottom to the top, consists of dolarenite/micritic dolomite, thrombolite, laminar stromatolite, and karst breccia. The interval between 3765.9 m and 3858.5 m in the representative Well T78 can be subdivided into four distinct sedimentary cycles, each exhibiting a thickness ranging from 1.3 m to 2 m, with exposed surfaces at the top of each cycle (Figure 3②). Cycle S1 is a descending half cycle, which includes three overlapping sets of reverse-graded grain dolomite, interspersed with thin layers of stromatolite and oolitic dolomite between the lower and upper shoals. Cycle S2 demonstrates a transition to micritic dolomite at the bottom, accompanied by local argillaceous stripes and an overlying layer of grain dolomite. Cycle S3 retains micritic dolomite at the bottom but transitions to microbial thrombolite and stromatolite in the middle to upper sections. Finally, Cycle S4 exhibits a facies sequence of argillaceous dolomite, micritic dolomite, and karst breccia, with the uppermost bedrock having been fragmented into breccia by karstic fluids. Compared with Zone A, Zone B displays a greater frequency and scale of development for both grain dolomite and microbial dolomite, as well as an increased intensity of karstification.

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Zone C

Zone C is predominantly characterized by the presence of anhydrite. The interval between 3643.8 m and 3636.7 m in the representative Well JT1 can be subdivided into four similar complete sedimentary cycles (Figure 3③). Each cycle is manifested by a lithological association that includes micritic dolomite, thrombolite, and stromatolite in the lower section, transitioning to layered dolomitic anhydrite and mosaic anhydrite in the upper section, occasionally interspersed with thin layers of gypsiferous rudaceous dolomite. Zone C demonstrates a notably increased percentage of layered anhydrite in the facies sequence, whereas the formation of karst features in this area appears to be comparatively limited.

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Zone D

The interval ranging from 3379.7 to 3371.0 m in the representative Well T112 comprises four distinct sedimentary cycles, which can be categorized into two types based on their lithological combinations (Figure 3④). The first type includes Cycle S1 and Cycle S4, characterized by a lithological sequence that features thick thrombolite at the base, followed by stromatolite, and culminating in layers of lamellar dolomitic anhydrite and mosaic anhydrite at the top. In contrast, the second type, represented by Cycle S2 and Cycle S3, is predominantly composed of anhydrites. These cycles typically display a progression from thinly layered micritic dolomite or dolomitic anhydrite to mosaic anhydrite, interspersed with dolarenite. The microbial dolomite in Zone D has a considerable thickness, with individual layers measuring approximately 2 m. Moreover, this zone contains karst features, including channels and karst breccia, within the microbial mounds, and the thickness of the karst development section surpasses that observed in Zone B. In addition, mosaic anhydrites are formed at the top of each cycle.

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Zone E

Zone E is distinguished by the symbiotic relationship between microbial rocks and evaporites. The interval between 3544.4 m and 3539.7 m in the representative Well J5 comprises five sedimentary cycles that exhibit similar lithological characteristics, with individual cycle thicknesses ranging from 0.5 to 1 m (Figure 3⑤). Each sedimentary cycle is characterized by a lower section consisting of a few layers of micritic dolomite, a middle section composed of thrombolite or bonding dolarenite, and an upper section featuring mosaic anhydrite. Notably, Zone E displays a smaller thickness of individual cycles compared with other zones and is also marked by karst features, including halite moldic pores, small irregular cavities, and anhydrite breccia.

