Figure
1.
Geogram of coal-bearing strata of SQB.
| Citation: | Zheng Zhang, Yong Qin, Xuehai Fu, Zhaobiao Yang, Chen Guo. Multi-layer superposed coalbed methane system in southern Qinshui Basin, Shanxi Province, China. Journal of Earth Science, 2015, 26(3): 391-398. doi: 10.1007/s12583-015-0541-z
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Coalbed methane (CBM) reservoiring process under condition of multi-coalbed developing has unique characteristics (Lan et al., 2012; Qin et al., 2012, 2008; Yang, 2011). Multiple coal seams have developed in coal-bearing strata of Carboniferous–Permian in Qinshui Basin, one of the two main CBM surface scale-development bases in China. The CBM resource of Qinshui Basin mainly occurs in Shanxi Formation of Lower Permian and Taiyuan Formation of Upper Carboniferous, and coal seams No. 3 and No. 15 are dominant occurrence coal seams. The CBM geological resource of Taiyuan Formation occupies 60% of that of the entire basin (Che, 2006); however, CBM development in this area is currently over-dependent on the gas recovery of the single coal seam No. 3 (Lü et al., 2012; Ye et al., 2009). Although exploratory tests of double-layer (multi-layer)-combined drainage were carried out (Li and Li, 2012; Ni et al., 2010a; Zhang and Tong, 2007), most of the effects were unsatisfactory (Zhang et al., 2011; Mu et al., 2009). The reason lies in the lack of knowledge about characteristics of vertical fluid pressures of multi-layer CBM system and their matching relationships (Qin et al., 2013; Seidle, 2011). Therefore, the southern Qinshui Basin (hereinafter referred as SQB), was taken as a case in this paper, in which the vertical development characteristics of CBM system were discussed. This paper is expected to provide reference for the efficient exploitation of the CBM in Taiyuan Formation.
The concept of the CBM system has not yet reached a consensus at present. Liu et al. (1998) considered that CBM system included coalbed methane, coalbed, cap rock, overlying rock and all geological processes and their reasonable time and space configuration during the formation of CBM reservoir. Ayers (2002) held that CBM system differed from conventional petroleum system in several ways, including source rock and gas origin, migration paths, and gas storage and trapping mechanisms, yet he did not provide a clear definition. Ye (2002) introduced the idea of the "groundwater system" into the field of CBM geology to analyze the relationship between groundwater and CBM from the viewpoint of "hydrodynamic system", based on which the concept of "gas-water two-phase flow system of coal reservoir" was put forward. He suggested that the coal seam and aquifers that had hydraulic connection with the coal seam were a whole, and they were a water- bearing rock series with a unified aquifer system; the underground water presented unified spatiotemporal ordered structure in this system.
Taking the concept of "petroleum system" defined by Magoon and Dow (1994) as reference, Zhu et al. (2006a) thought that CBM system is a natural system composed of coal seam, coalbed methane and all the essential geological elements and processes to form a CBM reservoir. Zhu et al. (2006b) further pointed out that a CBM system could be a single coal seam or several coal seams (coal seam group) with the same hydrodynamic contact, but it must have a relatively independent hydrodynamic system. Qin et al. (2008) deemed that CBM reservoir was a basic geological unit storing considerable quantity of natural gas, and a unit of coal-rock mass within the same fluid pressure system; thus, they put forward the academic viewpoint of "unattached multiple superposed CBM systems". Qin et al. (2008) held that "unattached superposed CBM systems" were superposed in the vertical direction, but due to the short of the exchange between gas and water among different CBM systems, the pressure coefficient and gas-bearing properties became relatively independent, presenting fluctuated change in the vertical direction. Qin (2012) further pointed out that CBM system was an energy dynamic equilibrium system, whose reservoiring process was a geological selection process, and the fluid pressure system gradually adjusted itself during the configuration process.
Based on previous discussions about the definition of CBM system, the authors think that: the essence of an unattached CBM system is to possess a unified fluid pressure system, which is composed of four key elements: firstly, gas-bearing coal-rock mass, namely, CBM reservoir; secondly, formation fluid, including CBM, coal seam water, and other formation water hydraulically connected with coal reservoir; thirdly, independent hydrodynamic system; fourthly, the surrounding sealing condition of the system: the upper and the lower are often capping layers, and the lateral is often the lithology sealing caused by sedimentary phase transition. Based on the above understanding and taking SQB as a case study in this paper, the vertical distribution characteristics of CBM system under the condition of multi-coalbed developing are discussed.
