Ordos Basin, with an area of ​​25×10⁴ km², has the Archean and Paleoproterozoic metamorphic rocks as basement, sedimentary systems including Changcheng System, Jixian System, Sinian, Cambrian, Ordovician, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Tertiary and Quaternary upwardly, and no Silurian and Devonian, with a total thickness of 5 000 to 10 000 m. The formations have a two-layer structure, marine facies in the lower part and continental facies in the upper part. If calculating according to the deepest gas reservoir (3 900 m) which has been developed on a large scale, nearly 1 100 to 6 100 m thick deep formations in the basin haven’t contributed natural gas reserves and production yet. Therefore, deep gas reservoir is a new domain still worth exploration in Ordos Basin. Based on the analysis of the exploration strata, the new deep domains are mainly Paleozoic and Proterozoic in the Basin. In this paper, on the base of the discovery of Qingyang deep coal-formed gas field, the accumulation conditions of gas reservoirs in Upper Paleozoic have been examined.

The Upper Paleozoic in Ordos Basin consists of Upper Carboniferous Benxi Formation, Lower Permian Taiyuan and Shanxi Formations, Middle Permian Upper and Lower Shihezi Formations and Upper Permian Shiqianfeng Formation upwardly. It is a complete sedimentary cycle of transgression-regression from marine to the continental facies. The lower Upper Carboniferous and Lower Permian are coal series of marine to marine-continental transitional facies, and the upper Middle and Upper Permian strata are continental facies. The coal series in the lower part are major source rocks through thermal evolution, and the tight sandstone layers within and adjacent to the source rocks are the primary gas reservoirs, which are universally gas-bearing. Currently, tight sandstone gas reservoirs have been found in all members and formations of Upper Paleozoic, but they are uneven in distribution. Taiyuan Formation, Shanxi Formation and Lower Shihezi Formation have the most abundant reserves, and the reservoirs are mostly middle-shallow gas reservoirs less than 3500 m deep. On the plane, the proved large and medium-sized gas fields, such as Sulige and Yulin, are mainly distributed in the north and east of Yimeng-Shaanxi slope structure belt in the basin, and no large gas field had been found in the southern part of the basin before the discovery of Qingyang Gas Field. Across the basin, the Upper Paleozoic coal series in Yimeng-Shaanxi slope belt increases in depth from the east to the west and from the north to the south. Therefore, in the southwestern part of the basin, the coal series is larger in burial depth, with the maximum depth revealed by exploration well of over 4 500 m. By comparing the burial depth of the proven gas fields in Upper Paleozoic with the depth of coal-series source rocks, it can be found that no large gas field had been found in the 1 000 m thick coal-series source rock at burial depth of over 3 500 m in the basin. This is contradictory with the geological understanding that the coal-formed gas source rock is characterized by “universal hydrocarbon generation and short distance migration and accumulation”^[1,2]. Therefore, the exploration and discovery of the large gas field in the deep formation of southwestern part of the basin has provided scientific basis for verifying the geological understanding of coal-formed gas.

At present, deep oil and gas exploration has become one of the hot issues in the global oil and gas basins^[3,4]. In the past ten years, with the continuous development of the geological theory of natural gas accumulation and advancement of engineering technology in China, a series of major breakthroughs in natural gas exploration have been made in three major fields, deep carbonate rocks, clastic rocks and volcanic rocks, and several large gas provinces with trillion cubic meters of reserves in the deep areas have been found, including the central and northeastern Sichuan Basin and Kuqa Depression in Tarim Basin. However, in terms of the deep formation, there is no uniform definition at home and abroad. Currently, there are various division schemes for deep formation, such as 3500 m, 4 000 m, 4 500 m and 5 000 m^[5,6]. In Ordos Basin, the gas reservoir more than 4 000 m deep is called deep gas reservoir. Beyong this depth, the degree of natural gas exploration is relatively low, and the basic natural gas accumulation conditions and enrichment laws such as hydrocarbon generation, storage, and accumulation haven’t been fully understood, and the costs of exploration and development are also great. The discovery of Qingyang Gas Field provides favorable conditions for studying the geological conditions for accumulation of deep gas reservoir.

Qingyang Gas Field was discovered in the eastern part of the Yimeng-Shaanxi slope in the basin in 2018 (Fig. 1). Compared with the large fields such as Sulige in the north of the basin, Qingyang Gas Field not only has the common characteristics of tight reservoir, low porosity, low permeability, low abundance and low pressure, but also has its particularities: (1) The exploration degree of deep formations is low, and the sand-bodies are not as stable as those in the northern part of the basin, with great variation; in addition, the evolution law of sedimentary facies is not clear, therefore, it is necessary to make clear the sedimentary characteristics. (2) The source rock, high in thermal evolution degree, has unclear hydrocarbon generation potential, and the gas-rich zones vary widely, so it is necessary to look deep into hydrocarbon generation mechanism of high thermal evolution coal-series source rock. (3) The gas reservoir is deep in burial depth, the production of vertical well is low, and exploration and development cost is high, posing great challenges to economic and effective development. Therefore, it is necessary to innovate development mode and tackle key technologies.