4.2. Types and Physical Properties of Reservoirs

4.2.1. Types and Characteristics of Reservoir Rocks

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Grain dolomite

The grain dolomite in the study is primarily categorized into two types: dolarenite/rudaceous dolomite (Figure 4a–c, Figure 5a and Figure 6a,b) and oolitic dolomite (Figure 5b). The smooth surfaces of the cores are dark gray to yellow-brown, while the natural cross-sections display a rough texture. The dolarenite/rudaceous dolomite reveals two notable features under the microscope: (1) the grains are well structured and preserved (Figure 4e,f, Figure 5e and Figure 6e,f), with a fabric that is grain supported. The sand-sized carbonate particles constitute approximately 60%–75% of the composition, with a grain size ranging from 0.1 to 0.5 mm, and exhibit moderate sorting. Additionally, dolomite and calcite cements locally infill the interstitial spaces between grains; (2) the grain structure appears blurred as a result of recrystallization (Figure 4g, Figure 5f and Figure 6g). At the microscopic level, the oolitic dolomite exhibits well-preserved ooid circle structures, mainly consisting of normal ooids, which may be either suspension supported (Figure 5g) or grain supported (Figure 7g,h).

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Microbial dolomite

Microbial dolomite includes three types: binding dolarenite (Figure 5c), thrombolite (Figure 5c and Figure 7b,c), and stromatolite (Figure 6c and Figure 7b). The binding dolarenite is formed by the binding and capture of sand-sized particles by the extracellular polymers of microorganisms [27]. It macroscopically appears gray-brown and develops anhydrite clumps and microscopically contains sand-sized particles mostly wrapped by microbial clumps (Figure 5h,i). The thrombolite macroscopically appears as brownish-gray speckles and microscopically contains dark growth-framework, cluster, and laminar textures and is infilled with multiple stages of cements (Figure 5j, Figure 6h and Figure 7e,f). The stromatolite displays a laminar texture with alternating bright and dark layers (Figure 6i). The dark layers are enriched in organic matter, with microbial fabric resulting from the calcification of microbial hyphae [27]. In contrast, the bright layers consist of brown-gray micritic-crystalline dolomite, with non-microbial fabric. The stromatolite in the study area mainly contains nearly horizontal and micro wavy laminae, with some areas exhibiting moundy laminae.

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Crystalline dolomite

The core of crystalline dolomite shows yellow-brown and gray-brown patches and exhibits karst features such as small irregular cavities (Figure 7d and Figure 8a–c) or layered dissolution (Figure 4d). Microscopic examination reveals that dolomite primarily occurs in a fine- to medium-grained form, displaying a xenomorphic to hypidiomorphic mosaic texture, with some particle ghosts and well-developed intercrystalline (dissolved) pores. The crystal grains within the crystalline dolomite, which have been altered by karstification, are generally loose and generally dark in color (Figure 4h,i, Figure 5k, Figure 7k and Figure 8d–f).

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Karst breccia

Karst breccia is observed to develop on a relatively small scale within the study area, primarily infilling karst channels (Figure 5d). Under the microscope, the breccias exhibit angular characteristics, with some displaying indistinct boundaries as a result of subsequent dissolution processes. The predominant composition of these breccias consists of crystalline dolomite, infilled with discrete carbonate silt, as well as saddle-shaped dolomite or calcite cements (Figure 5l, Figure 7l and Figure 8h,i).

By analyzing the typical reservoir rock types in the geomorphic units in the study area, it is found that the reservoir rocks are distributed differentially. Typically, the grain dolomite mainly occurs in the Taolimiao highland (Zone A and Zone B).

4.2.2. Types and Characteristics of Reservoir Spaces

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Interparticle (dissolved) pores

This type of pore is predominantly observed in dolarenite/rudaceous dolomite and oolitic dolomite (Figure 4e,f and Figure 5e). The interparticle pores are primary pores formed by grain support, while residual interparticle pores develop subsequent to the infilling of cements between the grains. Interparticle dissolved pores arise from the expansion of interparticle pores due to the dissolution processes facilitated by diagenetic fluids. Some of these pores are often infilled with vadose silt (Figure 4f and Figure 5e), fine-grained dolomite (Figure 4e,f and Figure 5e), and anhydrite cements (Figure 6e,f and Figure 7g,h). This type of reservoir space has irregular shapes and varying pore sizes (0.1–2 mm).

