The Ordos Basin is the largest gas producing area in China. Since the discovery of the Jingbian gas field in 1986, a lot of researches have been conducted on the exploration of the Ordovician gas^{[1⇓⇓⇓⇓⇓⇓-8]}. In recent years, the exploration field has been extended, for the distribution area of weathered crust gas reservoirs in the first and second sub-member of the fifth Member of Ordovician Majiagou Formation (Ma 5₁₊₂sub-members) around Jingbian gas field is expanded^[5-6], and the exploration in the vertical reservoirs is also expanded. In addition to weathered crust gas reservoirs, several high-yield enrichment areas and favorable gas-bearing areas are also discovered in the Ma 5_5-10 sub-members of Ordovician^[7], forming the Lower Paleozoic carbonate gas field with the resource of 10¹² m^3[8]. These achievements indicate the potential of substantial exploration in the Ordovician. Previous researches mainly focus on Ma 5_1-4 sub-members (upper assemblage) and Ma 5_5-10 sub-members (middle assemblage), but rarely on hydrocarbon generation and accumulation in Ma 1-Ma 4 members (lower assemblage). Recently, the low-yield gas flow has been obtained in 11 exploration wells drilling into the lower assemblage. Among them, the gas production of 2.16×10⁴ m³/d was obtained from Ma 4 Member in Well TA119 during the test. The gas flow has been found in the lower assemblage, but no large-scale reserve area has been formed. This is due to that the potential of hydrocarbon generation in the Ordovician lower assemblage is still unknown. The gas in the lower assemblage is far from the Upper Paleozoic, and whether the hydrocarbon source rocks are from Ordovician marine source rocks is still unknown. Whether the hydrocarbon generated in Ordovician strata, if possible, is sufficient to form the commercially exploitable gas reservoirs, and the distribution location of the major source rocks are still obscure. Moreover, the development of the favorable salt-related reservoirs in the Ordovician lower assemblage carbonate rocks-gypsum- salt rock strata, which are relatively dense, and reservoir types for the major target layers are still unclear. The high-yield enrichment areas have not been found. The mechanism and mode of gas accumulation is still not yet known due to deep burial and complex lithofacies of Ordovician strata.

It is noticed that the gypsum-salt-carbonate interbedded symbiosis characteristics similar to those in the upper and middle assemblages have been observed in the lower assemblage. A breakthrough has been achieved in the upper and middle assemblages, indicating the significant exploration potential in the lower assemblage. As we all know, the formation of large oil and gas fields is closely related to development of large areas of gypsum salt rocks^[9⇓-11]. The Ordovician gypsum salt rocks and carbonate rocks are developed as the rhythmic interbeds in the Ordos Basin, where there are abundant natural gas resources. The gypsum salt rocks play a role in controlling and promoting gas accumulation. Apart from acting as the sealing, the gypsum-salt rocks promote the evolution of source rocks and reservoirs^[12-13]. In the evaporative tidal flat and lagoon environment controlled by climate and biological factors, carbonate rocks, gypsum salt rocks and organic matter interbedded structures dominated by cyanobacteria algae mats can be formed, which have a certain potential of hydrocarbon generation^[14]. The gypsum salt rocks are also conducive to dolomitization to form favorable reservoir space. The previous studies mostly focus on the gas accumulation related to gypsum-salt rocks in the upper and middle assemblages, but pay no attention to the salt-related gas accumulation in the lower assemblage.

To study the key elements of gas accumulation in the lower assemblage, this paper evaluates source rock and reservoir, based on drilling core, logging and seismic data; analyzes gas accumulation and favorable zones, and explores the hydrocarbon accumulation rules of paleo-carbonate rocks in the environment of salinized restricted epeiric sea.

The sedimentary evolution of carbonate rocks is controlled by the paleo-tectonic framework of the deep strata in the Ordos Basin. At the end of the Cambrian, the tectonic framework was distributed along nearly NS^[15-16]. Controlled by the Yimeng paleo-uplift, the Wushenqi- Jingbian area occurs as an uplift area, which gradually plunges into the open sea in the periphery. At the end of the Huaiyuan Movement, the Lower Ordovician Yeli Formation-Liangjiashan Formation was deposited around the platform. In this period, due to the subduction of the Qinqi oceanic crust, the central paleo-uplift began to uplift, and together with the Yimeng paleo-land and the Lüliang low-uplift, controlled the Mizhi-Zizhou low to form a large-scale restricted intra-platform depression. Under arid evaporation conditions, a belt-like lithofacies sequence of gypsum salt rock, gypsum dolomite and argillaceous dolomite was formed, and low-energy argillaceous rock deposits were formed in the tidal flat environment around the belt (Fig. 1).

