The research and progress of coal-measure gas has a positive and profound influence on promoting the development of natural gas industry. In the 1940s, German scholars realized that coal-measure strata could generate a large amount of natural gas, and thus forming large industrial gas fields^[1]. Since the 1950s, the former USSR began to study coal-measure gas and guide natural gas exploration, and successively discovered a number of large and super large gas fields in the West Siberian Basin and Karakum Basin, consequently, the former USSR leaped from a gas-poor country to a major gas-producing country in the world, and had kept its number one position in the world for a long time^[2]. The Siberian Basin is also the basin with the largest coal-measure gas production and reserves in the world today^[3]. The research and exploration of coal-measure gas in China began in the late 1970s. As the beginning of natural gas geology, the paper "Petroleum and natural gas generation in the process of coalification" by Dai Jinxing is a milestone in the theoretical research of coal-derived gas in China^[4,5,6]. The theory of coal-derived gas broke the traditional understanding that coal-measure strata were forbidden areas for natural gas exploration, and provided important scientific guidance for opening up new terrains for natural gas exploration and achieving leap-forward development of natural gas industry in China. In 2017, natural gas reserves and production of China were 157 748.13×10⁸ m³ (with 58.7% of coal-measure gas) and 1 467×10⁸ m³ (with 61.5% of coal-measure gas) respectively. Compared with 1978 before the emergence of coal-measure gas theory, they have increased 96 times and 10.7 times respectively. Meanwhile, China has changed from the country poor in natural gas to the sixth largest gas production country in the world^[7]. By combing the exploration and development progress of coal- measure gas and its formation and distribution characteristics during coal accumulation period, this paper clarifies the resource types, formation conditions and distribution rules, and evaluates and envisions the development prospects of coal- measure gas, in the hope of providing reference for scientific researchers engaged in the research of coal-measure gas.

Coal-measure gas refers to natural gas generated by coal, carbonaceous shale and dark shale in coal-measure strata^[7], including coalbed methane and shale gas held in coal-measure source rocks such as coal, carbonaceous shale and dark shale, and coal-measure gas reservoirs formed by gas migrated out of coal-measure source rocks inside or outside coal-measure strata. Coal-measure gas includes unconventional continuous-type and conventional trap-type. They can be subdivided into several resource types, including unconventional coalbed methane, shale gas, tight gas and conventional trap gas, which are obviously different in accumulation form and mechanism, distribution characteristics, exploration and development mode (Fig. 1). The main coal-measure gas resources described in this paper are CBM inside source, tight gas outside source and conventional trap gas.

The exploration of coal-measure gas can be traced back to the 1940s. Thirty-six coal-measure gas fields were found in the area from the Ames River Basin to the west of the Weiser River in the northwest basin of Germany, where the gas source is the underlying Upper Carboniferous coal-measure strata^[1]. In 1959, in the northeastern part of the West Netherlands Basin, Groningen gas field, the world's first coal-measure gas field with reserves exceeding a trillion cubic meter, was discovered, with recoverable reserves of 2.7×10¹² m^3[8] (Fig. 2). Forty gas fields were found in the northwest basin of Germany, about forty gas fields were found in the West Netherlands basin, and 10 and 52 gas fields were found respectively in the Upper Carboniferous and Red Dimension in the offshore part of the English Basin. The gas sources of these gas fields are all coal-measure strata of the Upper Carboniferous (Fig. 2)^[8]. Most of the natural gas produced in the Siberian Basin of Russia is coal-measure gas sourced by coal-measure strata of the Cretaceous Bokur Formation (Fig. 2). Ten large gas fields with reserves of more than a trillion cubic meter have been discovered, including the Urengoy gas field, the world's largest gas field at that time with recoverable reserves of 10.75×10¹² m³, and the Yamburg gas field, the second largest gas field in the world with recoverable reserves of 5.24×10¹² m³. The natural gas output and recoverable reserves in 2011 were 5 392.8×10⁸ m³ and 332.1× 10¹² m³, accounting for 84% and 72% of Russia’s output and reserves respectively. By 2015, they had produced 6.3×10¹² m³ of natural gas. Tight sandstone gas reservoirs have been found in 23 basins in the United States mostly in the Rocky Mountains, where the Cretaceous coal and coal- measure mudstone are the major gas source rocks (Fig. 2).