Coal-bearing strata in SQB mainly include Taiyuan Formation of the Upper Carboniferous and Shanxi Formation of the Lower Permian. The thickness of the strata varies from 132.44 to 166.33 m, with an average of 150 m; the strata contains more than 10 seams of coal with a total thickness of the coal seams ranging from 3.65 to 23.8 m (Fig. 1). Taiyuan Formation was formed in transitional facies depositional environment and is mainly composed of medium and fine-grained sandstones, siltstone, mudstone, limestone and coal seam; the thickness of Taiyuan Formation is between 76 and 127 m, most of which is around 100 m; the sedimentary cycle is either complete or incomplete. Shanxi Formation was formed in the near-shore-delta depositional environment and is mainly composed of sandstone, sandy mudstone, mudstone and coal seam; the thickness is between 34 and 59 m (Qin et al., 2012). The primary target coal seams of CBM exploration and exploitation in this region are coal seam No. 3 (the upper main coal seam) of the Shanxi Formation and coal seam No. 15 (the lower main coal seam) of the Taiyuan Formation; the thickness of the two seams is large, and the lateral distribution is stable.
Coal seam No. 3 is in the lower part of the Shanxi Formation and lies above K7 sandstone (Fig. 1). The thickness of this seam varies from 5.07 to 7.22 m, with an average of 6.03 m. It is mainly composed of bright coal, often with 1–3 layers of mudstone or calcareous mudstone interbeds. The roof rocks mainly consist of mudstone and silty mudstone, while fine and medium-grained sandstones are found locally. The floor rocks are mostly siltstone and mudstone.
Coal seam No. 9 which is above K4 limestone is located at the bottom of the Group Ⅲ of Taiyuan Formation. The distribution of the thickness is unstable: the seam is thicker in the southern part, splitting and thinning out locally. The thickness of this coal seam in Panzhuang region varies from 0.2 to 1.9 m, with an average of 1 m; the thickness in Fanzhuang region ranges from 0 to 2.5 m, with an average of only 0.47 m. The roof rocks are mainly mudstone, siltstone, while fine and medium-grained sandstones or limestone appears locally. The floor rocks are mostly mudstone and siltstone.
Coal seam No. 15 is situated in the lower part of the Taiyuan Formation and widely distributed in the whole region. The thickness varies from 2.06 to 3.8 m, with an average of 2.59 m; this seam generally includes 3–6 mudstone or carbonaceous mudstone interbeds. The immediate roof rocks are mainly mudstone or calcareous mudstone, and K2 limestone is often regarded as the immediate roof as well. The floor is mostly mudstone.
The study area is located in the rising end of the south of Qinshui syncline, and the overall structural setting is a monoclinic structure, dipping to the northwest (Fig. 2). The eastern edge of the area is NNE trending Yihou Mountain fault-fold belt; the western edge is NE trending Sitou-Tuwo arc-shaped fault system; the southern boundary is near-EW trending Heling-Nanling fault system; the northern boundary is NW trending Hexi fault zone. A series of gentle subsidiary folds with axial trends towards NNE, NE and SN are widely distributed in the area. The formation dip is generally 5°–15°, and both wings are basically symmetry.
According to the adsorption principle of CBM, in a unified reservoir pressure system, the gas content of the coal seams appears an ascending or descending (under the critical depth of absorptive saturation) law with the burial depth increasing or stratohorizon dipping and the coalbed reservoir pressure rising (Qin et al., 2005). However, the phenomenon that the change of gas content is inconsistent with the "adsorption principle" or presents "fluctuated" change in nature is not rare (Ye et al., 1999), especially in the area with multi-coalbed occurrence (Qin et al., 2012, 2008; Yang, 2011; Zhao et al., 2010).
