Fund Project: Supported by the National Science and Technology Major Special Project 001(201kloc-0/zx05033-001).

Brief introduction of the first author: Wei Hou, male, Ph.D., mainly engaged in geological research of petroleum and coalbed methane. Mailing address: China Petroleum Coalbed Methane Co., Ltd. Email: houweinan @163.com. 。

(1. China Petroleum Coalbed Methane Co., Ltd. Beijing 1000282. School of Energy, China Geo University, Beijing 1000833. China Petroleum Exploration and Development Research Institute Beijing 10083).

Abstract: Linfen Block is located in the south of Hedong Coalfield Development Zone on the eastern edge of Ordos Basin, which is an important part of coalbed methane field in eastern Hubei. Based on the new round of coalbed methane exploration results, the geological conditions of coalbed methane are systematically analyzed, the distribution of coalbed methane resources is carefully evaluated, and the favorable target area is optimized, which provides an important basis for the next exploration, development and deployment of coalbed methane. Linfen block has the characteristics of a large-scale coalbed methane reservoir, and has the geological conditions and resource base for forming a large-scale coalbed methane field: (1) There are many coal seams with large thickness and stable distribution; (2) simple structure and moderate burial; (3) Large gas production and strong gas storage capacity; (4) Good storage conditions and high gas content; (5) The coalbed methane is rich in resources and reserves. Comprehensive evaluation of coalbed methane geological conditions, resource conditions, trial production results and exploration degree shows that Taoyuan and Mingzhu well areas are Class I areas (the most favorable areas) and high-quality coalbed methane enrichment areas, which are the key areas for submitting proven reserves and productivity construction in the near future.

Keywords: favorable target area for coalbed methane resource evaluation in Linfen block, eastern margin of Ordos Basin

Evaluation of coalbed methane resources and optimization of favorable targets in Linfen area

HOU Wei 1 Wen Shengming 1 Wen Guihua 1 Zhang Yue Bridge 2,3 Li Shuxin 1 Xu Hanlin 1 Sean 1

(1. China Petroleum Coalbed Methane Co., Ltd., Beijing100028; 2. School of Energy, China Geo University, Beijing100083; 3. Petroleum Exploration and Development Research Institute; Development Department, Beijing, China 100083)

Abstract: Linfen block is located in the south of Hedong coalfield in the east of Ordos basin, and it is an important part of coalbed methane field in the east of Ordos. According to the latest exploration results of coalbed methane, the geological conditions are systematically analyzed, the distribution of coalbed methane resources is carefully evaluated, and favorable targets are optimized. This study provides an important basis for the next exploration, development and deployment of coalbed methane. Linfen block has the characteristics of a large-scale coalbed methane reservoir, and has the geological conditions and resource base for forming a large-scale coalbed methane field: (1) There are many target layers, large thickness and stable coal seam distribution; (2) The construction is simple and the buried depth is moderate; (3) Large gas production and strong gas storage capacity; (4) Good preservation conditions and high gas content; (5) High abundance and abundant coalbed methane resources. Comprehensive evaluation of coalbed methane geological conditions, resource conditions, trial production results and exploration degree shows that Taoyuan and Mingzhu areas are first-class areas (the most favorable areas), high-quality coalbed methane enrichment areas, and also key areas for the recent submission of proven reserves and productivity construction.

Key words: East Ordos Basin; Linfen area; Coal bed methane; Resource assessment; Favorable goal

1 Introduction

Linfen block is located in the south of Hedong coalfield in the eastern margin of Ordos basin, and it is an important part of coalbed methane field in eastern Hubei. The main part of the plot is located in Shanxi Province, starting from Xixian County in the north, reaching Xiangning in the south, connecting Luliang Mountain in the east and crossing the Yellow River in the west into Shaanxi Province. The total area of the whole region is 5784. 175km2, the self-operated area of PetroChina (south-central part of the block) is 4267.66 1km2, and the cooperative area (north part of the block) 15438+06.56438+04km2. The structural position belongs to the south section of Jinxi Flexural Belt in the east of Ordos Basin (Ma, 2006). The typical strata in North China are Archaean Sishui Group (Ars), Paleozoic Cambrian (ε), Ordovician (O), Carboniferous (C2b), Permian (P 1t, P 1s, P2s, P3s) and Mesozoic Triassic. Among them, the Lower Permian Taiyuan Formation (P 1t) and Shanxi Formation (P 1s) are the main coal-bearing strata, which are widely distributed and well preserved in the work area and are the main horizons for coalbed methane exploration (Bin Sun, 2008). The exploration and research of coalbed methane in Linfen block began in the 1990s, and predecessors have done a lot of basic research work on the basic geological characteristics, reservoir-forming control factors and exploration and development potential of coalbed methane (Bin Sun, 2003, 2008; Wang Hongyan, 2005; Ma, 2006; Sun, 2006; Chen Fei, 2007; Chen Gang, 2009). However, due to the limited accumulated data and various conditions, the characteristics of coalbed methane reservoirs in this area need to be further deepened, and the distribution and exploration direction of coalbed methane resources need to be further clarified. Based on the new round of coalbed methane exploration results, the geological conditions of coalbed methane are systematically analyzed, the distribution of coalbed methane resources is carefully evaluated, and the favorable target area is optimized, which provides an important basis for the next exploration, development and deployment of coalbed methane.