During the deposition of Majiagou Formation, the tectonic episodic rise and fall led to the periodic rise and fall of the central water body in the depression and the rhythmic deposits of epeiric sea carbonate rocks and restricted marine gypsum salt rocks were developed. Generally, Ma 1, Ma 3 and Ma 5 Members are dominated by gypsum dolomite, gypsum salt rock and salt rock, and Ma 2, Ma 4, and Ma 6 Members are dominated by dolomite and limestone. The Ma 6 Member is missing in the most parts of the central-eastern Ordos Basin. Controlled by short-term transgression and regression, 10 rhythmic sub-members are developed in the Ma 5 Member. Previously, the Ordovician strata were divided into upper, middle and lower assemblages^[7] based on the exploration degree and the vertical stratigraphic relationship (Fig. 2). The upper assemblage is composed of Ma 5_1-4 sub-members, which are dominated by the weathered crust gas reservoirs. The middle assemblage consists of Ma 5_6-10 sub-members, where a breakthrough in exploration has been achieved in recent years. The lower assemblage includes Ma 1-Ma 4 Members, which are study objects and current key exploration target strata.

The major gas-producing reservoirs of the Ordovician lower assemblage in the Ordos Basin are Ma 4 and Ma 3 Members, characterized by small thickness, tight reservoirs and dry gas. The low-yield gas flow is obtained from Ma 4 Member, mainly Ma 4² sub-member, in 8 wells in the Wushenqi-Jingbian sub-uplift zone and Shenmu-Zizhou salt uplift zone. The low-yield gas flow is also found from Ma 3 Member in 3 wells in the Wushenqi secondary paleo-uplift, and the major gas-producing reservoir is Ma 3₂ sub-member. In addition, the CO₂ gas flow of 5.6×10⁴ m³/d is also obtained from Ma 3 Member in Well LT2. Generally, the upper and lower assemblage gas reservoirs are relatively thin, and the single layer effective thickness is 0.8-3.0 m in logging interpretation. The gas reservoir is dominated by dolomite with stable lateral distribution. It is inferred from well-to-well correlation that the gas reservoir extends more than 20 000 m. Reservoirs are in transition to the limestone or gypsum-salt rock in the east upward-dip direction, forming the lateral barrier. The gas reservoirs have good physical properties with well logging porosity of 1.72%-7.59%, and good gas-bearing conditions with well logging gas saturation of 26.8%-75.32%. The gas components in the lower assemblage of the Ordos Basin (Table 1) are dominated by the hydrocarbon gas with the content of 82.284%-99.474%, with high methane content ranging from 82.241% to 97.861%. The dryness coefficient is 0.935-0.999 indicating the dry gas of high-thermal evolution. The non-hydrocarbon gases are mainly by CO₂ and N₂. Recent study shows the relatively high H₂S gas content in the sub-salt^[17], which is different from the gas component characteristics in the weathered crust of Ordovician upper assemblage.

The gas source of the Ordovician lower assemblage in the Ordos Basin was studied based on previous research^{[17⇓⇓⇓⇓-22]}, and the results show the gas resources from marine source rocks according to gas isotopic composition, gas component, sulfur crystal and hydrogen sulfide, and hydrocarbon inclusion.

(1) The gas isotopic composition shows the characteristics of the oil-type gas. The sub-salt deep gas is generally characterized by relatively light methane isotope (Table 1). The latest natural gas test data show the carbon isotopic composition of methane ranges from -45.09‰ to -37.29‰ and that of ethane is relatively heavy. This phenomenon is illustrated with the thermal simulation of kerogen hydrocarbon generation in the golden tube. First, two samples from Ordovician marine source rock were heated from room temperature to 250 °C under a constant pressure of 50 MPa within 8 h, and then they were heat up to 600 °C by fast heating (20 °C/h) and slow heating (2 °C/h) respectively and analyzed at 12 temperature points. The results show that for the gas hydrocarbons generated from the marine mud shale, the carbon isotopic composition of the individual hydrocarbons gets heavier with the increase of thermal evolution degree (Fig. 3). The carbon isotopic composition of ethane becomes heavier more rapidly compared with that of methane, which indicates that the sulfate thermochemical reaction (TSR) may happen during the evolution of alkane gas and the light components of ethane tend to be cracked into methane during isotope fractionation, so that the isotopic composition of residual ethane is positively drifted.

