Scholarly article on topic 'Selection of strategic replacement areas for CBM exploration and development in China'

Selection of strategic replacement areas for CBM exploration and development in China Academic research paper on "Earth and related environmental sciences"

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Natural Gas Industry B
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{China / "Coalbed methane (CBM)" / "Exploration and development" / "Strategic replacement area" / "Selection of favorable CBM areas" / "Exploration direction" / "Main controlling factors" / "Multi-layered fuzzy mathematics"}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — Longyi Shao, Haihai Hou, Yue Tang, Jing Lu, Haijun Qiu, et al.

Abstract At present, the increase in proved reserves and production of coalbed methane (CBM) in China depends mainly on several CBM bases in the southern Qinshui Basin and in the eastern margin of the Ordos Basin. Therefore, it is urgent to find new batches of exploration and development strategic replacement areas. For this purpose, we investigated the resources, reservoir properties, preservation conditions, and development conditions of CBM in China, and put forward eight general factors and four critical factors that can be used in establishing selection criteria of strategic replacement areas for affecting the CBM exploration and development in China. The eight general factors are resource abundance, coalbed thickness, gas content, original permeability, burial depth, hydrogeological conditions, coalbed depositional environment, and landforms, and the four critical factors include genetic type, stability type, later reservoir reformation, and damage degree of coal structure. Comparison shows that the low-rank coal area in the northwestern area, the northeastern medium-low rank coal area, and the southwestern medium-high rank and structurally-complex coal area are the major replacement areas for CBM following the northern area of China. Key factors affecting CBM enrichment in each of the three key areas, respectively, are: a) genetic type and stability of coal seams, b) genetic type and reconstruction of the reservoir by volcanic intrusion, and c) stability of coal seams and damage degree of coal structure. Based on these factors, a system for selection and evaluation of strategic replacement areas for CBM development was established. Fifteen blocks in the above three areas were evaluated by using multi-layered fuzzy mathematics, selecting eight favorable areas and seven relatively favorable areas that contain 1.8 trillion m3 of predicted CBM geological resources. The eight favorable areas include the Wucaiwan-Dajing coal exploration area in the Zhundong coalfield (eastern Junggar Basin), the Hami-Dananhu mining area in the Turpan-Hami Basin, the Longdong coalfield (eastern Gansu Province), the Yilan and Hegang coalfields in eastern Heilongjiang Province, the Hunchun coalfield in Jilin Province, the southern Sichuan coalfield, and the Shuicheng coalfield in Guizhou Province. These favorable areas were recommended to be the CBM exploration and pilot development target areas in the near future.

Academic research paper on topic "Selection of strategic replacement areas for CBM exploration and development in China"

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ELSEVIER Natural Gas Industry B 2 (2015) 211-221

Natural Gas Industry B

www.elsevier.com/locate/ngib

Research article

Selection of strategic replacement areas for CBM exploration and

development in China

Shao Longyia*, Hou Haihaia, Tang Yueb, Lu Jinga, Qiu Haijunb, Wang Xuetiana, Zhang Jiaqiangb

a College of Geoscience and Surveying Engineering, China University of Mining & Technology, Beijing 100083, China b Oil & Gas Resource Survey Center, China Geological Survey, Ministry of Land and Resource, Beijing 100029, China

Received 10 February 2015; accepted 8 April 2015 Available online 10 September 2015

Abstract

At present, the increase in proved reserves and production of coalbed methane (CBM) in China depends mainly on several CBM bases in the southern Qinshui Basin and in the eastern margin of the Ordos Basin. Therefore, it is urgent to find new batches of exploration and development strategic replacement areas. For this purpose, we investigated the resources, reservoir properties, preservation conditions, and development conditions of CBM in China, and put forward eight general factors and four critical factors that can be used in establishing selection criteria of strategic replacement areas for affecting the CBM exploration and development in China. The eight general factors are resource abundance, coalbed thickness, gas content, original permeability, burial depth, hydrogeological conditions, coalbed depositional environment, and land-forms, and the four critical factors include genetic type, stability type, later reservoir reformation, and damage degree of coal structure. Comparison shows that the low-rank coal area in the northwestern area, the northeastern medium-low rank coal area, and the southwestern medium-high rank and structurally-complex coal area are the major replacement areas for CBM following the northern area of China. Key factors affecting CBM enrichment in each of the three key areas, respectively, are: a) genetic type and stability of coal seams, b) genetic type and reconstruction of the reservoir by volcanic intrusion, and c) stability of coal seams and damage degree of coal structure. Based on these factors, a system for selection and evaluation of strategic replacement areas for CBM development was established. Fifteen blocks in the above three areas were evaluated by using multi-layered fuzzy mathematics, selecting eight favorable areas and seven relatively favorable areas that contain 1.8 trillion m3 of predicted CBM geological resources. The eight favorable areas include the Wucaiwan-Dajing coal exploration area in the Zhundong coalfield (eastern Junggar Basin), the Hami-Dananhu mining area in the Turpan-Hami Basin, the Longdong coalfield (eastern Gansu Province), the Yilan and Hegang coalfields in eastern Heilongjiang Province, the Hunchun coalfield in Jilin Province, the southern Sichuan coalfield, and the Shuicheng coalfield in Guizhou Province. These favorable areas were recommended to be the CBM exploration and pilot development target areas in the near future.

