Symbiotic Combination and Accumulation of Coal Measure Gas in the Daning–Jixian Block, Eastern Margin of Ordos Basin, China

Abstract

Coal measure gas resources, including coalbed methane (CBM), shale gas, and tight gas are abundant in the Daning–Jixian Block. The complexity of the source–reservoir–cap relationship in the coal measure strata leads to unclear symbiotic characteristics and gas accumulation, which in turn, restrict the exploration and exploitation of the coal measure gas. In this study, the enrichment and accumulation of coal measure gas are discussed and summarized in detail. The results show that there are eight lithofacies and six reservoir combinations in the superposed strata of the coal measures in the study area. Controlled by the tidal flat-lagoon facies, the “sand-mud-coal” type mainly distributes in P₁s² and P₁t, showing a good gas indication. Based on the variation of the total hydrocarbon content, key strata, and pressure coefficient of the coal measure gas reservoir, four superposed gas-bearing systems are identified in the vertical direction. According to the relationship between the gas-bearing system and gas reservoir, the enrichment of coal measure gas in the study area can be divided into three modes, including an intra-source enrichment mode, a near-source migration enrichment mode, and a far-source migration enrichment mode. The symbiotic accumulation of a coal measure gas model is further proposed, that is, an “Adjacent to co-source reservoir” type superimposed coalbed methane and shale gas reservoir model, a “Three gas symbiosis” superimposed reservoir model in the local gas-bearing system, and a “Co-source far reservoir” tight sandstone gas reservoir model. Clarifying the symbiotic relationship of coal measure gas reservoirs is beneficial to the exploration and further production of unconventional gas in the study area.

1. Introduction

Coal measure gas refers to the unconventional natural gas in the coal system, mainly including coalbed methane, shale gas, and tight gas. Effective utilization of coal measure gas is important for energy supplementation, which has received extensive attention [1,2,3,4]. In recent years, due to continuous exploration and development, trail tests and commercial productions of coal measure gas have been successfully carried out in the Piceance Basin in the United States, the Surat Basin in Australia, and the eastern margin of the Ordos Basin in China, indicating that the comprehensive exploration and development of coal measure gas has great potential [5,6,7,8]. At the same time, it also puts forward clearer requirements for the full understanding of the basic theory and accumulation mechanism of coal measure gas reservoirs, so as to effectively ensure the improvement of adaptive commingled production technology and the economic development benefits of coal measure gas reservoirs [9,10,11].

Coal measure gas is mostly developed in coal measures with marine–continental transitional facies because thick sand bodies could be formed to accumulate CBM and tight gas [12,13]. The symbiotic combination of coal-bearing strata can be divided according to the vertical lithologic configuration [14,15]. The coal-bearing source rocks are mainly coal seams and dark organic-rich shales, and the sedimentary system, structure, and fluid dynamics determine the gas potential [16,17]. In addition to the generated gas in the source rock, the natural gas in coal reservoirs can be redistributed due to the continuous migration and further sealed by shale or clay formations. Gas is sealed in shale and sandstone by sedimentation, compaction, and densification, where it is enriched to form corresponding coal measure gas reservoirs under the sealing effect of the surrounding rock [18,19,20,21]. Therefore, the source–reservoir spatial combination relationship could be used to classify the gas reservoir types as it determines the distribution of gas and gas types [22]. As a whole, coal measure gases belong to a kind of wide-cover continuous accumulation system, and its accumulation elements are the ordered space–time configuration of generation–reservoir–cap, which is controlled by the adjustment of later structure, sedimentation, and thermal evolution, jointly creating the diverse accumulation types and accumulation scale in the same basin [23,24]. In addition, the superposition of rock strata with different physical properties and internal sealing conditions leads to complicated connectivity among gas reservoirs in the vertical direction, which affects the co-production capacity of coal measure gas [25].

Studies on clarifying the types of coal measure gas reservoirs, reservoir characterization, and production compatibility have been widely carried out [26,27]. However, the symbiotic relationship and accumulation mode of coal measures are seldom discussed, and the characteristics of co-generation accumulation are unclear. The Daning–Jixian Block in the eastern margin of the Ordos Basin is one of the important areas for the exploration and development of coal measure gas in China. The gas reservoir characteristics of this block are discussed in detail in this study to clarify the enrichment and accumulation of coal measure gas and to provide some theoretical support for the following exploration and production.

