Microbial carbonate rock can serve as a critical reservoir rock and also an important source rock^[1,2]. Microorganisms refer to tiny organisms, such as cyanobacteria, bacteria and small algae, which can be observed under a microscope. They can form mineral deposits dominated by carbonate - microbial carbonates, through processes like capturing, bonding, direct mineralization or induced mineralization^[3,4]. Since the Phanerozoic, due to the competition between metazoans and other eukaryotes, microorganisms could hardly survive in normal seawater environment, making microbial carbonates impossible to deposit, but only preserved in some special environments, like the ultra-salinity Shark Bay in Australia^[5,6], where microorganisms can survive and grow freely to contribute to the development of microbial carbonates. Therefore, a carbonate-evaporite symbiotic system is often associated with a large quantity of microbial carbonate deposits^[7,8,9,10].

The Ordovician Majiagou Formation in the mid-eastern Ordos Basin contains extremely thick cyclic deposits of carbonate-evaporite, which has always been a key target for oil and gas exploration. In the late 1990s, Jingbian gas field, the largest monolithic gas field in China, was discovered. Its reservoir has gypsum mold pores from the dissolution of anhydrite nodule dolomite in the gypsiferous dolomitic flat facies belt^[11,12]. Researches and exploration practices over 30 years have supported an increasing gas-bearing area and proved reserves in a giant gas province with nearly a trillion meters of reserves^[13]. In recent years, Ordovician gas exploration in the mid-eastern Ordos Basin has expanded to deeper formations, and major breakthroughs have been made in the Ordovician middle assemblage, especially the formation beneath the thick evaporite Ma5₆^[14,15]. Previous studies have shown that the Ordovician middle assemblage (including the Ordovician subsalt) is significantly different from the weathering crust reservoir at the Ordovician top. The former mainly contains microbial carbonate reservoir with microbial framework pores and microbial fabric dissolved pores as the pore space. Such studies analyzed the stratigraphic characteristics, dolomitization mechanism, pore type, diagenetic evolution and reservoir-caprock assemblage of microbial carbonates in the Ordovician middle assemblage in the mid-eastern Ordos Basin^{[16,17,18,19]}. However, the distribution law of such microbial carbonates remains unclear, hindering the gas exploration in the Ordovician middle assemblage. In this paper, based on outcrop sections, cores and cast thin sections, the types, microfacies combination and distribution law of microbial carbonates in the Ordovician middle assemblage are symmetrically analyzed, and the development model of microbial carbonates is constructed, aiming to provide a reference for future gas exploration in the Ordovician middle assemblage in the Ordos Basin.

The Ordos Basin was a part of the North China Craton in the Early Paleozoic. It was composed of carbonate deposits, but very unique compared to the overall sedimentary features of the North China Craton. Especially in the Ordovician, a large-scale evaporite sedimentary layer was developed in the east of the basin, due to the barrier effect of the Central paleo-uplift, Yimeng uplift and Lvliang uplift^[20] (Fig. 1). This suggests that the Ordos Basin began to show obvious structural and sedimentary differentiation from the North China Craton regarding structural pattern and lithofacies paleogeographic setting in the Ordovician. Recent drilling results have confirmed that the Cambrian paleo-uplift is developed in the Wushenqi-Jingbian area in the central Ordos Basin, where the Cambrian sediments are absent, allowing the Middle Proterozoic Changcheng System to directly contact the Middle Ordovician Majiagou Formation. The paleo-uplift has a certain successive impact on the deposition of the Middle Ordovician Majiagou Formation. It divides the central-east of the basin into two subsags, namely, the Mizhi subsag in the east and the Jingxi subsag in the central part^[21] (Fig. 1a).