© 2015 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: China; Coalbed methane (CBM); Exploration and development; Strategic replacement area; Selection of favorable CBM areas; Exploration direction; Main controlling factors; Multi-layered fuzzy mathematics

The total coalbed methane (CBM) geological resources of the areas with a burial depth of less than 2000 m in the 42 main gas-bearing basins in China are 36.81 trillion m3, ranking the third in the world [1]. However, at present, sustainable

* Corresponding author.

E-mail address: Shaol@cumtb.edu.cn (Shao LY).

Peer review under responsibility of Sichuan Petroleum Administration.

growth of CBM proved reserves and production mainly comes from a few industrial bases in the southern Qinshui Basin and the eastern margin of Ordos Basin [2]. Thus, there is an urgent need to find some strategic replacement areas for CBM exploration and development in China. The low-rank coal region in Northwest China, the medium-low rank coal regions in Northeast China, and the medium-high rank and structurally-complex coal regions in Southwest China have total CBM geologic resources of about 15.9 trillion m3,

http://dx.doi.org/10.1016/j.ngib.2015.07.013

2352-8540/© 2015 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

accounting for 43% of the national CBM resources [1]. With the drilling of a number of exploitation wells in these regions, some new achievements and understandings in CBM exploration technology, basic exploitation data, enrichment conditions, and main controlling factors (such as geochemical features and genetic types) have been obtained [3—8]. The authors examined the CBM resources exploration and development situation, evaluation methods of CBM favorable areas, and occurrence pattern of different coal-bearing regions in China when undertaking relevant projects. In this study, the exploration orientation in the three key research regions is presented, and target areas are proposed based on a system for strategic CBM replacement area selection and evaluation.

1. Current situation of CBM exploration and development, and geological survey of the three key regions in China

By the end of 2013, the total number of CBM wells in China was 14041, with proved reserves of 565.4 x 109 m3 in total. However, inputs and outputs are not balanced. Total CBM production in China was 13.813 x 109 m3 in 2013, in which only 2.926 x 109 m3 was produced from ground wells [5]. CBM discoveries have been made in 82 areas since 2007, and 94 projects of CBM exploration have been registered in the National Ministry of Land and Resources, 53.2% of which are located in the coal-bearing regions of northern China, while CBM commercial development hasn't been realized in the northwestern, most northeastern, and southwestern areas of China (Fig. 1).

1.1. The low-rank coal region in Northwest China

The low-rank coal region in Northwest China refers to the west of the Helan-Liupan Mountains, including the Tarim, Tianshan, Junggar, Turpan-Hami, Qaidam, and Hexi Corridor gas-bearing basins.

In Northwest China, the coal, formed in the Early-Middle Jurassic, contains 10.35 trillion m3 of CBM resources in total, accounting for 35.5% of the total CBM resources in China [1]. In some typical large-scale inland basins with low-rank coal reservoirs, such as the Junggar, Turpan-Hami, and Tarim basins, coal-bearing formations were formed in river—delta—lake depositional systems, with multiple layers of thick coal seams. For example, the coal seams in the Sha'erhu sag of the Turpan-Hami Basin is 217.7 m thick in total, and a single coal seam is over 100 m thick at maximum [9—11]. Generally low in metamorphic degree, the coal is mainly long-flame coal. Macroscopic coal lithotypes in the northwestern region consist mainly of bright and semi-bright coal, and secondarily semi-dull coal; the coal is made up of bright coal, and vitrain and dark coal; the coal maceral has a vitri-nite content of 45.1%—88.8%, inertinite content of 7.0%— 9.3%, and exinite content of 0—9.3%. The gas content of coalbed varies greatly in the range of 0.02—12.10 m3/t, and methane concentration is in the range of 1.67%—93.00% [12,13].

1.2. The medium-low rank coal region in the old industrial area in Northeast China

The northeastern old industrial area refers to the area east of the Greater Hinggan Mountains, including the Hei-longjiang, Jilin, and Liaoning provinces, where the coal-bearing strata are mainly Upper Jurassic, Lower Cretaceous, and Paleogene, principally at medium-low rank. With a long history of coal mining, this area suffers severe environmental damage and is facing increasing stress in energy supply and demand, so it is necessary to make resources and industry transformation in these coal mines and surrounding areas.

