Geological survey of coalbed methane in Shibaxiang Zhongnan basin group

Central South Hunan Basin Group is a sedimentary tectonic basin after the transformation and deformation of Paleozoic South China Craton Basin. Carboniferous and Permian coal-bearing series developed, forming Carboniferous-Permian coal-bearing basin.

The southern Hunan basin group is located on the Jiangnan block of the Yangtze block. A set of marine carbonate rocks and clastic rocks were deposited in the South China Craton Basin from Sinian to Middle Triassic. The Fenshui Formation in the middle of Early Carboniferous and Longtan Formation in the early of Late Permian are coal-bearing sedimentary rocks. Jiangnan block uplifted in the early Middle Triassic (Anne period), and the Xianggan Guihai Bay on the east side of the block continued to deposit. Jiangnan block uplifted in the late Middle Triassic (Latin period) and was connected with Cathaysian ancient land. The Yangtze block of Indosinian movement was pushed northward by Yunnan-Tibet plate and Indosinian block, pushed southward by North China block, and strongly squeezed by the paleo-Pacific plate from the southeast. Due to the NW-trending compression in the southeast of the mainland, an arc fold belt protruding westward is formed between the Nanhua block and the Yangtze block. During Indosinian period, magmatism was intense, and granite covered the whole fold belt. During the Yanshan Movement, the South China continent, located in the southeast of the Eurasian continental plate, was collided and squeezed by the unbalanced or alternating surrounding plates, and the deformation at the plate edge and inside the plate was very complicated. Because the land mass has hardened, the main forms of structural deformation are basement wave uplift, crustal melting and pulping and fracture development. From Yanshanian to Himalayan, the compressive stress from the southeast of the mainland not only continued to strengthen the fold deformation, but also developed NNE faults and invaded the granite body, forming the Cretaceous-Tertiary fault depression superimposed on the fold belt. The fold system in central Hunan has been transformed and deformed by Yanshan movement and Himalayan movement, forming a broad and gentle composite fold belt extending from north to south and west. The pre-Triassic strata uplifted by folds were denuded, forming Lianyuan sag, Shaoyang sag, Lingling sag (discontinuous) and Chen Lei sag trapped by Carboniferous strata, and overlapping with Permian strata remaining in the core of Carboniferous composite syncline to form a Carboniferous-Permian coal-bearing basin group.

The coal-bearing strata of the Central South Hunan Basin Group are the Lower Carboniferous Fenshui Formation and the Upper Permian Longtan Formation. In the early Carboniferous, the shallow sea area of Hunan and Guangxi was characterized by epicontinental sea deposits, and the seawater retreated at the initial stage of water measurement, forming a large tidal flat sedimentary environment and peat swamp deposits. In the early Longtan period of Late Permian, the sea area was reduced, forming an open coastal plain with warm and humid climate, lush vegetation and developed peat bogs. Delta plain and coastal lagoon become favorable coal-accumulating facies belts, with the best coal content. Delta, front sandbar and sandbar have also formed a good coal-accumulating facies belt, but due to low terrain, long-term flooding, lack of material sources, coal measures are thin.

The Lower Carboniferous Fenshui Formation is a coal-bearing clastic rock deposit alternating between land and sea, which is continuously deposited with the underlying Shidanzi Formation and the overlying Zimenqiao Formation, and is divided into upper and lower sections by the middle timely sandstone or gravelly Shi Ying sandstone. The lower coal-bearing section is grayish white fine sandstone, grayish black mudstone, sandy mudstone, siltstone and coal seam, with a thickness of 100 ~ 350m, and coal-bearing 1 ~ 8 layers, with 3 and 5 coals developed and a thickness of 0.7 ~ 4.70m m. The upper coal-free section is purplish red, grayish green siltstone, sandy mudstone and mudstone. Sedimentary facies can be divided into four facies areas: Lengshuijiang-Taiping Temple Bay lagoon swamp facies area. At the initial stage of water measurement, there were many islands and underwater uplifts around, which blocked the intrusion of seawater from south to north, and formed a relatively closed weakly reduced coastal lagoon environment, which was a favorable sedimentary environment for coal accumulation. Changsha-Liuyang coastal delta facies area has large subsidence, coarse sandstone and weak oxidation sedimentary environment, which is not conducive to coal accumulation. The coastal facies of Chenzhou-Leiyang continental margin is a coastal environment dominated by weak reduction, which is not conducive to coal accumulation. Shaoyang-Ningyuan coastal facies area is a coastal sedimentary environment with flat paleotopography and strong reduction. Because sediments are underwater for a long time, peat bogs are not developed, which is not conducive to coal accumulation.