The Ordovician Majiagou Formation is typically characterized by deposits of regressive-transgressive cycles of 400-900 m thick. It is the sediments of alternative carbonate and evaporite forming the restricted platform facies. According to the depositional cycle and lithologic association, the Ordovician Majiagou Formation is divided into 6 members, Ma1-Ma6, from the bottom to the top. Ma1, Ma3 and Ma5 are regressive evaporites, mainly composed of halite and gypsum deposits, and intercalated with carbonate deposits. Ma2, Ma4 and Ma6 are transgressive carbonates, mainly composed of limestone and dolomite deposits. Ma5, with a lower-order carbonate-evaporite cycle, is divided into 10 sub-members, Ma5₁ to Ma5₁₀, from the top to the bottom. Ma5₄, Ma5₆, Ma5₈ and Ma5₁₀ are regressive evaporites, dominated by gypsum, halite and argillaceous dolomite. Ma5₅, Ma5₇ and Ma5₉ are transgressive carbonates^[22,23], mainly composed of micrite dolomite, limy dolomite, and dolomitic limestone. Recent researches have divided the Ordovician Majiagou Formation into three gas-bearing assemblages (Fig. 1b): Ma5₄-Ma5₁ as the upper gas-bearing assemblage (the upper assemblage in brief), Ma5₁₀-Ma5₅ as the middle gas-bearing assemblage (the middle assemblage), and Ma4 and the underlying as the lower gas-bearing assemblage (the lower assemblage).

By texture and structure, microbial carbonates can be classified to four scales, i.e. giant sedimentary structure, large sedimentary structure, medium sedimentary structure, and micro sedimentary structure^[24]. A giant sedimentary structure is used to describe the spatial distribution of microbial carbonates within a formation to be a few to tens of meters, such as microbial mound and microbial biostrome. A large sedimentary structure is used to describe the macro morphological characteristics of microbial carbonatities to be tens of centimeters to a few meters, in shapes of mound, column and dome. A medium sedimentary structure is used to describe the macro fabric characteristics of microbial carbonatities to be such as a laminated structure (stromatolite), or a clotted structure (thrombolite). A micro sedimentary structure is the microscopic structure that can only be observed under a microscope, such as microbial fabric. For oil and gas exploration, medium and micro sedimentary structures are mostly used to study the types of sedimentary microfacies and the genetic mechanism of reservoirs, while giant and large sedimentary structures are of more practical significance to understand the spatial distribution of microbial carbonates on a macro stratigraphic framework.

The microbial carbonates in the middle assemblage can be divided into two types of giant sedimentary structures: microbial biostrome flat (layer) and microbial mound. The microbial biostrome flat (layer) was mainly formed in a tidal flat environment, which has stable, extensive and continuous distribution in the horizontal direction, but lack of obviously uplifted paleo-geomorphic shape. For example, the microbial carbonate at the bottom of Ma5₅ is a microbial biostrome structure developed in the early stage of transgression. At the bottom of Ma5₅ in Guanjiaya section in the Xingxian County, eastern Ordos Basin, oncolite microbial biostromes are found, only 0.6 m a layer (Fig. 2a, 2b and 2e). Toward the west, in the Mizhi-Shenmu area, the lithofacies changes to columnar stromatolite microbial biostrome. The microbial mound is mainly composed of microbial mounds or mound-shoal complexes, which is very thick, up to 25 m a layer. Although microbial mound structures have not been found on the outcrop section of the middle assemblage in the mid-eastern basin, the morphological characteristics of the microbial mounds can be understood through the Ordovician section at the south margin of the basin in Jinsushan, Fuping^[25]. According to the measured section, 6 stages of superimposed stromatolite mounds have been developed in Ma6, with a cumulative thickness of up to 80m, and obvious hummocky bulges (Fig. 2c, 2d). They are mainly laminar-columnar stromatolites (Fig. 2f).