The coal in continental fault basins in the northeastern region was deposited during Early Cretaceous and Paleogene, in alluvial fans, rivers, deltas, and lakes. The coal seams are 0.7-80.0 m thick, 1.5-18.0 m3/t in gas content, 27%-79% in gas saturation, and 67%-94% in methane concentration. CBM geological resources in this area reach 0.5 x 1012 m3 [1], with local high abundance. For example, the resource abundance of the Paleogene Fushun coalfield reaches 695 x 106 m3/km2 [14,15]. Coal types are mainly brown coal, long-flame coal, gas coal, and coking coal. Macroscopic coal lithotypes consist mainly of semi-dull and semi-bright coal, and secondarily bright and dull coal. Petrography composition of coal consists mainly of vitrain and clarain. The maceral has a vitrinite content of 64.0%-89.6% and an exinite content of 0-6.5% [16,17].

1.3. Medium-high rank and structurally-complex coal region in Southwest China

The southwestern structurally-complex region is located in the east of the Longmenshan fault and Jinshajiang-Honghe fault belt, including the Yunnan, Guizhou, and Sichuan provinces, and Chongqing Municipality. Gas-bearing basins in this region include the Sichuan-Chongqing, northern Guizhou, eastern Yunnan-western Guizhou, and Dukou-Chuxiong basins. Although CBM exploration in this region is more difficult due to the complex reservoir-forming conditions and the intensively deformed structure of coal formations, it is still a key area for CBM exploration and development.

The CBM resources in this region reach about 3.88 trillion m3 [1,18]. The coal-bearing seams are mainly the Upper Permian Longtan Formation with lithotype changing from transitional limestone to continental clastic rock, deposited in transitional environments from carbonate platform, tidal flat, lagoon, delta, to fluvial plain from east to west. The coal seams are 0.5-50.0 m thick, 54.98%-147.14% in gas saturation, 55.00%-87.11% in methane saturation, and maximum gas content in Zhina Coalfield of Guizhou Province reaches 23.52 m3/t [7,19]. High in metamorphic degree, the coal is mainly meagre coal, lean coal, and anthracite. Macroscopic coal lithotypes consist mainly of semi-dull and semi-bright coal, and secondarily dull coal. The maceral has a vitrinite content of 41.7%-96.0%, and an exinite content of 1.8%-16.9% [7,18]. Since coal seams are thin and numerous,

Fig. 1. Exploration and development progress and strategic replacement zones distribution of CBM in China.

simultaneous exploration and development of CBM, shale gas, and tight gas is worth trying [20,21].

2. Selection and evaluation system of CBM strategic areas

2.1. Selection of evaluation parameters

Resource abundance, seam thickness, gas content, permeability, burial depth, and terrain are some widely accepted factors in the selection of CBM replacement areas [22—25]. Moreover, through analyzing the occurrence and accumulation conditions of low-rank CBM in China, Sun Ping et al. proposed three reservoir-forming models: a) deep overpressure, b) secondary biogenic gas in the gently sloping basin margin, and c) conventional hydrodynamic entrapment at structural highs [26]. In light of the CBM occurrence and accumulation conditions in the Wangying-Liujia area in the Fuxin basin (Liaoning Province), Wang Bo et al. advanced the CBM accumulation model of combined hydrodynamics and dike sealing [27]. Thus, hydrodynamic condition is also a common influential factor in the selection of CBM replacement areas. Furthermore, as sedimentary environment controls the

thickness, ash content, and gas-producing macerals of coal and also the lithologies of the sealing rocks, and thus influences the generation, accumulation and preservation conditions of CBM [28], it is another common influential factor in the selection of CBM replacement areas.

Based on the main control factors and accumulation features of CBM in the three key regions, critical factors supplementary to common factors are not fully suitable for some replacement areas. Critical factors are equivalent in the assignment of evaluation parameters, but their weight during evaluation is different. Secondary biogenic gas and seam stability are deemed as critical influential factors for CBM accumulation in the low-rank coal areas in the northwestern region. In addition to secondary biogenic gas, the Yanshanian magmatic movement is another critical influential factor for the northeastern region, because it plays a pivotal role in the reformation of coal seams. Coal reservoirs in the southwestern region are mostly medium-high rank coal reservoirs, featured by high gas content and strong adsorption, so looking for high-permeability zones is the key in CBM exploration, besides coal reservoir stability and degree of coal structure destruc-tiveness in this region should be paid attention to.