Longtan Formation of Upper Permian is divided into upper member and lower member. The lower coal-free section is feldspathic sandstone, siltstone, sandy mudstone, mudstone and carbonaceous mudstone, and the mudstone or sandy mudstone rich in siderite nodules at the top is called "turtle-shaped nodule mudstone", which is the boundary marker layer of the upper and lower sections. This section is located in the south of Shaoyang-Zhuzhou line, with the characteristics of large single-layer thickness, uniform mudstone lithology and loose sandstone, and no coal mining. The thickness of Shaoyang-Youxian line is 66-540 m, and that of southern Hunan 1, 5 1-563m gradually decreases from south to north and from east to west. The upper coal-bearing section consists of gray-dark gray timely sandstone, siltstone, sandy mudstone, mudstone, carbonaceous mudstone and coal seam. This section is divided into "south type" and "north type" by Lianyuan-Zhuzhou line. The southern coal seam section is well developed, with clear cycle and complete structure. The eastern boundary ridge is 220 ~ 6 15 m thick, the central Lei Yong and Baisha are 92 ~ 539 m thick, and the western Shaoyang and Longhui are 22 ~ 6 15 m thick, which gradually decreases from east to west and from south to north. There is a lack or only a very thin coal-free section between the northern coal seam section and the underlying Maokou Formation. Lianyuan is 20 ~ 27 m thick, Xiangtan is 19 ~ 88 m thick, and Liuyang is 43 ~ 52 m thick, which gradually becomes thinner from east to west. Longtan coal measures have different coal-bearing properties due to the difference of sedimentary types between north and south. North briquette 1 ~ 6 layer, maximum 12 layer, exploitable 1 ~ 2 layer. The structure is simple and stable, and the area from Lianyuan to Zhuzhou is the best, with a thickness of 0 ~ 12.94 m, generally 2 ~ 3 m, and the area around Liuyang in the east is relatively thin, generally1m. The briquette in the south varies from place to place, and it is divided into three coal-bearing areas, the coal-bearing area in Chen Lei in the south, with 7 layers of coal and 2 layers of main coal mining, with a thickness of 0.94 ~/. Shaoyang coal-bearing area in the west contains 3 ~ 7 layers of coal, and 2 layers of coal can be mined, with a thickness of 0 ~ 5.82 m. Longtan Formation is a set of coal-bearing deposits dominated by transitional facies, and the coastal plain sedimentary environment after regression is conducive to the development of peat bogs. It can be divided into three facies areas: Changsha-Lianyuan coastal swamp facies area, where the paleotopography is low in the south and high in the north, the lower strata gradually disappear to the north, and the upper strata overlap with Maokou limestone in pseudo-conformity, and the overall topography is flat, with a weak oxidation-weak reduction environment, forming peat swamp deposits conducive to coal accumulation. Loudi-Shaoyang and Liling-Chenzhou Bay lagoon facies areas are blocked by ancient land, ancient island and underwater uplift, with calm waters and lush vegetation, which are strongly reducing environments and are very conducive to coal accumulation. In Qidong-Lingling Qianwan lagoon facies area, due to the water depth, peat swamp is not easy to form, which is not conducive to coal accumulation.