Based on the abovementioned difficulties and problems, a comprehensive study has been conducted on the hydrocarbon generation, storage and accumulation conditions of deep, medium and shallow coal-formed gas reservoirs through the field geological outcrops and core observation, identification of ordinary, cast and cathodoluminescence thin sections, Ro and other geochemical indicators, carbon isotope composition, inclusions, X-ray diffraction, electron microscopy and energy spectrum and other supporting tests, to find out the characteristics of deep gas reservoirs, geochemical characteristics of its source rock and the gas reservoirs. In addition, the quality of the deep reservoirs was evaluated, and the characteristics and controlling factors of deep coal-formed gas accumulation were examined. In the aspect of natural gas development, the integration of exploration and development method has been innovated according to the geological characteristics of deep tight sandstone gas reservoirs. The key technologies focus on improving single well production are researched, and the economic and effective development model of “dissecting sandbody with skeletal vertical wells and centralized development with horizontal wells” is established for deep tight sandstone gas fields. In short, the discovery and the deepening of geological understanding on deep coal-formed gas tight sandstone in Qingyang gas field have provided geological basis and technical support for the construction of 100 billion cubic meter scale natural gas field in the southwestern part of Ordos Basin, in both the development of coal-formed gas geological theory, and the exploration method, technical support and economic development of gas reservoirs.

Oil exploration stage (before 2000). Two large comprehensive exploration sections from Wuqi to Zhenyuan (Wells Pou 1 - Pou 14) and from Tuqiao-Qingyang to Taibai (Wells Pou 18 - Pou 26 - Qing 36) were completed in 1973 by Changqing Oilfield, and a total of 24 exploration wells were drilled, which made major contribution to geological understanding and wellbore technology research of the subsequent discovery of oil and gas. At this stage, the breakthrough of oil exploration was marked by the discovery, confirmation and development of Maling Oilfield, with million-ton production capability constructed. In the past 50 years, the exploration in the Longdong Area had been focused on oil, until Xifeng Oilfield was discovered in 2000. At present, the annual oil production of this area is nearly 1000×10⁴ t.

Regional exploration stage of natural gas (2000-2012). Since 2000, guided by the theory of large-scale tight sandstone gas accumulation^[2], regional exploration of natural gas was conducted in the southwest of the basin, while some large gas fields, including Jingbian, Yulin, Sulige and Shenmu were successfully explored and developed in the northern part of the basin. In 2003, Well Zhentan 1 tested an industrial gas flow of 5.5×10⁴ m³/d from Shan 1 Member, and the natural gas exploration was then extended to Qingcheng, Zhenyuan, and Ningxian Counties. From 2004 to 2012, two exploration wells (Qingtan 1 and Qingtan 2) were drilled the 50 km northeast and north of Well Zhenshen 1. Well Qingtan 1 tested an industrial gas flow of 6.62×10⁴ m³/d in Shan 1 Member, and Well Qingtan 2 tested an industrial gas flow of 5.46×10⁴ m³/d in He 8 Member, further proving good exploration potential of coal-formed gas in the deep Upper Paleozoic of the region, and confirming that Shan 1 Member and He 8 Member are major gas layers (Fig. 2).

Exploration and development integration stage (2013- 2017). At this stage, while strengthening research on geological conditions and evaluation of targets, gas-rich areas such as Qingtan 1, Qingtan2 and Chengtan 3 well-areas were evaluated to select exploration and development targets. Geological reserves of over 2000×10⁸ m³ were confirmed in He 8 and the Shan 1 Members, greatly enhancing the confidence of natural gas exploration in the southwest of the basin. At the same time, the gas-rich enrichment block of Well Qingtan1 was selected as the initial development evaluation target. Vertical wells were primarily used to develop this block at the initial stage.

Due to deep burial depth of the gas reservoir, high cost of single-well, and lack of effective development technologies for deep tight sandstone gas reservoir, some vertical wells had poor economic benefits. From 2014, to reach the goal of increasing single-well production, the gas enrichment blocks were further sorted, and the adaptive development test of fracturing stimulation in horizontal well was carried out in the deep tight sandstone gas reservoir, forming the model of “dissecting sandbody with skeleton vertical wells and centralized development with horizontal wells”, and realizing the economic development of the block. From 2017 to 2018, the development test and production with horizontal wells were carried out around the gas-rich development block of Well Qingtan 1. The 9 horizontal wells had open flow rate of (15.2-102.9)×10⁴ m³/d, and 61.42×10⁴ m³/d on average, showing horizontal well can increase production and enhance economic benefit significantly.

In 2018, checked by China State Oil Reserve Committee, the proved geological reserves of natural gas (OGIP) of 318.86×10⁸ m³ were obtained in Shan 1 Member of Qingtan 1 well-area. The average burial depth of the middle part of the gas reservoir is 4276.0 m, and the gas field was named as Qingyang Gas Field. In addition, Qingyang Gas Field has possible geological reserves of over 100 billion cubic meters. Therefore, Qingyang Gas Field can be regarded as a large deep gas field based on the division scheme of deep gas reservoir of more than 4000 m, geological reserves (OOIP) greater than 300×10⁸ m^3[7] or technical recoverable reserves greater than 250×10⁸ m³.

Qingyang Gas Field is located in the downdip of the southwestern Yimeng-Shaanxi slope unit in Ordos Basin. The top of Shan 1 Member, the major pay, is a westward gentle monocline, with a slope of (6-10) m/km and a dip of less than 1° (Fig. 3). Several rows of northeast-trending low and gentle nose-like uplift appear on the monocline. These nose-like structures, 10 m in amplitude, 4-5 km wide, and 25-30 km long, have no control on the gas reservoir. The reservoir layers are sandbodies of meandering river point bar. Multi-phase sandbodies of point bar stack over each other, forming a sandbody belt, which is irregularly distributed in the southwest-north strike. Toward both sides of the sandbody belt, the sandbodies become thinner, and poorer in physical properties and gas-bearing properties, showing strong heterogeneity. The gas reservoir is characterized by large changes in lithology and physical properties. It was difficult to determine the boundaries using the existing technical methods when submitting proven reserves. The boundaries of gas reservoir were often artificial boundaries (Fig. 3), with the basic characteristic of vague boundaries for tight sandstone gas reservoir. Above the reservoir sand body is medium thick-layered sandy mudstone and mudstone, acting as the caprock of the gas reservoir. The water production wells in the gas field are scattered on plane, and the water is the local stratum retention water of CaCl₂ type, with a maximum salinity of 120.17 g/L. The gas reservoir is an elastically driven layered gas reservoir constant in volume and inactive in hydrodynamics with no edge or bottom water through the development testing by horizontal wells (Fig. 4).