2.2. Classification and assignment of evaluation parameters

According to the CBM geological conditions and main control factors (including resources, generation, accumulation, preservation, and basic exploitation conditions) in the three regions mentioned above, common and critical factors influencing the formation of CBM reservoirs in each region were sorted out. Combined with related research results in the literature [23—25,27,29—34], these factors were examined comprehensively to establish an evaluation index system for CBM strategic area selection (Table 1).

2.2.1. Classification of evaluation factors

CBM areas in Northwest China are mainly characterized by large coal-bearing areas, thick seams, low gas content, high permeability, and poor coalbed stability. CBM areas in Northeast China are characterized by small residual resources, medium-thick seams, medium-low gas content, and medium-high permeability. In contrast, CBM areas in Southwest China are characterized by high resource abundance, multiple thin seams, low permeability, and severe destructiveness of coal structures. Therefore, the replacement areas are classified as

unfavorable, relatively favorable, and favorable ones according to the following control factors: resource abundance, coal seam thickness, gas content, and original permeability. Boundaries of resource abundance, coal seam thickness, gas content, and original permeability between unfavorable, relatively favorable, and favorable replacement areas in the northwestern, northeastern regions are 0.5 x 108-1.0 x 108 m /km and 1.0 x 108—2.0 x 108 m3/km2, 5.0-15.0 m and 1.5-3.0 m, 2-4 m3/t and 5-8 m3/t, and 0.5-1.5 mD and 0.1-0.5 mD, respectively (Table 1).

Coal seam stability is quantitatively characterized with coal seam recoverable coefficient (Km) and coal seam thickness variation coefficient (Z). The cutoff value of the two coefficients for favorable, relatively favorable, and unfavorable areas in the northwestern, northeastern and southwestern regions are: Z < 30% and Km > 0.9, 30% < Z < 70% and 0.7 < Km < 0.9, and Z > 70% and Km < 0.7, respectively. It should be noted that Km is the major index, and Z minor index for thin seams, whereas Z is the major index, and Km minor index for medium-thick and thick seams [35].

Burial depth of coal seams is one of the basic factors influencing CBM preservation and exploration. Favorable zones are less than 1000 m deep at present stage. Medium-low

Table 1

Indicator system of CBM strategic replacement areas selection in China.

Evaluation indicator Region Indicator meaning Favorable Relative favorable Unfavorable

Common Resource abundance A, B CBM resource abundance >1.0 0.5—1.0 <0.5

factors (108 m3/km2) C >2.0 1.0—2.0 <1

Seam thickness (m) A, B Single seam thickness >15.0 5.0—15.0 <5

C >3.0 1.5—3.0 <1.5

Gas content (m3/t) A, B Ton-coal gas content in place >4 2—4 <2

C >8 5—8 <5

Original permeability A, B Characterized permeability of >1.5 0.5—1.5 <0.5

(mD) C coal reservoir fractures and pores >0.5 0.1—0.5 <0.1

Burial depth (m) A, B, C Serious methane diffusion due to <1000 1000—1500 >1500

small burial depth, and otherwise

poor permeability and poor

adsorption

Hydrogeological A, B Hydrodynamic strength and Low salinity and weak Medium-low salinity and weak High salinity and

condition salinity level (mg/L), High runoff runoff—runoff strong runoff

C salinity >10000, low High salinity, weak Medium-high salinity, weak Low-salinity,

salinity <2000 runoff-stagnant, easy runoff—runoff and large water strong runoff and

depressurization flow large water flow

Coal measure A, B, C Coal measures sedimentary Barrier-coastal, Non barrier—coastal or fluvial Shallow sea

sedimentary system, seam surrounding rock shore-delta or lacustrine system or alluvial fan

environment lithology and its combination system system

Terrain A, B, C Characterized basic exploitation Plain or tableland Hill or Gobi Mountain or desert

conditions of CBM

Critical Origin type A, B Occurrence of secondary biogenic Yes Unclear No

factors gas or interfusion of oil-type gas

Coal seam stability A, C Coal seam recoverable Z < 30%, Km > 0.9 30% <Z < 70%, 0.7 < Km < 0.9 Z > 70%,Km < 0.7

coefficient (Km), coal seam stable-relative stable Relative stable-unstable unstable-extremely

thickness variation coefficient (Z) unstable

Magma metamorphism B Dyke and sill intrusion and coal Yes Unclear No

seam reformation

Coal structure C Characterized ground stress, Simple structure, coal Few faults, coal structure Rich in faults, coal

destructiveness permeability and structural structure intact or slightly moderately damaged structure severely

complexity damaged damaged

Note: A-low-coal rank regions in Northwest China; B- medium-low rank old industrial coal regions in Northeast China; C- complex tectonic medium-high rank and structurally-complex coal regions in Southwest China.

rank CBM enrichment areas are generally located in low salinity (<2000 mg/L) and weak runoff zones, whereas medium-high rank CBM enrichment areas are generally located in high salinity (>10000 mg/L) and weak runoff-stagnant zones. Coal structure destructiveness is a crucial factor affecting the permeability of seams and their subsequent exploitation. A favorable area should have simple structure, slightly damaged or intact coal seams [31].