By the end of 2016, 39 large coal-measure gas fields (Fig. 3) had been discovered in China, including the Sulige gas field with the largest proven reserves and gas production in China. The large coal-measure gas fields accounted for 66% of the gas fields in China (59), and are mainly distributed in the onshore Ordos Basin, Sichuan Basin, Tarim Basin and Qaidam Basin, as well as offshore Yinggehai-Qiongdongnan Basin and East China Sea Basin. By the end of 2017, the reserves and annual output of coal-measure gas in China were 92 538.51× 10⁸ m³ and 902.14×10⁸ m³, respectively. In the past 12 years, 6 620.53×10⁸ m³ of coal-measure gas have been produced^[7].

Coalbed methane resources are abundant in the world. The United States, Australia, Canada and Russia are the main producers of coalbed methane (Fig. 2, Table 1). The United States is the country explores and exploits coalbed methane earliest and most successful, with CBM recoverable resources of 21.19×10¹² m³. The San Juan, Black Warrior, Powder River, Euinta, Laton, Appalachia, Akma and Pischens basins are the eight main basins for commercial production of coalbed methane in the US. Coalbed methane production in the United States started in the 1970s. In 2008, the peak output of coal-bed methane was 556.7×10¹² m³. In recent years, the development of coalbed methane has slowed down. In 2018, the CBM production of the US was 289×10⁸ m³, accounting for 3% of its total natural gas production, half of which came from the San Juan Basin. Australia is another major producer of coalbed methane, with CBM estimated resources extent of (8-14)×10⁸ m³, which are mainly distributed in the Surat and Bowen basins. In recent years, coalbed methane production in Australia has increased rapidly. In 2017, its total output was 397.7×10⁸ m³, of which the output of Surat Basin was 307.4×10⁸ m³ and that in Bowen Basin was 90.3×10⁸ m³. The geological resources extent of coalbed methane in Canada are 76×10¹² m³, which are mainly distributed in sedimentary basins in Western Canada such as Alberta. The coalbed methane industry in Canada began in the same time as the United States. Large-scale exploration and development of coalbed methane began in 2000, and CBM production declined in recent years. In 2018, CBM production was about 51×10⁸ m³.

Coalbed methane development in China has made great progress in recent years. Dating back to the 1970s, China began to carry out gas drainage and utilization in coal mines. In recent years, state-owned oil and gas enterprises, coal enterprises and local enterprises have actively involved in the exploration and development of coalbed methane. By the end of 2017, the cumulative proven CBM geological reserves in China were 6 911.77×10⁸ m³. Two large coalbed gas fields, the southern Qinshui Basin and eastern margin of Ordos Basin, were discovered, and forty-eight exploration areas were opened up, including Fuxin, Shenyang, Jiaozuo, Fengcheng, Liupanshui and Fukang, etc. A total of 110×10⁸ m³/a of CBM production capacity has been built. In 2018, total CBM production of China was 53.5×10⁸ m³. Two major CBM industrial bases, Qinshui basin and the eastern margin of Ordos Basin, have been built.

Coal-accumulation period is closely related to paleotectonic, paleoclimate, paleogeography and paleovegetation in geological history^[9,10]. Globally, coal resources with industrial value began to form in the Late Devonian. Since then, coal accumulation has never been interrupted, but the coal accumulations in different periods differ in strength, with a certain timeliness. Late Carboniferous-Permian, Jurassic and end of Late Cretaceous-Paleogene are the three main periods with the strongest coal accumulation^[9]. More than 99% of coal resources are distributed in the sedimentary strata of these three coal-accumulation periods^[11]. About 40% of the world's coal resources are distributed in the Upper Carboniferous-Permian, about 50% in the Upper Cretaceous-Neogene, and about 10% in the Upper Triassic-Jurassic^[9,10].

Late Carboniferous-Permian is the first coal-accumulation period around the world, when the terrain was relatively flat, plants flourished, and coal accumulation became strong, giving rise to widely distributed coal-accumulation basins and coal-measure strata. Late Triassic-Jurassic is the second coal- accumulation period in the world, when gymnosperm was most prosperous, and Jurassic is the most important coal-accumulation period in China. Late Cretaceous-Tertiary is the third important coal-accumulation period, which witnessed the development of advanced plant, stronger tectonic activities and more obvious climatic zoning. In a word, from the Late Devonian to the Permian, the intensity of coal accumulation increased continuously. Early and Middle Triassic was a low tide period of coal accumulation. From the Late Triassic, through the Jurassic, Cretaceous to Neogene, the intensity of coal accumulation increased continuously. In addition, it can be inferred from the peat deposits widely distributed in the world that the intensity of coal accumulation during the Quaternary was also very high.