(2) Component identification indicates the oil cracked gas. The gas component data of the lower assemblage were projected on the identification chart of kerogen degradation gas and oil cracked gas at different evolution stages, which is established by Li Jian et al.^[18] (Fig. 4), and most of data fall in the oil cracked gas zone.

(3) Sulfur crystals are discovered in the cores from Ma 4 Member, which indicates hydrocarbon fluids and gypsum salt rocks once interacted in the lower assemblage. Light yellow sulphur crystals occur in the dissolved cavities semi-filled with calcite (Fig. 5a), and the sheet sulphur crystals filling the fractures occur in the core fractures. The presence of sulfurs indicates TSR reaction between the hydrocarbon fluid and the gypsum salt. The chemical process is expressed as follows.

In fact, H₂S is also observed in fluid inclusions (Fig. 5b). The gas produced through the low-yield wells in the low assemblage generally has the high H₂S content. Previous study shows the H₂S content of 9.016%-23.230%, averaging 11.58%, and comparison of δ³⁴S shows a good matching between the sulfur isotopic composition of the sub-salt gas with high hydrogen sulfide and the sulfate^[17]. The limestone coexists with the gypsum-salt rock in the lower assemblage, which experiences the over-maturity thermal evolution, and the positive drift of the carbon isotopic composition of ethane appears, confirming the TSR reaction in the lower assemblage.

(4) A large number of hydrocarbon-bearing fluid inclusions are discovered in the secondary dolomite or the calcite minerals in the deep sub-salt reservoirs in the Ordos Basin. Hydrocarbon fluid migration occurred during the formation of these inclusions. Laser Raman detection confirms abundant inclusions in the hydrocarbon fluids. For example, a large number of hydrocarbon-bearing inclusions are discovered in Well SH473 (Fig. 5c, 5d), whose gas is determined to be methane by means of the laser Raman detection.

The above data show the relatively light methane isotopic composition and the relatively heavy ethane isotopic composition. The gas component identification result indicates the oil cracked gas. The abundant sulfur crystals in the cores of the lower assemblage, H₂S in the gas and wide distribution of H₂S and hydrocarbon inclusions indicate that the gas of the Ordovician lower assemblage is mainly sourced from the marine source rocks.

Two types of source rocks are developed in the Ordovician lower assemblage. The first type is the gray-black argillaceous rock band or the thin laminae interbedded with the gray carbonate rocks (Fig. 6a, 6b), and they generally coexist with the gypsum dolomite or the gypsum-bearing dolomite. The gray-black argillaceous rock bands have higher organic matter abundance and are characterized by high gamma on the logging curve (Fig. 2). The second type is dominated by the algae mass and the algae dolomite (Fig. 6c). Microscopic observation indicates the black organic matter filling sutures, micro-fractures and pores (Fig. 6d).

A lot of efforts have been made to study the lower limit of TOC of carbonate rocks. Wang Zhaoyun proposed TOC=0.3% as the evaluation lower limit for carbonate gas source rocks^[23], and Qin Jianzhong proposed the lower limit of hydrocarbon expulsion as about 0.08% in the high maturity and over-maturity marine carbonate rocks, and the lower TOC for hydrocarbon expulsion as about 0.3% in the mature marine hydrocarbon-rich carbonate rocks^[24]. According to the linear relation between the Ordovician argillaceous rock content and TOC in Ordos Basin^[25], the lithology with the shale content higher than 20% is considered as the source rocks, which corresponds to TOC of 0.3%. The XRD (X-Ray Diffraction) mineral content analysis was conducted on the lower assemblage in the Ordos Basin based on the data of the fully cored well, and statistical analysis was carried out on the relationship between shale content and TOC, which is also linear. TOC=0.3% roughly corresponds to the shale content of 19%, so TOC=0.3% is adopted as the lower limit for source rocks.

The TOC values of source rocks analyzed by means of conventional methods are generally lower. The algae lamina and algae mass are darker colored, and have the relatively high TOC. Statistical analysis of 805 samples from Ma 1-Ma 4 Members shows TOC of 0.07%-3.24%, averaging 0.31%. The samples with TOC of 0.1%-0.5% account for about 89.1% of all samples, and those with TOC higher than 0.3% account for about 28%. For the rocks with interbedded light-gray carbonate rocks and gray-black argillaceous rock bands, the gray-black argillaceous bands were sampled for analysis (Fig. 6a). The results show that TOC decreases as the band is lighter colored. The maximum TOC of the dark-black argillaceous bands is up to 0.7% and its average is 0.45%. The average TOC of the black and gray-black argillaceous bands reaches 0.32% and 0.19% respectively, and their sample number accounts for 74.14%. The samples of dark black and black source rocks with TOC greater than 0.3% account for 44.83%. The TOC value of the dark black and black mudstone bands reaches 0.78%. The TOC value of algae lamina and algae mass is generally higher than 0.5% and can reach 3.24%.