The reservoir in Shan 1 Member is mainly composed of quartz sandstone and lithic quartz sandstone, with an average content of quartz and quartz debris of 84.5%, average content of rock debris of 15.3%, little or no feldspar. The test results of 708 core samples from 17 wells show that the reservoir has a porosity between 4% and 10%, 6.0% on average, and a median porosity of 5.7%. The average porosity is similar to the median porosity, indicating that the porosity distribution is relatively uniform and less discrete. The reservoir has a permeability of (0.08-1.00)×10^-3 μm², 0.64×10^-3 μm² on average, and a median permeability of 0.56×10^-3 μm². According to the current standard, the reservoir in the area is tight sandstone one. The reservoir has a good correlation between porosity and permeability (Fig. 5), with the permeability of 0.08×10^-3 μm² corresponding to the porosity of 4%, while for the reservoir of Shan 1 Member in the south of Sulige area, the same permeability corresponding to the porosity of about 5%. In comparison, for the Qingyang gas reservoirs, the unit porosity contributes higher permeability, and the reservoirs have higher seepage capacity. It is worth mentioning that for Shan 1 Member reservoir, the porosity of 8% corresponds to the permeability of more than 1.0×10^-3 μm², 2% lower than the required upper limit of porosity and permeability in the general concept of tight sandstone (with porosity of less than 10% and permeability of less than 1.0×10^-3 μm²). This further indicates that the reservoir in Shan 1 Member has stronger seepage capacity. Further analysis (Fig. 5) shows that there are conventional reservoirs with permeability greater than 1.0×10^-3 μm² in the deep formations, which account for more than 5%.

Since it is difficult for the actual samples analyzed to fully reflect the contribution of fractures to permeability, the deep conventional reservoirs could take up an even higher proportion.

The reservoirs have three types of pores (karst pore, intergranular pore and intercrystalline pore), mainly secondary dissolved pores, and microfractures and macro fractures in some wells. The reservoir has a total plane porosity of 1.96%, of which the debris dissolved pore, intergranular pore, and intercrystalline pore account for 87.1% (Fig. 6). The pores generally have large pore and throat, good sorting, and coarse skewness, with a displacement pressure from 0.34 to 3.73 MPa, 1.51 MPa on average, median pressure of 1.09 to 10.59 MPa, 4.55 MPa on average, and median throat radius of 0.02 to 0.91 μm, 0.20 μm on average. The pores are of small pore and thin throat structure, with lower discharge pressure and stronger seepage capacity.

The average depth of the middle gas reservoir of Qingyang Gas Field is 4276 m, which is 2000 m deeper than the burial depth of Mizhi Gas Field in the eastern part of the basin. The gas reservoir has an actual measured pressure of 33.92-40.16 MPa, an average formation pressure of 37.49 MPa, and a pressure coefficient of 0.88, representing abnormally low pressure gas reservoir. Similar in pressure to the tight sandstone gas reservoirs such as Sulige in the northern part of the basin, all the gas reservoirs have abnormally low pressure. The gas reservoir has an average methane content of 96.90% and no H₂S, belonging to sulfur-free dry gas reservoir.

Based on the analysis of structural position, Qingyang Gas Field is located in the triangular region held by two tectonic units, Weibei Uplift and the western margin thrust zone with relatively strong tectonic activity. Due to the stronger tectonic activity, the Upper Paleozoic is complex in characteristics of sedimentation, burial and denudation.

The stratum thickness reduced during the deposition period. At the end of the Ordovician period, North China Block was overall uplifted and denuded due to the Caledonian tectonic movement, and the Silurian, Devonian and Lower Carboniferous were lost. Starting from the Late Carboniferous, Ordos Basin ended the uplift and denudation of 130-140 million years and began to receive sediments. Because Yimeng- Shaanxi Slope was epeiric sea of Craton Basin at that time, featuring slow tectonic subsidence, gentle paleotopography, and shallow water body. Controlled by these factors, the sedimentary thickness of Upper Paleozoic varies little in Yimeng-Shaanxi Slope. But due to the influence of the central paleo-uplift, Benxi Formation and Taiyuan Formation in Lower Carboniferous in the southwestern part show southwestward overlapping characteristics. Benxi Formation is completely absent in the gas field range and Taiyuan Formation is also absent in the southwest of the block (Fig. 7). The depositional period of Shanxi Formation was the period of filling and leveling. Compared with the central and eastern regions, the sedimentary thickness of Shanxi Formation reduced only 10-20 m, indicating that the influence of the central paleo-uplift on sedimentation basically disappeared in this period. In general, the Upper Paleozoic in the southwestern part is 130 m thinner than in the central and eastern parts of the basin, and is only 900 m thick, clearly, the southwestern part is a region with thinner Upper Paleozoic in Yimeng- Shaanxi slope.