The sedimentary environment in which the coal measure deposited controls the preservation conditions of CBM to some extent through coal accumulation features, lithology, and lith-ofacies composition and spatial combination of coal-bearing rocks [32]. The barrier coastal systems, shore delta, and lacustrine systems have the best sealing capacity. Non-barrier coastal and fluvial systems are average in sealing capacity, and alluvial fan systems are the poorest in sealing capacity.

As for terrain, a basic condition for CBM development, from the perspective of costs of ground investment and exploitation difficulty, plains and loess tablelands are classified as favorable areas, hills and Gobi as relatively favorable areas, and mountains and deserts as unfavorable areas. The occurrence of secondary biogenic gas, interfusion of oil-type gas and reformation of coal seams by magma are favorable phenomena, but the absence of these phenomena is unfavorable, and the unawareness of these phenomena is considered relatively favorable.

2.2.2. Processing method and assignment of evaluation factors

The quantitative factors can be classified into two categories. One is positive correlation factors, i.e. the larger the factors, the greater their contribution to the evaluated layer, including resource abundance, seam thickness, gas content, original permeability, and so on; The other is neutral correlation factors, i.e., the middle index value (for example, burial depth) makes the largest contribution to the evaluated layer. Normalization of these two categories of factors was conducted separately as follows:

1) Processing function for the positive correlation factors

X" (1)

max {xij}

where, Uj is the normalized index value; xij is the original index value; i = 1, 2, ..., n; j = 1,2, ..., n (similar hereafter).

2) Processing function for the neutral factors

= < Xj - min{ Xj } Vj - min{ j

where, Vj is the ideal value of the neutral index. In the research regions, favorable burial depth is approximately between 500 m and 800 m.

Some factors are difficult to quantify, such as hydrogeolo-gy, sedimentary environment of coal measures, and terrain etc, so they are assigned to qualified values. i.e., favorable, relatively favorable, and unfavorable are assigned to the value range of 0.7 and 1, 0.3 and 0.7, and 0 and 0.3, respectively. Indexes without data are assigned to 0.3 and 0.4.

3. Quantitative ranking of CBM evaluation indexes

Researchers have to deal with the overlap of evaluation indexes when selecting and evaluating CBM favorable areas. For instance, resource abundance, gas content, and coal seam thickness influence each other, and hydrogeological conditions affect gas content and gas generation type. Therefore, it is necessary to quantify the weight of each index. Analytic hierarchy process (APH) was applied to rank all the indexes and a decision-making system for CBM strategic area selection and evaluation was set up.

According to the principle and algorithm of APH [33,34], a hierarchical structure was set up for CBM evaluation. Considering numerous factors affecting generation, migration, and accumulation of CBM and their relationship between each other, three first-class evaluation factors were sorted out, namely resource condition, generation-accumulation condition, and preservation condition. The influence of the first-class evaluation factors on CBM exploitation is determined by several second-class factors (Table 2).

Secondly, following the principles of objectiveness and particularity of evaluation subject, a judgment matrix of the factors in a same class was established, based on the assignment of comparison factors by relevant experts. Subsequently, the maximum eigen value (1max), the eigenvector of the judgment matrix, and the weight of each factor were calculated with MATLAB software (Table 3). Checking the consistency of the judgment matrix is necessary to ensure the reliability and accuracy of the calculated results, and avoid unreasonable errors caused by the judgment matrix. Random consistency index (C.R.) is adopted to check the consistency of the judgment matrix in the study, which equals the ratio of consistency index (C.I.) to average random consistency index (R.I.), where, C.I. = (1n"7n), C.R. = JJ, n is matrix dimension, and R.I. equals 0.52 (3-factorial matrix) or 0.89 (4-factorial matrix). If, the C.R. is less than 10%, the inconsistency of the judgment matrix is considered acceptable, otherwise reassignment and correction calculation is required until getting through consistency check. Finally, importance factor of each index of an index layer relative to the criterion layer was determined using a weighted synthetical method to calculate the relative weight of the index layer to object layer (Table 4).

4. Selection of CBM exploration and development strategic replacement areas

According to the CBM strategic replacement area selection and evaluation system mentioned above, the evaluation factors

xj > V

xij < vj

Table 2

Selection index system for CBM exploration and development replacement areas.