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Intraparticle dissolved pores and moldic pores

This type of reservoir space is mainly found in dolarenite, oolitic dolomite, and bonding dolarenite (Figure 4g, Figure 5g and Figure 8e). The pores are mostly rounded, with pore sizes ranging from 0.1 to 0.4 mm. Both pores develop less frequently in the study area, and they are formed by fabric-selective dissolution of carbonate particles by meteoric freshwater during the penecontemporaneous period [28]. These pores are often infilled with vadose silt (Figure 4g and Figure 5g), and they exhibit geopetal structures (Figure 8e). However, they are poorly interconnected and often remain isolated.

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Growth-framework pores

They are primary pores formed by the construction of biological growth frameworks during the sedimentary period of microbial dolomite (Figure 5h–j and Figure 6h,i). This type of pore is often infilled with cements such as fine-grained dolomite, calcite, or anhydrite. The dissolved pores formed by the dissolution and expansion of diagenetic fluids display embayed dissolution characteristics at their edges. The sizes of these pores range from 0.1 to 0.3 mm (Figure 6i and Figure 7e). This type of pore is mainly distributed in various forms of microbial carbonate rocks in the study area, exhibiting a high development frequency. Furthermore, these pores can interconnect with other pore types, thereby forming network pore spaces that enhance connectivity.

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Intercrystalline (dissolved) pores

This specific type of reservoir space is present within the crystalline dolomite (Figure 4h,i, Figure 5k, Figure 7k and Figure 8d). The process of karstification, which has occurred in multiple stages, has led to the dissolution and enlargement of intercrystalline pores, resulting in the formation of intercrystalline dissolved pores. In certain areas, vadose silt that partially infills these pores diminishes the quality of reservoirs.

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Micropores between vadose silt in karst channels

This type of pore belongs to non-fabric-selective secondary pores, which are extensively present in the study area, particularly within dolarenite, microbial dolomite, and karst breccia dolomite. The reworking of reservoir rocks through multiple phases of karstification has resulted in early dissolved pores or karst breccia being infilled with vadose silt, which possesses relatively loose and porous characteristics. This process leads to the formation of micropores within the vadose silt (Figure 5l and Figure 7k,l). Additionally, the karst channels are infilled with calcite, anhydrite, and halite cements (Figure 5k,l, Figure 7i–l and Figure 8f–i).

4.2.3. Comparison of Reservoir Physical Properties in Zones A–E

The analysis of reservoir porosity and permeability statistics for geomorphic units in O₂m₅⁶ (Figure 9), alongside the examination of reservoir petrological characteristics, indicates notable variations in reservoir properties across different zones.

In Zone A, the reservoir rocks are represented by grain dolomite, with a minor presence of crystalline dolomite. Specifically, the dolarenite reservoir exhibits the highest development frequency. The primary reservoir spaces consist of interparticle (dissolved) pores and intercrystalline dissolved pores, which are infilled with dolomite cements and vadose silt. Zone A is generally characterized by low porosity and by low-permeability reservoirs with moderate physical properties (

Table 1. Reservoir rocks, reservoir spaces, infilling features, and physical properties of reservoirs in geomorphic units in O₂m₅⁶ in central–eastern Ordos Basin.

Geomorphic Unit	Reservoir Rocks	Main Reservoir Space Types		Infilling Features	Physical Properties
Zone A	Mainly grain dolomite, along with a few crystalline dolomite and microbial dolomite	Mainly interparticle pores, with a few moldic pores (intraparticle dissolution) and intercrystalline dissolved pores	Pore-type space	Calcite and dolomite cements, and a minor quantity of vadose silts	Moderate
Zone B		Mainly interparticle dissolved pores, with a few growth-framework dissolved pores and intercrystalline dissolved pores		A minor quantity of carbonate cements	Excellent
Zone C	Grain dolomite and microbial dolomite	Interparticle pores and growth-framework pores, with a few dissolved pores	Pore- to fracture-type space	Anhydrite cements almost infilling	Poor
Zone D	Mainly microbial dolomite, along with a few binding dolarenite	Mainly growth-framework (dissolved) pores, with a few interparticle pores and residual dissolved micropores		Anhydrite or calcite partially infilling	Good
Zone E		Moldic pores (anhydrite moldic pores/salt moldic pores), growth-framework pores, strong karst channels	Fracture-type space	Anhydrite and halite almost infilling	Poor

). The porosity within this zone ranges from 0.45% to 12.56%, with an average of 3.76% ± 3.20%. Notably, a significant proportion of the total samples, specifically 38.82%, demonstrates porosity values below 2% (Figure 9a–c).