Coal accumulation area is the result of various coal-forming controlling factors such as regional background, sedimentary environment and climate conditions jointly^[9,12]. Coal resources in the world spread in many regions of all continents widely, but unevenly. On the whole, the northern hemisphere has more coal resources than the southern hemisphere, especially the mid-temperate and sub-frigid regions of the northern hemisphere (Fig. 2). The most important coal-accumulation belt in the world lies between 30 and 70 degrees of north latitude in the northern hemisphere, which has more than 70% of the world's coal resources (Fig. 2). In comparison, the three continents in the northern hemisphere are richer in coal resources, of which Asia has coal resources of about 8.65×10¹² t, accounting for more than 56% of the global coal resources, North America about 4.06×10¹² t, accounting for more than 26%, and European 1.56×10¹² t, accounting for more than 10%. The continents in the southern hemisphere have smaller coal resources, of which Oceania has about 7 800×10⁸ t, accounting for 5.1% of the global coal resources, Africa about 2 100×10⁸ t, accounting for 1.4%, and South America the least, accounting for less than 0.4%. In addition, coal resources are also found in Victoria and other areas of Antarctica, but it is difficult to estimate the exact resources amount there. Of all the countries in the world, about 80 countries and regions in the world have coal resources. Central Asia-Russia, the United States and China have the most abundant coal resources, with combined resources accounting for more than 83% of the global coal resources.

The geographic distribution of coal resources in the world is most prominent in the two huge coal-accumulation belts. One belt stretches across Eurasia, from Britain in the west to Germany, Poland, Central Asia-Russia and to North China area. The other one stretches north-south across the central part of North America, covering coalfields in the United States and Canada. Coal resources in the southern hemisphere are also mainly distributed in temperate zone, and Australia, South Africa, Botswana and Brazil are richer.

4.1.1. San Juan Basin, USA

The San Juan Basin is located in southwestern Colorado, northwest New Mexico and eastern Colorado Plain in the western Rocky Mountains of the United States, with an area of 7×10⁴ km². It is a bowl-shaped depression, with estimated coal resources of 3 248×10⁸ t, coalbed methane geological resources of 2.38×10¹² m³ and coalbed methane recoverable resources of 0.29×10¹² m³. Formed in the Mesozoic Laramian orogeny, it is a foreland basin in the Rocky Mountains. The Basin is an asymmetric syncline with the syncline axis trending NW-SE located in the north-central part. The strata dip steeply in the northern margin of the basin and tend to be flat in the central and southern parts. Coal-bearing strata deposited in Late Cretaceous and Paleocene, and Fruitland Formation of Upper Cretaceous was the most important coal-bearing strata. The coal seams of the Fruitland Formation are generally 21.3 m thick in total, bituminous coal of high volatile to low volatile in rank. Buried at the depth of 84-1 050 m, they have a gas content of 0.11-16.98 m³/t, generally 13.44 m³/t. They are thickest in total (more than 12 m) and single layer and most in number in the north-central part. Groundwater receives recharge from the northern coal outcrop and flows along the coal seam to the deep. At the same time, groundwater carries coalbed methane and accumulates near the structural hub line (non-flow boundary), forming overpressure reservoirs in the north-central part. The Meridian 400 area, with an average gas production of 8 500-85 000 m³/d and water production of about 48 m³/d per well, is the area with the highest production capacity in the San Juan Basin. The Blanco Mesaverde gas field is located in the center and deepest part of the San Juan Basin with an area of 3 467 km². Natural gas is mainly produced from transgressive sandstone of Point Lookout Formation and regressive sandstones of Cliff House Formation, and a small amount of natural gas is in channel sands of the Menefee Formation. The source rocks are carbonaceous shale and coal formed in river and swamp environments of the Menefee Formation^[13], with type III humic kerogen. The natural gas is generated by the source rock in mature and over-mature stages. The hydrocarbon generation peaked during the formation of the deepest strata in the Oligocene. Blocked by dynamic water pressure from the basin margin and expansive clay from non- reservoir rocks, the natural gas accumulates near the source.