First-class CBM strategic area selection and evaluation system A

evaluation factors Resource B1 Source and reservoir B2

Northwestern region B2 Northeastern region B2 Southwestern region B2

Second-class Abundance C11 Original permeability C21 Original permeability C21 Original permeability C21

evaluation factors Coal seam thickness C12 Origin type C22 Origin type C22 Seam stability C22

Gas content C13

Seam stability C23

Magmatic metamorphism C23 Coal structure C23

Preservation B3

Burial depth C31 Hydrogeological condition C32 Coal measure sedimentary environment C33

Note: A represents exploitation potential of a replacement area, B1, B2, ...Cn, C12,... represent evaluation factors and parameters.

of fifteen CBM strategic replacement areas in the three key regions were collected from literature and related experimental results [7,17,18,35-43] (Tables 5-7). With 0.5, 0.6 and 0.7 as the boundaries between favorable areas (class I) and relatively favorable areas (class II) in the northwestern, northeastern, and southwestern regions, 15 strategic replacement areas in the three key regions were selected accordingly. The evaluation results show that there are 8 class I favorable areas and 7 class II relatively favorable areas in total. Class I favorable areas include the Wucaiwan-Dajing coalfield, Hami-Dananhu area in the Turpan-Hami Basin, Longdong coalfield, Yilan coalfield, Hegang coalfield, Hunchun coalfield, southern Sichuan coalfield, and Shuicheng coalfield, which have a total area of about 10.9 thousand km2, and total CBM geologic resources of 1.32 trillion m3. Class II relatively favorable areas include the Tiaohu-Malang sag in the Santanghu Basin, Kubai coalfield, Chaoshui coalfield, Jixi coalfield, northwestern Guizhou coalfield, Guishan coalfield, and Zhenwei coalfield, which have a total area of about 10.1 thousand km2, and total

CBM resources of 0.48 trillion m . Class I favorable areas are pilot areas for CBM exploration and development in the near future, whereas class II relatively favorable areas are medium-long term pilot areas for CBM exploration and development.

Evaluation results in Tables 5—7 show that class I favorable areas basically conform to the critical evaluation factors in Table 1. Geological factors (especially critical factors) of class I CBM favorable areas are summarized as follows.

4.1. The low-rank coal regions in Northwest China

(1) Wucaiwan-Dajing area is one of the coal-rich zones in the Zhundong coalfield, where seam inclination is between 15° and 30° in general, and less than 10° in local area, the d13Ci value of methane in the Dajing area is between —72.8%o and —39.2%o, and dD value of methane is between —211% and —245%, indicating that the gas is derived mainly from carbon dioxide reduction of biogenic gas [36], which conforms to

Table 3

Importance coefficient of the index layer relative to the target layer.

Evaluation index and matrix Eigenvector Maximum eigen value Random consistency ratio (C.R.)

A-B A B1 B2 B3 B4 Wb 4.0057 0.22%

B1 1.00 1.20 1.50 5.00 0.3697

B2 0.85 1.00 1.25 4.20 0.37102

B3 0.65 0.80 1.00 3.40 0.2462

B4 0.2 0.24 0.30 1.00 0.0739

B1-C1 B1 C11 C12 C13 WB 3.0339 3.26%

C11 1.00 1.20 0.90 0.3362

C12 0.85 1.00 0.80 0.2882

C13 1.20 1.25 1.00 0.3755

Northwestern region B2 C21 C22 C23 WB 3.0267 2.57%

B2-C2 C21 1.00 0.70 0.90 0.2802

C22 1.40 1.00 1.30 0.3992

C23 1.25 0.75 1.00 0.3206

Northeastern region B2 C21 C22 C23 WB 3.0066 0.63%

B2-C2 C21 1.00 0.85 0.70 0.2770

C22 1.20 1.00 0.85 0.3315

C23 1.40 1.20 1.00 0.3915

Southwestern region B2 C21 C22 C23 WB 3.0173 1.66%

B2-C2 C21 1.00 0.85 0.5 0.2387

C22 1.25 1.00 0.6 0.2885

C23 2.00 1.62 1.00 0.4727

B3-sC3 B3 C31 C32 C33 WB 3.0230 2.21%

C31 1.00 0.80 0.90 0.2954

C32 1.25 1.00 1.10 0.3664

C33 1.20 0.90 1.00 0.3382

Table 4

Ranking of evaluation index weight of all the factors.