In Zone B, the predominant reservoir rocks are composed of grain dolomite, accompanied by some microbial dolomite. The reservoir spaces include interparticle (dissolved) pores and growth-framework (dissolved) pores. Compared with Zone A, the dolomite cements within the pores are comparatively lower in this zone. The porosity ranges from 0.03% to 11.99% with an average of 4.50% ± 2.85%, indicating a relatively stable distribution of porosity values. Specifically, the samples with a porosity of 2%–8% account for 66.96% (Figure 9d–f). Furthermore, Zone B is classified as a pore-type reservoir, which is characterized by medium porosity and medium permeability, demonstrating the best reservoir performance among all zones analyzed in the study area (Table 1).

In Zone C, the predominant reservoir rocks consist of grain dolomite and microbial dolomite. The reservoir spaces include interparticle pores, growth-framework pores, and a few dissolved pores. The original pores are well developed, but most of the pores are fully infilled with late-stage anhydrite cements. The porosity values observed range from 0.61% to 9.25% (3.15% ± 2.23%), predominantly falling below 2%, as indicated by 41.18% of the total samples analyzed (Figure 9g–i). Zone C is identified as a pore-to-fracture-type reservoir, which is characterized by low porosity and medium permeability, exhibiting poor physical properties (Table 1). Compared with Zones A and B, Zone C exhibits an increase in infillings within the pore spaces.

In Zone D, the predominant reservoir rocks are mainly microbial dolomites, including binding dolarenite, thrombolite, and stromatolite. The reservoir spaces consist of growth-framework (dissolved) pores, with a limited presence of interparticle pores and micropores between infillings. Some larger pores are infilled with anhydrite or calcite, while residual micropores are retained. The observed porosity ranges from 0.74% to 14.11%, with an average of 3.92% ± 2.68% (Figure 9j–l), mainly 2%–4% (as 33.73% of the total samples). Zone C is identified as a pore-to-fracture-type reservoir, which is characterized by medium porosity and medium permeability, along with good physical properties (Table 1). Compared with Zone B, the reservoir rocks in Zone D have undergone a transformation into microbial dolomites, accompanied by an increase in the infillings present within the pores.

In Zone E, the reservoir rocks consist of microbial dolomite and micritic dolomite with moldic pores. The reservoir space is composed of growth-framework pores, which are either partially or completely infilled with anhydrite or halite. The observed porosity ranges from 0.16% to 16.05% (1.39% ± 1.72%), mainly <2% (as 87.25% of the total samples). Zone E is identified as a fracture-type reservoir, which is characterized by low porosity and medium permeability, reflecting the worst physical properties in the study area (Table 1).

4.3. Types of Infillings Within Pores

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Vadose silt

Vadose silts are defined as the minute fragments of bedrock and insoluble residues that result from the disintegration of bedrock during the process of karstification. These silts encompass fine dolomite powder, sand-sized particles, and fragments of growth frameworks, which are frequently deposited in karst channels (roughly) in situ [29,30]. Vadose silts exhibit a relatively loose structure and contain a significant number of micropores (Figure 5, Figure 7 and Figure 8). This type of infilling is a typical byproduct of karstification and is particularly prevalent in the study area, especially Zones B, D, and E (Figure 5, Figure 7 and Figure 8).

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Calcite and dolomite cements

This type of infilling contains medium-grained calcite and fine- to medium-grained dolomite. These cements partially infill primary and secondary karst pores and are frequently associated with vadose silts. These infillings predominantly occur in Zones A and B in the western region of the study area (Figure 4 and Figure 5). The dolomite cements exhibit a foggy center, a clear rim, and a dissolved edge, and they locally infill the interparticle pores in Zone A (Figure 4).