2 Geological conditions of coalbed methane

2. 1 There are many coal seams with large thickness and stable distribution.

There are two sets of coal-bearing strata in the Upper Paleozoic in Linfen Block, namely, the Lower Coal Formation (Taiyuan Formation) and the Upper Coal Formation (Shanxi Formation), and several sets of coal seams, such as 9#, 8#, 5# and 4#, are developed from bottom to top (Figure 1). 9# coal and 8# coal are located at the top of the lower coal series (Taiyuan Formation) and formed in the bay-lagoon environment. 5# coal and 4# coal belong to the bottom of Shangmei Formation (Shanxi Formation) and formed in river-delta environment (Bin Sun, 2008). 5# and 8# coal seams are the main exploration targets, and 9# and 4# coal seams are the exploration targets.

4# coal seam is locally developed and stably distributed, with a total coal thickness of 0.5~3.5m and a distance of 20m from the top of 5# coal. It has a simple structure and consists of 1~2 layers of coal and mudstone. There are two thick coal areas in the north and south, the north is located in Well A9-C 14, and the south is located in Well B2- 1 after provincial inspection.

The 5# coal seam in the whole region is developed and stably distributed, with a total thickness of 3 ~13.5m. The coal seam structure is relatively simple, consisting of 2~3 layers of coal, sandstone and mudstone. There are two thick coal areas in the north and south, the north is located in C 1 1-A5 well area, and the south is located in C30-C23 well area.

Figure 1 Cross-sectional view of coal seam of connecting well in Linfen block (see Figure 2a for location)

The 8# coal seam in the whole region is developed and stably distributed, with a total thickness of 4.8~23m. The coal seam structure is simple, generally single-layer coal, without dirt band. Two thick coal areas in the north and south. The north area is located in well area C 18-C 12, and the south area is located in well area C24-C22.

9# coal seam is locally developed and its distribution is unstable, with a total thickness of 0.5~2.5m and a distance of 15m from the bottom of 8# coal seam. The coal seam structure is simple, with a single layer of coal and no dirt band. There are two thick coal areas in the east and west. The East Area is located in Well Area C 19-C24, and the West Area is located in Well Area C 14-A5.

2.2 The simple structure is beneficial to the large-scale enrichment of coalbed methane.

Linfen block as a whole is a monoclinal structure that strikes northeast and gently dips northwest. Influenced by the Guan Xue thrust fault that runs through the whole area, the monocline is divided into two parts (Figure 2a). The western part of monocline, that is, the upper wall of reverse fault, can be divided into two secondary structural units: Taoyuan anticline and Hukou slope. The eastern part of monocline, namely the footwall of reverse fault, can be divided into two secondary structural units: Puxian sag and Mingzhu slope. Puxian sag and Taoyuan anticline are bounded by Guan Xue fault. The natural transition between Hukou slope and Taoyuan anticline is roughly bounded by the structural isoline of-100m on the top surface of 5# coal. The natural transition between Mingzhu Slope and Puxian Depression is roughly bounded by the 0m structural isoline at the top of 5# coal seam. Generally speaking, the structure of the study area is relatively simple, showing a structural pattern of "one uplift, one depression and two slopes", which is conducive to the large-scale enrichment of coalbed methane.

2.3 Moderate burial depth is beneficial to coalbed methane exploration.

The buried depth of coal seam in the study area is controlled by both structure and topography. The buried depth characteristics of 5# and 8# coal seams are similar. The overall buried depth of coal seams gradually increases from east to west, and the middle part rises due to the influence of Guan Xue fault, and the buried depth becomes shallow, and the buried depth line tends to the northeast as a whole (Figure 2b). 8# coal seam is about 50~60m deeper than 5# coal seam on average. The buried depth of main coal seams in Linfen block is mainly distributed between 800~ 1500m, which belongs to medium buried depth and is relatively favorable for coalbed methane exploration. The buried depth of coal seam is a key parameter affecting the exploration and development of coalbed methane, and it should not be too deep or too shallow.