Evaluation index Weight Evaluation factor Weight

Resources 0.37 Resource abundance 0.12

Coal seam thickness 0.11

Gas content 0.14

Source and reservoir Northwestern 0.31 Original permeability 0.09

Gas generation type 0.12

Seam stability 0.10

Northeastern 0.31 Original permeability 0.09

Origin type 0.10

Magmatic metamorphism 0.12

Southwestern 0.31 Original permeability 0.07

Seam stability 0.09

Coal structure destructiveness 0.15

Preservation 0.25 Burial depth 0.07

Hydrogeological condition 0.09

Coal measure sedimentary environment 0.09

Basic exploration and development conditions 0.07 Terrain 0.07

the factors of biogenic gas origin and stable seams in Table 1.

(2) Long-flame coal and gas coal dominate in the Hami-Dananhu area of the Turpan-Hami coalfield which is characterized by a relatively stable regional structure according to seismic exploration results. Melt water from Mount Tianshan infiltrates into the sediments and forms low-salinity formation water suitable for the reproduction of methanogens in coal seams at certain depth. d13Ci value of Well DaMei No.4 in the Dananhu sag ranges between —66.5%o and —71.7%o [40], indicating the gas origin of secondary biogenic gas, which is consistent with factors of biogenic gas supplement and stable coal seams in Table 1.

(3) In the Longdong coalfield in the western margin of the Ordos Basin, the coal seams are less than 10° in inclination, and 19.6% in thickness variation coefficient. The main minable measures, M5 and M8 of the Jurassic Yan'an Formation have a maximum vitrinite reflectance of 0.83%, average methane concentration of 79.91%, and average permeability of 0.32 mD, average reservoir pressure of 11.48 MPa, and pressure gradient between 0.83 MPa/100 m and 1.07 MPa/100 m, which is within the low pressure-normal pressure range [42]. Although the origin of biogenic gas has not been proved, the area conforms to the critical factor of stable seams.

4.2. Medium-low rank old industry coal regions in Northeast China

(1) The coal-bearing measure in the Yilan Basin is the Paleogene Dalianhe Formation, which is simple in texture with few partings, and dominated by long-flame coal and gas coal. Acidic granite developed at the bottom of the major seam, about one hundred meters of oil shale overlying the coal measures provides perfect sealing conditions [31,36], and there is probably oil-type

gas interfusion in the coal reservoirs. These features conform to such critical factors as reservoir reformation by magma and oil-type gas interfusion.

(2) Major coal-bearing measure in the Hegang coalfield is the Lower Cretaceous Chengzihe Formation, where coal types are mainly gas coal and fat coal. Frequent magmatic activities have reformed the coal reservoirs strongly, so metamorphic grade in the areas where magma intruded reaches lean coal and anthracite. The coal seams have a vitrinite content between 52.2% and 82.3%, an inertinite content between 2.3% and 26.3%, and a liptinite content between 0.2% and 6.2% [36]. These features are consistent with the critical factor of reformation by magma intrusion.

(3) Coal-bearing measure in the Hunchun coalfield is the Paleogene Hunchun Formation, which is dominated by lignite and long-flame coal, and develops on Permo-Carboniferous metamorphic rocks and Jurassic pyro-clastic rocks basement. Coal reservoir was reformed by a diabase intrusion into the coal measure in the Baliancheng area. M19 coal seam, the major target, is mostly overlaid by siltstone and silty mudstone with good sealing ability. These features are consistent with the critical factor of reformation by magma intrusion too.

4.3. Medium-high rank and structurally-complex coal regions in Southwest China

(1) Coal-bearing measure in southern Sichuan favorable area is the Upper Permian Longtan Formation, which has the maximum vitrinite reflectance of over 2.5%, and one to eight are minable seams, of which Coal 19 and Coal 25, with relatively developed cleats, are relatively stable. The coal seams are simple to medium complex in structures, and the average vitrinite content in the Guxu mining district is 79.38%. Well test data show that the

Table 5

Evaluation factors and results of low-coal rank regions in Northwest China.

No. Replacement area Coal Resource Total seam Gas Original Biogenic gas Seam stability Burial Hydro-geological

measure abundance thickness content permeability occurrence depth (m) condition

(108 m3/km2) (m) (m3/t) (mD)