For the Lower Paleozoic source rocks with low TOC from the conventional analysis, the low residual organic carbon content after hydrocarbon generation in the high maturity and over maturity evolution and the loss of organic acid salts in conventional TOC analysis should be taken into consideration. A lot of basic researches have been conducted on organic acid salts and their hydrocarbon generation characteristics^{[26⇓⇓⇓⇓⇓⇓⇓-34]}. It is indicated that the carbonate rock-gypsum salt rock sedimentary environment is conducive to generation of organic acid salts in the algae organic matter. In addition, a detailed analysis procedure is established. Organic acid salts are generated from reaction of the organic acids, which is discharged during transformation of sedimentary organic matter into kerogen during the diagenesis process and in the kerogen hydrocarbon generation stage, with the calcium, magnesium and other metal ions in the marine limestone. The organic acid salts remain stable at low temperature and have the strong hydrocarbon generation capacity at high temperature, indicating the potential of large-scale transformation to gas hydrocarbons in the high and over-maturity stages.

In order to investigate the effect of the organic acid salts in the Ordovician low assemblage in the Ordos Basin on detection of organic carbon, the organic acid salt analysis of 12 samples from the lower assemblage was performed by means of the previous analysis method^[31-32,34] from the perspective of application, and then the results were compared with the TOC value obtained by means of the conventional method. The results show that the TOC value obtained by conventional methods is lower, with an average of 0.26%. The total TOC value was recalculated after the organic acid salt was analyzed. It is basically higher than 0.3%, with the average of 0.58% and the maximum of 1.36%, indicating the occurrence of organic acid salt and the high hydrocarbon generation potential in the lower assemblage gypsum salt strata (Fig. 7).

The kerogen maceral identification and division were carried out with 22 source rock samples from the Ordovician lower assemblage. The results show no exinite and inertinite in these samples. The components are mostly amorphous components in the sapropelinite (Fig. 6e), accounting for more than 96%, with a few marine vitrinites, which is classified as type I kerogen. It is indicated that no terrestrial organic matter is introduced into the hydrocarbon-generating parent material of the lower assemblage and the hydrocarbon generation conditions are better.

The kerogen marine vitrinite and asphalt reflectance of 30 source rock samples from the Ordovician lower assemblage were tested (Fig. 6f), and the equivalent vitrinite reflectance (R_o) was calculated with the previous correction formula^[35]. The calculated R_o is in the range of 1.62%-2.16%. R_ois slightly lower near Wushenqi in the north and slightly higher in Fuxian-Huangling area in the south, indicating the organic matter in the source rocks of the lower assemblage is in the high-over maturity stage of thermal evolution and can generate a large amount of gas.

Generally, the source rocks of Ordovician lower assemblage are relatively thin. However, the relatively thick argillaceous rock bands are developed to varying degrees surrounding the salt depression, with a single layer thickness up to about 22 m, and the source rock is in a relatively large scale. The total source rock distribution of the lower assemblage is mapped by combining partial seismic inversion profiles and drilling source rock data (Fig. 8).

The source rocks of Ordovician lower assemblage are distributed around the gypsum-salt rock depressions in the shape of girdle and mainly developed in Jungarqi-Wushenqi-Wuqi-Fuxian-Huanglong, etc. The carbonate source rocks with the relatively high organic matter abundance and larger thickness are developed near Mizhi-Yulin-Shenmu in the middle of the basin. The source rock thickness generally reaches 20-80 m, and locally is up to 100 m. In the girdle belt, the argillaceous rocks of Ma 3 Member are associated with the gypsum salt rock, dolomite and limestone, and is dominated by dolomitic mudstone or argillaceous dolomite characterized by high gamma logging response (Fig. 2). The dark black argillaceous rock band is featured with the high organic carbon content and provides a larger scale of hydrocarbon supply. In the central area of the depression, the source rocks of Ma 4 Member are mainly alga-gobbet rocks and crumby algae dolomite, with overall large scale.