Increase of sedimentary thickness in the burial period. Contrary to the thickness reduction during the deposition period, the thickness of overlying strata of Upper Paleozoic increased in the burial period in the southwestern part. From the analysis of sedimentary evolution, the Upper Paleozoic and Mesozoic Triassic deposited continuously, and Triassic, Jurassic, Cretaceous and Cenozoic deposited above it in turn. Influenced by the gathering and orogenic activities of Qinling Orogenic Belt^[8,9,10], flexural settlement occurred in the south of the foreland to the basin. From Triassic-Upper Jurassic, the basin had subsidence center located in the south, relatively sufficient source supply, strong hydrodynamic, large structural subsidence and great sediment thickness, the whole basin was generally thick in the south and thin in the north, and deep in the south and shallow in the north. During Late Jurassic-Cretaceous, the tectonic pattern turned into going up in the east and going down in the west of the basin, but the southwestern part was in the western subsidence area of the basin, where the Cretaceous of over 1000 m deposited^[11]. Now the average thickness of the Mesozoic is 3800 m in the southwestern region, making it the part with the largest Mesozoic residual thickness in Yimeng-Shaanxi slope.

The southwestern part of the basin has the minimum denudation thickness. Ordos Basin is a multi-cycle superimposed basin. After the deposition of Upper Paleozoic, the Mesozoic experienced four major tectonic changes^[12], at the end of the Triassic, the Middle Jurassic, the Late Jurassic and the Early Cretaceous respectively. Among them, the most intense uplifting and denudation of the whole basin happened after the deposition of Zhidan Group in the Early Cretaceous since the Triassic. The tectonic uplift and denudation second in intensity took place after the deposition of Middle Jurassic, the other two denudations are weaker. Estimates of Mesozoic denudation thickness by different researchers with different methods show^{[13,14,15,16]} that the eastern part of the basin has the maximum denudation thickness, with a maximum denudation of more than 2000 m; the southwestern part of the basin has denudation thickness of less than 800 m in general, so it is the region with the smallest denudation thickness of Mesozoic in Yimeng-Shaanxi slope.

The Cenozoic sediments don’t occur in the southwestern part of the basin. The Quaternary loess directly covers on Huanhe Formation of the Cretaceous Zhidan Series, with a general thickness of about 150 m. Influenced by the incision of loess gully, the topographic height difference varies greatly. The burial depth of gas layer at the bottom of the gully reduces, but the burial depth of gas layer on the loess tableland ridge increases. But the variation in topographic height difference has no impact to the fact that the gas reservoir is in the deep formation. In short, the Mesozoic is large in sedimentary thickness and small in denudation during the later periods, making the burial depth of the Upper Paleozoic increase in the southwestern part of the basin, so the large coal-formed gas field now is deep in burial depth.

The Upper Paleozoic Benxi Formation, Taiyuan Formation and Shanxi Formation in the Ordos Basin all have two to five coal seams. The coaliferous source rock composed of coal seam, carbonaceous mudstone and dark mudstone is characterized by extensive hydrocarbon generation^{[1-2, 17-20]}, which has been confirmed by the discovery and exploration of several gas fields. It has been confirmed by hydrocarbon generation simulation experiment and the calculation of hydrocarbon generation amount^{[21,22,23,24,25]} that the coal rock is the major source rock among the three types of source rocks, contributing 87% of the hydrocarbon generated by all of them. The statistics on cumulative thickness of coal rock in discovered large gas fields show that the cumulative thickness of coal rock is 6-25 m in the Upper Paleozoic gas fields, specifically, 6-20 m in the Sulige gas field, 10-15 m in the Yulin and Zizhou large gas fields, 15-25 m in the Shenmu large gas field. In the Longdong area where the Qingyang Gas Field is located, due to the influence of the Caledonian tectonic movement and the central paleo-uplift of the basin, in the Upper Paleozoic, the Benxi Formation coal seam is absent, and the Taiyuan Formation coal seam thins. The coal rock mainly occurs in the second member of the Shanxi Formation, and is 2-4 m thick per layer. The dark-colored mudstone is 20-60 m thick, and thick in the northeast and thin in the southwest. Compared with other large gas fields, the coal layers are thinner in Qingyang Gas Field. But the coal rock of Qingyang Gas Field has higher R_o of 2.17%-3.02%, 2.48% on average, while the source rock of Shenmu gas field in the northeastern basin shallower in burial depth has a R_o of only 1.2%-1.4%; the source rock larger in burial depth in Yulin gas field has a R_o of 1.2 % to 1.8%, the source rock of Sulige gas field larger in burial depth has a R_o of 1.4% to 2.0%. Clearly, although the coal rock in the Qingyang Gas Field is thin, it is highest in thermal evolution level, reaching the stage of over-maturity, with R_o 0.87%-1.02% higher than other large gas fields. Because coal rock is characterized by continuous gas generation, it still has 25% of gas generation capacity when reaching R_o of 2.0%- 5.0%^[26]. Therefore, the thin coal layers have large gas generation potential due to the high thermal evolution degree. The discovery of the Qingyang Gas Field supports the conclusion of laboratory experiment.