Coal measure Terrain

sedimentary

environment

Weighted Comprehensive evaluation evaluation value grade

1 Wucaiwan-Dajing J1-2 area

2 Tiaohu-Malang J1-2 depression

3 Kubai coalfield J1-2

4 Hami-Dananhu J1-2 area

5 Chaoshui coalfield

6 Longdong coalfield

J1-2 J2

1.04 0.8 0.99

1.05 0.04 0.45

7.0-37.0 0.2-12.1 0.09-9.33 Yes

5.0-55.0 2.1-2.7 / Unclear

5.7-61.7 3.0-7.0 / Unclear

11.2-169.0 0.9-3.0 5.0-9.47 Yes

1.4-15.3 0.4-0.7 / Yes

0.2-45.6 0.7-4.1 0.04-3.43 Unclear

Relative

stability- stability Relative

stability- stability Instability-extremely instability Relative

stability- instability Stability- relative stability

Stability- relative stability

500-1500 500-2000 200-2000 300-2000 400-2000 800-1200

runoff- runoff Weak

runoff- runoff Weak

runoff- runoff Weak

runoff- runoff runoff

runoff- runoff

Braided river Hill—gobi 0.726 delta—lake

Alluvial Hill—gobi 0.441 fan—fan delta

Fan delta Hill—gobi 0.464

Braided river Gobi—desert 0.689 delta—lake

Fan delta Mountain— 0.353 desert

Delta—lake Loess 0.519 tableland

Note: '—' means no data.

Table 6

Evaluation factors and results of medium-low rank coal regions in Northeast China.

No. Replacement Coal Resource Total seam Gas Original Biogenic gas Magma Burial Hydrogeological Coal measure Terrain Weighted Comprehensive

area measure abundance (108 m3/km2) thickness (m) content (m3/t) permeability (mD) occurrence metamorphism depth (m) condition sedimentary environment evaluation value evaluation grade

1 Yilan coalfield E2 1.06 9.5-30.0 2.5-12.0 3.87-5.52 Yes Yes 600-1000 Weak runoff Fan delta—lake Plain-hill 0.695 1

2 Jixi coalfield K1 0.38 0.7-14.0 1.8-14.6 0.02-0.10 Unclear Yes 500-1000 Weak runoff—runoff Braided river delta— lake Plain-mountain 0.571 N

3 Hegang coalfield K1 1.65 2.0-20.0 7.0-16.0 0.10-0.40 Unclear Yes 100-1500 Weak runoff River—delta Mountain-hill 0.617 1

4 Hunchun coalfield E 2.92 7.4-34.5 2.0-6.0 2.65-4.38 Unclear Yes 400-1200 Weak runoff—runoff Fan delta— lake Plain-mountain 0.701 1

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coal reservoir pressure gradient ranges between 0.96 MPa/100 m and 1.29 MPa/100 m, and is in normal pressure-slightly overpressure range [39]. These features conform with the critical factors of undamaged structure and stable coal seams.

(2) The coal-bearing measure of the Shuicheng mining area is also the Upper Permian Longtan Formation, which contains up to 39 coal seams. Macroscopic coal rocks are mainly semidull, semibright, and bright with fragmented structures. Coal seams with original structures are seen in some areas. They have a vitrinite content of between 57.24% and 97.2%, average porosity between 3.52% and 9.48%, and pressure gradient between 0.4 MPa/100 m and 1.0 MPa/100 m in under-pressure-normal pressure range [38]. Underground mine survey suggests that coal in the Dahebian and Shenxianpo coal mines is in original (27.2%) and fragmented (72.8%) structures, which also basically conforms with the critical factor of slightly damaged structures.

5. Conclusions

(1) Based on the geological information of CBM collected from the low-rank coal region in Northwest China, medium-low rank old industrial coal region in Northeast China, and medium-high rank structurally-complex coal region in Southwest China, eight common factors and four critical factors (covering resource conditions, generation and storage conditions, preservation conditions, and basic development conditions) affecting CBM accumulation were identified. The eight common factors include resource abundance, seam thickness, gas content, original permeability, burial depth, hydrogeological condition, sedimentary environment of the coal measures, and terrain; the four critical factors include gas generation type (in the northwestern and northeastern regions), stability (in the southwestern region), reservoir reformation (in the northeastern region), and coal structure damage degree (in the southwestern region). A system for selection and evaluation of strategic CBM replacement areas in China was set up, and the weight of the evaluation factors of the three key regions were quantitatively ranked by using multi-layered fuzzy mathematics.

(2) Fifteen CBM exploration and development replacement areas in the three key regions were evaluated and selected. The results show that eight of these areas are class I favorable areas, with a total area of 10.9 thousand km2 and total CBM resources of 1.32 trillion m3, and seven areas are class II relatively favorable areas with a total area of 10.1 thousand km2 and total CBM resources of 0.48 trillion m3. The class I favorable areas including the Wucaiwan-Dajing area of the Zhundong coalfield, Hami-Dananhu area of the Turpan-Hami Basin, Longdong coalfield, Yilan coalfield, Hegang coalfield, Hun-chun coalfield, southern Sichuan coalfield, and Shuicheng coalfield, are areas for CBM exploration and pilot development in the near future.

Acknowledgments

This work has been supported by the China Geological Survey Scientific Research Project 1212011220794 and the National Science and Technology Major Project 2011ZX 05033-002.

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