The coal-bearing strata in central and southern Hunan are the Early Carboniferous Fenshui Formation and the Late Permian Longtan Formation, which are widely distributed, with thick coal seams and stable horizons. Wenshui Formation is mainly distributed in Lianyuan, Shaoyang, Lingling and Chen Lei Sags, with Lianyuan Sag being the best and Xiangtan and Ningyuan Sags also being deposited. The coal-bearing series enrichment zone of Fenshui Formation is located in Jin Zhushan and Taiping Temple, with an area of 2000 km2, 1200 km2 and coal-bearing 18 layers. The thickness of No.3 coal and No.5 coal is greater than 1 m, the thickness in the west is 1.6 ~ 5.2 m, and the thickness in the east is 1.2 ~ 4.65438+. The area of Leiyang and Zixing coal-rich belts 150 ~ 700km2 and 1 ~ 2 layers are relatively stable, with a general thickness of 1 ~ 1.7m and a maximum single-layer thickness of1.80m. Longtan Formation is the main coal-bearing series, which is similar to Jianshui Formation. The development of coal measures strata in Longtan Formation is not as good as that in Fenshui Formation, but the coal seams larger than 1 m are widely distributed and the thickness of coal seams is also large, especially in southeastern Hunan. It can be divided into four coal-rich zones: Chenlei coal-rich zone includes synclines such as Baisha, Lei Yong and Mei Tian, with 5 ~ 17 layers of coal, 7 layers in general, and 2 ~ 6 layers of coal seams greater than 1 m, among which 6 layers are generally developed, and the total thickness of coal seams is 0.94 ~ 12.28 m, which is greater than 65438. Chaling-Youxian coal-rich belt includes synclines such as Jiehualong, Huang Feng Bridge and Huatang, with 8-22 coal-bearing layers, generally 10, and 2-8 coal seams larger than 1 m, with a total thickness of 2.55-8.31m. The coal-rich belt is distributed in the northeast. Lianyuan coal-rich belt, including Enkou, Doulishan, Qiaotou River and Coal Dam syncline, belongs to the northern coal-bearing belt, with coal seam 1 ~ 6, up to 12, coal seam thickness 1.2 ~ 5.40m or more, and total thickness1.2. Hongshandian-Shaoyang coal-rich belt includes synclines such as Hongshandian, Niumasi, Baohetang and Short Pit Bridge, with 3-6 coal-bearing layers, 2-3 coal seams larger than 1m, with a total thickness of 2. 1-4.30m, and all coal seams larger than 1m are/kloc-0.

The coal ranks in the central and southern Hunan basin group are developed, ranging from lignite to anthracite, of which anthracite accounts for about two thirds. The composition of water measurement is widely distributed, and it is a single anthracite. Jiangyong has only a small amount of coking coal and lean coal. Longtan Formation has complete coal rank, Chenzhou and Leiyang synclines are anthracite, and Douling syncline in the west is thin coking coal. In Hunan Province, the coal rank is annularly zoned, and five annularly metamorphic belts are formed in Xiangtan, Lianyuan, Shaodong, Shaziping and Qiyang. The edge of the belt is anthracite or lean coal, and the center is low metamorphic fat coking coal. Among them, Shaziping ring metamorphic belt has small scope, monotonous coal rank and high metamorphic degree. Because of the different formation and evolution history of coal-bearing series, the metamorphic degree of coal and rock is different in different coal-bearing basins, even in the same coal-bearing basin. The coal seams of Fenshui Formation are all anthracite with deep burial and high metamorphic degree. The Longtan Formation coal seam is shallow buried, the crust uplifts in the later period, and the influence of magmatic activity on coal and rock has a wide range of coal steps, and the coal steps in Hunan are well developed, and most of them are highly metamorphic anthracite in the southeast of Hunan. The subsidence range of coal seam in Longtan Formation is generally less than 3500m, and the distribution trend of coal rank in the belt is mostly consistent with the overall trend of coal-accumulating sag, but inclined to the folds in the sag. The highly metamorphic coal measures strata are mostly distributed along the uplift area, with high metamorphic degree of coal and rock at the edge of the belt and low metamorphic degree in the center, and intrusive rocks and volcanic rocks are developed around the belt. From this analysis, the metamorphism of coal and rock in Longtan Formation is related to plutonic metamorphism, magmatic thermal metamorphism and tectonic stress metamorphism.