2.2.1. Stromatolite

Stromatolite dolomite has permeability of (0.14-0.92)× 10^-3 μm² (averaging 0.36×10^-3 μm²) and porosity of 2.11%-7.01% (averaging 5.31%). With a laminar structure, it is diverse morphologically, such as in laminar, dome, hummock, or column shapes. The different morphology is mainly related to the sheeting microbial community and sedimentary environment^[26], especially the relevant ecosystem. Laminar stromatolite presents ripples with a relief of less than 2 cm generally, and it is mostly formed in the upper part of an intertidal zone or supratidal zone (Fig. 3a, 3b). Dome-shaped stromatolite is small (6-8 cm high), isolated or overlapped (Fig. 3c), and it may be formed in the "satellite mound" growing at the top or flanks of a microbial mound. Hummocky stromatolite is like a hemisphere, closely arranged with other spheres or isolated, which is generally 4-5 cm high and 5-10 cm wide, and often appears in an intertidal zone (Fig. 3d). Columnar stromatolite has the most complex and changeable morphology, like columns in dumbbell (Fig. 3e), dendritic (Fig. 3f), and spindle shapes (Fig. 3g), etc. In addition, stromatolites in a relatively open environment are obviously affected by biological disturbance, and the edges of the stromatolites are damaged by epigenetic organisms to be irregular (Fig. 3h).

The microscopic characteristics of stromatolites are simple and mainly shown as bright-dark laminas. The dark laminas are rich in organic matter, and the bright laminas are composed of micritic pellets or micritic aggregates, with various fenster structures (Fig. 4a, 4b). These fabrics are mostly interpreted as the residue of oxygen bubbles produced by cyanobacteria photosynthesis^[27,28,29]. The genesis is further explained to be related to the calcification of microbial membranes.

2.2.2. Thrombolite

Clotted dolomite has permeability of (1.46-28.08)×10^-3 μm² (averaging 9.82×10^-3 μm²) and porosity of 5.62%-14.60% (averaging 9.02%). With a clotted structure, it is mainly formed from the lower part of an intertidal zone to a subtidal zone. According to the morphological characteristics, it can be divided into laminar, reticular and dendritic shapes^[30], representing different sedimentary environments or water energy. Laminar thrombolite is found with alternative intermittent horizontal bacterial laminae and agglomerates, and fenster pores, which represents lower water energy (Fig. 3i). Reticular thrombolite has obvious porphyro granulitic textures. They are composed of massive agglomerates^[31], darker and several millimeters to centimeters. Most cysts among the clots have undergone strong dolomitization, so the grains are coarser and lighter in color, and show a strong contrast with the clots. This kind of agglomerates have a strong barrier capacity and moderate water energy (Fig. 3j). Dendritic agglomerates mostly grow vertically and branch upward. The environment where they settle has deeper water and higher energy (Fig. 3k, 3l).

Stromatolite has relatively complex micro characteristics. It is composed of clots and the microspar filler between the clots. The clots are composed of dark micrite and micritic aggregates, or polymerized by cyanobacteria, dark micrite and microspar. The clots are very irregular in shape, and variable in size, with a diameter of 100-1000 μm. It is worth noting that some typical calcified cyanobacteria can be seen in the clots composed of larger dark micrite. These calcified cyanobacteria, mainly cake-shaped bacteria, Hydatid bacteria and Lycopodium algal calcified microbes, are also the direct evidence of the microbial origin of the clots^{[32,33,34,35,36,37,38]} (Fig. 4c to 4e).

2.2.3. Oncolite

Oncolite is only found on the Guanjiaya section in Xingxian County and in the Ma5₅ in Well Tao 112 at the east margin of the Ordos Basin (Figs. 2a, 2e and 4f). It is mainly limestone, with fewer matrix pores, and generally 0.4-1.0 m thick. It is mostly oval-shaped or kidney-shaped, with a diameter of 2-6 cm, and filled with calcsparite. Its inner circle is an irregular concentric striated cladding around the core. The outer circle is a sheet-like radial structure, about 500 μm wide a sheet. Some filamentous fungi are faintly visible in the oncolite, which may be related to the calcification of cyanobacteria.