4.1.2. Surat Basin, Australia

The Surat Basin is a craton basin of the Upper Jurassic- Cretaceous in Australia. It extends from central Queensland in the north to northern New South Wales in the south, with a maximum sedimentary thickness of about 2 500 m and an area of about 30×10⁴ km^2[14]. The Walloon subgroup of Middle Jurassic is a meandering river and meandering river delta-lake sedimentary system with low energy. Coals mainly occur in the floodplain environment of meandering river and swampy environment of meandering river delta plain. There are 10 coal seam groups. The coal seams are thin individually, mostly 0.1-0.5 m, interbedded with thin carbonaceous mudstone, and 20-40 m thick cumulatively. In the northeastern margin of the basin, the strata are gentle, the large faults are few, and the nose-shaped uplifts and small adjusting faults exist in the structural transfer zone, improving the permeability of coal reservoirs. The regional hydrogeological conditions and the degree of coal rock evolution determine that the coalbed methane is dominated by secondary biogenetic gas and supplemented by thermal genetic gas. After nearly ten years of exploration and development, three dessert areas with high CBM enrichment and high production have been identified, i.e. Mimosa syncline in the north, Undulla nose uplift in the northeast and Chinchilla-Geoondiwindi slope in the east, covering an area of nearly 5 000 km^2[14]. Different coal groups in the three tectonic zones are different in accumulation types. The Mimosa syncline in the north is fault-hydrodynamic plugging type, while the Undulla nose uplift in the northeast and part of the eastern slope are anticline-water drive plugging gas reservoirs. Successful development of coal-measure gas in the Surat Basin is closely related to the frequent interbedding of thin coal seams and clastic rocks and the favorable development geological conditions derived from them.

4.1.3. West Siberian Basin, Russia

The West Siberian Basin has an area of about 250×10⁴ km². The basement is Hercynian fold at the burial depth of generally 8 km and 12 km at most in the natural gas area in the north of the Basin. The basin has undergone two development periods, rift period (Upper Paleozoic-Early Triassic) and depression period (Late Triassic-Quaternary). The depression period is the main development stage of coal-measures strata. From bottom to top, there are three sets of main coal-bearing strata, Middle-Upper Triassic-Lower Jurassic Cheliabine Formation, Middle-Lower Jurassic Tyumen Formation and Cretaceous Pokur Formation. It is one of the world's largest coal-bearing basins^[15,16]. The multi-cycle coal-bearing deposits and abundant coal resources have laid a good material base for the formation of a large amount of coal-measure gas. The Bokur Formation is the main source rock of the Cretaceous natural gas in the basin. It is a set of coal-bearing and sub- coal-bearing strata. The mudstone has an average TOC of 1.31% and 6% at maximum. The mudstone strata with high TOC value are mainly distributed in the middle and northern part of the basin, with TOC value of 1.5%-2.0%, which plays an important role in controlling the planar distribution of natural gas in the Cenomanian of Upper Cretaceous. The proven recoverable reserves of the Cenomanian gas reservoirs account for 70% of the total basin, most of which are concentrated in areas with methane abundance greater than 30×10⁸ m³/km². The Urengoy gas field is located in the area with the largest methane abundance and the thickest formation containing humic organic matter. In addition, the basement uplift placanticline trap, thick reservoirs with good physical properties, thick mudstone caprock have all created conditions for the enrichment of coal-measure gas. Coal-measure gas in the West Siberian Basin is the main body of Russian natural gas industry. The basin has the super-large gas field with the largest gas production in the world. Ten super-large gas fields with original recoverable reserves of more than 1×10¹² m³ have been found. In 2011, the natural gas reserves and production in the basin accounted for 72% and 84% of the total in Russia, respectively.

4.1.4. Ordos Basin, China

The Carboniferous-Permian strata in the Ordos Basin of China are a set of coal-bearing clastic deposits with abundant natural gas resources. Up to now, six large gas fields of coal-measure tight sandstone with proven geological reserves of more than 1 000×10⁸ m³ have been discovered, namely, Sulige, Yulin, Daniudi, Wushenqi, Zizhou and Yan'an gas field. Sulige area is located in the northwest part of Ordos Basin, with an exploration area of 4×10⁴ km². Carboniferous-Permian have multiple gas-bearing strata. The main gas-bearing strata are the eighth member of the Lower Shihezi Formation in Permian and the first member of the Shanxi Formation, which feature large exploration area, many gas-bearing strata, high tightness, low pressure and gas abundance, and great exploration and development potential^{[17,18,19,20,21,22]}. It had proven geological reserves of 1.65×10¹² m³ in 2017, and is the largest gas field in China at present. The coal seams in Benxi Formation, Taiyuan Formation to Shanxi Formation are 6-15 m thick combined and generally higher than 70% in TOC. Dark mudstone is widely distributed with a thickness of 30-50 m and a TOC value of 2%-3%. The tight sandstone reservoirs are distributed in continuous large pieces on the plane in a wide range. Longitudinally, multi-layered sand bodies are superimposed, with a cumulative thickness of 30-100 m and a long north-south extension distance of 150- 200 km. The reservoirs are gentle, unclear in trap boundaries, complex in gas-water relationship, showing dynamic accumulation characteristic of continuous hydrocarbon charging.