The Upper Paleozoic coal layers in the Longdong Area have TOC values ​​from 40.23% to 91.32% (Table 1), specifically Shan 2 Member and Taiyuan Formation coals have an average TOC of 66.35% and 64.11% respectively. The dark-colored mudstone in Shan 2 Member and Taiyuan Formation have an average TOC of 3.15% and 5.28%, respectively. In terms of soluble organic matter content, the organic matter content of coaliferous source rock is lower. The chloroform bitumen “A” contents of coal seams in Shan 2 Member and Taiyuan Formation are 0.105 8% and 0.108%, respectively. The chloroform bitumen “A” contents of the dark-colored mudstone in Shan 2 Member and Taiyuan Formation are 0.0055% and 0.0105%, respectively. Compared with the Upper Paleozoic coaliferous source rock in the whole basin, the hydrocarbon generation potential in the Longdong area is significantly lower as well. The average hydrocarbon generation potential of coal rock in the whole basin is about 100 mg/g, but that of the coal seam of the Shan 2 Member in the Longdong is only 12.62 mg/g. The hydrocarbon generation potential of the Upper Paleozoic dark-colored mudstone in the whole basin is 3 mg/g on average, while that is 0.4 mg/g in Longdong area. There are several reasons for the obviously low soluble organic matter content and low hydrocarbon generation potential. On the one hand, it is related to the high thermal evolution stage of organic matter. On the other hand, it also indicates that the Upper Paleozoic coaliferous source rock has experienced a process of strong hydrocarbon generation and hydrocarbon expulsion, resulting in obviously decline in residual liquid hydrocarbon content and hydrocarbon generation potential.

The source rock of the Shan 2 Member in Longdong area has reached the over-mature stage (the coal is lean coal-anthracite). The high thermal evolution characteristic is the combined result of the thermal increasing by normal sedimentary burial and the Early Cretaceous tectonic thermal anomaly^[27,28]. The thermal anomaly area is mainly distributed in the Qingyang-Fuxian-Yan’an-Wuqi area in the south of the basin, and the Longdong area is located in the western part of the thermal anomaly area. This stage of tectonic thermal event had obvious controlling effect on the gas generation of the source rocks. With the deepening of metamorphism, the chloroform bitumen “A” of coal rock generally varies in the following pattern: extremely low at turf stage, higher at fat coal stage (R_o 0.9%-1.2%), and almost no variation from the fat coal to coking coal stage (R_o 1.2%-1.5%), being at peak stage, and lowest at anthracite stage (with R_o greater than 2.5%)^[29]. According to the hydrocarbon generation mechanism of coal rock, on the one hand, the soluble organic matter in coal rock is formed by the degradation of Type III kerogen; on the other hand, it is cracked into natural gas and be consumed. When the thermal evolution of coaliferous source rock reaches the mature stage, the liquid hydrocarbon can’t be newly generated and supplied from coal rock. The liquid hydrocarbon in the whole system is largely thermal cracked, and the soluble organic matter is hugely consumed to generate natural gas, resulting in significant drop in the content of soluble organic matter. The liquid hydrocarbons in the coal seam is another source gas than cracking of kerogen, resulting in increasing in the amount of gas in the system. Therefore, the content of chloroform bitumen “A” in current coal rock is not representative for the evaluation of source rock, and the hydrocarbon generation potential of coaliferous source rock can’t be evalu-ated with parameters of the coaliferous source rock at high thermal maturity now^[21]. The correct results should be obtained by restoring according to the experimental data of the hydrocarbon generation simulation data^[22,23,24].

According to the test results of carbon isotope composition of natural gas components (Table 2), the natural gas of the Shan 1 Member in Longdong area has δ¹³C₁ values from -29.5‰ - -24.1‰, -27.14‰ on average; wider distribution range of δ¹³C₂ from -33.6‰ to -23.0‰, on average -29.46‰. Among the 5 groups of tested samples, 3 groups of samples have δ¹³C₂ values of less than -28.5‰, accounting for 80%. The δ¹³C₃ value is slightly lighter, ranging mainly from -30.7‰ to -25.5‰, with an average of -30.24‰. And 80% of the test samples have δ¹³C₃ of less than -26.5‰.

Compared with other blocks in the basin (Table 2), the carbon isotope composition of the Upper Paleozoic natural gas in the Longdong area is similar to that of natural gas in the southern Sulige, Gaoqiao and Yichuan-Huanglong areas, all of them have higher δ¹³C₁ value and typical features of coal-derived gas, but significantly lighter δ¹³C value of C₂₊ heavy hydrocarbon components. In the northeastern part of the basin and most part of Sulige area, the carbon isotope composition of the natural gas components in the Upper Paleozoic is generally higher, and the averages of δ¹³C₁ value, δ¹³C₂, and δ¹³C₃ are -30.2‰, -24.54‰, and -24.4‰ respectively.

The fractionation pattern of carbon isotope composition of the alkane gas components in the Upper Paleozoic of the Ordos Basin is closely related to regional thermal evolution degree. In the northeastern part of the basin, the Upper Paleozoic coaliferous source rock is low in thermal maturity (with Ro of less than 1.7%), and the alkane gas components show a series of positive carbon isotope pattern, namely δ¹³C₁ < δ¹³C₂ < δ¹³C₃. The thermal evolution of organic matter in the Sulige area is higher (R_o 1.7%-2.0%), so the gas has δ¹³C₂ > δ¹³C₃, and partial reversal generally^[32]. The thermal evolution of coaliferous source rock in the Longdong area in the southeastern basin has reached the mature stage (R_o 2.17%-3.02%), and the δ¹³C₁, δ¹³C₂ and δ¹³C₃ in natural gas completely reverse (δ¹³C₁ > δ¹³C₂ > δ¹³C₃), with characteristic of a series of negative carbon isotope composition, which is mainly related to the evolution of over-mature or high-mature (greater than 200 °C) source rock^[30,31]. This change pattern also indicates that the over-mature evolution of coal rock is the main controlling factor for the formation of negative carbon isotope composition series, and the negative carbon isotope composition series should be one of the signs of deep coaliferous gas.