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Anhydrite cements

In the core samples, the anhydrite cements are observed as isolated or dispersed patches. Microscopic examination reveals that the crystals predominantly exhibit a clotted morphology (Figure 7), indicative of secondary anhydrite [31]. This type of cement fully infills primary pores, dissolved secondary pores, and micropores between vadose silt and is primarily found in Zones C–E, where anhydrites are widely deposited (Figure 6, Figure 7 and Figure 8).

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Halite cements

The halite cements are transparent (Figure 8e,f) and infill the dissolved pores and vugs with embayed dissolution edges or distinct particle contours, frequently in conjunction with minor quantities of anhydrite minerals. The low solubility of halite results in the dissolution of halite present on the core surfaces, leaving behind irregularly shaped pores (Figure 8b,c). This type of infilling is highly concentrated and mainly found in Zone E to the west of the Hengshan highland, which is the halite sedimentary zone of the study area (Figure 8).

As one moves from Zone A to Zone E, there is a transition in the reservoir rocks from grain dolomite to microbial dolomite. Additionally, the infillings within pores shift from carbonate minerals to evaporite minerals. Zones B and D exhibit the best reservoir properties, with interparticle dissolved pores and growth-framework dissolved pores. In contrast, Zones A and C exhibit secondary reservoir properties, where the reservoir space is composed of primary pores and a few dissolved pores. Zone E has the worst reservoir properties, containing pores entirely infilled with evaporite minerals.

5. Discussion

5.1. Main Controlling Factors for Reservoir Development

5.1.1. Coupling of Sea-Level Fluctuations with Sedimentary Landforms Controls Reservoir Types

The O₂m₅⁶ carbonate formations in the central–eastern region of the Ordos Basin are classified as restricted–evaporative platform sediments [17]. Within this restricted shallow marine environment, the sea level fluctuates periodically at a high frequency (Figure 1b) [12], resulting in the development of a thin upward-shallowing cycle with a small thickness of shoals and microbial mounds [14,15]. In addition, high-quality reservoirs are distributed regularly and cyclically below the exposure surfaces of these cycles (Figure 3). During the initial phase of transgression, thin to thick micritic dolomites (approximately 0.4–1.2 m in thickness) were deposited at the bottom of the cycles, occasionally featuring intercrystalline pores, which corresponded to a porosity typically below 1%. In the early regression phase, when the hydrodynamics were strong, high-energy shoals and microbial mounds were deposited, with thicknesses ranging from 0.5 to 2.0 m, and the presence of abundant primary pores, including interparticle and growth-framework pores, resulted in a porosity exceeding 1%. As sea levels continued to decline, the restricted seawater underwent evaporation and salinization, facilitating the deposition of anhydrite and halite at the uppermost sections of the cycles. The reservoir properties improve upwards with the development of shoals/mounds in a single cycle, while the upper evaporite serves as a tight cap rock. Therefore, the combination of high-frequency cycles results in an alternation of high-permeability layers and low-permeability layers, thus enhancing the heterogeneity of reservoir properties in the carbonate–evaporite symbiotic system.

During the sedimentary period of O₂m₅⁶, the prevailing strong hydrodynamic conditions above the normal wave base facilitated the formation of shoals and microbial mounds in the Taolimiao and Hengshan highlands [17,32,33]. Zone B records the sedimentary environment of dolomitic restricted lagoons, shoals, and microbial mounds. Zone D reflects a transition in sedimentary conditions from a microbial mound/dolomitic–gypsiferous evaporative lagoon to a gypsiferous evaporative lagoon. Zone E is marked by microbial mounds and gypsiferous evaporative lagoons. In contrast, the sedimentary environment in Zone A transitions from a dolomitic restricted lagoon to a shoal, ultimately evolving into a dolomitic–gypsiferous evaporative lagoon. Zone C is characterized by a combination of dolomitic restricted lagoon and gypsiferous evaporative lagoon environments. Notably, the thickness of the shoals and mounds within Zones B and D–E are considerably larger than those found in other zones, whereas the lowland zones predominantly consist of micritic dolomite or layered anhydrite deposits (Figure 3). Therefore, the reservoir quality in Zones B and D is significantly superior to that of the other zones (Figure 9 and Figure 10).