4.2.1. Genesis mechanism of coal-measure gas

The hydrocarbon generated by coal-measure strata consists of mainly gas and a small amount of oil (R_o= 0.5%-1.5%), with three aspects of evidence. Firstly, the coal is mainly humic coal, and the content of lignin and cellulose with low H/C value in its original substance is more than 60%. Secondly, the chemical structure of humic organic matter is composed of mainly methyl groups and condensed aromatic rings, which is conducive to gas generation. Thirdly, in the hydrocarbon generation simulation experiments, coal and coal-measure mudstone generate gas primarily^[7]. The ethane carbon isotope composition of natural gas has strong matrix inheritance, and thus is the most commonly used effective index to distinguish coal-measure gas from oil type gas^[23]. After a lot of data statistics, different researchers have found that the carbon isotope composition of ethane in coal-measure gas is generally higher than -28‰^[15,24-29]. The natural gases of Sulige gas field in the Ordos Basin, Hechuan of the Sichuan Basin and Kela 2 gas field in the Tarim Basin of China have ethane carbon isotope composition significantly higher than -28‰, indicating they are typical coal-measure gas. The natural gases of Northwest basin in Germany, Cooper basin and Gippsland basin in southeast Australia also have ethane carbon isotope composition higher than -28‰, indicating that these natural gases are also generated by coal-measure source rocks. The natural gas of the Urengoy super gas field in Russia is the product of coal-measure strata of the Pokur Formation in the low evolution stage^[30,31,32]. According to the kerogen type of natural gas, the natural gas in this gas field is also coal-measure gas. The δ¹³C₁-δ¹³C₂-δ¹³C₃ plot^[3] (Fig. 4) is usually used to identify the genetic types of natural gas. It can be seen from the figure, large gas fields in China such as Sulige, Hechuan and Kela 2 are all coal-measure gas fields. Natural gases of the Northwest basin in Germany, Cooper basin and Gippsland basin in southeast Australia fall in the coal-measure gas area, indicating that they are all coal-measure gas. Galimov et al.^[31] published part of carbon isotope values of ethane in Russian Urengoy gas field (Fig. 4). According to the ethane carbon isotope composition of these gas samples, it can be concluded that these gases are typical coal-measure gas.

4.2.2. Accumulation models of coal-measure gas outside source

According to the differences in natural gas accumulation mechanism, coal-measure gas reservoirs are divided into two types, i.e., large-area near-source continuous-type tight gas reservoir with medium and low abundance and local high abundance trap-type gas reservoirs far from source (Fig. 1). (1) The continuous-type tight gas is usually located in the low part of relatively stable structure, with weak sedimentary differentiation, lower gas generation intensity, large-area continuous distribution, usually near-source accumulation, ambiguous trap boundaries, and often no uniform gas-water interface or with inverted gas water position, and large reserves scale, low reserves abundance and low single well production. Through a thorough study of the tight sandstone gas fields such as the Carboniferous-Permian in the Ordos Basin and Triassic in the Sichuan Basin, it is found that the coal-measure source rocks and tight sandstone reservoirs are in sandwich structure, the coal-measure source rocks in layers expel hydrocarbon in evaporation manner, the delta sandstone layers of multi-source are widely distributed, and the source and reservoir are in close contact on a large scale, with the marked signature of large-scale continuous distribution. The gas-bearing areas of the Upper Paleozoic in the Sulige area of the Ordos Basin and the Triassic Xujiahe Formation in central Sichuan Basin are estimated at 4×10⁴ km² and 6 800 km² respectively. The widely distributed nano-scale pore-throat network system is the main controlling factor. The pore-throats, mainly 20-500 nm, connect the pores. The continuous accumulation law of tight sandstone gas in continental coal-bearing strata in China was revealed and demonstrated systematically. In addition, a geological evaluation method for tight sandstone gas has been developed. (2) Trap-type gas reservoirs tend to gather in favorable traps in high parts of active tectonic areas, with high gas generation intensity, normal gas-water relationship, gas above water, and high reserves and high single well production, for example, coal-measure gas fields of Kuqa depression, West Sichuan depression and Yinggehai basin. Taking the Kuqa sag-Krasu structural belt in the Tarim Basin of China as an example, trap-type gas reservoirs generally possess four reservoir-forming conditions: (1) Cretaceous sandstone reservoirs are located in the gas-generating center of Triassic-Jurassic coal-bearing gas source rocks with gas generation intensity of over 100×10⁸ m³/km² ; (2) braided delta sandstone reservoirs of Cretaceous Bashkirchik Formation deep under salt rock are distributed in large area; (3) the stably distributed gypsum salt caprock is conducive to the accumulation and preservation of oil and gas; (4) gas charged and accumulated in reservoirs in high efficiency in late Himalayan period.