3.1.1. Sequence cycles control the vertical distribution of microbial biostromes

Through the analysis of the microfacies sequences, it is found that the microbial biostrome in the Ordovician middle assemblage is mainly developed at the bottom of Ma5₅ and Ma5₆, and it is sandwiched in the evaporite layers. The single-layer thickness is generally 2-3 m, and the cumulative thickness may reach 70 m. It is symbiotic with evaporite vertically (Fig. 5). The microbial biostrome is mainly composed of stromatolite and thrombolite, as well as oncolite locally found in Ma5₅. The difference of microbial carbonates between the bottom of Ma5₅ and Ma5₆ is mainly reflected on the medium sedimentary structure. The microbial biostrome at the bottom of Ma5₅ is based on the evaporite of Ma5₆, and it was formed in the early stage of transgression transiting from evaporite to carbonate in Ma5₆ and Ma5₅. It grows vertically and is mainly composed of columnar stromatolite and reticular, dendritic thrombolite (Fig. 3e-3l). The medium sedimentary structure of microbial biostrome in Ma5₆ is mainly transverse topology, the stromatolite is mainly laminar and hummocky, and the thrombolite is mainly laminar and reticular.

The microbial biostrome development is apparently controlled by sequence cycles. The rise and fall of sea level lead to barrier paleo-highs to submerge or expose, resulting in the change of salinity within the platform, and depositing (crystallizing) different types of rocks^[39]. The vertical facies follow the sequence of "halite-gypsum rock-microbial carbonate-bioturbated limestone", which represents the evolution process of salinity from higher to lower, and vice versa. The lithology transition from evaporite to carbonate is the key period for the development of microbial carbonates, indicating that the sedimentary environment is narrower and mainly controlled by seawater salinity, which further affects the number and abundance of microbial population. In a relatively open water environment, microorganisms are often difficult to survive due to the competition of metazoans and other eukaryotes for living space. In the sedimentary environment of hypersaline water, the higher salinity of seawater leads to the precipitation of evaporite minerals, which also greatly reduce the number and abundance of microbial population. This is also not conducive to the development of microbial carbonate that is mostly millimeter-sized microbial mats sandwiched in evaporites. The water environment with medium salinity (70‰- 220‰) is the most suitable for microbial survival and prosperity, which is similar to the sedimentary environment of microbial carbonate in the Shark Bay, Australia^[40].

3.1.2. Sedimentary facies change controls the regional distribution of microbial biostromes

Based on the observation of drilled cores and rock electrical property relationship, the microfacies combinations related to the microbial carbonate in the Ordovician middle assemblage were analyzed (Fig. 6). The results show that the microbial biostrome in Ma5₆ is composed of more thrombolite and less stromatolite in several sequences shallow upward. The major microfacies associations include oolitic shoal and thrombolite, thrombolite and hummocky stromatolite, laminar stromatolite dolomite + anhydrite nodule dolomite + karst breccia (Figs. 5, 6a and 6b), representing the typical microfacies associations of subtidal, intertidal and supratidal zones, respectively. The lithofacies paleogeographic analysis of the Ordovician middle assemblage in the mid-east of the basin (Fig. 7) shows that the lateral lithofacies change of the regressive evaporative layer in Ma5₆ is significant, and with orderly facies change in the order of halite-gypsum rock-gypsiferous dolomite-microbial carbonate from east to west. Further analysis shows that seawater salinity controls the regional lithofacies distribution of the carbonate-evaporite system. The Mizhi subsag in the east of the basin is mainly halite deposits blocked by the Wushenqi-Jingbian paleo-uplift in common with the Lvliang uplift. The deposit of the Wushenqi-Jingbian paleo-uplift is dominated by halite. With the gradual retreat of sea water to the west, the Jingxi subsag became a tidal flat environment, and microbial biostromes developed on a large scale in the carbonate-evaporite transition zone.

According to the Walther's law, in the continuous stratigraphic sequences, vertically continuous sediments are laterally adjacent^[41]. Vertically, microbial carbonates are mainly developed near the carbonate-evaporite interface, which can be preliminarily determined to have similar microfacies changes in laterally adjacent areas. The microbial biostrome in Ma5₆ is developed in the thick evaporite section, while the microbial biostrome in carbonates is only limited to the initial stage of transgression or regression. Although their single-layer thickness is smaller, they can extend to an extensive area. Taking the microbial biostrome at the bottom of Ma5₅ overlying Ma5₆ as an example, it is continuously distributed in a large area in the mid-east of the basin (Fig. 6c-6f). The middle of the basin is dominated by reticular, dendritic thrombolite limestone, and the east of the basin is dominated by columnar stromatolite dolomite.

