Geological characteristics and reservoir-forming conditions of coalbed methane

Geological characteristics of (1) coalbed methane

1. Coal-bearing strata and coal seams

The coal-bearing strata in Ordos Basin are mainly Carboniferous-Permian and Jurassic. The Triassic coal-bearing stratum is Wayaobao Formation, and only No.5 coal seam is the main minable coal seam, which is only distributed from Zichang to Panlong. The internal subsidence of Ordos block in Benxi period is very small, and the sedimentary thickness is only about 10 ~ 25m. The sedimentary thickness of Taiyuan Formation of Upper Carboniferous is 50 ~100m, and it contains 5 ~ 8 coal layers. The thickness of coal seam varies from place to place. For example, the main minable coal seams of Taiyuan Formation in Hedong Coalfield are No.8, No.9 and 10, with an average total thickness of 6.66 meters ... The area from the south to Xiangning becomes thinner and even unacceptable. The Jingyuan Formation and Yanghugou Formation in the western margin of the basin have large sedimentary thickness, with as many as 50 thin coal seams and coal lines. In Taiyuan period, the depression amplitude decreased, but the sedimentary thickness was still greater than that in the east, and there were more than 0 layers of coal 10, which was one of the main coal-bearing strata. Shanxi Formation of Lower Permian is 60 ~ 100 m thick, which is a thick minable seam. The average total thickness of No.4 and No.5 coal seams in Hedong coalfield is 7.82m. The coal thickness in Weibei coalfield in the south is thinner from east to west, and the No.3 coal seam is generally 0.8 ~ 5m (Figure 6-8 and Figure 6-9).

Figure 6-8 Distribution Map of Carboniferous-Permian Coal Seam Thickness in Ordos Basin

The Jurassic coal-bearing strata belong to Yan 'an Formation, which is divided into 5, 4, 3, 2 and 1 coal formations from bottom to top. There are 5 ~ 7 main minable seams, and the cumulative thickness of minable seams is generally 15 ~ 20m. The main minable coal seams are developed in the south and north of the basin, and only the coal lines are developed in the middle. Coal accumulation is controlled by the lake-delta-river sedimentary system, forming a huge coal accumulation zone around the center of the basin, and the number and thickness of coal seams gradually increase from the coal-free area to the periphery.

Fig. 6-9 Histogram of Carboniferous-Permian Coal Measures in Liulin Area in the Middle Section of Shanxi Flexural Belt

2. Quality characteristics of coal and rock

(1) coal and rock characteristics

The vitrinite content of Shanxi Formation and Taiyuan Formation of Carboniferous-Permian is between 765,438+0%-90%, with an average content of 79%. The vitrinite content of Jurassic Yanan Formation coal varies from 65438 09.4% to 95.2%, with an average of about 58.5%. From the change trend of maceral content in two groups of coal seams, the maceral characteristics of Shanxi Formation and Taiyuan Formation are obviously better than that of Yan 'an Formation. Judging from the macroscopic coal petrographic types of the two groups of coal seams, the Jurassic Yan 'an Formation is hard and dark, and easy to get dirty, while the Carboniferous-Permian Shanxi Formation and Taiyuan Formation are soft and bright, with metallic luster, and their coal characteristics are obviously better than Yan 'an Formation.

(2) Coal quality characteristics

The ash content of Carboniferous-Permian coal in northern and eastern coal-bearing areas has little change, and it is basically medium ash coal. Middle Jurassic coal in northern Shaanxi coal-bearing area is mainly low-ash coal, with ash generally less than 10%, while Huanglong coal-bearing area is low-ash coal. The late Triassic coal in Zichang coal-producing area in northern Shaanxi coal-bearing area is medium ash coal.

Carboniferous-Permian coal seams in Ordos Basin are mainly medium-high metamorphic bituminous coal and anthracite, and the distribution of coal rank in different regions is quite different. In the eastern part of the basin, the coal seam is mainly affected by plutonic metamorphism, and the coal rank gradually increases from north to south. From Junggar coalfield, Hedong coalfield to Weibei coalfield, the vitrinite reflectance increases from 0.65% to 1.95%. From east to west, the coal rank increases with the increase of depth, reaching more than 2.8% in the middle of the basin. In the western margin of the basin, the coal rank distribution is complex. From Shitanjing Mining Area-Shizuishan Mining Area-Hengcheng Mining Area-Weizhou Coalfield syncline to the east wing, there is a low metamorphic coal distribution area with a north-south distribution, and the vitrinite reflectance is about 65438 0%. In the west of this belt, the reflectivity of Mabiantan, Yuanyanghu mining area and vitrinite increases sharply, reaching more than 4% in Rujigou mining area. It reflects that magmatic thermal metamorphism is superimposed on the basis of western plutonic metamorphism.