Gas generation intensity is the core index for evaluating and predicting coal-measure gas fields. By dissecting the key strata of coal-bearing basins in China, seven main controlling factors of accumulation of coal-bearing gas field have been sorted out, including gas generation intensity, late-stage accumulation, porous-type reservoirs, paleo-uplift traps in gas-generating areas, in, above or below coal-measures strata, lower gas potentials area and abnormal pressure sealing^[3]. Among them, the gas generation intensity is a comprehensive reflection of the thickness of source rock, and the abundance, type and maturity of organic matter. The high gas generation intensity area can not only obtain gas source with high abundance, but also avoid hydrocarbon loss during long-distance migration. It is generally accepted that the gas generation intensity greater than 10×10⁸ m³/km² is essential for the formation of large coal-measure gas field. The areas with gas intensity greater than 20×10⁸ m³/km² are the dessert areas in the coal-measure gas fields of China. Putting this knowledge to the evaluation and exploration of coal-measure gas fields in China has achieved good results: Firstly, 12 coal-measure gas fields, such as Sulige, Jingbian and Daniudi, have been discovered in the Ordos basin, making this area the largest gas production area in China. The Sulige gas field here is also the largest gas field in the country with the largest reserves and annual gas production. By the end of 2017, this gas field had cumulative proven geological reserves of 1.65×10¹² m³ and produced 212.5×10⁸ m³ of gas this year, which provided natural gas source guarantee for four Shaanxi-Beijing gas pipelines. Secondly, fourteen coal-measure gas fields, including the Kela 2, Keshen and Dabei gas fields have been found in the Kuqa depression, of which Kela 2 coal-measure gas field has the highest reserves abundance (59.05×10⁸ m³/km²) and the highest single well production (Kela 2-7 has cumulative production of over 125×10⁸ m³), and the Keshen gas field is the first ultra-deep coal-measure gas field with a depth of more than 6 000 m in China. These large gas fields contribute more than 90% of gas of Tarim gas area, and has produced enormous social and economic benefits.

4.2.3. Accumulation model of coalbed methane inside source

Medium and high rank coal-bearing basins are characterized by synclinal enrichment of coalbed methane, such as San Juan Basin in the United States and Qinshui Basin in China. Coalbed methane content increases from basin margin to basin center. Synclinal enrichment of coalbed methane is the result of comprehensive action of structural evolution, hydrodynamic conditions and sealing conditions. Under the background of a regional syncline structure, towards synclinal axis, due to the convergence of atmospheric water along the edge outcrop to low water potential area at the axis, a syncline water catchment area would be formed, with the increase of salinity, lateral hydraulic sealing and thus good preservation environment can be formed, which is conducive to the enrichment of coalbed methane.