On the basis of the desorption data of nearly 40 wells in SQB, the gas content (air dried basis, the same below) of coal seam No. 3 is generally lower than that of coal seam No. 15, which differs from northern and central region of Qinshui Basin and even other area of central and southern parts of Late Paleozoic Era basin of North China (Fig. 3). The main reason lies in that there are differences between the two coal-bearing sections in terms of hydrodynamic, roof and floor conditions. The roof rock of coal seam No. 15 is mainly limestone formed in carbonate platform environment; the closure effect of limestone on CBM is much weaker than that of mudstone on CBM. However, in southern Qinshui Basin, the fractures development of limestone is poor, and most of the fractures are compressed and filled, which is conducive to CBM preservation, making the preservation condition of coal seam No. 15 excel that of coal seam No. 3 (Ni et al., 2010b; Liu et al., 1998). In addition, the special hydrodynamic conditions of "hydrodynamic trap" of coal-bearing strata in southern Qinshui Basin also result in that the gas content of coal seam No. 15 is higher than that of coal seam No. 3 (Qin et al., 2012). That is, the increase of burial depth is not the primary cause for the higher gas content of the lower coal seam.
In addition, theoretically, the gas content of coal seams should be No. 3 < No. 9 < No. 15 with the increase of burial depth. However, according to the desorption data (Li et al., 2010) of drilling coal cores of CBM wells of Chengzhuang Block in southern margin, only 8 of the 25, less than 1/3 of the total number of wells, comply with this rule (Fig. 3b). Further analysis indicates that a turning point of the changing trend of the gas content with the dropping of layers emerges around coal seam No. 9, and the gas content presents a "fluctuated" change which first decreases and then increases or first increases and then decreases (Fig. 3b). It implies that in the same vertical well coal seam No. 9 may not be controlled by the same reservoir fluid pressure system that controls the overlying coal seam No. 3 or the underlying coal seam No. 15, that is, multiple gas-bearing systems exist in the vertical direction.
Generally reservoir pressure gradient curve is a straight line, but different hydrodynamic systems correspond to different curves (Li, 2007). Furthermore, in the same CBM system, the formation fluid pressure is unified, and the slope of burial depth-reservoir pressure curve appears constant; in contrast, between two superposed (independent) fluid pressure systems in the vertical direction, the relationship curve of burial depth and reservoir pressure presents non-linear or "jump" change.
Well test data of 40 CBM wells in SQB, acquired through injection/fall-off well test method (Cui and Zheng, 2009), show that the reservoir pressure of coal seam No. 15 is significantly higher than that of coal seam No. 3 in the same vertical well, with the exception of well g14 (Fig. 4). Through comparative analysis of pressure gradient of the two main coal seams (Fig. 5), it is discovered that the pressure gradients are overall low in this region, and the reservoir pressure gradients of coal seams No. 15 and No. 3 in the same vertical well are not equal in most cases, where the former is evidently higher.
To further discuss the vertical distribution characteristics of multi-coalbed reservoir pressure state, a new geological variable named equivalent reservoir pressure gradient is introduced in this paper. It represents reservoir pressure increment of unit burial depth drop between adjacent coal seams, which can quantitatively describe the variation characteristics of continuity of reservoir pressure in the vertical direction. The equivalent reservoir pressure gradient is represented by Pg
|
|
(1) |
where Pg stands for equivalent reservoir pressure gradient, in MPa/hm; P2 is reservoir pressure of the lower main coal seam, in MPa; P1 is reservoir pressure of the upper main coal seam, in MPa; H2 represents burial depth of the lower main coal seam, in m; H1 represents burial depth of the upper main coal seam, in m.
If the two fluid pressure systems of the coal reservoir in the vertical direction are in the same CBM system, Pg should maintain better consistency with the reservoir pressure gradient of the upper coal seam. However, it can be seen that there exists significant difference between Pg and coal seam No. 3 from their comparison chart (Fig. 6), where Pg is higher evidently, showing that the reservoir pressure gradient takes on obvious "jump". That is, although the reservoir pressure meets the general rule of rising with the depth increasing, the relation between the reservoir pressure and the burial depth is a non-linear one, indicating that the pressure coefficients of coal seams No. 3 and No. 15 are inconsistent. On condition that the reservoirs are not connected (proved in the part of "Discussion"), it is indicated that coal seams No. 3 and No. 15 may not be in the same reservoir pressure system.