Generally speaking, vitrinite is the main maceral of coal and rock, followed by inertinite. The inorganic composition (> 10%) of the coal seam in Shuishui Formation is higher, the vitrinite content in Longtan Formation is lower than that in Shuishui Formation by about 10%, and the inertinite content is higher 10% ~ 20%. The inorganic composition is mainly clay rock, and the vitrinite content tends to increase with the increase of coal rank.

Semi-bright coal and semi-dark coal are the main types of gas coal; Fat coal is mainly semi-bright coal, followed by semi-dark coal, and locally dark coal; Coking coal has both semi-bright coal and semi-dark coal, with more semi-bright coal; Thin and lean coal is mainly semi-bright coal, followed by bright, semi-dark and dim coal; Anthracite is mainly bright coal and semi-bright coal. The same coal seam is relatively stable laterally, with different coal and rock types in different intervals, and dark coal appears at the top and bottom. Affected by structural changes, coal and rock often appear dark coal or semi-dark coal with high degree of damage. The analysis of coal quality shows that the coal quality of different coal rank is quite different, and the volatile matter decreases with the increase of coal rank. Gas coal is low-medium ash, medium sulfur-rich and low phosphorus coal, chemical fertilizer and coking coal are low-ultra-low ash, low phosphorus and low sulfur coal, lean coal is ultra-low ash coal, and lean coal is generally medium-high ash and medium-high sulfur coal. The anthracite of Shuishui Formation or Longtan Formation is medium ash coal, which has higher ash content and lower sulfur content than bituminous coal.

The coalfield data show that the outburst intensity of Nevas in the area is high, the pressure is high and the times are many. From1965 to1991year, gas outburst occurred for 327 times, coal outburst was 23740t, the maximum outburst intensity was 1020t, and the average gas emission per ton of coal (rock) was11. Since 1975, underground gas drainage has been started in this area, with the maximum annual drainage capacity of 172× 104m3 and the drainage rate of 8.7% ~ 14%. The measured gas content in 204m deep roadway is 2 1.70 m3/t, and the pressure is 2.36mpa.. According to the data in the area, in the section from coking coal to anthracite coal, the well with strong gas outburst is the well with high coalbed methane content. The measured gas content in 7 boreholes and the collected data of gas content in 107 coal boreholes show that the gas content is generally greater than 10 m3/t, generally between 15 ~ 20 m3/t, and the highest is 59.05 m3/t ... For example, the average gas content of anthracite in Shuishui Group is 24.05 m3/t, Lengshuijiang is 16.63 m3/t, Jin Zhushan is 16.63 m3/t and Lumaojiang is18.90m3/t. The average gas content of Longtan Formation is: Aibeiqiao lean coal 19.28 m3/t, and Doulishan coke. According to the enrichment degree of gas in coal seam, it can be divided into high enrichment area, high enrichment area and low enrichment area. Longtan Formation coal seam (south type) is distributed in the south of Lianyuan sag and the north of Shaoyang sag. For example, Hongshantang in Lianyuan sag belongs to thin and lean coal with gas content of18.48m3/t; Aibeiqiao is a lean coal with gas content19.28m3/t; Fengjiang River belongs to anthracite with gas content of 20.31m3/t; Doulishan belongs to coking coal, with gas content of13.73m3/t; Two ponds are fat coking coal, the gas content is 13.86m3/t, and the gas enrichment intensity in this area is 0.3 1× 108m3/km2, which belongs to the gas enrichment area. Matian in Shaoyang sag belongs to anthracite, with gas content of19.30m3/t; Baisha is anthracite, with gas content of19.10m3/t; Yuanjia lean coal, anthracite, gas content of 22.5 m3/t; Salt Lake belongs to anthracite, with a gas content of 20.77m3/t .. The gas-rich intensity in this area is 0.32× 108m3/km2, which belongs to a gas-rich area. Longtan Formation coal seam (north type) is distributed in Lianyuan sag and belongs to gas-rich area, with gas-rich intensity of 0.2 1× 108m3/km2. Yanjiapu is lean coal with gas content19.66m3/t; Qiaotou River belongs to chemical fertilizer coking coal, with gas content of 7.23m3/t; Enkou belongs to chemical fertilizer coking coal, with gas content of 4.60m3/t; Maanshan belongs to coking coal, with gas content of 13. 15m3/t, the coal seam of Shuishui Formation belongs to anthracite, Lianyuan sag has high gas content areas, and Shaoyang and Chen Lei sags have low gas content areas. The gas-rich intensity in Lianyuan sag is 0.5 1× 108m3/km2, the gas content in Zhadu is 24.05 m3/t, the gas content in Taiping Temple is 12.77 m3/t, and the gas contents in Jin Zhushan, Lengshuijiang, Maoyi and Jianxin are all16 ~.