Fig. 2 Structural map and buried depth map of No.5 coal seam in Linfen block

2.4 The geological conditions of the main coal seam are good, which is conducive to the enrichment and accumulation of coalbed methane.

The coal rocks in the study area are mainly coking coal, lean coal and lean coal, and the Ro value is between 1.43%~2.69%, which belongs to the coal rank with medium and high metamorphic degree and large gas production. The main coal seam is dominated by bright coal, and joints and cracks are developed. The vitrinite content of coal and rock is relatively high, the average vitrinite content of 5# coal is 73.9%, and that of 8 # coal is 765,438 0.38%, which indicates that it is deposited in a closed reducing environment, and it is easy to form a composite coal reservoir with cracks and matrix pores, which has strong gas generation potential, gas storage potential and permeability. The main coal seam is low in ash, volatile matter and water content, which is beneficial to the enrichment and accumulation of coalbed methane. The gas content in the main coal seam is high, with the gas content in 5# coal being 4~24m3/t and that in 8# coal being 4 ~ 20 m3/t.. The gas content of 4# coal is 6 ~15m3/t, and that of 9# coal is 6 ~13m3/t. ..

2.5 The overall sealing of coal seam roof is good, which is beneficial to the preservation of coalbed methane.

The roof of 5# coal seam is mainly mudstone with sporadic dense sandstone (Figure 3a). The roof of 8# coal seam is mainly limestone mixed with mudstone, with mudstone locally (Figure 3b). The sealing of coal seam roof is very important for the preservation of coalbed methane. Generally speaking, mudstone roof has the best sealing performance, followed by limestone and sandstone. However, the structure in this area is simple, faults are undeveloped (Figure 2a), limestone and sandstone have dense lithology and high argillaceous content, which has little effect on coalbed methane preservation. The overall sealing of the main coal seam roof in Linfen block is good, which is beneficial to the preservation of coalbed methane.

2.6 Simple hydrogeological conditions are conducive to the enrichment of coalbed methane.

Linfen block has simple hydrogeological conditions, bounded by Guan Xue thrust fault, and can be divided into two independent hydrogeological units, east and west. In the east, a complete hydrogeological unit is formed from coal seam outcrop area → Mingzhu slope → Puxian sag: water supply area → runoff area → weak runoff area → pressure bearing area → runoff area → drainage area; In the west, a relatively complete hydrogeological unit is formed from Taoyuan anticline to Hukou slope: runoff area → weak runoff area → pressure-bearing area, and the characteristics of water supply area are not obvious, which may be that the eastern basin has played a role in water supply area in the west. An independent and complete hydrogeological unit can prevent the lateral migration of coalbed methane and form a confined water sealed coalbed methane reservoir, which is beneficial to coalbed methane enrichment.

Fig. 3 Lithologic map of coal seam roof in Linfen block

3 Resource assessment

3. 1 evaluation method

This evaluation adopts the volumetric method (Jia Chengzao, 2007). Volume method is the basic method to calculate the geological reserves of coalbed methane, which is suitable for the calculation of geological reserves of coalbed methane at all levels. The calculation formula of volumetric method is:

Gi=0.0 1×A×h×D×Cad

Where: a is the gas-bearing area to be controlled, km2h is the thickness of coal seam, m; D is density, t/m3; Cad refers to the gas content, m3/t;

According to the development, structural characteristics and buried depth of coal seam, there are two calculation units in the plane, namely Taoyuan-Jixian and Mingzhu-Xiangning, and there are two calculation units in the longitudinal direction, which are 300-800 m, 800- 1000 m, 1000 m and 1200 m respectively.

3.2 Evaluation results

A new round of coalbed methane resource evaluation reveals that Linfen block is rich in coalbed methane resources (table 1) and has the resource base for forming a large coalbed methane field. The shallow sea area of 2000m in the whole region is 3259km2, and the resources are about 650 1 100 million m3. The main target strata (5# coal and 8# coal) have the most resources, accounting for 93% of the whole region; The resources of exploration target layers (4# coal and 9# coal) are small, accounting for only 7% of the whole area. The shallow area of the whole area is 1.200m, and the resources are about 3 1.54 million m3, which has great exploration and development potential. Among them, 5# coal with shallow resources 1200m is177.6 billion m3, and 8 # coal with shallow resources 1200m is108.3 billion m3, which is the main exploration object in the whole region. 4# coal with shallow resources 1200m is 24 1 100m3, and 9 # coal with shallow resources 1200m is 5.4 billion m3, which can be used as a supplementary force in local areas.