Previous study shows that the development of mound- shoal reservoirs in the carbonate platform is controlled by the paleomorphology of uneven and high-low differentiation^[36-37]. The development and distribution of Ordovician dolomite reservoirs in the study area are also controlled by sedimentary setting and paleogeomorphic environment. Before the sedimentation of Majiagou Formation, the central paleo-uplift and the Wushenqi-Jingbian secondary paleo-uplift were developed during the Huaiyuan Movement and controlled the macroscopic distribution of Majiagou Formation beach dolomite reservoirs, and the uplift and depression patterns were inherited. For example, Cambrian Sanshanzi Formation and Ordovician Ma 1 and Ma 2 Members are missing near Well DT1 in the central paleo-uplift (Fig. 9), indicating occurrence of the inherited paleo-uplift in this period. The uplift with large amplitude and long period separated the North China Sea from the Qilian Sea and controlled the lithological characteristics of the overlying Ma 3 and Ma 4 Members.

The Cambrian is missing locally in the Wushenqi- Jingbian secondary paleo-uplift belt, and the Ordovician Ma 1 Member directly overlies the Changcheng System (around Wells TG100, JN14 and JN6 in Fig. 9). Cambrian Zhangxia Formation and Sanshanzi Formation are drilled successively in Wells QI44, MI131 and YU9 to the east. The Ordovician Ma 2 Member directly overlies the Maozhuang Formation, and the Middle-Upper Cambrian Xuzhuang, Zhangxia and Sanshanzi formations are lost near Well TA59 to the west. This secondary uplift is an inherited paleo-uplift that occurred in the Cambrian. The Cambrian strata are gradually thickened to both sides of the uplift zone and are completely lost near the uplift. During the deposition of Ordovician, the water was relatively shallow and the sedimentary strata are relatively thin. For example, the thickness of Ma 1 Member is 23.6 m and 15.4 m respectively in Wells JN14 and JN6 in the uplift and increases to 55.0 m, 67.0 m and 104.4 m respectively in Wells QI44, MI131 and YU9 to the east.

For the perspective of sedimentary cycles and lithology, Ma 1 Member in the uplift is dominated by mudstone and muddy dolomite with interbedded thin gypsum rock, and is transformed into thick halite near Wells QI44, MI131 and YU9 to the east, where the argillaceous rocks are gradually thinned. During the deposition of Ma 2 Member, transgression was dominant, and three sets of dolomites were developed separated by two sets of gypsum-salt rocks deposited in short-term regression. During the deposition of Ma 3 Member, it was an arid evaporation environment, where gypsum dolomite, gypsum and halite deposits were mainly developed. Gypsum and dolomite were mainly developed in the secondary paleo-uplift areas near Wells TG100, JN14 and JN6. Thick gypsum salt rocks were locally developed in the Shenmu-Zizhou area. During the deposition of Ma 4 Member, the Ordos area was in the maximum flooding period, and relatively thick limestone deposits were developed in the deeper water of North China sea to the east of the central paleo-uplift, while thick dolomite with the maximum thickness of 450 m was developed in the shallower water body in the central paleo-uplift and its periphery. The dolomites is dominated by the porphyritic dolomite formed under the condition of biological disturbance in a relatively shallow water environment and is featured with large crystalline and good physical properties, but it mainly produces water. The thin dolomite occurred mainly in the Wushenqi-Jingbian secondary paleo-uplift. During the late sedimentation of Ma 4 Member, the thick dolomite body was developed associated with the occurrence of thin gypsum rock, which marks the fall of high water body and local formation of an evaporation environment, which is conducive to development of quasi-contemporaneous dolomite. In the Shenmu-Zizhou area, a low uplift is formed due to the salt uplift of local thick gypsum-salt rocks in Ma 3 Member, so the dolomite reservoirs of Ma 4 Member are relatively developed as well.

The gypsum-salt layers promote the formation of dolomite^[38⇓-40]. The genesis of the dolomite reservoirs below the gypsum salt rock is mainly reflux seepage dolomitization, e.g. the thick massive dolomite and bio turbulent porphyritic dolomite in Ma 4 Member near the central paleo-uplift. The specific process is that at the end of the Ma 4 Member deposition and in the early Ma 5 Member deposition, the regression caused the super-tidal evaporation environment near the uplift and its periphery and local precipitation of gypsum, resulting in a significant increase in the ratio of magnesium to calcium ion concentration in the formation water. Moreover, when the surface periphery was basically ended, the brine flowed to the lower loose sediments, forming the thick dolomite. In addition, the promotion of gypsum salt rock on the reservoir evolution is also reflected in the transformation of anhydrite to gypsum, and the water loss led to a decrease in rock volume, which is conducive to the formation of intercrystalline pores. The sulfate reduction reaction consumed calcium sulfate and promoted the gypsum dissolution, and the released acidic fluids (e.g. CO₂ and H₂S)^[27] are conducive to mineral dissolution and formation of secondary pores.