The key to enrichment and accumulation of low rank coalbed methane lies in the combination of multiple gas sources and good sealing conditions. Low rank coal seams have low degree of thermal evolution, thus, thermogenic gas is insufficient, and various other gas sources must be available as supplement, especially biogenic gas. Typical low-rank coalbed methane basins in foreign countries, such as Powder River basin and Surat Basin, have mainly biogenic gas. The methane carbon isotope values of Erlian Basin CBM in China are generally -60.3‰ - -65.3‰, indicating the gas is biogenic gas. The large amount of surface water infiltrating into coal seam in the late stage made the hydrochemical conditions of coal seam suitable for biogenic gas generation. Generally, CBM water is characterized by lower salinity, low sulfate, neutral or weak alkalinity of pH value. Infiltration of surface water also brings about coal-rock degrading bacteria group, which is conducive to biogenic gas generation; coupled with good sealing conditions, CBM enrichment area can come about in low-rank coal layers^[36,37]. In addition, in the Euinta Basin of the United States, there are several kinds of supplementary gas sources, including primary thermogenic gas, secondary thermogenic gas and biogenic gas. The isotopic compositions of CBM in the southern margin of Junggar Basin and in traps near faults of Baijiahai area of eastern Junggar Basin in China show that there are supplementary mature sapropelic gas, indicating that nearby and deep thermogenic gas migrated along faults to coal seams to enrich and form reservoirs.

5.1.1. Coal-measure gas resources outside source

The United States, Russia and Canada haven’t separately evaluated the resources of coal-measure gas, but most of the tight sandstone gas in the Rocky Mountains foreland basin group and the natural gas in the West Siberian basin are coal- measure gas^[3,38-39]. According to the fourth resource evaluation, China's coal-measure gas resources amount to 29.82×10¹² m^{3[7, 39]}, and the natural gas in the onshore tight sandstone gas fields of China is mostly coal-measure gas^[3]. The predicted geological resources of tight sandstone gas are (17.0-23.9)×10¹² m³, and recoverable resources are (8.1-11.4)×10¹² m³, the Ordos, Sichuan and Tarim basins are main contributors, providing 81% of the total recoverable resources.

5.1.2. Coalbed methane resources inside source

The global CBM resources exceed 270×10¹² m³, and are mainly concentrated in 12 countries, Russia, Canada, China, the United States, Australia, Germany, Poland, the United Kingdom, Ukraine, Kazakhstan, India, and South Africa (Table 2). The major coal resource countries are also major coalbed methane resource countries. Russia has 6.5×10¹² t of coal resources and (17-113)×10¹² m³ of coalbed methane resources, ranking first in the world. According to the latest prediction results, CBM resources in China amount to 30.05×10¹² m³, surpassing the United States, ranking the third in the world, of which technically recoverable resources of CBM are 12.5× 10¹² m^{3[7, 39]}. Russia, Canada, China and the United States are the top four countries with a total CBM resources of 240×10¹² m³, accounting for about 89% of the world's total CBM resources.

The global coal resources are abundant, and the coal-measure gas resources are huge, too. With the progress in understanding and exploration and development technology, the resources potential of coal-measures conventional gas, tight sandstone gas and coalbed methane will have large room to rise, so coal-measure gas will have broad prospect of development and utilization and take an important position in the future development of oil and gas industry. Exploring and exploiting coal-measure gas resources vigorously is of great practical significance for guaranteeing the world energy supply, reducing air pollution and improving the quality of life.

At present, the unconventional oil and gas breakthroughs in North America have led to a rapid increase in production. Especially, the shale gas revolution has changed the global natural gas production pattern. Since 2009, the U.S. natural gas production has surpassed that of Russia by a large margin, and the substantial increase in shale gas production has played a decisive role. With the great breakthrough in shale gas exploration and development, the investment and workload of coalbed methane industry in North America have decreased sharply, and the production of coalbed methane has reached a new low since 1997. Coalbed methane wells put into operation in North America (USA, Canada) are gradually declining. It is expected that by 2030, coalbed methane business will further shrink and production will decrease year by year. The large-scale development of LNG in Australia mainly comes from coalbed methane, the production of which is expected to remain stable and rise slightly by 2030.

