Deposit Characteristics of Coalbed Methane and Evaluation on Target Area in Hancheng Mining Area Wang Xiaogangª¬ª©£Û£±£Ýªª, Wang Shuangming, Wang Shengquan Long Rongsheng, Chen Lianwu, Duan Mu Heshun Abstract Through the studies of fundamental geology, generating and storing conditions, the controlling factors of coalbed methane and enrichmen t in Hancheng mining area,the authors regard that the region is a fav orable exploration and development area for coalbed methane because th e generating and storing conditions of coalbed methane are excellent, methane content is high, the occurrence and accumulation characteristi cs are clear and permeability is excellent. The paper for the first t ime divides the region into three CBM exploration and development area s based on the thickness and burial depth of coal seams, texture of co al body and regional structure conditions, etc. Six principles for det ermining CBM types were made and six exploring and developing target a reas for coalbed methane and development prospect areas in this area were selected. 1 Introduction to Geology of the Mining Area Coal reserves in the minable No.2, 3, 5 seams are tremendous and t he coal quality is excellent. The thickness of coal seams are stable. The resources of coalbed methane are abundant with good generating a nd storing conditions and the potential for development is great. The coal measures in this area from bottom to top are the Upper Ca rboniferous Benxi formation, the Taiyuan formation and the Lower Permi an Shanxi formation respectively. Among which Benxi formation does not usually bear coal seams, Taiyuan and Shanxi formations are the main c oal bearing strata. Coal seam Noª±11 is quite stable being 3-5m thick in general. The Noª±3 seam in the southern area belongs to stable mid- thick coal seam of 1ª±5 m thick. In the northern area, its thickness c hanges regularly being 5-7 m thick in general. Coal seam Noª±5 is appr oximate 3 m thick. Coal seam Noª±2 is £Û£±£ÝLecturer,Geological Department of Xi¡äan Mining Institute,Xi¡äan ,Shaanxi provinceextremely unstable,general thickness is 0ª±8 m. Macerals are dominated by vitrinite (including pseudo-vitrinite) w hich generally approachs 85% or so. Fusinite content (including semi-f usinite) is normally lower than 10% and the contents of liptinite comp onents are very small. Apart from the No. 11 seam, which has a relativ ely high fusinite content, vitrinite content is high in other seams. Structures in the mining area are controlled by the periphery mega tectonic zone, the direction of tectonic line in the area is dominated by north-eastern direction (including north-northeast direction), nea r east west direction (containing northeast east and northwest west di rection) and north west direction. The hydraulic connections of aquifers in the roofs of coal seams and overlayers of coal measures are not close. They don¡ät have notabl e influence on the coalbed methane. But after the coal or methane is e xtracted, the pressure in the coal seam will be released, and the unde rground water in the Ordovician limestone under the coal measures will affect the coalbed methane markedly. 2 Generating and Storing Conditions of Coalbed Methane Deposit and S ealing of the Surrounding Rocks Of the main recoverable coal seams in this area, the average conte nts of organic carbon is up to 70ª±6%, the degree of metamorphism is s ituated in the main phase of hydrocarbon generation. The minable coals are good source rocks for gas generation. The microtextures of the ma in mining coals are various, including gas pores, organization pores o f plant residue, corrosion pores, mold pores, intergranular pores, and fractures, etc., among which gas pore and fracture are well develope d. The coals belong to cleat-pore reserviors. The medium and large por es are dominant among the gas pores, occupying 61% on an average. Micr opore and ultra-micropores occupy 39%. Parameters of the pore physical character, such as porosity, pore volume and the surface area, etc., indicate that porous structure is better developed in the northern coa l seams compared to those in the south. coal seam Noª±3 has well devel oped porous structure in the north while coal Noª±5 is higher in the s outhern area. Looking through the adsorption capacity, the gas-storage capacity of coal seam Noª±2 in northern area belongs to medium to lo w storing type; coal seam Noª±3 belongs to medium storage model, coal seam Noª±11 belongs to low storage type. In the southern area, coal s eams Noª±3,5 and 11 are medium to low storage type. According to lithology and pore texture, the surrounding rocks of coal seams in the area may be divided into mudstone (sub-divided into mudstone, siltstone), sandstone (subdivided into graywacke and sandsto ne) and limestone. Mudstone is a shielding layer for coalbed methane, its sand content is geneal less than 25%, tectonic disruption is small . Siltstone is a semipermeable stratum with sand content ranging from 25% to 50%, tectonic disruption is also stronger. Sandstone and limes tone are permeable with sand content higher than 50%.The floor of each coal seam in the region is mainly shielding layer, so the sealing abi lity is very good. The roof of the coal seam Noª±3 is mainly shielding layer, the sealing ability is the best. All three lithologies are pre sent in the roof of coal seam Noª±11 but mainly shielding layers, the sealing ability is also good. The roofs of coal seams Noª±2 and 5 are mostly river channel sandstone phase belonging to permeable and semi-p ermeable strata, so the sealing ability is worse than that of coal sea ms Noª±3 and 5. 3 Accumulation Characteristics of Coalbed Methane and Controlling Factors (1) Northern area Coalbed methane content of coal seam Noª±2 ranges from 1ª±19 mª¬3/ t to 13ª±33 mª¬3/t, with an average of 6ª±06 mª¬3/t. The bulk of the coal seams has CBM content between 2-11% mª¬3/t, in which areas of lo w methane content occupy 35ª±48%, medium methane content 56ª±97%, and rich methane areas 7ª±15%. The methane contents in the shallow part of the mining area is lower and the methane contents increase gradually toward the middle to deep part. Coalbed methane content of coal seam N oª±3 ranges from 4ª±61 mª¬3/t to 15ª±52 mª¬3/t, average value 8ª±18 m ª¬3/t. 87ª±6% of the seam have CBM content between 6-11 mª¬3/t, 7ª±3% below 6 mª¬3/t,only 5% of the area having CBM content >11 mª¬3/t. Over all, areas of medium CBM content account for 85ª±4% rich methane area 13%. The methane content is relatively low in the boundary and shallow parts of the mining area, there is a trend that it increases graduall y toward the mid and deep parts of the mining area. The minimum methan e content of coal seam Noª±13 is 1ª±86 mª¬3/t (Noª±77 borehole), maxim um value 15ª±86 mª¬3/t, average value 8ª±08 mª¬3/t, the distribution o f methane content is mainly between 6-10% mª¬3/t, taking up 68ª±1%. Ar eas of medium CBM concent is dominant accounting for 72ª±3% of the tot al with low and rich methane areas 11ª±7% and 16% respectively. (2) Southern area The minimum methane content of coal seam Noª±3 is 2ª±56 mª¬3/t, ma ximum value 14ª±20 mª¬3/t, averaging 6ª±42 mª¬3/t. 88ª±5% of the seam has methane content from 4 to 9 mª¬3/t. Overall, low methane area acco unts for 25ª±9%, mid methane 72ª±2% and rich methane 1ª±9%. The minimu m methane content of coal seam Noª±5 is 2ª±13 mª¬3/t, maximum 13ª±38 mª¬3/t, averaging 7ª±83 mª¬3/t, in which the contents between 4-12 mª¬ 3/t occupy 88ª±88%. The dominant methane concentration is 7-8 mª¬3/t,, low methane points occupy 14ª±82%, mid methane 66ª±65% and rich metha ne 18ª±52%. The main part of the seam is medium methane area. The mini mum content of coal seam Noª±11 is 1ª±31 mª¬3 /t, maximum value 13ª±17 mª¬3 /t, average value 7ª±35 mª¬3 /t, mostly between 5-9 mª¬3 /t, occ upying 61ª±6%. Areas of medium methane concentration occupy 69%, low m ethane 13ª±9% and rich methane 16ª±3%. Methane content generally incre ases with increasing depth. Main geological factors of controllong coalbed methane contents an d accumulation are geological structure,burial depth, thickness and te xture of the coal seams, the thickness of coal measures, the thickness of sandstone in the coal measures, lithology of the roof and floor of the coal seam, etc., in which the decisive factors are geological str ucture,burial depth, thickness and texture of coal seams and the litho logy of the roof and floor of the coal seam. 4 Gas Control by Tectonics The outcropped large-mid tectonic tracks on the surface mainly occ ur in several tectonic zones of the boundary and shallow part. Medium and small scale structures are common in the coal seams. Main tectonic types include folds, flexure structure, fault structure, interlayer g liding structure, stratum fracture, joints and fissurs etc. 4ª±1 Determination of gas-controlling structure 4ª±1ª±1 Gas-resisting structure (1) Anticline and syncline structures. Mainly of compressive natur e, the coal body in the axial part and two limbs are powdered in major ity, associated broadly with compressive interlayer gliding structure. The structures have a sealing ability because the powdered coal fills or obstructs the tensional fracture zone, though tensional fractures are well developed in the axial part. Methane contents are general low in boreholes, only at some individual points on the axial part of the syncline methane contents are higher. (2) Interlayer gliding structure (compression model and shear mode l). The interlayer gliding structure is mainly formed by lateral compr ession and shear actions. The confining pressure surrounding the inter layer gliding plane is high, shearing glide strong, so that the phenom enon of powered coal are very common. The powered coal obstructs the c onduction action of other structures, so the closing ability of the i nterlayer gliding structure is very strong. They occur mostly at the l imbs of the fold and the pitching zone of coal seam, methane contents are general low in the boreholes. (3) Reversed fault. It is a sealing fault determined by its mechan ical property, it is not favorable to transport underground water and coal seam methane. For example, it has been proven that F-2 reversed f ault in the boundary of the mining area plays a role of water-resistin g action. Most reversed faults discovered in the coal seams do not exi st drip phenomenon on the fault zone. 4ª±1ª±2 Gas conducting structure (1) Extendible normal fault. Controlled by the regional extension action, usually appears in a group and forms a step fault group or gar ben and horst fault combination. Cataclasis rock is well developed in the fault zone. The fractures or secondary small faults are well devel oped in the two block strata of the fault (especially in the upper sid e), the fractures are generally open and only filled incompletely. Mos t fault zones observed in the field conduct water. If a fault cuts coa l seam up to the surface, it also plays a role in the release of coalb ed methane. As the Dongze village fault group extend towards the east and cut coal seam Noª±11. Methane contents measured adjointing the fau lt zone are clearly low. This kind of normal faults developed in the c oal seams are conducive for CBM accumulation and migration since the w ell developed fractures in the coal increases the conductance of the c oal seam cleat. (2) Extendible interlayer gliding structure. The gliding is contro lled by regional extension action. In the gliding process along the co al seams, coal body became thinner and powered to a different degree. Since extension action is very strong, fractures in the coal seam are very extensive to a degree that many of these structures, have an app earance of leaching and dripping. (3) Axes of a few of the folds and limbs of broad gentle folds. C oal seams and rocks located at the convex side of the neutral plane of a fold are in an extension stress status, and play a role in transpor ting and storing gas. For example, higher methane contents are encount ered in the axis of fª¬ª©12ªªª­ª©3nªª syncline and the northern limb of fª¬ª©12ªªª­ª©3nªª anticline in the Xiayukou mine field. (4) Near EW, NE and SN fracture groups and intensive fracture zone . The final period of tectonic evolution was tensive and tensive torsi on nature.When observed in field and underground, the fractures are v oid and can act as excellent CBM migration channels. 4ª±2 Determination of gas control tectonic zone 4ª±2ª±1 Sealing type tectonic zone It includes mainly: the first flexure zone and the boundary parts and the shallow parts of the gentle incline zone(1-2 km from the first flexure center line). The stress in the zone is mainly compressive to rsion resulting in large degree of deformation. Inversion folds, recum bent folds, S model and reverse faults are well developed. Coal seams also suffered strong corrugating. Although many faults were formed und er the late period of extension action, the interlayer gliding was so strong the coal was broken and also close to the surface methane relea sed and dispersed quickly, therefore, the average value of methane is very low. It should be pointed out that as the zone is a compressive t ectonic zone, it obstructed leaking of the mid and deep methane, playi ng a sealing role. This kind of fold zones includes: (1) the fold zone at the norther n region of Xiayukou mine field and the northern of Zuokaihe; (2) Wen jialing fold zone; (3) Jushuihe fold zone. 4ª±2ª±2 Open type tectonic zone It mainly includes: (1) The mid and deep parts of gently inclined zone. It is a NE sti rp within 2-5 km of the central line of the first flexure zone. In the zone, there are mainly tensional or tensional torsion structures. Fol ds are broad, gentle, weak and the degree of deformation is low. The t exture of coal body is dominated by fragmented and unbroken coal bodie s, cleat and fracture are well developed. Methane contents measured i n boreholds are relatively high. (2) The second flexure zone. Tension and torsion action are domina nt in the zone. Final result of tectonic evolution is expressed as te nsive structures and degree of tectonic deformation in the zone is s mall. The texture of coal body is mainly fragmented and primary textur e coal, tension fracture is well developed in the coal. (3) Fault zone extending near EW direction; This includes the frac ture zone which was compressed at first and extended later in south, e xtended fracture zone in the middle part and Longguling extension fra cture zone. It is composed of a step normal fault group or graben and horst normal fault group. The final result of tectonic evolution is ex pressed as extension fault zone action. The fracture of the strata in the zone is very well developed and filled to a low degree and th faul t zone is generally water conductive. Fault cutting into coal seams an d extending to the surface may become an escape channel for coalbed me thane. For example, the eastern part of Dongze village normal fault gr oup cut in the coal seam and the methane contents measured in the bore holes are clearly low, which could be caused by CBM release along the fault zone. 4ª±2ª±3 Vertical structural conducting zone This mainly includes the Dongze village extending fault zone and t he first flexure zone. Because extending fault or fracture structure w as very well developed and the burial depth of coal seam was very smal l even exposed on the surface in part, the structure zone was a vertic al conducting zone. Methane released quickly in the zone therefore, me asured methane content is the lowest in the zone (5ª±87-6ª±4). If the south fracture zone compressed firstly and then extended and Longgulin g fracture zone cut the coal seam, they may become a zone from which a great volume of methane was released. 4ª±3 Gas controlling tectonic network The intersect and compound of various gas controlling structures a nd gas controlling tectonic zones constitute a complexed gas controlli ng tectonic network. 4ª±3ª±1 Methane escape area Several extended fracture zones in the mid and deep parts of the m ining area, generally cut through the coal seams, enabling methane to escape from deep part along the opening fracture zone to the surface o r to other porous rocks to form poor methane areas. The first flexure zone and areas to the east of the gently inclined central part are poo r methane areas because the uplift was so strong that the coal seams w ere close to surface, dip angle also deep, that methane was strongly w eathered and oxidized. Methane escaped very quickly forming low methan e areas. 4ª±3ª±2 Methane retention areas In the south, they are located in the central and eastern part of Xiayukou mining area and to the east of Noª± 78 and Noª±91 boreholes i n Sangshuping mine area. All are fold developed areas. Coal body text ure is dominated by fine coal. Powdering of coal makes the cleat and fracture disappear basically.Coalbed methane can migrates only in the extension fracture zone in the vicinity of anticline or syncline axes and latteral passages are blocked to form CBM retention areas. 4ª±3ª±3 Methane seepage flow area This area is divided into four methane seepage zones. One is a nea r east-west strip between the Qingshui fracture zone and the Tilachua n-Ying mountain extension normal fault zone. It is primary structural coal, coal seams were generally subjected to extension opening action , methane seepage condition is good. The second is located in a rectan gular area between Ying mountain to Houchangxia fold zone and Jiangzhu anggou to Wanglingshang fold zone. It is composed of the western part of central gently inclined zone and the second flexure zone as well as deeper zones. Many coal seams in the district have primary texture. T he status of opening and relaxation of the coal seams is excellent, it is a good area for methane seepage flow. The third is located in a t riangular zone, that is, the south to the line from Fanjia mountain in the south of Xiayuping mine field to Xiachiping in the deep part of t he mine area, the north to the line from Beimiao village to Bujialing, which is constituted by the second flexure zone as well as it¡äs deep er zone and areas of the western sector of the gently inclined zone. I n the interior there exist north-east extended structural tracks in th e middle to shallow part and possible east-west structures in the deep er part. Opening and relaxation of coal seams are also excellent bein g an area for methane seepage. The fourth is located in a broad region north of Xiayukou mine field and the west of methane retention area, which is constituted by the western part of central gently inclined zo ne and the second flexure zone as well as it¡äs west deep sector. It i s a zone with a strong south and north extended action, the status of opening and relaxation of the coal seams may also be good, however,the south north compressed structure may have an influence on the opening and relaxation of the coal seams. Due to lack of information, the extent is difficult to evaluate. The area could be classified as a met hane seepage zone. ª©5 Determinationªª of Methane Exploration and Development Types and Selection of Target Area. 5ª±1 Principles for determining methane exploration and development ty pes Based on the various indexes and their relationships,coal seams in the area can be divided into three types: type ¢ñis favorable methane exploration and developmetn area, type ¢ò is common methane explorati on and developmetn area, type ¢ó is unfavorable methane exploration an d development area. Specific principles for determining respective type are as follow ing: (1) Thickness of the coal seam. The coal seams are divided into th ree types <1ª±3m, 1ª±3-3ª±5m and 3ª±5-8m. Type ¢ñ: the thickness of th e coal seam is general more than 3ª±5 m ( the thickness of coal seam 3# in the southern area is 1ª±3-3ª±5m), type ¢ò: the thickness is 1ª±3 -3ª±5m, type ¢ó: the thickness is smaller than 1ª±3m. (2) Methane content. Methane contents of individual coal seams are generally from 4-10 mª¬3/t. Referring to the requirement for coalbed methane content in USA and the situation in the area, methane content s are divided into three types: type ¢ñ>8mª¬3/t, type ¢ò8-6 mª¬3/t and type ¢ó < 6 (usually 6-4) mª¬3/t. (3) Burial depth. Burial depth of the various coal seams in the ar eas is normally more than 250m. The depth of weathering and oxidizing zone varied from 150m to 500m. For classification factors of methane weathering and oxidizing zone normal methane content area or location deeper than the boundary line of the maximum mining depth are consider ed. It is divided into 3 types: ¢ñ> 500m, ¢ò500-300m, ¢ó <300m ( local ly may be about 500m). (4) Coal texture. Coal texture could generally be divided into thr ee types of ¢ñ block coal, ¢òblock and powder coal and ¢ó powder coal. (5) Sealing ability of roof and floor. Mainly refers to lithologie s of the roof and floor of coal seams. They can be divided into mudsto ne or mudstone + siltsone, siltstone + sandstone, sandstone + limesto ne or limestones. Type ¢ñ: is mainly mudstone, locally mudstone and si ltstone, being a shield layer. Type ¢ò: is mainly mudstone and siltsto ne and a few sandstones (or limestone), that is mainly a shield layer and semi-permeable layer. Type ¢ó: is mainly mudstone, siltstone and s andstone or limestone, it is a composite of shield strata and semi-per meable strata and perameable strata. (6) Structure. The classification is mainly based on the structure areas determined on the tectonic analysis results. