Arab J Geosci (2015) 8:619–630 DOI 10.1007/s12517-013-1246-1
ORIGINAL PAPER
Coalbed methane reservoir formation history and its geological control at the Shuigonghe Syncline Laicheng Li & Chongtao Wei & Yu Qi & Jia Cao & Keying Wang & Yuan Bao
Received: 11 October 2013 / Accepted: 11 December 2013 / Published online: 3 January 2014 # Saudi Society for Geosciences 2014
Abstract Coalbed methane (CBM) commercial developments have made breakthroughs in a few areas in China, including Qinshui Basin and Ordos Basin. For years, Eastern Yunnan Province and Western Guizhou Province have been the new hot spots of CBM study in China. Predecessors have discussed CBM reservoir characteristics, CBM system, and geological process evolution. Basing on previous research, we studied the formation history of the CBM reservoir by numerical simulation at the Shuigonghe Syncline in Western Guizhou. Data were obtained from geological survey and laboratory testing. According to the simulation results, the CBM reservoir formation history can be divided into five stages. In addition, a plane distributive contour map of CBM reservoir formation-related data was constructed to recognize the change in CBM content and CBM dissipation quality at different stages in the region. Each stage has its feature on CBM generation, dissipation, and accumulation speed. Geological process controlling factors, including burial history, tectonic history, geothermal history, and maturation, were analyzed. All factors acted together and formed the CBM reservoir in the Shuigonghe Syncline. Among these factors, tectonic evolution history is the most important because it determines the whole generation–preservation–dissipation process of CBM. The other factors affect the process in various ways. L. Li : C. Wei (*) : Y. Qi : K. Wang : Y. Bao School of Resource & Geoscience, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China e-mail:
[email protected] C. Wei : Y. Qi : K. Wang : Y. Bao Key Laboratory of Coalbed Methane Resource & Reservoir Formation Process, Ministry of Education, Xuzhou, Jiangsu 221008, China J. Cao Jiangsu Mining Engineering Group, Xuzhou, Jiangsu 221000, China
Keywords Coalbed methane . Reservoir formation history . Numerical simulation . Shuigonghe Syncline
Introduction Since the late 1980s, experiments on coalbed methane (CBM) exploration, development, and related research work have been performed in China (Song et al. 2012). Nowadays, CBM development in China mainly concentrated in Qinshui Basin, Ordos Basin, and other areas. The reserved area for CBM exploitation is insufficient (Fu et al. 2013). Eastern Yunnan Province and Western Guizhou Province in China have rich CBM resource and well CBM exploitation prospect (Xiong 2009; Xiong and Qin 2009). Thus, these areas are potential spots for CBM development. Lei et al. (2012) analyzed the factors controlling the distribution of gas content in the Qingshan Syncline of Western Guizhou Province. Lan et al. (2012) studied the cause of the abnormally high concentration of heavy hydrocarbon in Upper Permian Coal Seams from Enhong Syncline, Eastern Yunnan, China. Fu et al. (2013) designed progressive drainage sequence and pressure control scheme for multiple superposed CBM systems according to static liquid surface pressure, reservoir pressure, and critical desorption pressure. Wu et al. (2013) studied coal reservoir permeability in the Gemudi Syncline in Western Guizhou, China. Li et al. (2013a, b) studied the CBM reservoirs in Faer Coalfield, Western Guizhou. Eastern Yunnan Province and Western Guizhou Province have been new hot spots of CBM study region in China. The Shuigonghe Syncline, which is located in Western Guizhou Province, China, has also been well studied by many scholars. Zhu et al. (2008) analyzed the impact of structural features and evolution of geo-stress field on CBM reservoir formation. Qin et al. (2008) suggested that the CBM reservoir
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in the Shuigonghe Syncline is composed of three independent superposed CBM systems based on the phenomena that the gas content and gas content gradient vary vertically. Jiang and Wu (2011) and Gao et al. (2012) analyzed the reservoir energy system in the Zhjin–Nayong coalfield to which the Shuigonghe Syncline belongs and investigated the formation of CBM reservoir. Li et al. (2012) concluded that the physical properties of coal reservoirs in Western Guizhou are determined by different evolution processes and physicochemical structures that result from different depositional and burial history or tectonic thermal events. These studies helped understand the geological evolution and CBM reservoir characteristics. However, the process of CBM reservoir formation requires further study. This study examined the CBM geological characteristics and the whole evolution process of CBM reservoir formation, through which the quantity of CBM genesis, dissipation, and accumulation at different geological times can be clarified quantificationally. The formation of CBM reservoir is a complex dynamic process. It includes gas generation of coal organic matter, CBM preservation in coal reservoir, and CBM dissipation from coal reservoir. The whole process is controlled by many factors, such as tectonic, burial, and geothermal field evolution history, and coal and cap features, etc. This study aims to understand the whole evolution process and to provide a basis for the CBM exploitation in the area.
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maturity of the organic matter in the coal. The CBM storage submodel calculates the amounts of adsorbed and free gas in the coal reservoir based on the reservoir pressure and the pore and adsorption parameters of the reservoir. The gas dissipation submodel calculates the diffusion, permeation, and cap outburst dissipation quantities based on the gas content, the reservoir pressure, and the physical characteristics of the reservoir and the cap rock. The basic parameter submodel calculates the coal seam thickness and the reservoir’s physical characteristics in real time during the evolutionary process based on their recent values, burial depth, geo-temperature, and coal rank. Four steps are involved in the simulation of CBM reservoir formation history. First is to study the geological background after the basic data of strata, geological structure, coal seam spatial dimension, CBM geological, and hydrogeological features are obtained. The second is to study the geological evolution history including the history of tectonic deposit, burial, and geothermal fields. The works of the two steps form the basic database for the simulation. The third step is simulation calculation. In this step, gas generation, retention, and dissipation of coal reservoir are calculated by using the simulation program. The resulting data are processed for further analysis. The last step is to analyze the diagrams of gas generation, gas content, and dissipation versus time that reflect their evolution history and lateral distribution. The entire CBM formation process can be understood by completing all four steps.
Model and methodology The CBM reservoir formation history is one of the important aspects of CBM geologic study. Theories and technologies of basin modeling are widely used in the study of oil and gas formation history. Tectonic evolution history, maturation and hydrocarbon generation history, hydrocarbon expulsion, and accumulation and dissipation history must be studied to elucidate the formation of oil and gas reservoir. Computer software packages can be used in the study. However, unlike conventional hydrocarbon reservoir, CBM reservoir has particular features. The above theories and technologies are not completely suitable for studying the formation history of CBM reservoir. Wei et al. (2004, 2007a, b, 2008, and 2010) built a model to describe CBM generation, retention, migration, accumulation, and dissipation in geological history. A computer program was developed based on the model. In addition, the CBM reservoir formation histories of several CBM basins in China, such as Qinshui Basin and Ordos Basin, were numerically simulated. The key principle of the model is the mass conservation under which a dynamic equilibrium of CBM generation, retention, and dissipation has been maintained. The model includes four submodels. The gas generation submodel calculates the amount of gas generated based on the content and
Geological settings The Shuigonghe Syncline lies within the Zhijin–Nayong coalfield and is located in Southern Nayong County, Guizhou Province (Fig. 1). Its area is approximately 120 km2, approximately 15 km long and 8 km wide. A coal geology survey project was performed by the Guizhou Bureau of Coal Geology in the 1990s. Some CBM geological studies were performed in the project. These works provide important information for studies on CBM reservoir formation. Strata and coal seams The Cambrian, Carboniferous, Permian, Triassic, and Quaternary systems occurred in the syncline (Table 1). The coal-bearing stratum in the area is the upper Permian Longtan Formation (P3l). It is a deposition of transitional facies with an average thickness of 318.0 m. The Longtan Formation is composed of siltstone, mudstone, and fine-grained sandstone. Several bioclastic limestone layers also occur in the formation. The formation includes 33 to 62 coal seams, of which 35 are normally present. Eleven of these are recoverable or partly recoverable (Fig. 2). Features of part of these coal seams are
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Fig. 1 Location and structure framework of the Shuigonghe Syncline (the elevation of the ground surface is approximately 1,700 to 2,000 m)
listed in Table 2. These coal seams are also potential CBM exploration targets. Geological structure Regional structure The Zhijin–Nayong coalfield lies within the Central Guizhou Uplift. As shown in Fig. 3, the Machang fault, Zunyi fault, Ziyun–Yadu fault, and Guiyan–Zhenyuan fault form the
north, east, west, and south boundaries of the coalfield, respectively. These boundary faults control the distribution of the late Paleozoic deposition and the occurrences of middleand small-scale folds and faults in the coalfield. These middleand small-scale faults mainly strike NE or NNE, whereas others strike NW, NS, and EW. Most of these faults are high-dip angle reverse faults with multiple active stages (Jin and Tang 2010). The syndepositional faults in the coalfield striking NE dominantly control the deposition of the late Permian system.
Table 1 Strata of the Shuigonghe Syncline Strata
Average thickness (m)
Lithology
System/series
Formation
Symbol
Quaternary Lower Triassic
– Yonningzhen Feixianguan
Q T1 yn T1 f
– 49.00 595.54
Slope, residual, flood alluvial sediments Limestone, muddy limestone Siltstone, muddy siltstone and limestone
Upper Permian
Changxing–Dalong Longtan Emeishan Maokou Qixia Liangshan – –
P3c+d P3l P3β P2m P2q P2l C Є1
41.70 318.00 115.00 336.00 258.00 42.00 68.00 –
Siltstone and limestone Mudstone, silt stone, limestone, and coal Basalt Limestone Dolomitic limestone Conglomerate, quartz sandstone, and mudstone Dolomite and limestone Silty sandstone, siltstone, and limestone
Lower Permian
Carboniferous Lower Cambrian
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average concentration of CH4 is greater than 95 %, whereas those of N2 and CO2 are 2.48 and 0.73 %, respectively. All gases are dry gas. High gas content region lies along the axis of the syncline. From the axis to the two sides, gas content drops gradually; at the outcrop edge, no gas occurs in coal seams (Xiong et al. 2007). The total CBM resource of the syncline is 699.45×1012 m3, among which 229.13×1012 m3 is recoverable. The resource abundance of the syncline is 2.63×1012 m3/km2. Coal reservoirs and caps
Fig. 2 Stratigraphic column of the Longtan Formation (K7 is a marker bed of limestone)
Some coal reservoir parameters are listed in Table 2. All coals in the syncline are anthracite. Bright coal is the main composition, and a few bended anthraxylous coal and dull coal may occur in it. Studies using microscopy show that vitrinertite takes the majority. Pilot CBM well test in the neighboring area shows that the permeability of coal reservoir ranges from 0.11 to 0.49 mD, with an average value of 0.30 mD. The sedimentary environment of the Longtan Formation is tidal flat and delta. Mudstone or silt mudstone forms the roof and floor of coal reservoir. These rock layers are often thick, stable, of low permeability, and very good caps for the preservation of CBM.
Geological evolution histories The movement of other syndepositional faults that strike NEE and approximately E lead to the differential subsidence of the area and result in a complex depositional pattern (Xiong et al. 2006). Structure of the Shuigonghe Syncline The Shuigonghe Syncline (Fig. 1) is a roughly symmetrical and gently curving syncline. Its axis strikes NNW–SSE. The limbs of the syncline dip between 20° and 40°. The northern edge of the syncline is bound by a normal fault (F1). Several small-scale folds and faults are present within the syncline, including the Zuogong Anticline and the Zhangwei Anticline, which are approximately vertical to the axis of the Shuigonghe Syncline and the F4 normal fault and some others that dip between 70° and 80° with the fault throw smaller than 25 m. CBM geology Gas bearing features The gas content of 11 main coal seams ranges from 5.47 m3/t to 32.30 m3/t (Table 2), with an average value of 17.51 m3/t. The gas is composed of CH4, N2, CO2, and minor C2H6. The
Basing on the geological survey data and open literature, we studied the tectonic evolution history, deposition–burial history, geothermal history, and maturation history. The study follows the framework of before, during, and after the formation of coal bearing stratum. Before the deposition of coal bearing stratum The Caledonian Orogenesis occurred in the late Ordovician and Devonian Period (Chen et al. 2010). It formed an unconformity between the early and late Paleozoic Erathem in the upper Yangtze Plate where the Zhijin–Nayong coalfield is located. A series of faults or fault belts was also formed. Among these faults, the Guiyang–Zhenyuan fault, and the Ziyuan–Yadu Fault (Fig. 3) controlled the deposition of the Palarozoic Earthem. The subsequent Dongwu Orogenesis caused the uplifting of the whole upper Yangtze Plate and the development of a wide eluvial plain. In the early stage of the late Permian Period, the ancient fractures in the Sichuan–Yunnan ancient land became active again, and the extensional movement caused the uplift of the Emeishan mantle plume and the eruption of the Emeishan basalt (Jin and Tang 2010; Xu et al. 2010).
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Table 2 Basic data of 11 coal seams of the Shuigonghe Syncline Coal seam no.
Thickness (m)
Ash yield (%)
Sulfur content (%)
Moisture (%)
Ro, max (%)
Gas content (m3/t)
3 5−2 5−3 6−3 8 9 14u 16 20 32 33
0.20–2.79/1.21 0.14–3.99/1.06 0.09–4.65/1.80 0.09–6.67/1.87 0.57–3.70/1.78 0.09–1.63/0.86 0.19–2.10/0.89 0.30–2.82/1.28 0.10–1.69/0.88 0.25–2.36/0.87 0.24–2.87/1.37
18.11–39.46/25.49 19.22–39.42/26.63 10.99–34.11/20.58 14.50–34.83/21.83 10.89–37.40/20.41 10.63–36.53/24.22 18.35–39.44/28.91 13.49–37.77/22.58 17.67–34.49/27.33 13.47–39.27/24.11 13.70–36.43/18.70
1.43–9.45/3.86 0.28–6.85/2.55 0.39–9.47/3.28 0.33–2.86/1.04 0.46–5.52/2.41 0.66–5.87/3.23 1.52–7.21/4.46 0.31–4.50/2.76 1.32–8.08/3.88 0.26–2.16/0.90 0.45–4.78/2.09
0.40–3.33/1.49 0.50–3.62/1.62 0.39–3.24/1.55 0.47–3.99/1.62 0.45–3.41/1.63 0.43–3.50/1.57 0.54–3.01/2.22 0.68–2.24/1.53 0.78–3.00/2.10 0.57–4.02/2.28 0.50–3.19/2.24
2.73–3.03/2.82 2.76–2.94/2.86 2.75–3.76/3.02 2.59–3.05/2.86 2.74–3.17/2.97 2.81–3.00/2.90 2.96–3.12/3.02 3.09–3.27/3.18 2.98–3.26/3.14 2.95–3.49/3.31 3.05–3.38/3.18
12.54–24.88/18.64 11.33–20.22/14.73 7.02–20.04/13.65 4.55–21.40/13.11 12.18–17.14/14.37 8.97–24.77/19.33 5.47–19.94/12.05 7.59–23.99/13.23 11.66–31.69/21.95 6.33–14.57/10.60 8.18–22.92/14.71
The formation of the data represents minimum–maximum/average
During the deposition of coal-bearing stratum The coal-bearing stratum deposited at the middle and late of the late Permian Period after the eruption of the Emeishan basalt. Under generally stable conditions accompanied by frequent and small uplift and subsidence tectonic environment, coal seams of the Longtan Formation are deposited (Xiong et al. 2006). Heavy transgression occurred consequently, and muddy and carbonate shallow sea sediments of the Dalong Formation and the Changxing Formation replaced the coal-bearing strata deposition. At the end of the Permian Period, all coal seams were shallowly buried with very low maturity.
After the deposition of coal-bearing stratum After the deposition of coal-bearing stratum, a series of deposition and deformations occurred. This occurrence significantly affected the formation of CBM reservoir in the Shuigonghe Fig. 3 Regional structure framework of the Zhijin–Nayong coalfield (modified from Jin and Tang 2010)
Syncline. The evolution process can further be classified into three stages. The Triassic and Jurassic Period In the early and middle Triassic Period, shallow sea deposition was maintained in the area and Feixianguan and Yongningzhen Formation appeared. The Anyuan Orogenesis that occurred in the late Triassic Period changed the entire area from extension and shallow sea environment to compression and continental environment. In the Jurassic Period, the entire area remained subsiding and terrestrial facies deposition was formed and later eroded. As shown in Fig. 4, the coal seams reached their maximum depth at the end of the Jurassic Period. The temperature that coal seams bore kept increasing. By the end of the Jurassic Period, the temperature of No. 33 coal seam reached 198 °C, which is the first peak temperature that coal seams bore. Coalification occurred through burial metamorphism. Maturity of coal organic matter increased continuously. At the
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Fig. 4 Burial, geo-temperature, and maturity curve of No. 33 coal seam
end of the Jurassic Period, the vitrinite reflectance (Ro, max, %) of No. 33 coal seam was 2.30 %. Early Cretaceous Period The Yanshanian Orogenesis occurred from the late Jurassic Period to the early Cretaceous Period. During that time, the whole area suffered NE–SW compression that caused the compressive-shearing displacement of the Ziyun–Yadu fault zone and the formation of folds striking NW and a few faults on the NE side of the fault zone. Part of these folds and faults were cut by folds and reverse faults striking NE or NNE formed later. By that time, the structural framework of the study area was formed. In the remainder time of the Yanshanian Orogenesis, compressive stress field struck NS and NW-SE developed one after the other. Deformation overlay on the early formed folds and faults striking NE or NNE in large angle and formed superimposed folds and faults (Jin and Tang 2010). In this stage, coal-bearing stratum turned from subsiding to uplifting under the background of compressive stress field. Strong tectonic movement triggered regional magma and hydrothermal fluid intrusion. Palaeo-geothermal heat flow and geo-temperature gradient reached 76.9 mw/m2 and 3.92 °C per 100 m, respectively. Then, 229.88 °C is the second peak temperature that coal seams of the Longtan Formation suffered. Under the abnormally high geothermal field, all coals changed to anthracite.