Meanwhile, from the test data of the only Well P2 (a well in Panzhuang Block) that has been tested on coal seam No. 9, it is found that the reservoir pressure gradient of coal seam No. 9 reaches 1.2 MPa/hm, while its underlying coal seam No. 15 is only 0.9 MPa/hm. The former is in abnormally high pressure state, while the latter is close to the normal pressure state; this relation does not conform to the general law, and it takes on the "jump" change, manifesting that they belong to different reservoir pressure systems.
Multiple sets of aquifers develop in the coal-bearing strata in the vertical direction. The mudstone, distributed in the top of Shanxi Formation, and the aluminum mudstone, distributed in the bottom of Taiyuan Formation, cut off the hydraulic connection between coal-bearing strata of Carboniferous–Permian and its overlying and underlying aquifers, making the hydrogeological system of coal-bearing strata relatively independent and the water-abundance of the coal-bearing strata generally weaker in the study area. The data of drill pumping test show that the water inflow ranges from 0.012 to 0.283 L/s, unit water inflow from only 0.000 11 to 0.019 8 L/(s·m), permeability coefficient from 0.000 65 to 0.1 m/d (Table 1).
| Coalfield | Bore No. | Aquifer | Static level (depth/elevation) (m) | Unit water inflow (L/(s·m)) | Water inflow (L/s) | Permeability coefficient (m/d) |
| Daning | 031 | Shanxi Formation | 116.39/593.36 | 0.000 145 | 0.001 5 | |
| Taiyuan Formation | 170.35/539.15 | 0.000 111 | 0.000 65 | |||
| Fanzhuang | 0404 | Shanxi Formation | 9.15/703.02 | |||
| Taiyuan Formation | 0/683.00 | |||||
| Sihe | 102 | K5 limestone | Dry hole | 0.001 3 | 0.06 | 0.096 |
| K2+K3 limestone | 142.63/669.15 | 0.019 8 | 0.283 | 0.051 | ||
| Panzhuang No. 1 | 9-5 | K8 sandstone | 68.45/558.23 | 0.000 4 | 0.061 | 0.002 |
| K2+K3 limestone | 72.11/554.57 | 0.000 8 | 0.101 | 0.008 | ||
| 7-3 | K8 sandstone | 563.45 | Pump dry | |||
| K5 limestone | 57.93/547.59 | 0.000 12 | 0.007 | |||
| 317 | K8 sandstone | 52.16/569.98 | 0.000 5 | 0.023 | 0.073 | |
| K5 limestone | 82.35/539.79 | 0.000 3 | 0.012 | 0.031 | ||
| K2+K3 limestone | 56.55/565.59 | 0.000 4 | 0.12 | 0.10 | ||
| Panzhuang No. 2 | 0606 | K2+K3+K5 limestone | 160.21/527.36 | Pump dry | ||
| K7+K8 sandstone | Dry hole |
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From the analysis of the coal-bearing strata itself, it can be seen that the vertical aquifers and aquicludes are separated with each other, and in the macroscopic view, the combination of them belongs to parallel composite structure type. The hydraulic connection between aquifers is cut off by the intervening aquicludes; generally, if there is no connecting fault, there is no hydraulic connection between them (Fig. 1). The main water- filled aquifer of coal seam No. 3 of Shanxi Formation is its roof K8 sandstone fractured aquifer. There are multiple sets of marine limestone in Taiyuan Formation, and among them, K2, K3, K5 and K6 limestone aquifers are relatively stable. K2 and K3 limestone aquifers lie in the lower of Taiyuan Formation and are the main water-filled aquifers of coal seam No. 15; K5 and K6 limestone belong to the upper aquifers of Taiyuan Formation and lie above coal seam No. 9.
Through further analysis of the drill pumping data, it is found that in the same drill, the water level of aquifers of Shanxi Formation and Taiyuan Formation is not unified. There are also obvious differences among several major sets of limestone aquifers of Taiyuan Formation (Table 1). For example, the height of hydraulic head of K8 sandstone, K5 limestone, and K2+K3 limestone were observed in subsection through No. 317 bore in Panzhuang Block, and significant differences were found in the water level elevation of these three water-bearing segments (Fig. 7). Thereby, it is indicated that there is almost no hydraulic connection among the three layers mentioned above, in which No. 3, No. 9 and No. 15 coal seams belong to different water-bearing systems respectively.