The test results of coal and rock adsorption characteristics show that the methane adsorption range of coal seam is between 22.73% and 47.32% at a constant temperature of 30℃, and the adsorption capacity of coal increases with the increase of coal rank. Isothermal adsorption curves can be divided into three types: one is Doulishan and Qiaotouhe coking coal type, with gentle curve and high Langmuir pressure; Short North Bridge and Hongshandian are lean coal types, with gentle curve, small fluctuation, rising trend at the end and high language pressure; Slag is anthracite type, with steep curve and low Langmuir pressure. The gas content from coking coal to anthracite was measured through three boreholes in Doulishan, Hongshantang, Zhadu and Matian syncline, which has the characteristics of short gas release time and fast desorption speed. The desorption time is generally less than 24 hours, and the proportion of escaping gas to the total gas is relatively high, ranging from 28.4% to 59.2%, with an average of 46.2%.

The storage properties of coal reservoirs include porosity and permeability, and the factors that affect the pore structure of coal and rock are metamorphic degree, coal and rock type, maceral and coal body structure. The matrix porosity and pore size variation chart compiled from the test data shows that the porosity variation range in this area is 0.0 13~0.666 mm3/g mm3/g, and there are two peaks in the range of fat-coking coal and lean-anthracite. The average pore size is about 10000 nm, and there are also two peaks at coking coal and lean coal. The proportion of various pores shows that with the increase of metamorphic degree of coal and rock, the proportion of micropores increases, while the proportion of small pores and large pores decreases correspondingly, and the distribution of mesopores in coke and lean coal tends to increase. Electron microscope observation shows that secondary pores are more developed than primary pores in coal and rock pore types. The main pore types are stomata, dissolution pores, water loss pores, intergranular pores, plant cell cavities and intercellular pores.

The composition of coal and rock and the structure of coal affect the porosity of matrix, and the different contents of vitrinite, inertinite and crust in maceral will affect the porosity of bedrock. No.3 coal in Lengshuijiang syncline has high content of filaments, but low porosity. For coals with the same metamorphic degree, the porosity of bright coal-semi-bright coal-semi-dark coal-dark coal without silk charcoal decreases and micropores are not developed. Compared with other coals, there are more microcracks and smaller openings in vitrinite, bright coal and semi-bright coal in Doulishan syncline. The influence of inorganic mineral composition on bedrock porosity is that low ash content leads to high porosity. The influence of coal structure on bedrock porosity is reflected in the dense cementation of coal matrix, which is in dense block and strip structure and micropores are not developed. Lengshuijiang syncline has a high degree of metamorphism, dense matrix and undeveloped pores. Coal and rock with silty structure have developed pores but are not connected. There are pores and micro-cracks in the coal seam with network fracture structure of Zhadu syncline and Jinzhushan syncline, but gas outburst is easy to occur.