dropped back to normal. Coal organic matter stopped maturation at this stage.
Simulations After the basic data were collected and the evolution histories were resumed, simulation can be conducted. Eleven coal seams, which include the recoverable coal seams mentioned above, are involved in the simulation. A network of 1,000 m× 1,000 m that includes 126 simulation nodes were created, and basic parameters on every node were collected and inputted into the database of the simulation program. These parameters include coal seam thickness, coal seam space, recent maturity, ash yield, sulfur content, cap diffusion coefficient, etc. These data were obtained from the interpolation of more than 140 coal geological survey well bore data. Data of burial, geothermal, and maturation evolution history were also input into the database based on the study in “Geological evolution histories.” Table 3 gives an example of the database for simulation node No. 28 (Fig. 1). The simulation program output data include burial depth, Ro, max, cumulative gas generation, gas content, reservoir pressure, and the strength of cumulative diffusion, permeation, and cap outburst. The term strength here means the dissipation quantity in one unit area, and its unit is cubic meter per square meter. Through the calculations, we generated 1,386 documents (47.5 Mb) in the *.txt format for further study.
Late Cretaceous Period and Cenozoic Era Results and discussion In this stage, the Himalayan Orogenesis performed a deformation on the early formed structures. Uplifting and erosion were the major movements of the area. The cumulative erosion thickness reached approximately 2680 m (Zhu et al. 2008; Jin and Tang 2010; Dou et al. 2012). The geothermal field reverted to normal after the Yanshanian Orogenesis, and geo-temperature gradient
After the simulation, the resulting data of each node were carefully studied. Xiong et al. (2007) showed the CBM content contour map of No. 8 coal seam, which coincide to our simulation results, especially in the Wulunshan coal field. The study includes three aspects. The first is to figure out the features of CBM reservoir formation history from the time
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Table 3 Basic data of simulation node No. 28 Parameter
Value
Parameter
Value
X Thickness (m)* Ash yield (%)* Moisture (%)* Turning point of Ro, max (Ma)
35,532,000 1.40 18.64 1.97 250, 241, 220, 205, 145, 132, 115.5, 0 250, 241, 220, 205, 145, 132, 18, 1, 0 132, 111.5, 1, 0
Y Recent burial depth (m)* Sulfur content (%)* Recent Ro, max (%)* Ro, max at turning point (%)
2,949,000 504.07 2.56 3.23 0.28, 0.50, 0.79, 1.09, 2.30, 2.30, 3.29 580, 2100, 3400, 4370, 5788, 4042, 1734, 571 2.94, 3.92, 2.77, 2.13
Burial depth turning point of bottom coal seam (Ma) Turning point of geo-temperature gradient (Ma)
Burial depth of bottom coal seam at turning point (m) Geo-temperature gradient at turning point (°C/100 m)
Parameter with “* ” is specified to No. 16 coal seam
coal seam deposited to recent. The second is to find out the lateral diversities in the whole syncline. The third is to analyze factors that affect the process. Stages of CBM reservoir formation history The CBM reservoir formation history (Fig. 5) of node No. 28 can express the entire process. As shown in Fig. 5, the entire CBM reservoir formation history can be divided into five stages. The first stage is shallow buried-immature stage that lasted from coal seams deposited to the end of the Permian Period within geological time from 260.0 to 250.0 Ma. In the stage, coal seams were buried in shallow depth and coal organic matter was immature. Only biogenetic gas was generated and almost completely dissipated because of the shallowly burying and the thin and loose cap. The generation, accumulation, and dissipation of CBM were very weak within the whole stage. The biogenetic gas was not considered in the model; thus, simulation results showed no such indication. The second stage can be referred to deep buried-primary CBM reservoir formation stage that covers the