Table 1 Statistics of coalbed methane resources in Linfen block

4 Preferred favorable target area

4. 1 Division of exploration target area

According to the geological conditions, resource conditions, test production results and exploration degree of coalbed methane in Linfen block, the six shallow targets within 2000m of the self-operated area are divided into three categories: two in Class I (the most favorable area) and two in Class II (the favorable area). There are 2 Class III areas (favorable areas) (Table 2 and Figure 4).

Table 2 Table of Comprehensive Evaluation Parameters of Linfen Block

Class I area: Taoyuan well area (Ⅰ1) and Mingzhu well area (Ⅰ 2), which are high-quality coalbed methane-rich areas and key exploration areas in the near future, have recently submitted realistic areas of proven reserves and production scale. Class II area: Jixian well area (II1) and Xiangning well area (II 2), which are rich in coalbed methane resources, are key areas to be explored in the near future, and will be submitted to the proven reserves replacement area in the future. Class Ⅲ area: Puxian sag (Ⅲ1) and Hukou slope (Ⅲ 2), with a buried depth of1200 ~ 2000m, are the prospective areas and open exploration areas for coalbed methane exploration.

4.2 Recent key exploration and development target areas

4.2. 1 taoyuan well area (Ⅰ1)

Taoyuan well area (I1) is located in the middle of Taoyuan anticline structural belt (Figure 4), with an area of 325km2 and a buried depth of 800~ 1200m. The exploration degree in this area is high, and the results of drilling and seismic interpretation reveal that the coal seams in this area are thick, and the cumulative thickness of 5# coal seam and 8# coal seam is 8~20m, with good continuity, stable distribution and good coal quality, and the coal body structure is mainly blocky. The gas content of coal seam is high, generally 12 ~ 24m3/t, 20 10 years. The exploration of coalbed methane in this area has made a major breakthrough, and the newly put into operation C4 and C5 wells have successively obtained stable and high-yield industrial gas flow. Well C4 reaches1425m3/d; The maximum well height of Well C5 is 2,639 m3/d, which is currently stable at1435 m3/d. At present, the trial production of well groups in this area has also achieved good results. Well B 1- 10, which has been newly put into production, has reached 1500m3/d at present. The comprehensive evaluation shows that this area has superior geological conditions, strong coalbed methane recoverability and high exploration degree, and is the first choice target area for coalbed methane exploration and development.

Fig. 4 Comprehensive evaluation map of Linfen block

4.2.2 Mingzhu Well Area (Ⅰ 2)

Mingzhu well area (Ⅰ 2) is located in the north-central part of Xiangning slope belt (Figure 4), with an area of 458km2 and a buried depth of 500 ~1200m. The exploration degree in this area is medium-high, and the results of drilling and seismic interpretation reveal that the coal seams in this area are relatively thick, and the cumulative thickness of 5# coal seam and 8# coal seam is 6~ 14m, with good continuity, stable distribution, good coal quality and massive coal body structure. Coal seam gas content is high, generally 4 ~ 22 m3/t; At the same time, the newly discovered 9# coal seam is also distributed in this area, which has gas production capacity. Both Well A 18 and Well C 19 obtained industrial gas flow, and the daily production of single well reached and stabilized at 1300m3/d and 2000m3/d respectively. The comprehensive evaluation shows that this area has good geological conditions, good coalbed methane recoverability and high exploration degree, and it is also the first choice target area for coalbed methane exploration and development.

5 conclusion

Linfen block (1) has the characteristics of a large-scale coalbed methane reservoir, and has the geological conditions and resource base for forming a large-scale coalbed methane field: (1) There are many coal seams with large thickness and stable distribution; (2) simple structure and moderate burial; (3) Large gas production and strong gas storage capacity; (4) Good storage conditions and high gas content; (5) The coalbed methane is rich in resources and reserves.

(2) According to comprehensive evaluation, Taoyuan and Mingzhu well areas are Class I areas (the most favorable areas), high-quality coalbed methane-rich areas, and key areas for large-scale proven reserves and capacity building in the near future; Jixian and Xiangningjing areas are Class II areas (favorable areas), which are rich in coalbed methane and are replacement areas for strengthening pre-exploration in the near future and submitting proven reserves in the future; Puxian sag and Hukou slope (buried depth1200 ~ 2000m) are three-level areas (favorable areas), which are prospective areas for coalbed methane exploration and restricted exploration areas.

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