The distribution and evolution of dolomite reservoirs are controlled by paleotopography in the sedimentation period and the activity of brine fluids related to gypsum-salt rocks. Reservoir development is controlled by dolomitization, fluid dissolution and other geological processes^{[41⇓⇓⇓⇓-46]}, and there are three types of dolomite reservoirs, namely intercrystalline pore type, dissolution pore type and fracture type.

The intercrystalline pores are mostly developed in coarse powder crystal and fine-medium crystal dolomites. The irregular polygonal pores are mostly the euhedral intercrystalline pores, which occur in a small number between euhedral-anhedral crystals (Fig. 6g). For example, the euhedral crystal and intercrystalline pores occur locally in the bio turbulent porphyritic dolomites, semi-euhedral crystals and intercrystalline micro-pores occur mostly in the limy dolomites, and small-diameter intercrystalline pores occur in the oolitic dolomites.

Dolomite reservoirs of dissolution pore type are further classified into two types. The first one is related to the gypsum nodule dissolution in the micrite dolomite or granular shoal dolomite. It is common in the weathered crust reservoir of the upper assemblage due to the leaching of atmospheric fresh water, and is also observed in the algal pellet gypsum dolomite of Ma 4 Member. The second one is mostly distributed in the algal clastic shoal dolomite and microbial framework dolomite in the local paleo-uplifts and paleo-highlands, and its distribution is related to the black organic matter belts. It is laterally distributed in the shape of irregular belt, which is common in the Ma 4 Member of low assemblage (Fig. 6h). Its formation is mainly due to the interaction of the fluid with dolomite when flowing along the intercrystalline pores and micro-fractures with a low degree of dolomitization, forming the intercrystalline dissolution pores or intracrystalline dissolution pores. Due to the strong fluid dissolution along the intercrystalline pores formed earlier, the intercrystalline dissolution pores are transformed into the dissolution pores of slightly large scale. The calcite growth and filling is often observed in the dissolution pores. The intra-crystalline dissolution pores are limited in scale due to the low dissolution degree.

Fractured reservoirs are the major reservoirs in carbonate rocks. The micro-fractures are relatively developed in Ma 4 and Ma 3 Members of the lower assemblage, including micro-fractures formed by tectonic action and dissolution fractures formed by dissolution, such as pressured dissolution fractures and suture seams. Generally, the reservoir performance is improved by the fracture development belt due to its communication with the pores. Moreover, the dissolution pores are relatively developed near the fractures due to active fluid migration and strong dissolution along the fractures (Fig. 6i). In the late stage, some dissolution fractures are filled with calcite, which destroys the reservoir space.

The distribution of carbonate reservoirs is closely related to paleomorphology and sedimentary environment. During the deposition of Ma 4 Member, the central paleo-uplift and the Wushenqi-Jingbian secondary paleo-uplift were developed in sequence from west to east in the Ordos Basin. The secondary paleo-uplift can be recognized on the well-to-well correlation section (Fig. 9). In the early and middle deposition stages of Ma 4 Member, large-scale transgression occurred in the uplift with the great water body energy, so high-porosity grain beaches were easily formed in the high position. In the late stage, regression began and the sedimentary environment was mostly in a state of continuous-intermittent exposure, causing strong dolomitization and gypsification, coupled with the strong evaporation. The reservoirs in the eastern Shenmu-Zizhou area are also relatively developed for lime-bearing dolomites are developed in the Ma 4 Member due to the local salt uplift of Ma 3 Member (Fig. 10).

According to core measurement results, the porosity of the dolomite reservoirs of Ma 4 Member in the Wushenqi-Jingbian secondary paleo-uplift is 0.23%-14.34%, and that of the high-quality reservoirs is mainly 1%-3%, averaging 1.8%. And according to logging interpretation, the porosity is 0.71%-10.26%, averaging 4.26%. For the dolomite reservoirs of Ma 4 member in the Shenmu- Zizhou regional low uplift zone in the east, the measured porosity is 0.11%-5.93%, and that of the high-quality reservoirs is mainly 1%-2.8%, averaging 1.6% while the logging interpretation porosity is 2.02%-8.65%, averaging 4.51%. There are few wells drilling in Ma 3 Member. Statistics shows that the measured porosity of the core is 0.11%-10.35%, averaging 1.65% while the logging interpretation porosity is 0.01%-10.33%, averaging 3.75%.