3.2.1. Ma5₁₀-Ma5₇ is the main interval of microbial mounds

The Ma5₁₀-Ma5₇ interval in the east of the Ordos Basin is dominated by deposits of interbedded carbonite-evaporite. Specifically, Ma5₇ and Ma5₉ are chiefly calcareous dolomite and dolomite, while Ma5₈ and Ma5₁₀ are mainly gypsum-salt rock. The lithofacies association represents the transition type from carbonate dominated by macular limestone of Ma4 to evaporite dominated by halite of Ma5₆. From the perspective of vertical lithofacies association, the depositional period of Ma5₁₀-Ma5₇ was the transition period from normal sea water to hypersaline sea water, which represents the medium-salinity water environment. From the perspective of eustacy, the Ma5₁₀-Ma5₇ interval was equivalent to the high-system tract of the third-order sequence, and the sea level was in a relatively static stage. The medium-salinity water environment is the basis for the development of microbial carbonate, and the depositing rate in the high-system tract is higher than the growing rate of accommodation, which is conducive to the accretion growth and large-scale development of mound-shoal bodies.

The mound-shoal complexes in Ma5₁₀-Ma5₇ are composed of clotted dolomite and algal arenaceous dolomite. The clotted dolomite shows reticular and dendritic with strong adhesion. The algal arenaceous dolomite is formed by wave winnowing and crushing of the microbial mound. It has a typical algal coating structure, and generally it is at the top and wing of the microbial mound, representing a relatively high-energy sedimentary environment. The pores in the microbial mounds are mainly thrombolite lattice pores and intergranular dissolution pores. The reservoir is very heterogeneous. Taking Well Tao 112 in the gentle slope of the Wushenqi- Jingbian paleo-uplift as an example, Ma5₁₀-Ma5₇ developed in thrombolite mounds in the sedimentary environment with lower water energy. The thrombolite lattice pores are mostly cemented by radial dolomite, so the porosity is low (Fig. 5). But the top of the upward-shallower cycle is prone to be exposed and corroded, so the pores there are generally better developed.

3.2.2. The Wushenqi-Jingbian paleo-uplift is conductive to microbial mound development

In addition to a long-term and continuous medium- salinity water environment, paleogeomorphology is also an important factor controlling the development of large microbial mounds. The paleogeomorphologic high is at shallow water and high energy, so it is conducive to the development of microbial mounds. As mentioned above, the Cambrian paleo-uplift was developed in the Wushenqi-Jingbian region in central Ordos Basin. The whole Cambrian system is absent in this region, then the Middle Ordovician Majiagou Formation directly contacts with the Changcheng System. Influenced by the inheritance of the Cambrian Wushenqi-Jingbian paleo-uplift, this paleo-uplift was still a low-relief underwater paleo-uplift during the deposition of the Ordovician middle assemblage, and laid a foundation for the development of microbial mounds and shoals, consequently extensive microbial mounds (Fig. 8).

The paleotectonic pattern included three uplifts (the Central paleo-uplift, the Lvliang uplift, and the Wushenqi- Jingbian paleo-uplift) and two subsags (the Mizhi subsag and the Jingxi subsag) in the mid-east of Ordos Basin. Taking the lithofacies paleogeography of Ma5₇ as an example, the Mizhi subsag in the east was dominated by calcareous dolomite; the water body in the Jingxi subsag was relatively open, and the initial rock was dominated by biodisturbed micrite. However, most gypsum-salt layers in Ma5₆, which is adjacent to the overlying Ma5₇, have been transformed into porphyritic micrite dolomite by seepage, reflux and dolomitization during the burial period. The paleogeomorphology of the Wushenqi-Jingbian paleo-uplift was relatively high, and was generally in the intertidal zone. Large-scale microbial mounds and shoals were developed around the Jingxi subsag in central basin, covering tens of kilometers from east to west and hundreds of kilometers from east to west. They are almost thrombolite mounds and algal arenaceous shoals.