Cracks in coal and rock include endogenous cracks formed in the process of coal and rock metamorphism and exogenous cracks formed by structural stress. The development degree of endogenous fractures is controlled by coal rank, coal quality and coal rock type. The secant linear density (bar /5cm) of coking coal measured in Doulishan syncline is bright coal 45, semi-bright coal 32, semi-dark coal 27 and dark coal 1 1. The measured density of thin and lean coal in Hongshantang syncline shows that bright coal is 35 and semi-bright coal is 20. The measured density of anthracite in Lengshuijiang syncline is bright coal 22 and semi-dark coal 10. The development of exogenous cracks is mainly influenced by the structure of coal. The exogenous fractures in Hongshandian syncline Shaba mine field are long and run through the whole coal seam.

Coal permeability is an important condition for coalbed methane enrichment. In view of the present situation of exploration and research, the permeability of coalbed methane has not been directly tested in this area, and the permeability coefficient tested in mining area is mainly used as the reference value. The tests of Hongshandian syncline in Lianyuan sag and Baisha syncline in Chen Lei sag show that the permeability is low, which is 0.0 1× 10-3μm2. After fracturing, the permeability of coal seam can be increased by dozens or even hundreds. The permeability coefficient (m2/ATM 2 day) of No.4 coal seam in Hongshandian syncline is 2.2×10-3 ~ 3.07×10-2, and the equivalent permeability is (0.55 ~ 0.768 )× 10-5 μ m2. After coal seam liberation, the permeability is (0.55). The permeability coefficient of Baisha syncline is (2.40 ~ 4.72) × 10-3, and the equivalent permeability is (0.6 ~1.18 )×10-5 μ m2. After liberation, the coal seam permeability is 4.86×/kloc. The permeability of coal and rock is controlled by many geological factors, including the degree of coal and rock metamorphism, in-situ stress, hydrogeological conditions and tectonic movement. The regional permeability of Lianyuan sag shows that cleavage and cracks in Longtan coal seam are more developed and have higher permeability than Jianshui anthracite and Longtan anthracite in Chen Lei sag. The permeability of different parts of the fold structure is different, and the cracks in the axis of the anticline structure are developed and the permeability is relatively good, but the coal seam in the anticline is often eroded. Cracks are developed near the fault zone, and hidden faults and tensile cracks are developed in the eastern part of the sag. The groundwater activity in the southern coal-bearing basin of Longtan Formation is weak, which has little effect on the permeability of coal seam, and the permeability between the northern coal-bearing basin and groundwater layer has improved. The whole depression has good permeability in synclines such as Hongshantang, Doulishan and Enkou in the central and eastern part. The anthracite in Shuishui Formation is mostly powdery and scaly structured coal with poor permeability.

As a coalbed methane reservoir, there are many factors that affect its dynamics. According to the comprehensive factors such as gas production, coal structure, porosity and permeability, adsorption performance and hydrogeology, coal reservoirs in this area are divided into five types.

Type I briquette is coke and lean coal, and its reservoir performance is the best. It is massive coal, with moderate degree of fragmentation and metamorphism, large porosity, well-developed cleavage, short extension of exogenous cracks, fragile coal body and more fragmentation components, which is easy to protect. The gas yield and adsorption performance of coal are good, the gas content is 15 ~ 20m3/t, and the isothermal adsorption curve is gentle. Hydrogeological conditions are simple-complex.

ⅱ briquette is coke and lean coal, which has good storage performance. The inverted wing or steep wing developed in the basin has a low degree of structural damage, good coal protection and less broken components, mostly massive to broken coal. The porosity is large, cleavage is developed, the density of exogenous fractures is large, the extension is long, and most of them span the whole coal seam, with good permeability and easy protection. Coal and rock have large gas production and strong adsorption performance, and the gas content is about 10 m3/t, and the isothermal adsorption curve is in good shape. Coal seam has simple hydrogeological conditions and is a good reservoir.