The coal reservoir pressure can be generally measured by the height of hydraulic head of the direct water-filled aquifer of the reservoir itself (Wu et al., 2008), therefore
|
|
(2) |
where h2 stands for the height of hydraulic head of the roof aquifer of the lower main coal seam, in m; h1 represents the height of hydraulic head of the roof aquifer of the upper main coal seam, in m.
Through the analysis of Eq. (2), it can be seen that the equivalent reservoir pressure gradient (Pg) can, to some extent, indirectly reflect the hydraulic connection between the upper and the lower main coal seam. If Pg is equal to hydrostatic pressure gradient, namely, Pg=ρg, then h2–h1=ΔH≈Δh, which means the upper and the lower aquifers possess the same water level elevation, reflecting that there may exist a certain hydraulic connection between them. However, Pg is less than the hydrostatic pressure gradient (0.978 MPa/hm, fresh water) as a whole in the study area (Fig. 6), namely, h2–h1 < Δh, showing that the water level elevation of the lower aquifer is overall lower than that of the upper aquifer, which is agreed with the data of drill pumping test in Table 1. Meanwhile, there are some points at which Pg is greater than the hydrostatic pressure gradient, which suggests that the water level elevation of the lower aquifer is higher than that of the upper aquifer in local area. In short, the water levels of the two coal-bearing sections are not consistent.
The distribution characteristics of gas content, reservoir pressure and hydraulic head discussed above show that there exists no fluid connection among No. 3, No. 9, and No. 15 coal seams. This may constitute three sets of relatively independent CBM systems. In the vertical direction, they are mutually superposed and independent, but jointly controlled by sedimentary and structural conditions.
Carboniferous–Permian coal-bearing strata in SQB were formed in the interactive marine & terrestrial depositional environment, including offshore carbonate shelf, barrier-lagoon and lower delta plain sedimentary system from bottom to top (Shao et al., 2008). Taiyuan Formation is generally divided into three segments by lithological association: the lower segment mainly consists of mudstone, siltstone, fine-sandstone and coal seam; the middle segment is mainly composed of bioclastic limestone, marlite, mudstone and thin coal seam; the upper segment is mainly formed of fine-sandstone, siltstone, mudstone, limestone and thin coal seam (Fig. 1). Shanxi Formation mainly includes sandstone, sandy mudstone, mudstone and coal seam. That is, tight mudstone and silty mudstone aquicludes with varied thickness, distributed among the major coal-bearing sections of coal-bearing stratum in this region, block the vertical fluid exchange, which is the depositional controlled reason of the formation of multi-layer superposed CBM system. In other words, the sedimentary process lays the material foundation for the formation of multiple superposed CBM system.
Tectonic stress field has control action on water and gas conductivity of the fault (Li et al., 2013). The large faults are mostly located at the boundary of the study area; however, they are quite rare within the boundary faults (Fig. 2). After coal forming period of Late Paleozoic Era, SQB was heavily influenced by the tectonic stress fields of Indosinian, Yanshanian and Early Himalayan, resulting in the formation of the main body of current tectonic framework (Qin et al., 2012, 1999; Liu et al., 1998). Horizontal compression tectonic stress field of SN direction during the period of Indosinian brought the reverse faults and folds trending nearly EW into existence. During the period of Yanshanian, tectonic stress field was manifested as horizontal compression of NNW-SSE direction, forming a series of large-scale reverse faults and folds trending NNE-NE. During the period of Early Himalayan, tectonic stress field was manifested as horizontal compression of NE-SW direction, which generated a few small size folds and fractures, trending NW and superposed on the NEE-NE structure of the earlier stage. Since Neogene, tectonic stress field has been mainly manifested as horizontal compressive stress of nearly NEE-SWW (Meng et al., 2010). Compressive stress field of NEE-SWW direction is perpendicular to the NNW-SSE faults that had been formed inside of the coalfield; therefore, most of the faults are basically closed. Influenced by the NEE-SWW compressive stress field process, the NE-NNE faults, widely distributed close to the boundary, while being affected by extrusion process, may also have strike-slip property. They are in a relatively closed state, and the characteristics of blocking gas and water are significant. Take Sitou fault at the western edge as an example, although its structural type appears as a normal fault, exploration results have found that, when drilling in the fault fractured zone, no major changes occur in the water level, and no corrosion happens on the calcite filled in the shatter breccia fracture. Meanwhile, obvious differences on gas-bearing property of coal seams and hydrogeochemical properties of aquifers between both sides of the fault were found (Qin et al., 2012). Take gas content as an example, it can reach more than 30 m3/t in Daning No. 2 coalfield and Panzhuang coalfield at the eastern side of Sitou fault, while it is usually less than 15 m3/t under the same burial depth at the western side. Another example is the southern section of Jinhuo fracture, which constitutes the eastern boundary, the difference on the water level of its two sides can reach more than 50 m; furthermore, the west side is the fully dissolved district of calcite, gypsum and dolomite, while the east side is the precipitation zone of calcite and the dissolution zone of gypsum and dolomite, and the difference between the two sides is also significant. In short, the tectonic stress field in SQB was mainly compressional stress since coal seams had formed, and the tectonic activities after coal forming period did not play a connecting or damaging role in the CBM systems; the sedimentary control is dominantly responsible for the formation of vertical independent CBM system.