Variations in reservoir characteristics across geomorphic units indicate that shoals were deposited in the western region and that microbial mounds were chiefly found in the eastern region. In the study area, the open seawater was indirectly replenished from the northwest (Figure 1c). The effective isolation of water by the highlands contributes to a reduction of kinetic energy [17], which in turn results in a more restricted environment and an increase in salinity towards the eastern areas. Furthermore, it is significant to note that the range of evaporites has expanded. The halophilic characteristics of microbial rocks [2] result in the massive deposition of microbial dolomite in the eastern regions as salinity increases (Figure 2 and Figure 3).

Overall, the coupling of high-frequency sea-level fluctuations and uplift–depression landforms during the sedimentary period determines the hydrodynamic intensity and salinity variations in the study area. This dynamic subsequently influences the distribution and extent of favorable reservoir rocks, including grain dolomite and microbial dolomite, and ultimately constrains the disparities in distribution and quality of various reservoirs [17,32,34,35].

5.1.2. Penecontemporaneous Karstification Controls Pore Morphology and Distribution

Penecontemporaneous karstification is widely acknowledged as a significant genetic mechanism contributing to the formation of high-quality carbonate reservoirs [36,37,38]. During the penecontemporaneous stage, as the strata had not yet entered the burial stage, the rock texture was loose. High-energy sedimentary environments, such as shoals and microbial mounds, exhibited elevated matrix porosity and permeability. Karst water flowed preferentially along the high-permeability layers in the microbial mounds and shoals, with distinct “facies-controlled karst” characteristics [39,40,41].

In the context of regression across the study area, high-frequency oscillations of the relative sea levels led to the preferential exposure of carbonate shoals and microbial mounds deposited on highlands to the water surface. This exposure subjected these formations to form fabric-selective or non-fabric-selective dissolution in the meteoric freshwater seepage zone (Figure 10). The intraparticle dissolved pores and moldic pores formed by fabric-selective dissolution of early metastable minerals (e.g., aragonite and high-magnesium calcite) are an effective indicator of penecontemporaneous karstification. On the other hand, karst water flowed along the high-permeability layers of shoals and microbial mounds and continuously expanded and dissolved the primary interparticle pores [42]. As the dissolution process continued to expand, a karst cavity system emerged [43]. Moreover, due to the arid climate in the sedimentary region and the relative decline in sea levels, the environment became restricted and gradually salinized. This process was conducive to the occurrence of penecontemporaneous dolomitization and the formation of intercrystalline pores [44]. The interplay of these two factors contributed to the formation of significant reservoir spaces, including intraparticle/interparticle dissolved pores, growth-framework pores, intercrystalline (dissolved) pores, and small karst vugs (Figure 10). These features are readily formed in reservoir rocks such as dolarenite, oolitic dolomite, thrombolite, stromatolite, and crystalline dolomite situated beneath the exposure surface, thereby enhancing the reservoir performance of these formations (Figure 3).

In the upward-shallowing cycle, as one progresses from bottom to top, there is a notable increase in the intensity of karstification within the shoals and microbial mounds, leading to improved reservoir properties (Figure 3). In addition, the high-permeability layers formed by karstification further enhance the reservoir heterogeneity during the vertical stacking of these cycles. During the sedimentary period of O₂m₅⁶, the Hengshan highland was slightly higher than the Taolimiao highland, The carbonate shoals and microbial mounds that have formed on the Hengshan highland are preferentially exposed above the water surface. This resulted in an extended and vigorous karstification process, which ultimately resulted in the formation of karst breccia and vadose silt at the top of the cycle, ultimately diminishing reservoir properties (Figure 3②,④). However, this phenomenon occurs locally in the study area. Overall, the karstification caused by high-frequency exposure during the penecontemporaneous period plays a crucial role in enhancing porosity and generating new types of reservoir spaces (Figure 10).