Ⅲ briquette is anthracite with good reservoir performance. The degree of coal and rock fragmentation is low, from massive to fragmented, which is easy to protect. The metamorphic degree is high, and the average value of Ro is 4.395%. The porosity of bedrock is about 6%, mostly fractured pores. Micro-cracks are not developed, exogenous cracks are developed, and permeability is good. Coal seam has large gas production, strong adsorption capacity, poor storage performance and simple hydrogeological conditions.

Ⅳ briquette is gas-fat coal, and its reservoir performance is average. The coal body structure is good, mainly blocky, with moderate and low metamorphic degree, porosity less than 5%, good isothermal adsorption curve shape, coal seam gas content less than 10%, and complicated hydrogeological conditions.

ⅴ briquette is thin anthracite with poor reservoir performance. High metamorphic degree, poor coal structure and poor preservation conditions, mostly powdery coal. The porosity of bedrock is greater than 10%, but the pores are not connected with each other, with few external cracks and poor permeability. The gas production is large, the adsorption capacity is strong, the gas content is more than 20m3/t, the coal is difficult to protect, the gas outburst is serious, the isothermal adsorption curve is steep, and the critical desorption pressure is low. The water-rich property of coal seam is weak, and the hydrogeological conditions are simple.

The prospective resources of coalbed methane in central and southern Hunan basin group are calculated. This parameter selection is based on the basic data of coal reserves, with the single-layer minable coal greater than 0.6 m as the basic unit, the coal seam buried depth of 300 ~ 1500 m as the calculation range, and the coal seam gas content adopts seven measured coal core data and 107 drilled coal core desorption method test data. There are 30 blocks in the election, including coal and rocks. Jianshui Formation has 8 basins and Longtan Formation has 22 basins. The calculation results show that the coalbed methane resources are1171.79×108m3, of which the Longtan Formation resources are 546.22× 108m3 and the horizontal group resources are 625.57× 108m3. From the analysis of resource abundance, there are 16 basins such as Hongshantang, Baisha, Lengshuijiang, Zhadu, Doulishan and Matian, which are more than 0.3× 108m3/km2. (0.2 ~ 0.3) × 108m3/km2 is the four basins of Liangshitang, Qiaotouhe, Enkou and Maanshan, and the others are less than 0.2× 108m3/km2.

Hongshandian syncline is a Permian coal-bearing basin in Lianyuan depression in central Hunan. Zhongnan Petroleum Bureau, an emerging oil company, drilled the CBM exploration well Xiangmei 1 in 1993, and evaluated the geological conditions of CBM in the Central South Hunan Basin Group. The well Xiangmei 1 was drilled in 1 June, 1993, with a depth of 626 meters, and1and 4 coal seams were drilled. The total thickness of 1 coal is 3.6 m, the tested gas content is 18.46 m3/t, and the thickness of No.4 coal is 0.4 m, so 1 coal is the target layer, and open hole completion is conducted. 1994 65438+1mid-October investigation, calculate instantaneous gas production1781.1m3/d, open hole cave completion,1April 1994, gas production of 5624.

Hongshandian syncline, located at the eastern end of Lianyuan sag, is a NE-trending compound syncline. Yang Meiling anticline divides it into two synclines, with Li Yutang syncline fault in the west and Nantang syncline in the east being gentle in the north and steep in the south. The depression depth (4 coal) is about 1350 m, and the coal-bearing area is 43 km2. The coal-bearing series is the Longtan Formation of Late Permian, which is a coal swamp deposit formed by the delta facies of land and sea. Longtan Formation is 400~470 meters thick and divided into upper and lower coal-bearing sections. The upper coal-bearing section is 122 m thick, with 5 layers of coal, 3 layers of coal can be mined, 4 coal is the main coal seam, and 1 and 3 coal are partially recoverable. Except Pengjiachong, the coal thickness in the whole region is stable, with an average thickness of 1.84m, mainly low-sulfur and low-ash bright coal, and the coal body is mainly broken coal with a small amount of lump coal.