On the whole, the physical properties of the Ma 4 Member dolomite reservoirs are better than those of Ma 3 Member, and the physical properties of Ma 4 Member near the Wushenqi-Jingbian secondary paleo-uplift are better than those in the eastern Shenmu-Zizhou area. The high-quality reservoirs with the porosity of 1% to 3% are dominated by dolomite intercrystalline pores with good local homogeneity. Some reservoirs with the porosity greater than 3% are related to dissolution pores or fractures, and their reservoir performance is better, but it is of strong inhomogeneity and large lateral change.

During the sedimentation of the Ma 3^rd Member of Ordovician lower assemblage, it was an arid evaporative environment of regression, where the gypsum salt rock and gypsum dolomite were mainly deposited. During the sedimentation of Ma 4 Member, it was a high-water sedimentary environment of transgression, where dolomite and limestone are mainly developed. Both Ma 3 and Ma 4 Members are dominated by the reservoir space of dolomite intercrystalline pores, intercrystalline dissolution pores and micro-fractures, but they are sealed by different cap rocks, so as to form different source-reservoir-cap assemblages.

The cap rocks are mainly classified into two types, i.e., tight carbonate rock and gypsum salt rock. Ma 4 Member is dominated by tight carbonate rock, including laminar argillaceous rock, marl, micrite limestone, etc. Generally, the cap rocks of Ma 4 Member are distributed in the deep-water sedimentary area between local uplifts, and covers the local uplift in the relatively high water level period, to interbed with the quasi-contemporaneous dolomite formed in the relatively low water level period. Generally, it is characterized by the tight lithology, the low porosity and permeability and strong sealing. The algae and argillaceous deposits occurring in Ma 4 Member are the key source rocks, forming an interbedded source-reservoir-cap assemblage with layered dolomite reservoirs and tight carbonate rocks.

The gypsum-salt rock cap is developed in Ma 3 Member, and is lithologically dominated by anhydrite, argillaceous gypsum rock, and salt rock deposited in an evaporation environment. It is characterized by low density, tight lithology and strong sealing. The lithology occurs as salt rock, gypsum salt rock, gypsum-bearing limestone, gypsum dolomite, marl and argillaceous rock from the central depression to its periphery. Among them, the organic-rich argillaceous rocks surrounding the depression mostly occur in the form of thin laminae and have a certain potential of hydrocarbon generation to form the source-reservoir-cap assemblage with dolomite between gypsum-salt rocks and gypsum-salt rock cap in an evaporative environment.

For the Ordovician lower assemblage, Etuokeqi-Qingyang paleo-uplift, Wushenqi-Jingbian secondary low uplift, and Shenmu-Zizhou salt uplift occur from west to east, and dolomite reservoirs are relatively developed. There is a relatively deep water environment between the uplifts, and the tight limestone is mostly developed (Fig. 10). The Yanshan Movement leads to the overall uplift of the eastern Ordos Basin, forming the lateral lithological up-dipping sealing traps and the local low-amplitude structural-lithological composite trap. In this tectonic setting, Ma 4 member is dominated Wushenqi-Jingbian and Shenmu-Zizhou plays, which take algae-bearing marl and algae dolomite as the major source rocks, the dolomite in the low uplift belt as the reservoir, and the tight carbonate rock as the cap rock. The dolomite reservoir is laterally transformed into the tight limestone to act as the sealing. The gas discovered in Ma 3 Member is mainly distributed in the Wushenqi-Jingbian low uplift belt and its play takes the laminar argillaceous rock surrounding the depression as the source rock, the thin inter-salt dolomite as the reservoir, and the gypsum-salt rocks developed laterally to the east as the sealing.

The Ordovician upper, middle, and lower assemblages in the central and eastern Ordos Basin are different in hydrocarbon accumulation modes, which are classified into three types (Fig. 11). Type I is the hydrocarbon accumulation mode of weathered crust in the upper assemblage, which is mainly distributed in Jingbian and the surrounding areas, corresponds to the Upper and Lower Paleozoic mixed source model dominated by the upper Paleozoic gas source, and is dominated by gypsum-molded pore reservoirs and stratigraphic traps. Type II is the hydrocarbon accumulation mode in the middle assemblage, and is mainly distributed in Wushenqi, Jingbian West and Wuqi areas surrounding the salt depression in the shape of girdle. The gas reservoirs of Ma5₄ and Ma5₅ sub-members above the Ma 5₆ sub-member gypsum salt rock are dominated by the Upper and Lower Paleozoic mixed source hydrocarbon supply. The gas reservoirs of Ma 5_6-10 sub-members occur as the self-source and self-reservoir oil type gas play with the argillaceous belt, algal dolomite and algal limestone as source rocks^[17,22]. Type III is the hydrocarbon accumulation mode in the lower assemblage and is mainly distributed in the Wushenqi-Jingbian secondary paleo-uplift and the salt uplift belt near Shenmu-Zizhou. The source rocks are acted mainly by the Ma 3 Member source rock dominated by thin argillaceous bands and the Ma 4 Member source rock dominated by algal dolomite and algal limestone. The reservoirs are dominated by grain beach and algae beach fine crystalline dolomite or calcareous dolomite in the uplift part, with dissolution pores and intercrystalline pores. Limestone and argillaceous limestone are mostly developed in low-lying areas around the uplift. The uplift in the east after the Yanshanian tectonic movement leads to the lateral lithological sealing trap or the low-amplitude structure-lithological composite trap. Vertical small fractures, micro-fractures and lateral thin dolomite form a transport system for gas migration and accumulation.