Triassic and Jurassic Periods (geological time from 250.0 to 145.5 Ma). The depth of coal seams rapidly increased, coal organic matter matured, and sufficient gas was generated in this stage. Part of the gas was stored in the coal reservoir, and the remaining was dissipated through diffusion. Taking coal seam No. 16 as an example, the cumulative gas generation was 194.56 m3/t, the gas content was 18.15 m3/t, and the cumulative diffusion strength was 264.45 m3/m2 at the end of the stage. The third stage is the early uplifting dissipation stage that occurred in the early Cretaceous Period. This stage lasted from 145.5 to 132.0 Ma. The Yanshanian Orogenesis caused the whole area changed from subsidence to uplifting. Geotemperature that coal seams bore dropped off and the maturation and gas generation of coal organic matter stopped.
Diffusion dissipation decreased the gas content to some extent. Taking No. 16 coal seam as an example, the cumulative gas generation did not change. The gas content fell from 18.15 to 15.03 m3/t, and the cumulative diffusion strength slightly increased to 270.61 m3/m2 at the end of the stage. The fourth stage is the secondary maturation-active CBM reservoir formation stage that occurred at the middle and late phases of the early Cretaceous Period (132.0 to 111.5 Ma). Although coal seams kept uplifting, the development of abnormally high geothermal field because of the Yanshanian Orogenesis caused the secondary maturation, strong hydrocarbon generation, and strong diffusion dissipation. The gas content of all coal seams reached the highest value at the end of the stage. For No. 16 coal seam, the cumulative gas generation was 270.45 m3/t, the gas content was 26.35 m3/t, and the cumulative diffusion strength was 374.66 m3/m2. The fifth stage is the dissipation stage that started since the beginning of the late Cretaceous Period until now (111.5 to 0 Ma). Coal seams uplifted at various speeds, and the geothermal field reverted to normal. The maturation and hydrocarbon generation stopped completely. Dissipation through diffusion lasted in this stage, and the gas content of each coal seam gradually decreased. In addition, Fig. 5c and d show that the dissipation rate of various coal seams differ obviously from each other in the stage. This result can be ascribed to lateral dissipation, which led to the formation of multiple superposed CBM system, which will be discussed in the authors’ next paper.
CBM reservoir formation history and geological control factors All simulation results were analyzed and reorganized (Table 4). The CBM reservoir formation history of the Shuigonghe Syncline can be well described and explained.
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Fig. 5 Curves show the CBM reservoir formation history
Shallow buried-immature stage In the late Permian Period, except for some biogenetic gas, almost no gas was generated, stored, and dissipated in the entire syncline. The key factors affecting the CBM reservoir formation in this stage are the shallow burying and low maturity of coal seams. Both factors are unfavorable for CBM accumulation. Deep buried-primary CBM reservoir formation stage In the stage, coal seams experienced a long period of deposition and deep burying. At the end of the stage, the average cumulative gas generation of all coal seams was 192.41 m3/t, with minimal lateral and vertical variation. The average gas content and cumulative diffusion strength were 18.23 m3/t and 226.19 m3/m2, respectively. The latter two parameters varied heavily both in lateral and vertical. Figure 6 shows the distribution of
the two parameters of No. 16 coal seams reveal this phenomenon clearly. Thus, deep burying is the main factor that controls the above process. Under the background of monolithic subsidence, combined with the action of palaeo-geothermal field, the burial metamorphism transfers the coals from brown coal to meagre coal accompanying the generation of a large amount of gas and forming the first accumulation of methane in the geological history. At the same time, gas accumulation and dissipation are of strong inhomogeneity which is mainly caused by the lateral variation of coal thickness, coal qualities, and features of cap layers. Early uplift-dissipation stage In early phase of the early Cretaceous Period, only diffusion dissipation occurred because of the monolithic uplifting of the whole syncline. The average gas content of all coal seams