2. Regional Geology Setting

The study area is located in the eastern margin of the Ordos Basin, Linfen City, Shanxi Province, and it is adjacent to Jixian to the south, Yonghe to the north, the Yellow River to the west, and Daning to the east. The main region of the study area is located in the east of Yanchang Block and the north of Shanxi Hedong coalfield. It is situated in the southern part of the Jinxi flexural fold belt and the southeastern part of the Yishan Slope. It was mainly formed in the Mesozoic period and shows as a transitional basin margin structure type [28]. The whole study area presents a monoclinic structure inclined from east to west and the structural deformation is relatively weak and less affected by faults.

The bottom of the coal measure strata in the block is Middle Ordovician Majiagou Formation, and from the bottom to the top there is Benxi Formation, Taiyuan Formation, Shanxi Formation, and Lower Shihezi Formation. The Upper Paleozoic is composed of Carboniferous and Permian, lacking Devonian strata. Limestone, mudstone, and sandstone co-exist within the coal seams in the Benxi formation (C₂b). The top layer is bounded by Pangou limestone with the Taiyuan formation (P₁t), and the bottom is stably developed bauxite rock as the identification marker layer at the bottom of the stratum, which is in unconformity contact with the underlying Ordovician system. The Taiyuan Formation is widely distributed in the study area, which has multiple sets of limestone marker beds and thick coal seams. Dongdayao limestone stably developed at the top, with Beichagou sandstone acting as the separation marker layer of the Shanxi Formation and Taiyuan Formation. The Shanxi Formation is a set of marine–continental transitional delta deposits, which can be subdivided into two lithologic sections from bottom to top, with Tieyinggou sandstone as the separation marker layer between the second (P₁s²) and first (P₁s¹) member in the Shanxi Formation. Sandstones and dark mudstones are superposed in the whole section, and coal seams are mainly developed in the second Shan member. The mudstone and sandstone are developed in the eighth member of the lower Shihezi Formation (P₂s⁸), and the Luotuobozi sandstone is the marker layer integrating with the P₁s¹ member (Figure 1). In general, multiple sets of coal measures are stably developed in the study area, and their vertical superposition relationship is obvious, indicating that the study area has great coal resources and exploration potential.

3. Symbiotic Combination and Type of Coal Measure Gas Reservoirs

3.1. The Lithofacies Combination Patterns and Sedimentary Environment of Coal Measure Strata

3.1.1. Lithofacies Combination Patterns

As an important part of sedimentary facies, lithofacies are rocks or a set of rock assemblages formed in a specific sedimentary environment, which can effectively indicate the changes of the sedimentary environment. The coal-bearing strata in the study area mainly include sandstone, mudstone, coal seam, and limestone, in which the source rocks are mainly coal and dark mudstone. The symbiotic change of each lithology determines the coexistence of gas reservoirs. The study area mainly develops 5# coal seam of P₁s² (the thickness is 2–12 m, average 7 m) and 8# coal seam of P₁t (the thickness is 4–12 m, average 8.7 m). Stable and thick mud shale is developed in each layer. Good gas content is discovered near the coal seam. Different types of source rocks are superimposed and alternately distributed, and the sandstone is mainly distributed in the P₂s⁸ and the P₁s¹, with a thickness of 27.8 m, which is a favorable reservoir for the occurrence of coal measure gas. In addition, the P₁t and C₂b in the study area have developed multi-layer gray-black limestone, which is a good capping layer (Figure 2).

Coal measure gas mainly occurs in coal seams, shales, and nearby sandstones. The particularity of the vertical lithologic superposition relationship determines the characteristics of coal-measure gas accumulation and reservoir types. Based on the sedimentary environment and lithologic superposition relationship, the lithofacies in the study area can be divided into eight types (Figure 3).

Type A includes mudstone and sandstone superimposed interbedded deposits, mostly developed in the P₂s⁸, and it is the main occurrence layer for gas. Type B includes sandstone, mudstone mixed with a thin coal seam, or carbonaceous mudstone. Type C is a combination of a thick coal seam and mudstone developed near the 5# coal seam with good hydrocarbon potential. Type D is embedded in sandstone, mudstone, and a coal seam, and it is a favorable coal measure gas reservoir type. Type E is a combination of limestone–sandstone–mudstone, and some areas contain thin coal seams which are tidal flat-lagoon sedimentary facies. Type F is developed near the 8# coal seam of P₁t, limestone is the roof, and the coal seam is the main gas source of the coal measure gas reservoir in the study area. Type G is the main reservoir, with the top being limestone, and the lower is the interbed of sandstone, mudstone, and a coal seam. Type H is developed in the C₂b Formation, without a coal seam and dominated by sandstone and mudstone interbeds with gray-brown limestone at the top.

3.1.2. Sedimentary Environment

The study area is located in the marine–continental transitional facies which is the combination of barrier coastal facies (Type E–H) and delta facies (Type A–D). The provenance is sufficient, which lays a rich organic matter foundation. Vertically, there are many sets of sedimentary systems in which the main coal seams are interbedded with sandstone and mudstone and reflects the cyclic evolution of coal-accumulating facies under the large sedimentary environment which further formed a differentiated space–time configuration relationship of the source–reservoir–cap (Figure 4).

Among them, the mixed flat under the barrier coast is well developed, and C₂b forms with limestone, mudstone, and a small amount of sandstone; the upper P₁t develops lagoon-tidal flat subfacies, and the lower mud flat deposits are composed of mudstone and siltstone, and they slowly transit to carbonate tidal flat to form multiple sets of thick limestone. With the gradual shallowing of the water depth in P₁t, and under the isolation of the barrier island, the plants that grew in the freshwater lagoon environment formed a peat swamp lagoon, which is favorable for coal accumulation. Therefore, the stable and continuous 8# coal seam and black shale were developed. Based on the analysis of the total hydrocarbon content and the percentage ratio (TG/ΣC) in the field gas logging, this set of strata has a strong gas-bearing display. At the same time, the development of barrier sand and tidal channel sand deposition indicate that the P₁t of the study area has good reservoir conditions.

The upper part is mainly delta front subfacies deposition, the 5# coal and many sets of thin coal rock are developed in P₁s², and the sandstone is not developed or poorly developed, indicating that the sedimentary environment was in the swamp facies of the reducing environment for a long time. Under the condition of static water, many sets of thick mudstone and black shale were formed and have good preservation conditions and good hydrocarbon content. In some areas with turbulent water bodies, terrigenous debris supply was better, forming staggered sandstone, mudstone, and coal seams. In addition, the superimposed sandstone and mudstone deposits were developed in the upper underwater distributary channel and the interdistributary bay, and the grain size gradually becomes finer, which created the lithological combination of P₁s¹ and P₂s⁸. In some areas, the mouth bar–far sand dam was developed, which provided favorable formation conditions for coal measure gas accumulation.

3.2. Combination and Distribution of the Coal Measure Gas Reservoir

Combining gas logging of the total hydrocarbon content and gas-bearing tests, the second member of the Shanxi Formation and P₁t are identified with rich coal resources, and the coalbed methane reservoir, shale gas reservoir, and tight gas reservoir are confirmed as widely developed in the study area (Figure 5). Among them, the coalbed methane reservoir is mainly composed of two sets of main coal seams. Shale gas reservoirs are widely distributed, except P₂s⁸ of the low Shihezi Formation, with a large thickness and high hydrocarbon content. The cumulative effective thickness of each well in P₁s² can reach 14.8 m, with an average of 6.56 m. Tight gas reservoirs are mostly developed in P₂s⁸ (1.72~21 m) and P₁s² (1~23.37 m), with an average cumulative effective thickness of 9.61 m and 9.37 m, respectively, which are also the main tight gas reservoirs in the study area.

The superimposed vertical lithofacies determine that the coal-bearing gas reservoirs have different spatial and temporal configuration relationships, resulting in the comprehensive development of independent gas reservoirs and interbedded gas reservoirs in each layer. Combined with the source–reservoir–cap combination, the study area can be classified into six types of coal-bearing gas reservoirs (Figure 6).

Type 1 is an independent coalbed methane reservoir which was developed in P₁s² and the bottom of P₁t. The number of layers is small, but the total thickness is large. The cap rock is mainly composed of mudstone and limestone with strong sealing capacity composed of Type C or Type F lithofacies. The whole gas reservoir has strong hydrocarbon generation and expulsion ability, and the internal gas logging peak value is high and continuous showing that the gas reservoir has good gas content and is the main gas reservoir.

Type 2 is an independent shale gas reservoir which is distributed below P₂s⁸ with different degrees. The hydrocarbon generation intensity and the gas content of this type of gas reservoir are lower than those in Type 1. However, the number of gas-bearing layers and their thickness are larger than those in Type 1. Most of the shales are close to the coal seam. The roof and floor are mostly mudstone and some limestone, being mainly Type C lithofacies.

Type 3 is an independent sandstone gas reservoir developed in the Shanxi Formation and above. It is composed of A and B lithofacies. The reservoir is tight sandstone with low porosity and permeability. The gas contained in Type 3 is mainly generated from other layers. Type 3 has general gas content with a large distance to the coal seams. The thickness of the individual layer is between 1 and 8 m.

Type 4 is coal seam–mud shale interbedded gas reservoir distributed near the main coal seam and consists of Type C and Type G lithofacies. There are 1–4 sets of such assemblages developed vertically with good self-storage conditions and a large and continuous gas content.

Type 5 is a shale–sandstone interbedded gas reservoir mainly developed in P₁s². The lithofacies combination is mainly Type D and E, in which carbonaceous mudstone is the main hydrocarbon generation layer, with medium thickness and general gas content.

Type 6 is a coal seam–shale–sandstone interbedded gas reservoir of P₁s² and P₁t. The lithofacies combination type is Type D or G and has a good source–reservoir–cap configuration. Hydrocarbon gas migrates to each layer through cracks and fractures. Gas logging shows the highest gas level and a continuous and cumulative thickness over 15 m.

4. Symbiotic Combination and Type of Coal Measure Gas Reservoirs

4.1. The Enrichment and Accumulation of Coal Measure Gas

A gas-bearing system refers to the combination of mature gas-producing source rocks and gas reservoirs comprised of three geological elements: source rock, gas reservoir, and caprock [30]. The symbiotic association of coal measures leads to the frequent interaction between organic-rich shale and coal seams. A large amount of gas is produced during the thermal evolution process. At the same time, the difference in the physical properties of coal-bearing strata will inevitably lead to the repeated occurrence of water and gas resisting layers or internal seals in the longitudinal direction resulting in multiple independent superimposed gas-bearing systems.

The vertical fluctuation of gas content shows a lack of connection between different coal-bearing reservoirs [31]. Overall, the gas survey near the coal seam and shale shows the highest gas content, followed by sandstone; the gas-bearing property in the whole interval from the coal seam of P₁t to the lower part of P₂s⁸ fluctuates with depth, which indicates that different superposed gas-bearing systems are developed in the study area.

The essence of forming an independent gas-bearing system is the existence of an internal unified fluid pressure system, and the division of the vertical gas-bearing system is closely related to the development of key sealing layers with low porosity and permeability in coal measure strata. For example, some scholars have suggested that the siderite-bearing mudstone can be used as the key identification mark, and its essence is low porosity and low permeability [25]. The development of key strata is controlled by a sequence stratigraphic framework and mainly develops near the maximum flooding surface, that is, it is easier to form stable key strata in the rising cycle. The logging of the A6 well is denoised by a one-dimensional discrete wavelet threshold, and the principal component analysis method is used to fuse multiple logging curves. The first principal component curve that can comprehensively reflect the characteristics of formation information is the GR curve. This curve is transformed by the one-dimensional continuous wavelet transform. According to the different scale factors obtained by modulus maxima, different levels of sequence strata are divided. Combined with the characteristics of lithologic assemblage, five long-term cycles and ten short-term cycles can be divided, and then five sets of key strata with good regional continuity are identified. Based on the comprehensive sequence division, the key intervals and the pressure system show abnormal pressure coefficient intervals. Further, combined with the vertical gas-bearing changes, the target intervals of coal measure strata in the study area are divided into four gas-bearing systems (Figure 7), which are the bottom of P₁t (I), the middle bottom of P₁s² (II), the middle bottom of P₁s¹ (III), and the bottom of P₂s⁸ (IV).

4.2. Enrichment of Coal Measure Gas

4.2.1. Enrichment of Coal Measure Gas Based on the Superimposed Gas-Bearing System

The configuration of source–reservoir space determines the enrichment of coal measure gas. Vertically, multiple sets of gas-bearing systems and various coal-bearing gas reservoirs are superimposed in the study area, and the non-connectivity between different gas-bearing systems leads to significant differences in the overall enrichment and gas content. Furthermore, considering the relationship between gas-bearing systems and accumulation, i.e., the source–reservoir–cap assemblage, the enrichment of coal-bearing gas reservoirs in the study area can be divided into three modes, including in-source enrichment mode, near-source migration enrichment mode, and far-source preservation enrichment mode (Figure 8).

• In-source enrichment mode

This enrichment mode is characterized by “self-generation and self-storage”. Under the good sealing capacity of roof and floor, adsorbed and/or free gas are stored in pores and fractures in the reservoir within the same gas-bearing system. It can be further subdivided into two types depending on whether the gas source is shale or coal (Figure 8c,d), which correspond to the Type 2 and Type 1 coal-bearing gas reservoirs, respectively. The overall reservoir configuration conditions are good with a high gas content.

• Near-source migration enrichment mode

In this mode, the regional caprock is stable, and the coal seam and shale layer are acting as source layers. When the gas concentration reaches a certain value, it migrates through the pore–fracture network and regional faults to the adjacent layers. As a result, the coal seam, shale, and sandstone can all be reservoirs. The more frequent the superimposed interaction is, the more favorable it is for the enrichment of coal measure gas (Figure 8b). After enrichment, Types 4, 5, and 6 coal measure gas reservoirs can be formed within one gas-bearing system and have the characteristics of “self-generation and self-storage” and “self-generation and other storage”.

• Far-source preservation enrichment mode

This enrichment mode has the characteristics of long-distance migration of free gas. The reservoirs are mainly tight sandstone layers with favorable preservation conditions and are far away from source rocks (underlying coal rock or carbonaceous mudstone) (Figure 8a). On one hand, fractures and highly permeable sandstone provide pathways for hydrocarbons to migrate to the overlying strata. On the other hand, due to the influence of structure and faulting, the original pressure system of the formed gas reservoir in some gas-bearing systems is destroyed, and the hydrocarbon gas in the gas reservoir is transported to the reservoir with good sealing capacity through faults or fissures to form three types of coal measure gas reservoir combinations. In general, this mode is strongly affected by tectonic movement, and most of the escaped gas is difficult to be reclosed, resulting in low gas content.

4.2.2. Characteristics of Coal Measure Gas Enrichment

Based on the well profile in the north–south direction of the study area, it can be seen that from the large-scale deposition of the main 8# coal seam and 5# coal seam, the source enrichment is mainly distributed in P₁t and P₁s² of the gas-bearing system I and II. The stable source is dominated by coalbed methane reservoirs, supplemented by shale gas reservoirs. The enrichment mode of the near-source migration type is mainly distributed near the coal seam of P₁s² with good lithological superposition development; this is mostly adjacent enrichment of coalbed methane–shale gas and adjacent enrichment of coalbed methane–shale and sandstone gas, with good gas-bearing property, but less in P₁t where limestone is widely developed. P₁s¹ and P₂s⁸ are mainly far-source preservation enrichment mode, and tight sandstone gas reservoirs present multiple sets of stable distribution in the study area (Figure 9).

4.3. Evolution and Symbiotic Accumulation of Coal Measure Gas

Hydrocarbon charging in various historical periods is normally the prerequisite for the enrichment and accumulation of coal-bearing gas [32]. The study area mainly experienced two gas generation periods, including the first gas generation in the Late Triassic–Early Jurassic and the second gas generation in the Early Cretaceous. The study area continuously received relatively rich sediments from Permian to Triassic. The source rock was rapidly buried in the Late Triassic–Early Jurassic period, and then the first hydrocarbon generation began. The generated natural gas quickly filled the surrounding sandstone layers, and the gas expansion force was the main driving force for hydrocarbon migration. Because the consolidation of sandstone was earlier than the hydrocarbon expulsion time, a large amount of natural gas accumulated in the sandstone with good physical properties, while sandstone layers far away from the source rock cannot be filled with gas. The Late Jurassic–Early Cretaceous period was affected by the eastward subduction of the Pacific plate. The Yanshanian magmatic intrusion caused the temperature of the coal seam to be higher than the temperature of the first occurrence of deep metamorphism, and then the second deep metamorphism occurred, followed by the second hydrocarbon generation. At this time, affected by the Yanshan movement in the Cretaceous period, the heat flow of the regional base generally increased, the ground temperature increased, and the magmatic thermal metamorphism was superimposed, which aggravated the thermal evolution of coal. The degree of coal metamorphism reached the lean coal–anthracite stage, and a large amount of gas was generated in the Yanshan period. At the end of the Cretaceous period, due to the influence of the Yanshan movement, the strata in the study area were greatly uplifted. The uplift of the strata caused the overlying strata to be eroded and cooled, the cap rock of the coal measures became thinner, the metamorphism of the coal seams stopped, the preservation conditions of the coalbed methane became worse, and the coal measure gas began to escape in a large amount into the nearby favorable strata. However, the fracture structure caused by strata uplift promotes the development of cleats in coal, improves reservoir permeability and adsorption capacity, and forms high-abundance coal measure gas reservoirs. During the strata uplift in the late Yanshan–Himalayan period, the coal measure gases experienced the process of migration, dissipation, and re-accumulation. During these processes, the coal measure gas reservoirs were constantly destroyed and regenerated, and finally, the Late Paleozoic gas reservoirs were finally formed in the Himalayan period.

The enrichment and accumulation of coal-measure gas depend on the key geological factors of hydrocarbon generation, evolution, migration, and reservoir formation, that is, the structural uplift and re-subsidence leading to the adjustment of gas reservoirs and the configuration of source–reservoir–caprock [33]. In other words, the formation of the current gas reservoirs is the result of the comprehensive action of various historical periods. From the perspective of spatial and temporal distribution, the Daning–Jixian block has a good configuration of accumulation factors. The current comprehensive model of coal measure gas symbiotic accumulation can be summarized as follows: an “Adjacent to co-source reservoir” type superimposed coalbed methane and shale gas reservoir model, a “Three gas symbiosis” superimposed reservoir model in the local gas-bearing system, and a “Co-source far reservoir” tight sandstone gas reservoir model (Figure 10).

The two sets of main coal seams in the study area are the main gas-generating source rocks, and the coal seams nearby are the main gas reservoir. The cap layers are mostly thick mudstone and part limestone. The reservoir combination is mainly composed of mudstone and coal seams, and most of them are characterized as Type 4. Locally developed sandstone reservoirs can form Type 5 reservoirs. In addition, due to sedimentation, the tight sandstone with good reservoir performance is mainly developed in P₂s⁸ and P₁s¹. With the supply of the gas source from the underlying source rock, the third type of gas reservoir is mainly formed, that is, the current “Co-source far reservoir” tight sandstone gas accumulation model. In general, the study area of Daning–Jixian has a good accumulation of coal measure gas, especially in the gas-bearing system II of P₁s², which is a favorable layer for the exploration and exploitation of coal measure gas in this block.

5. Conclusions

This study determined the symbiotic combination relationship and gas reservoir characteristics of coal measure strata in the Daning–Jixian block, divided the superimposed gas-bearing system, and summarized the enrichment and accumulation of coal measure gas in the study area. The main conclusions are as follows:

• Based on the superposition relationship of coal measure strata in the study area, the symbiotic characteristics of coal measures can be divided into eight types of lithofacies and six types of gas reservoir. As the material basis of gas reservoirs, coal and shale are the main source rocks; controlled by the sedimentation of tidal flat-lagoon facies, the “sand-mud-coal” type is mainly distributed in P₁s² and P₁t, with good gas-bearing indication, and the tight sandstone gas is mainly distributed in P₂s⁸ in an underwater distributary channel.
• A differential display of the total hydrocarbon content and pressure coefficient of the coal measure reservoir gas survey, identification of key strata, and four superposed gas-bearing systems are developed vertically (the lower part of P₂s⁸, the middle and lower part of P₁s¹, the upper and lower part of P₁s², and the coal seam of P₁t). According to the relationship between the gas-bearing system and the gas reservoir, the enrichment of coal-bearing gas reservoirs in the study area are further divided into three modes: in-source enrichment mode, near-source migration enrichment mode, and far-source migration enrichment mode.
• The formation of coal measure gas reservoirs is the comprehensive result of enrichment–destruction–re-accumulation in each historical period. Models of coal measure gas symbiotic accumulation are further proposed in this study, including “Adjacent to co-source reservoir” type superimposed coalbed methane and shale gas reservoirs, “Three gas symbiosis” superimposed reservoirs in local gas-bearing systems, and “co-source far reservoir” tight sandstone gas reservoirs. The determination of the symbiotic relationship of coal-based gas reservoirs will facilitate the clarification of the exploration direction.

The aim of this study is to understand the regional distribution and formation of coal-measure gas. The findings will serve as a theoretical basis for predicting favorable areas for coal measure gas exploration and development, as well as selecting optimal production sections, leading to improved economic benefits through multi gas co-production and the unlocking of resources.