4.1.1. Microbial biostrome

Considering the paleotectonic pattern and the distribution of the sedimentary facies belts (Fig. 7), the sedimentary model of the microbial biostrome at the bottom of Ma5₆ and Ma5₇ is established (Fig. 9a, 9b). It is clear that the microbial biostrome of Ma5₆ is "facies-controlled". It is mainly developed near the carbonate-evaporite transition zone in the east of the Central paleo-uplift. The supratidal zone developed the microfacies association of lamellar stromatolite dolomite + anhydrite nodule dolomite + karst breccia, while the intertidal subtidal zone developed microfacies association of thrombolite + stromatolite and thrombolite + oolitic shoal. Vertically, microbial carbonate and evaporite are superimposed on each other. Laterally, from east to west, a supratidal zone transitions to a subtidal zone; and the types of microbial carbonate gradually change from lamellar stromatolite dolomite, then to columnar stromatolite dolomite, hummocky stromatolite dolomite, and finally to thrombolite (oolitic) dolomite. In comparison, the microbial biostrome at Ma5₅ bottom is obviously "stratum-controlled". It is mainly developed near the carbonate-evaporite transition zone. Vertically, the lithofacies changes from stromatolite dolomite to thrombolite dolomite and then to bioturbated limestone, reflecting the transgressive process of a gradually deepening water body. Laterally, the Jingxi area developed a microfacies association of multi-stage superimposed thrombolite; the Jingbian area developed a microfacies association of thin-layered wavy stromatolite + thick-layered thrombolite limestone (Fig. 6c); the Mizhi area developed a microfacies association of multi-stage superimposed columnar stromatolite (Fig. 6d); the Jiaxian area developed a microfacies association of lamellar-columnar stromatolite dolomite (Fig. 6e). They reflects the gradual transition from a subtidal zone to a supratidal zone from west to east and the characteristics of transgressive overlap.

4.1.2. Microbial mound

Microbial mounds were developed with the deposition of the Middle Ordovician Ma5₁₀-Ma5₇ in the mid-east of the Ordos Basin (Fig. 8). They are reticular and dendritic thrombolite mounds. The paleotectonic pattern was a double-barrier one in the mid-east during this period. The Central paleo-uplift was a platform margin barrier and the Wushenqi-Jingbian paleo-uplift was an intra-platform barrier. The Wushenqi-Jingbian paleo-uplift obviously controlled the sedimentary differentiation in the mid-east. During the short transgressive period (such as the depositional periods of Ma5₇ and Ma5₉), the Jingxi subsag was mainly composed of biodisturbed micrite, while the Mizhi subsag was mainly composed of dolomitic limestone and limy dolomite. The Wushenqi-Jingbian paleo-uplift had a higher paleo-geomorphic background and high water energy, which were beneficial to the development of large-scale mound-shoal complexes (Fig. 9c), and the cycle of the mound-shoal complexes and evaporite was developed vertically. However, in the short regressive periods (such as the depositional periods of Ma5₈ and Ma5₁₀), due to the slight fall of sea level, the sedimentary differentiation on both sides of the Wushenqi-Jingbian paleo-uplift was further intensified. The barrier effects of the Wushenqi-Jingbian paleo-uplift and the Lvliang uplift resulted in a sharp increase of the seawater salinity in the Mizhi subsag, so that halite deposits were dominant. In comparison, in the Jingxi subsag which was still in communication with the western open sea, micrite-rich bioclasts were dominant. The mound-shoal complexes inside the platform migrated westward to the gentle slope belt on the west side of the Wushenqi-Jingbian paleo-uplift (Fig. 9d), and the cycle of the mound-shoal complex and subtidal limy mudstone flat was developed vertically.