5.2. Differential Preservation Mechanism of Reservoirs

The favorable reservoirs within carbonate shoals and microbial mounds deposited in highlands serve as the foundational material for reservoir development. Concurrently, penecontemporaneous karstification is the diagenesis process that improves reservoir performance. The coupling of these two factors controls the formation of high-quality reservoirs. In the carbonate–evaporite symbiotic system, the key to reservoir preservation lies in the filling process [41]. Based on the petrological characteristics of aforementioned reservoirs, it is evident that variations in seawater properties influence the differential infilling of reservoirs across geomorphic units and determine the varying states of reservoir preservation (Figure 10).

The highlands exhibited a susceptibility to karstification, especially in Zones D and E (Figure 5, Figure 7 and Figure 8). The carbonate shoals and microbial mounds on the highlands were subjected to prolonged exposure and underwent extensive karstification that led to the formation of karst channels, with the pores and vugs infilled with vadose silt. However, these relatively loose infillings may contain a large number of residual micropores, which have a minimal impact on the reservoir quality. In addition, the medium-grained calcite cements are also clearly controlled by karstification, and they are the cements of residual meteoric freshwater in the pores during the burial stage [28]. Zones A and B in the western part of the study area (Figure 4 and Figure 5) record relatively weak karstification, slow pore water flow, and limited material exchange between pores. Following the dissolution of the matrix by residual meteoric freshwater, the saturation of calcium carbonate increases, which is conducive to the precipitation of low-magnesium calcite in primary and secondary karst pores [45], thereby forming abundant residual pores. The heightened intensity of karstification, coupled with accelerated pore water flow and material exchange, has led to only limited sedimentation of medium-grained calcite in certain areas (Figure 7k,l).

The phenomenon of evaporite mineral filling in reservoirs is prevalent in inter-salt and pre-salt formations [16]. The typical example is the Cambrian inter-salt reservoirs in Oman [7]. In the study area, the primary infillings are anhydrite in Zones C and D, while halite predominates in Zone E. The pores with embayed dissolution edges that are either partially or fully infilled suggests that evaporite minerals deposit in the burial stage, after early penecontemporaneous karstification (Figure 7 and Figure 8). Simultaneously, the acidic fluids generated during the evolution of hydrocarbons can lead to the dissolution and reprecipitation of evaporite minerals [7] (Figure 7i).

The Taolimiao highland and Hengshan highland act as a barrier to the supply of seawater from the northwest. This obstruction results in the gradual restricted sedimentary environment of the Taolimiao depression and the East salty depression, ultimately leading to an increase in seawater salinity [12,13,14,15,16,17]. Afterwards, gypsiferous evaporative lagoon and salty evaporative lagoon environments were successively formed. Hypersaline brine formed by dissolution of the evaporite layer at the top of the cycles, and evaporation of sedimentary seawater flowed back into the karst channels within the blown carbonate. Hypersaline brine is formed by the dissolution of the evaporite layer at the uppermost part of sedimentary cycles, as well as through the evaporation of seawater. This brine flows back into karst channels or high-permeability layers within the mounds and shoals. The evaporation and crystallization of this brine forms multiple types of evaporative minerals [10,16], which then infill the reservoir spaces reworked due to karstification. Compared with Zones C and D, Zone E records a more restricted sedimentary environment, with a significantly high concentration of halite cements, which severely damages the reservoir spaces and makes the reservoir preservation impossible.

5.3. Differential Evolution Paths of Reservoirs in Geomorphic Units

In the restricted–evaporative platform sedimentary environment, the ancient landform with alternating uplift and depression significantly influences the composition and salinity of seawater [17]. Accordingly, reservoir types, karstification, infilling features, and other controlling elements overlap regionally, resulting in pronounced heterogeneity within carbonate reservoirs and a relatively intricate distribution pattern.