The Ordos Basin was in the western part of the North China block in the Late Paleozoic, neighboring with the Xing’anling- Mongolia orogenic belt in the north, adjacent to Qinling fold belt in the south, forming a “dustpan-shaped” paleo-tectonic pattern high in the northwest and low in the southwest. Many researchers analyzed the coupling relationship between the Late Paleozoic provenances, sedimentary filling and the Xing’anling-Mongolia orogenic belt in the northern part of the basin^[33,34], and concluded that the provenance area of ​​the northern Xing’anling-Mongolia orogenic belt rapidly uplifted then, providing sufficient clastic debris for the formation of many large-scale NS direction sandbodies in the Sulige and other gas fields in the northern part of the basin. With the exploration of natural gas area in the southern basin, especially in the southwest, the coupling relationship between the evolution of the Qinling orogenic belt and the Paleozoic sediments in the southern part of the basin has drawn extensive attention^[35,36]. From the clastic components, heavy minerals, trace elements, and the likes, many researchers have confirmed that the Northern Qinling orogenic belt uplifted in the Late Paleozoic, providing source sediments for the southern part of the basin. However, it is still necessary to study the type of the provenance area and whether it is related to the Qilian orogenic belt further.

The authors have conducted LA-ICP-MS U-Pb dating analysis of the detrital zircons from the southwestern part of the basin^[37]. The results show that the age distribution of the 105 magmatic detrital zircons can be divided into four groups: (1) 260-340 Ma, accounting for 21.9% of the total number of samples, mainly sourcing from the denudation of the north and west Qinling Structural Belts in the Late Proterozoic. (2) 370-470 Ma, accounting for 24.8% of the total number of samples, mainly from the denudation of the north and west Qinling Structural Belts and partial denudation of North Qilian Orogenic Belt in the Early Paleozoic. (3) 1600-2000 Ma, accounting for 32.4% of the total sample. (4) 2300-2600 Ma, accounting for 15.2% of the total sample, mainly sourcing from the uplift and denudation zone of the basement crystalline rock series of the North China Plate, suggesting that the orogenic belt was deeply denuded. Combined with the research results of the tectonic evolution of the Qinling Orogenic Belt^[38], the sedimentary responses of the Permian basin-mountain coupling in the southern part of the basin are the following: at the Late Cambrian of about 500 Ma ago, the Shangdan Ocean closed, the arc and land collided, and the Erlangping Basin started to form; around 450 Ma ago, the northern Qinling rapidly uplifted, resulting in the uplifting and denudation of the southern part of the basin; about 420 Ma, the Erlangping Basin was closed, the northern Qinling began to uplift rapidly, after 360 Ma, the northern Qinling slowly and continuously uplifted, causing the denudation of the southern part in the basin, and the absence of the O₂-C₁ deposits; since the Late Paleozoic, the southern part of the South Qinling stretched and the Mianxian-Lueyang oceanic basin was formed in the Carboniferous. In the Early Permian, the Mianxian-Lueyang oceanic basin began to subduct and extrude, accelerating the uplift and denudation of the North Qinling orogenic belt, which provided provenance for the southern part of the basin. The analysis of sedimentary response to the basin-mountain coupling shows that the Late Paleozoic Northern Qinling orogenic belt, the Qinling structural belt in the southwest and the North Qilian orogenic belt provided sediment sources for the southwestern part of the basin, giving rise to the reservoir sandbody distributed in NE strike. Through the sedimentary response study of the Permian basin-mountain coupling in the basin, the macroscopic sandbody distribution mode has been established: the provenance belt in the northern Ordos Basin strongly uplifted, providing sufficient source material, while the provenance belt of the southern orogenic belt slowly uplifted, providing some source material (Fig. 8).

Detailed sedimentary microfacies study shows that the sedimentary system of the Shan 1 Member in the eastern part of the basin has the characteristics of meandering river- shallow gentle slope delta. (1) The Upper and Lower Paleozoic in the Ordos Basin are generally in conformable contact, and are in angular unconformable contact only in the northern Weibei uplift belt in the southern part of the basin^[39], reflecting the effect of uplifting stress of the Northern Qinling orogenic belt on it during the Caledonian Movement, and that fold uplifting and denudation occurred. Therefore, the Upper Paleozoic in the southwest began to deposit on the paleogeomorphology that had suffered weathering and denudation in the Late Caledonian period. As the paleogeomorphology was high in the southwest and low in the northeast, formations and members of the Upper Paleozoic successively overlapped to the southwest (Fig. 8). In the ​​Luanchuan-Qingyang-Huanxian- Zhenyuan area, Shan 1 Member is 40-50 m thick, thin and stable, indicating that the paleogeomorphic height difference didn’t vary greatly, reaching quasi-plain state. It reflected that the depositional paleogeomorphic background of the Shan 1 Member was a gentle slope. At the same time, the mudstone deposited in the delta of the Shan 1 Member was mainly grayish gray and gray, with visible vertical biological boreholes, indicating that the water body was shallow when it deposited. Therefore, the Shan 1 Member was gentle slope shallow water delta system. (2) The Shan 1 Member sediment has lower ratio of sand to stratum, and obvious “dual” structure of the river in vertical. The sandbodies have tabular cross bedding and parallel bedding, and fine grain size, reflecting that the river was a meandering river. (3) In Pingliang outcrop area, the mudstone of the Shan 1 Member is purple-red (Fig. 9a), with a thickness of 2 to 4 m, reflecting exposed environment above water, so it is the upper plain sediments of the meandering river delta. From wellblock Zhentan 2 to Qingyang Gas Field, the purple muddy mudstone (Fig. 9b) coexists with gray silty mudstone (Fig. 9c), reflecting the frequent variation of underwater and continental sediments, representing sediments of lower delta plain of shallow meandering river. (4) Whether it is above the water or underwater delta plain, the Shan 1 Member has river bottom conglomerate sediment in the lower part, and medium-coarse sandstone with tabular cross bedding in the middle and upper parts (Fig. 9d-9f), reflecting the upper and lower plain channel were similar in sedimentary characteristics and the hydrodynamics were strong, and these layers take on bell-shape and box-shape on the natural gamma curve. (5) The delta front facies isn’t well developed, and it is difficult to distinguish it from the underwater delta plain in sedimentary characteristics. Therefore, the Shan 1 Member in the southwestern part of the basin is generally a shallow delta sedimentary system of meandering river (Fig. 10).