The coal seam in this area has good reservoir performance, and the bedrock pores are developed, with porosity greater than 5%, generally reaching 8%, and the local porosity is 1 1.9%. The pore volume is 0.0658 m3/g, the pore area is 14. 147 m2/g, the average pore size is 12300 nm, and the mesopores (10000 ~ 100000 nm) account for 34. Small pores (1000 ~ 10000 nm) account for 19.47%, and large pores (> 100 micron) account for 12.24%. The bedrock pores are mainly primary pores. According to the development of coal seam cleavage, the linear density of cleavage in bright coal face is 35 ~ 40 /5 cm, and the length and opening are larger, with 4 coal being better. Measurement of coal seam permeability shows that 1 coal permeability is 5.43× 10 -6μm2, 3 coal is 1.88× 10 -6μm2, and 4 coal is 5.1×10. After the coal seam was liberated, the No.3 coal increased to 1.3× 10 -3μm2, and the No.4 coal increased to 0.73× 10 -3μm2.

According to the gas content in coal seam and the results of gas drainage by drilling holes in coalfield, the average gas content of 1 coal is 18.24 m3/t, that of No.3 coal is 14.8 m3/t, and that of No.4 coal is 22.02m3/t .. It is 1 measured from Meimei coal 1 well. When the depth of the coalfield drilling well is 28 1.29 m, the coal seam pressure is 2.77 MPa ... The gas drainage amount of Shexingshan mine field is 180× 104m3/a, and the initial gas flow of Xiangmei 1 well is178/kloc-. After drainage for 3 months, the gas production is 24 19 m3/d, and the calculated instantaneous production is 5624m3.

Hongshandian syncline minable coal seam reserves 8.1144×104t, predicted reserves 9700× 104t, total reserves 1.78× 108t, and average gas content of coal seam/kloc.

China Petroleum Group drilled a cold well test 1 in Lengshuijiang, Lianyuan Depression on 1994. At the depth of 550~565 m, 3 coal and 5 coal of Carboniferous Fenshui Formation were drilled with the thickness of 1.6 m and 1.4 m respectively, and the Ro of coal and rock was 3.97 1% and 4.099% respectively. The porosity is 8. 13% ~ 13.58%, the permeability is (2.3 ~ 2.4) × 10-3 μ m2, and the gas content of No.3 coal is17.33 ~18.4m3/. The coal seam section is lined with slotted open hole completion mixed layer sand fracturing, and the gas test output is 200m3/d. ..

Consult the Atlas of China Coalbed Methane Basin, Geological Map of Central South Basin Group, Lithofacies Palaeogeography Map of Central South Basin Group in Early Carboniferous Period of Water Measurement, Lithofacies Palaeogeography Map of Central South Basin Group in Late Permian Period of Longtan Period, Metamorphic Degree Map of Coal and Rock of Upper Permian Longtan Formation of Central South Basin Group, Classification Table of Coal Reservoir Types of Central South Basin Group, Table of Isothermal Adsorption Parameters of Different Coal Steps of Central South Basin Group and Table of Coal Adsorption Parameters. Parameter Table of CBM Resources in Central South Basin Group, Column Profile of Upper Permian Longtan Formation in Central South Basin Group (North Type), Column Profile of Upper Permian Longtan Formation in Central South Basin Group (South Type), Column Profile of Lower Carboniferous Liushui Formation in Central South Basin Group, D2-J 1 Column Profile of Central South Basin Group, and Geological Map of Hongshantang Syncline. Comprehensive coal seam interpretation of Xiangmei 1 Well, coal seam parameter table of Xiangmei 1 Well, initial drainage table of Xiangmei 1 Well, comparison table of well test data of Xiangmei 1 Well and Well 2, and histogram of Hongshandian Syncline.