257.25– 277.51/ 270.97 13.27–49.64/ 25.97 57.59–765.53/ 258.40 –
Gas 256.50– generation 276.93/ 270.33 Gas content 16.27–64.44/ 34.60 Dissipation 64.81–597.32/ strength 330.10
Gas – generation Gas content 0.59–14.94/ 6.53 Dissipation 70.19–723.65/ strength 388.34
4
5
All the data are in the formation of minimum–maximum/average
9.02–30.03/ 17.46 58.50–871.13/ 277.26
10.45–31.33/ 20.41 149.96– 939.40/ 492.21
11.16–32.27/ 20.17 104.15– 613.98/ 324.12 258.00– 278.10/ 271.61 19.90–50.41/ 33.47 143.63– 835.43/ 450.80 –
7.45–30.03/ 15.32 41.63–55.69/ 183.60
188.11
Gas 187.03 generation Gas content 6.30–40.12/ 19.47 Dissipation 45.99–451.25/ strength 237.73
12.83–38.19/ 23.39 103.14– 591.00/ 314.81 189.20
8.39–35.60/ 17.78 41.45–528.19/ 178.79
3
189.20
No. 5−3
188.11
No. 5−2
Gas 187.03 generation Gas content 10.66–47.49/ 24.10 Dissipation 44.59–427.12/ strength 228.53
No. 3
2
Stage Parameters
Table 4 Simulation result data of each coal seam
16.73–37.56/ 21.80 190.38– 435.69/ 301.42 191.35
191.35
No. 8
15.34–36.11/ 22.52 128.20– 1319.28/ 523.41
8.62–22.42/ 16.11 289.83– 707.10/ 470.41
14.22–31.79/ 18.89 195.97– 449.08/ 308.78 258.75–278.68/ 259.49– 279.24/ 272.25 272.88 19.23–3.80/ 24.45–54.10/ 33.13 31.26 125.70– 270.08– 1178.49/ 634.52/ 485.63 429.59 – –
11.01–34.86/ 19.83 88.29–862.13/ 350.68
190.27
12.06–41.28/ 22.77 87.88–821.32/ 341.05
190.27
No. 6−3
6.15–16.01/ 11.25 90.50–338.52/ 219.51
3.25–14.36/ 9.28 86.58–444.51/ 254.28
–
260.96– 280.37/ 274.12 8.29–34.61/ 17.43 83.66–412.67/ 240.01
260.23– 279.81/ 273.50 13.73–33.34/ 21.75 86.65–313.64/ 205.468 –
4.14–20.29/ 9.71 59.37–293.87/ 173.34
193.49
5.53–24.70/ 12.05 58.69–286.17/ 169.68
193.49
No. 14
7.28–20.31/ 12.23 60.86–226.35/ 148.52
192.42
9.39–24.78/ 15.21 59.81–219.93/ 144.88
192.42
No. 9
3.19–18.06/ 12.01 78.39–662.80/ 346.06
–
261.68– 280.92/ 274.73 7.92–47.41/ 22.61 76.00–607.17/ 323.67
3.97–27.36/ 12.9 54.40–452.52/ 234.71
194.56
53.82–437.97/ 229.29
5.34–33/15.71
194.56
No. 16
3.91–13.03/ 8.46 65.98–421.41/ 233.84
–
262.40– 281.47/ 275.33 8.55–35.34/ 17.84 64.15–391.29/ 220.04
4.04–20.17/ 9.87 46.17–283.89/ 159.72
195.62
5.53–24.51/ 12.29 45.66–277.13/ 156.53
195.62
No. 20
5.55–17.39/ 9.32 91.31–511.99/ 164.17
–
263.11– 282.01/ 275.93 13.76–32.85/ 20.19 86.80–467.93/ 153.37
7.51–19.96/ 11.39 64.33–349.96/ 112.21
196.68
9.80–24.24/ 14.29 63.21–336.58/ 109.54
196.68
No. 32
7.29–19.56/ 11.16 263.75– 598.86/ 483.21
–
10.57–33.54/ 16.66 173.33– 414.65/ 323.84 263.82– 282.55/ 276.53 20.93–55.69/ 29.32 242.8–543.04/ 438.9
14.71–38.99/ 21.10 170.58– 404.02/ 313.59 197.73
197.73
No. 33
Arab J Geosci (2015) 8:619–630 627
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Arab J Geosci (2015) 8:619–630
Fig. 6 Contour map of gas content (a) and cumulative diffusion strength (b) of No. 16 coal seam at the end of the Jurassic Period
decreased from 18.23 to 15.13 m3/t, and the cumulative diffusion strength increased to 232.48 m3/m2 (6.29 m3/m2 for this stage). The lateral distribution of the two parameters is similar to the former stage. Tectonic movement acts as a major role that controls the accumulation of CBM in the stage. The uplifting is unfavorable for CBM accumulation. The factors mentioned in the preceding section also control the lateral distribution of gas content and cumulative diffusion strength to some extent. Secondary maturation-active CBM reservoir formation stage In the stage, deep burial and magmatic metamorphism drove the coals from meagre coal to anthracite. The average cumulative gas generation and gas content were 273.47 (81.07 m3/t for this stage) and 26.14 m3/t, respectively, and the cumulative diffusion strength was 321.45 m 3 /m 2 (88.98 m 3 /m 2 for this stage). As the Shuigonghe Syncline was forming in the stage, the differentiation of burial depth between the axis and the wings was distinct. As shown in Fig. 7, both gas content and cumulative diffusion strength of No. 16 coal seam were higher in the area along the axis than in the two wings. Cap outburst occurred in this stage, but the region and coal seam were limited. After checking up all the data, we found that only No. 3 coal seam showed this