(1) The essence of an unattached CBM system is to possess a unified fluid pressure system, which includes generally- speaking four key elements, namely, gas-bearing coal-rock mass, formation fluid, independent hydrodynamic system, and sealing condition.
(2) The gas content of main coal seams in SQB presents a change rule of "non-monotonic function" with the increase of depth, and a turning point of the change appears nearby No. 9 coal seam, which indicates coal seams of No. 3, No. 9 and No. 15 belong to different gas-bearing systems. Well test reservoir pressure gradient of coal seam No. 15 is obviously higher than that of coal seam No. 3, and equivalent reservoir pressure gradient of No. 15 coal seam "jumps" significantly compared with the reservoir pressure gradient of No. 3 coal seam, that is, the relation between reservoir pressure and burial depth appears a characteristic of discontinuity. The vertical hydraulic connection among the aquifers of Shanxi Formation and Taiyuan Formation is weak, in which coal seams No. 3, No. 9 and No. 15 belong to different water- bearing systems.
(3) The formation of vertical independent CBM systems is jointly controlled by sedimentary and tectonic conditions, of which sediment control is the dominated one. Sedimentary process lays the material foundation for the formation of multi- layer superposed CBM system. Since Neogene, the tectonic stress field of SQB has been mainly manifested as compressive stress, most of faults are closed, which does not impact significantly on the CBM system.
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Zheng Zhang, Yong Qin, Xuehai Fu, Zhaobiao Yang, Chen Guo. Multi-layer superposed coalbed methane system in southern Qinshui Basin, Shanxi Province, China. Journal of Earth Science, 2015, 26(3): 391-398. doi: 10.1007/s12583-015-0541-z
| Coalfield | Bore No. | Aquifer | Static level (depth/elevation) (m) | Unit water inflow (L/(s·m)) | Water inflow (L/s) | Permeability coefficient (m/d) |
| Daning | 031 | Shanxi Formation | 116.39/593.36 | 0.000 145 | 0.001 5 | |
| Taiyuan Formation | 170.35/539.15 | 0.000 111 | 0.000 65 | |||
| Fanzhuang | 0404 | Shanxi Formation | 9.15/703.02 | |||
| Taiyuan Formation | 0/683.00 | |||||
| Sihe | 102 | K5 limestone | Dry hole | 0.001 3 | 0.06 | 0.096 |
| K2+K3 limestone | 142.63/669.15 | 0.019 8 | 0.283 | 0.051 | ||
| Panzhuang No. 1 | 9-5 | K8 sandstone | 68.45/558.23 | 0.000 4 | 0.061 | 0.002 |
| K2+K3 limestone | 72.11/554.57 | 0.000 8 | 0.101 | 0.008 | ||
| 7-3 | K8 sandstone | 563.45 | Pump dry | |||
| K5 limestone | 57.93/547.59 | 0.000 12 | 0.007 | |||
| 317 | K8 sandstone | 52.16/569.98 | 0.000 5 | 0.023 | 0.073 | |
| K5 limestone | 82.35/539.79 | 0.000 3 | 0.012 | 0.031 | ||
| K2+K3 limestone | 56.55/565.59 | 0.000 4 | 0.12 | 0.10 | ||
| Panzhuang No. 2 | 0606 | K2+K3+K5 limestone | 160.21/527.36 | Pump dry | ||
| K7+K8 sandstone | Dry hole |
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