Different from the classical delta sedimentary model, shallow delta has its special sedimentary characteristics^[40,41,42]. The basic characteristic of the shallow delta in the southwestern part of the basin is the lack of Gilbert-style three-layer structure (bottomset, foreset and topset), which can be divided into upper delta plain and lower delta plain. But it is difficult to divide delta front from prodelta subfacies, and the delta is basically characterized by great plain and small front. Researchers established a shallow delta system model in the southwestern part of the basin based on the basic characteristics of the shallow delta^[43,44]. However, in the actual exploration and production, the success of tracking underwater distributary channel sandbody according to shallow delta system sedimentary model is only about 20%. Therefore, the characteristics and genetic analysis of the distributary channel sandbody are the keys. In terms of river-controlled shallow delta sediments, it was the result of continuous balancing between river dynamics and lake water hydrodynamics. When the hydrodynamics of lake water was strong, and the sediments brought by the river was reconstructed, giving rise to lobate sandbodies. When the hydrodynamics of river was stronger, branched sandbodies would emerge generally, these are the ideal conditions of two end members. In fact, the shallow lake basin has large fluctuations in water depth. During the flood period, the water body becomes deep, and the lake area enlarges. During the dry season, the water body becomes shallow, and the lake area reduces. For example, in modern Poyang Lake with shallow delta, the lake area can reach 4647 km² at high water level during flood period, and the lake area can reduce to 146 km² at low water level during dry season, with water areas differing for nearly 32 times^[45]. Therefore, the river can pass through the bottom of the lake^[45,46] during the dry season, and the sandbody formed is essentially river facies rather than underwater distributary channel. Study on modern and ancient sediments shows that shallow delta underwater distributary channel sediments are generally dominated by medium-fine sandstone or silt^{[46,47,48,49]}. But the gas-bearing sandstone in the southwestern part of the basin is mainly medium-grained sandstone with conglomerate, and the major sandbodies are about 9 m thick. This is due to the shallow water and exposure in dry season. The area was a shallow lake in the flooding period, and the reconstruction of sandbody by lake water was not evident. Therefore, the sedimentary sequence of the point bar in meandering river during the dry season was basically preserved. Scouring surface is commonly observed at the bottom of the sandbody. The sandbody has conglomerate and medium-coarse sandstone with tabular cross bedding in the lower part, fine grain sandstone with pulsy bedding, current bedding, and horizontal bedding in the middle and upper part, and gray mudstone with horizontal bedding and plant leaf fossils in the upper part, and takes on bell-shape on natural gamma curve. On the plane, a number of composite sandbodies formed by the frequent lateral migration of a number of highly-curved active tributary channels in the lower delta plain of meandering river. Through the interpretation of seismic data and horizontal well drilling, the point bar sandbody of the Well Qingying 1 was finely described. The point bar sandbody is 11-14 km long and 3-5 km wide, and is composite sandbody formed by lateral superposition of multi-period sandbodies (Fig. 11). Point bar sandbodies generally appear in points, making it difficult to predict. It is also the reason of the low encountering rate of Shan1 sandbody in the regional exploration stage.

The maximum burial depth of the Shan 1 Member in the southwestern part of the basin is 4500 m at present. If the amount of denudation was about 800 m in the burial process during the geologic period, the reservoir used to be buried at a depth of 5300 m. The Ro value of the coaliferous source rock underlying Shan 2 Member is 2.3%-2.5%. The 22 samples of coaliferous source rock from 9 exploration wells except one (with 485 °C) have T_max values of greater than 490 °C, the highest value of 607 °C, and 555.1 °C on average. The highest diagenetic temperature from inclusion test is 170 °C. And the reservoir is a dry gas reservoir. These indicators demonstrate that the reservoir has entered into the late diagenetic evolution stage, and the characteristics of late diagenetic stage are also clearly reflected in the minerals. Through the identification of more than 1000 ordinary and cast thin sections, it is found that the skeleton particles of sandstone are mainly in line and suture contact (Fig. 12), the biotite particles are bended and in directional arrangement (Fig. 12a, 12b); and the quartz has overgrowth up to the IV level. Ferrous calcite cementation and microfractures are commonly observed in the thin sections (Fig. 12c, 12d). The clay mineral combination is I-S mixed layer, illite, kaolinite and chlorite, and the sericitization with low grade of metamorphism occurs in local part (Fig. 12e). According to the oil and gas industry standard of diagenesis classification of clastic rock (SY/T 5477-2003), the reservoir of the Shan 1 Member in the southwestern basin has entered the stage of late diagenesis to low-grade metamorphism. It is generally believed that the primary pores basically disappear at the late diagenetic stage. However, the reservoir in the Qingyang Gas Field still has a certain number of primary pores (Fig. 12f) in triangular and polygonal shapes with straight pore walls. The primary pores contribute 25% of the plane porosity. They mainly exist in coarse-grained quartz sandstone in the center of point bar in the meandering river facies. Since the quartz sandstone has mainly siliceous cement highly resistant to compaction, residual primary pores are retained between the enlarged edges of quartz grains incompletely cemented. The primary pores provided channels for diagenetic fluid flow, so the point bar sandbody had suffered stronger secondary dissolution, and a large number of secondary dissolved pores were formed.