With China's economic development entering a new stage, in order to build a beautiful China and protect the ecological environment, it is necessary to continuously increase the supply of clean energy. The proportion of natural gas to the primary energy structure will be greatly increased from 7.6% in 2018 to 15% in 2030, to guarantee the realization of the national carbon emission target, and the green transformation of oil and gas industry, and the transition to a low carbon society^[39,40,41]. In 2018, China's natural gas production was 1 580×10⁸ m³ and its consumption was 2 766×10⁸ m³. With the development trend of current theory and technology inertia, it is predicted that China's domestic natural gas production will be about 2 000×10⁸ m³ and its demand will be more than 6 000×10⁸ m³ by 2030. Domestic natural gas production won’t satisfy the demand of consumption, and the gap is expanding. To increase the development of coal-measure gas will improve the gas supply capacity. Conventional coal-measure gas, coal-measure tight gas and coalbed methane will be the main body of natural gas reserves and production growth, and will continue to play an irreplaceable role. Firstly, the conventional coal-measure gas makes up the most of the growth of coal-measure gas reserves and production. It is estimated that its production in 2030 will be (500-550)×10⁸ m³. The Cretaceous in Kuqa depression, Mesozoic-Paleogene in the southern margin of Junggar Basin, Upper Jurassic-Upper Cretaceous in Songliao Basin, Cenozoic in Ying-Qiong Basin, Cenozoic in Pearl River Mouth Basin and Cenozoic in East China Sea Basin are the important areas for increasing reserves and production. Secondly, comparing with the United States, China's coal- measure tight gas has the characteristics of tight reservoir, strong heterogeneity, difficulty in gas-water differentiation and dessert identification, and some of the tight gas reservoirs are low in pressure (with pressure coefficient of 0.7-0.9, against 1.1-1.4 in the United States). Under the same geological and technical conditions, the recovery rate is 20% lower than that of the United States and is more difficult to be improved. Effective utilization of tight gas is a complex system engineering. Tight gas is a practical resource with great potential for development. At present, on the basis of successful development in Sulige and other local areas, the development of China's tight gas should focus on solving the bottleneck problems, such as dessert prediction of complex type of tight gas, well pattern deployment and horizontal well trajectory optimization, seepage law and productivity evaluation, effective reservoir stimulation, and comprehensive improvement of recovery factor, etc. Exploration must mainly expand to new fields and new basins, in development, more effort should be put into the recovery of hard-to-use reserves and EOR improvement, and engineering is more inclined to multi-layer gas production and multi-well production increase, thus promoting the large scale development of China's tight gas on the whole. It is predicted that the tight gas of coal- measures in China will increase steadily in the next 10 years. It is expected that the proven reserves will reach 5.0×10¹² m³ and the production will be (400-450)×10⁸ m³ by 2030. The Upper Paleozoic in the Yishaan slope of Ordos Basin and the Xujiahe Formation of Upper Triassic in the Sichuan Basin will be the key areas for increasing reserves and production. Thirdly, compared with the United States, the development of CBM industry in China is more than 20 years late, and the exploration and development and operation management of CBM fields are still in the beginning stage. The exploration and development potential of CBM is still huge. It is necessary to change the conventional concept and use unconventional thinking to realize the technological revolution of efficient exploration, development and utilization of CBM. At present, the development of CBM industry in China will still be devoted to the development of medium and high rank CBM of the old areas in Qinshui Basin and eastern Ordos Basin, and actively expand and extend to the comprehensive development of low and medium rank CBM, and deep coal-measures (commingled production of CBM, coal-measure sandstone gas and coal-measure shale gas)^[42]. With the exploration and development of coalbed methane pushing to new areas, new strata and new fields, key technologies and equipment for coalbed methane exploration and development must be innovated, especially, the breakthroughs in dessert prediction technology, deep multi-gas co-production technology, automatic fracturing technology and intelligent drainage technology in low-rank complex structural areas will be important guarantees for the sustained and rapid development of China's coalbed methane industry. Coalbed methane (CBM) is expected to grow rapidly in the next 10 years. It is expected that the proven reserves will reach 1.2×10¹² m³ and the production will reach (150-200)×10⁸ m³ by 2030. Above all, underground coal gasification may realize industrial development^[43].

Coal-measure gas is a natural gas resource generated by coal, carbonaceous shale, and dark shale in coal-measure strata. It includes several resource types, continuous-type coalbed methane (CBM) and shale gas inside source, continuous-type tight gas outside source, and trap-type coal-measure gas, and is an important gas source in the natural gas industry.

Continuous-type and trap-type accumulation models of coal- seam gas are predominant. It is proposed that the gas generation intensity greater than 10×10⁸ m³/km² is essential for the formation of a large coal-measure gas field. The CBM is usually enriched in syncline in medium- to high-rank coal layers, while CBM enrichment in low-rank coal depends on good configuration of source rocks and caprocks.

It is predicted that the coal-measure gas around the world still has great remaining resources potential. Coal-measure gas outside source is concentrated in Central Asia-Russia, the United States, Canada and other countries/regions, while CBM inside source is concentrated in 12 major countries. The production of coal-measure gas in China is expected to exceed 1 000×10⁸ m³ by 2030. Technological progress, policy changes and market demand will affect the production.