Many wells produce low-yield gas flow from Ma 3 and Ma 4 Members in the Ordovician lower assemblage of the Ordos Basin during the test. It is indicated they are important exploration areas recently and their exploration potential in the future shall be concerned.

The Ma 3 Member is 140-180 m thick and covers the favorable exploration area of about 1.6×10⁴ km² near the Wushenqi-Hengshan-Jingbian secondary paleo-uplift. The source rocks in the sub-sag to its west are relatively developed and they are dominated by gray-black laminar argillaceous rock and algae-mass argillaceous dolomite interbedded with gypsum dolomite with high gamma logging response, which is similar to the sawtooth-shaped high gamma logging response of upper and middle assemblage (Fig. 2), so their hydrocarbon supply potential is greater. These argillaceous rocks are alternately interbedded with dolomite and gypsum salt rock, indicating shallow seawater, sufficient light and luxuriant algae during the deposition, which is conducive to the adhesion and preservation of organic matter. The Ma 4 Member is 160-220 m thick and covers about 2.4×10⁴ km² in Wushenqi-Hengshan-Jingbian secondary paleo-uplift and the Shenmu-Zizhou salt uplift. The argillaceous rocks and algal limestone are developed in the sub-sags between uplifts, and their hydrocarbon supply capacity is high. The algal dolomite developed in the secondary uplift is distributed in a large scale, indicating good reservoir capacity. Gas show and low-yield gas flow have been obtained from Ma 4 Member in several wells, indicating promising exploration prospect.

From the perspective of areal distribution, the favorable exploration targets are directed to the Wushenqi- Hengshan-Jingbian secondary paleo-uplift and the Shenmu-Zizhou salt uplift belt. The dolomite reservoir developed in the local low uplift and the surrounding dolomite and limestone interbedded zone are gradually transformed into tight limestone to the east (Fig. 10), which is conducive to the formation of up-dipping lithological sealing gas reservoirs, and the local low-amplitude structural traps are the best in enrichment conditions. Recently, Well JT1 and MT1 are drilled in these two favorable zones with good shows of gas, which indicates these two zones are important exploration directions and proves the Ordovician lower assemblage has the potential of large-scale hydrocarbon generation and is an important exploration field in the future.

The gas in the Ordovician lower assemblage of the Ordos Basin is the Ordovician self-generated dry gas with high-thermal evolution degree and is featured with the relatively light methane isotope and the relative heavy ethane isotope. The identification on the gas component chart indicates the oil cracked gas, which experiences TSR reaction with sulfate to form the sulfur crystals and H₂S gas.

The gray-black argillaceous lamellae and algae mass source rocks are developed in the lower assemblage and are distributed around the salt depression. The TOC is largely 0.10%-0.5%, the maximum 3.24%, averaging 0.31%. The average TOC recovered by means of the organic acid salt method reaches 0.58%, indicating enough conditions for large-scale hydrocarbon supply.

The distribution of dolomite reservoirs is controlled by sedimentary paleomorphology and local gypsum-salt uplift. They can be classified into three types, namely intercrystalline pore type, dissolution pore type and fracture type, and their evolution is closely related to the gypsum-salt rock.

The hydrocarbon accumulation mode in the lower assemblage is composed of marine source rocks for hydrocarbon supply, beach facies dolomites as reservoirs, small and micro-faults as transportation pathways, and structural-lithological traps for accumulation.

The dolomite reservoirs are developed in Wushenqi- Hengshan-Jingbian paleo-uplift and the Shenmu-Zizhou salt uplift and are transformed gradually into the tight limestone eastward, to form the up-dipping lithological sealing gas reservoirs, which are key exploration directions.