In terms of reservoir rocks, Zone B and D, as highlands with strong hydrodynamics during the sedimentary period, respectively develop high-quality reservoir rocks such as thick grain dolomite and microbial dolomite (Figure 11). These rocks exhibit rich primary pores such as interparticle pores and growth-framework pores, suggesting excellent reservoir properties. In contrast, the lowlands are mainly composed of evaporite and thick micritic dolomite deposits, along with thin grain dolomite and microbial dolomite, which generally reflect inferior reservoir properties.

In terms of karstification, O₂m₅⁶ is distinguished by restricted shallow marine environments and ancient landforms with alternating uplift and depression [17,46]. The highlands and the high-frequency oscillation of sea levels significantly induced the karstification process [30]. On this basis, high-energy lithologies, such as grain dolomite and microbial dolomite, coupled with karstification, controlled the formation of reservoirs [41]. The shoals and mounds in Zones B and D were preferentially exposed, underwent prolonged karstification, and developed extensive irregular cavities and karst channels, thereby enhancing improved reservoir properties. In contrast, the lowlands are primarily characterized by primary pores along with short-term fabric-selective dissolution, featuring intraparticle dissolved pores or moldic pores. These features contribute to an increase in reservoir porosity but have a limited effect on permeability (Figure 11).

In terms of infilling features, the progressive variation in seawater salinity from the western to the eastern regions has resulted in the deposition of anhydrite infilling in the pores within geomorphic units such as Zone C and D, with the most significant accumulation observed in Zone C. In Zone E, a large amount of halite in the primary pores, karst pores, and vugs resulted in the reduction of available reservoir spaces (Figure 11).

In summary, the varying degrees of influence exerted by key controlling factors, including characteristics of shoal and mound reservoirs, karstification, infilling features, and salinity of seawater, play a crucial role in the formation and preservation of the O₂m₅⁶ reservoirs. The statistical data from drilled reservoirs and the results of gas testing (Figure 12) provide additional validation for the identified distribution of reservoirs, which govern the concentrated development of high-quality reservoirs in Zones B and D.

6. Conclusions

This paper focuses on the Ordovician inter-salt reservoir developed in the Ordos Basin, North China. By integrating geomorphological analysis, the research investigates the influences of seawater energy, salinity, and meteoric freshwater leaching on the formation and distribution of the reservoir. This research systematically clarifies the origin of the north–south distribution of inter-salt reservoirs in O₂m₅⁶, which provides valuable insights for subsequent studies of inter-salt reservoirs with similar geological conditions. The specific research conclusions are as follows:

• Shoal and microbial mound reservoirs developed in highlands. However, the dolomitic restricted lagoon (Zone A) and the gypsiferous evaporative lagoon (Zone C) developed in depressions.
• The physical properties of reservoirs in highlands are evidently superior to those found in depressions. In Zones A and B, various reservoir spaces are present, including interparticle dissolved pores, growth-framework dissolved pores, and channels. Concurrently, these reservoir spaces are filled with calcite, anhydrite, and halite, with a distribution that transitions from west to east.
• In a restricted–evaporative platform environment, highlands are more prone to karstification, which results in the formation of numerous dissolution pores and vugs that enhance the physical properties of reservoirs.
• The geomorphology of alternating uplift and depression in the study area constrains the gradual increase in seawater salinity from west to east. This process controls the distribution of reservoirs, with mounds deposited in the east and shoals deposited in the west. Moreover, the high salinity of pore water leads to the precipitation of evaporite minerals, which infills the pores and weakens the physical properties of the reservoirs. This mechanism is crucial for the preservation of the reservoirs. In general, the formation and preservation of reservoirs in O₂m₅⁶ are influenced by a combination of factors, including reservoir rocks, karstification, and infilling features.
• An interesting phenomenon revealed in this study is that variations in the salinity of seawater from west to east lead to alterations in the types of cements found in reservoir spaces. This study mainly focused on the petrological characterization and the analysis of reservoir genetic models. The availability of abundant core samples in the study area facilitates a quantitative assessment of the formation and degradation of reservoirs under varying degrees of salinity of seawater in future investigations. Furthermore, when integrated with the geochemical characteristics of inter-salt reservoirs, this area presents an optimal subject for analyzing the properties of diagenetic fluids across different salinity environments.