The metamorphic strength of Jurassic coal is lower than that of Carboniferous-Permian coal. The thermal metamorphism of Jurassic Yanan Formation coal is dominated by regional plutonic thermal metamorphism, and the degree of coalification is low around the basin and high in the middle. The vitrinite reflectance ranges from 0.4 1%- 1.07%, and the coal rank is equivalent to lignite, long flame coal, gas coal and fat coal. The vitrinite reflectance of Qingyang, Heshui and Ningxian areas in the south of Huanxian county in the south-central part of the basin and Wushen Banner and Otog Banner in the north is more than 0.7%. The vitrinite reflectance of Dongsheng, Shanbei, Lingyan and Longdong coalfields is 0.42% ~ 0.6 1%. Huanglong coalfield is 0.5% ~ 0.75%.

3. Gas bearing characteristics

Weibei coalfield in the east and south of Ordos basin and Zhuozhanghe coalfield in the west are the distribution areas of Carboniferous-Permian coalfields. Coal exploration and coalbed methane exploration have accumulated a large number of data of coal seam gas content, as shown in Table 6- 1 1. As can be seen from the table, the gas content of the coal seam in the eastern margin of Ordos Basin increases with the increase of coal rank from north to south, and the gas content increases with the increase of the thickness of the overlying effective stratum. Affected by the thickness of overlying effective strata, the gas content in Weibei coalfield gradually decreases from east to west, and Hancheng mining area is rich in coalbed methane. Zhuozhanghe coalfield is rich in coal and gas.

The Jurassic coal seam in Ordos Basin has low rank, generally low gas content, and high gas content in some areas and deep coal seam. The gas content of coal in Changbin mining area of Jurassic coalfield in Huanglong is 0. 1 ~ 6.29 m3/t, and the gas content of a few coal seams in Huangling mining area and Jiaoping mining area is 4 ~ 6 m3/t. ..

Table 6- 1 1 Carboniferous-Permian Coal Seam Gas Content in Ordos Basin

sequential

(2) Reservoir forming conditions

(1) Coal seam continuously produces gas for a long time, and the gas production rate gradually increases, resulting in a large total gas production.

After the main coal measures of Carboniferous-Permian in Ordos Basin were deposited, they continued to subside for a long time, and the degree of coal metamorphism gradually deepened, resulting in a large number of coalbed methane. A large number of thermal simulation experimental data in the eastern region show that the cumulative gas production rate from lignite to long-flame coal is 138 ~ 168m3/t, that from fat coal is 199 ~ 230m3/t, and that from lean coal is 257 ~ 287m3/t.

(2) The cleavage of coal reservoir, the development of pores, and the development of secondary cracks in structural axis are favorable for coalbed methane production.

The coal and rock in this area are mainly bright-semi-bright, with high vitrinite content, mainly moderately metamorphic fat coal and coking coal, with appropriate metamorphic degree, well-developed cleavage of coal seam, dense favorable area network and good connectivity. According to incomplete statistics, there are two groups of cleavage in coal seam: one group is surface cleavage with density of 7 ~ 25 strips /5cm and crack width of 0.01~ 0.3 mm; The other group is end cleavage with a density of 7 ~ 22 strips /5cm and a crack width of 0.001~ 0.05 mm. Since Mesozoic, under the action of north-south torsional stress and east-west compressive stress, rows of compression-torsion fault nose structures or fault anticline structures have appeared in this area, with a few tensile faults along the structural axis and a series of tensile cracks in the coal seam. These secondary cracks dredge the end cleavage and surface cleavage of coal seam, and make the storage performance of coal seam better.

According to the observation of coal maceral, gas pores are particularly developed in this area, especially in matrix vitrinite, which are mainly gas pores left by gas escaping during coalification, and the pore size is generally 0.01~ 0.7 mm. These pores are not only one of the direct signs of coal seam gas generation, but also the main storage space for coal seam gas adsorption.

(3) The caprock has strong sealing ability and good hydrodynamic conditions, which is beneficial to the preservation of coalbed methane.

After the coal-forming period, the faults and folds of Yanshan movement and Himalayan movement in this area are very weak, and the coal-bearing strata are well preserved. No matter whether the top and bottom of coal seam are limestone or mudstone sealing layer, structural cracks rarely appear in the cores drilled. The variegated mudstone and silty mudstone of the Upper Shihezi Formation and the Lower Shihezi Formation are relatively thick (generally 5 ~ 10m), which has strong comparability and continuity. The porosity of mudstone samples is1.17% ~ 8.11%,and the permeability values are all close to zero, which is the area.

The relevant data of groundwater characteristics of Well Kloc-0/7 in the edge of Ordos Basin were collected. The self-flow of Triassic aquifer is 0.5 ~ 4. 19L/s, the salinity value is 20g/L ~ 60g/L, the hydrochemical type is CaCl2, and Na2SO4 is contained locally. The aquifer gravity flow of Shanxi Formation and Taiyuan Formation coal-bearing strata is 0.9 ~ 8.7 L/s, the salinity is 10 ~ 250 g/L, and the water type is mainly transitional NaHCO3. Majiagou limestone aquifer has a gravity flow of 28.5 ~ 6 1.05 L/s, a salinity of 1 ~ 100 g/L, and the water types are CaCl2 and MgCl2. It can be seen that the groundwater characteristics of these three groups are obviously different, and the hydrogeological characteristics are independent and closed, which is conducive to the preservation of coalbed methane.