The microbial biostrome models are of practical significance for discovering lithologic gas reservoirs. First, the distribution law of the microbial biostrome is clearer, which reveals that the carbonate-evaporite transition zone is the favorable zone of microbial biostrome dolomite reservoir. Second, the genetic mechanism of the microbial biostrome dolomite reservoir has been clearly understood. The microbial biostrome dolomite is interbedded in regressive evaporites in a multilayer style, and it is composed of multiple superimposed sequences that become shallower upward. Because it was exposed in a short term many times, it suffered leaching of atmospheric fresh water in the early diagenetic period. This is beneficial for the development of the reservoirs with dissolved pores, so the reservoir space is mainly microbial fabric dissolved pores (Fig. 4a, 4e). Third, the prediction of favorable reservoir-forming zones is more accurate. The eastern uplifting during the Yanshanian Movement formed a paleo-tectonic pattern with higher east and lower west. The updip direction of Ma5₆ was sealed by a lithological barrier. From west to east, the lithofacies changed from carbonate into evaporite, and there was a large regional lithotransition zone, which was distributed in a ring band around the east side of the Central paleo- uplift, and conducive to gas accumulation in traps (Fig. 7).

The establishment of microbial mound development models highlights the exploration potential of the intra-platform mound-shoal bodies. Previous exploration focused on the Central paleo-uplift and its surrounding areas. Although this area also has dolomite reservoirs in the platform margin (reef) shoal facies, it is difficult to form effective traps due to the lack of gypsum cap rocks and the Yanshanian uplifting in the east, which resulted in higher east and lower west. Exploration practice has proved that the dolomite reservoirs in this area generally produce more water during well test, so the exploration effect is worse. However, the Wushenqi-Jingbian paleo-uplift is located in the evaporite development area in the mid-east of the basin. The thick evaporite cap rock in Ma5₆ directly overlies the dolomite reservoir in the intra-platform mound-shoal of Ma5₇, thus it has a better sealing ability and good trap conditions. In recent years, new breakthroughs have been made in the subsalt exploration of the Wushenqi-Jingbian paleo-uplift in Ma5. Several wells have obtained industrial gas flows by well test, which further confirms that the reservoirs in the paleo-uplift traps contain gas in multi layers. Next three-dimensional exploration will be run around the Wushenqi-Jingbian paleo-uplift, and it is the key step to make breakthroughs in oil and gas exploration in the Ordovician microbial carbonate.

The Ordovician middle assemblage in the mid-east of Ordos Basin is microbial carbonate deposits formed by cyanobacteria calcification, including stromatolite, thrombolite, oncolite microbial biostromes and microbial mounds. The microbial biostromes were deposited in a tidal flat environment, with the single-layer thickness of only 2-3 m. The microbial mounds are mainly intra- platform mound-shoal complexes, with the single-layer thickness of 15-25 m.

Sequence cycle and sedimentary facies change control the spatial distribution of microbial biostromes. In the early transgressive stage of Ma5₅, microbial biostromes were developed near the carbonate-evaporite interface, and mainly distributed in the Mizhi subsag in the east of the basin. The microbial biostromes of Ma5₆ were developed near the carbonate-evaporate transition zone, and distributed in a ring band on the east side of the Central paleo-uplift. The paleogeomorphology plays a more prominent role in controlling microbial mounds. Ma5₁₀-Ma5₇ is the primary interval of microbial carbonate, mainly distributed in the Wushenqi-Jingbian paleo-uplift.

Combined with the regional lithofacies paleogeography and microfacies sequence, the development models of microbial carbonate in the Ordovician middle assemblage in the mid-east of Ordos Basin are established. The paleotectonic pattern of "three uplifts and two subsags" controlle the spatial distribution of microbial carbonate. Dissolution during early diagenesis further improved the reservoir properties of microbial carbonate. The carbonate-evaporite transition zone and the Wushenqi-Jingbian paleo-uplift are favorable zones for oil and gas exploration in microbial carbonates.