manner of dissipation and the region was located at the northeast part of the west wing. Its cumulative dissipation strength was lower than 50 m3/m2. In addition, simulation results show that permeation dissipation did not occur in the entire syncline. The abnormally high geothermal field controlled by the Yanshanian Orogenesis is the principal cause of the active CBM reservoir formation stage. High gas generation led to the high gas content and reservoir pressure and consequently high diffusion dissipation. Compared with the former three stages, geological structure (Shuigonghe Syncline) also controlled the lateral distribution to a certain extent. It controlled the burial depth of the coal seam, namely, the thickness of cap rock. The coupling of the above factors and processes resulted in the accumulation of CBM in this stage. Dissipation stage All the coal seams lost their CBM through diffusion at this stage. As shown in Fig. 8, the average gas content and the cumulative diffusion strength of all coal seams were 13.14 m3/t and 350.25 m3/m2, respectively. The patterns of their lateral distribution are similar to the former stage. Diffusion process is mainly controlled by the concentration of methane, the features of cap rocks, and the
Arab J Geosci (2015) 8:619–630
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Fig. 7 Contour map of gas content (a) and cumulative diffusion strength (b) of No. 16 coal seam at the end of the early Cretaceous Period
lasting time. Figure 5 reveals that the diffusion speed gradually slowed down. This result implies that methane concentration is more important than cap rocks. Thus,
gas content, cap rocks, and tectonic background are the factors that control the dissipation of CBM and the reservation of the CBM reservoir.
Fig. 8 Contour map of recent gas content (a) and cumulative diffusion strength (b) of No. 16 coal seam
630
Conclusions Basing on the studies of CBM geological background, tectonic, geothermal, and maturation evolution history, we performed numerical simulations to study the formation history of the CBM reservoir. The following conclusions were achieved: The CBM reservoir formation history can be divided into five stages: shallow buried, immature stage; deep buried, primary CBM reservoir formation stage; early uplifting, dissipation stage; secondary maturation, active CBM reservoir formation stage; and dissipation stage. At the second and fourth stages, CBM concentrated in the coal seams. At the third and fifth stages, CBM dissipated from the coal reservoir through diffusion and partly in cap outburst. The factors tectonic evolution history, geothermal history, maturation history, regional structure, coal seam spatial dimension and quality characteristics, and cap rock features control the formation of the CBM reservoir. Among these factors, tectonic evolution history is the most important because it can decide the entire generation–preservation–dissipation process of CBM. The other factors affect the process in different ways. Acknowledgments This study was funded by the Key National Science Foundation of China (Project No. 40730422), the National Science and Technology Major Project of China (Project No. 2011ZX05034), and the Program of Introducing Talents of Discipline to Universities of China (Project No. 13023). The authors wish to thank the staff of the Guizhou Bureau of Coal Geology, including Dr. Tongshen Yi, Mr. Menghui Xiong, and Mr. Fangfa Wang, for their assistance during the fieldwork and geological survey data collection.
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