The Upper Paleozoic tight sandstone reservoirs in the Ordos Basin are dominated by secondary dissolved pores. A large number of simulation experiments have been carried out to find out the dissolution mechanisms^[50,51]. The results show that organic acids produced by the maturation of organic matter in coal series dissolved feldspar or volcanic matter to form secondary pores. Further research shows that the feldspar in the Upper Paleozoic coal series in the basin has basically disappeared in the area where the R_o value is more than 1.3%^[52]. The sandstone reservoir in the southwestern part of the basin is also characterized by the absence of feldspar. The secondary dissolved pores in the stratum are mainly intergranular dissolution pores and kaolinite intercrystalline pores (Fig. 12g, 12h). The rock debris mainly consists of clasts of igneou rocks, volcanic material detritus and deep metamorphic rock. The intercrystalline pores of kaolinite are composed of book-like, lamellar recrystallized coarse intercrystalline tiny pores, which are interconnected and can form effective pores, with a porosity of up to about 3%.

Core observation and imaging logging show that the Upper Paleozoic sandstone strata in the Longdong area have high- angle fractures (Fig. 13a, 13b), and the fractures are mainly in NE-SW (Fig. 13c, 13d) and NW-SE. The two groups of fractures intersect with each other and form a complex network, which can effectively improve the permeability of the reservoir and serve as migration channels for natural gas. The existence of natural fractures can improve the permeability of reservoir to a certain extent, expand the gas storage space, and enable high-yield of well. For example, the gas-bearing sandbody of the Shan 1 Member is the coarse-grained quartz sandstone, with rich dissolved pores, intercrystalline pores of kaolinite, and fractures, thereby improving the permeability of reservoir. When stimulated by mixed water fracturing, the production of vertical wells can reach 5.8961×10⁴ m³/d. The high-angle fractures developed in the gas-bearing sandstone in the He 8 Member in the Well Chengtan 3 also improve the reservoir permeability to some extent. Therefore, after mixed water fracturing, the well had a production of 5.2414×10⁴ m³/d.

The discovery of Qingyang deep coal-formed gas field has important enlightenment for coal-formed gas exploration: (1) The thin coaliferous high mature source rock of a few meters thick has a large hydrocarbon generation capacity, and is favorable source rock for the formation of large gas fields. Therefore, the southern and western parts of the Ordos Basin have the basic source rock conditions for the formation of gas fields, and large exploration potential of coal-formed gas. (2) Multiple gas enrichment zones, like wellareas Zhentan 1, Qingtan 1, Qingtan 2, and Chengtan 3, have been found in the southwestern part of the basin. Through technology optimization, process innovation, the comprehensive costs of single wells and horizontal wells have been greatly reduced. At the same time, by using the integrated exploration and development method, guided by the correct sedimentation model, the skeleton straight wells have been used to finely dissect the sandbody, key problems in mixed water fracturing have been researched, and horizontal wells have been used to develop the gas field, by taking all these measures, the gas field is expected to reach the reserves of 100 billion cubic meters. (3) In southwestern part of the Ordos Basin, through 50 years of exploration, Qingyang deep coal-formed gas has been discovered for the first time. This fully illustrate that the geology understanding innovation, and advancement in exploration methods and technology are indispensable, which can provide precious experience for the areas with complex accumulation conditions, long history of exploration and difficulties in exploration breakthrough. (4) The coal series in the deep Bohai Bay Basin, the deep Upper Paleozoic in the southern North China Basin, and the Ordos Basin are all parts of the Late Paleozoic of large North China Basin, with large burial depth and high thermal evolution, with Ro value up to 5.0%. The discovery of Qingyang deep high-mature coaliferous gas field shows that the deep Upper Paleozoic in the deep Bohai Bay Basin and the southern China North Basin have basic conditions for the formation of coal-formed gas fields, and the exploration potential is great in these areas.

The average burial depth of the Qingyang large gas field is more than 4200 m, and the gas-bearing layer is the Lower Permian Shan 1 Member characterized by “four lows” of low porosity, low permeability, low pressure and low abundance. High thermal evolution thin layer coal source rock can generate enough gas to form coal-derived gas field. Some rock layers in deep burial, high-thermal evolution late-diagenetic stage can still have dissolved pores, primary intergranular pores, and intercrystalline pores, and thus act as high-quality sandstone reservoirs. Fractures improve reservoir seepage capacity. High natural gas drying coefficient and negative carbon isotope composition series are typical geochemical characteristics of deep coal-formed gas. By using integrated exploration and development method, and the development mode of “dissecting sandbody with skeletal vertical wells and centralized development with horizontal wells”, the deep gas field in the southern part of the basin can be economically and effectively developed. The discovery and successful exploration of the Qingyang Gas Field makes it possible to find a large-scale natural gas field with 100 billion cubic meters of reserves in the southwestern basin, and also provides references for the exploration of deep high-mature Upper Paleozoic coal-formed gas in the Bohai Bay Basin and southern North China Basin.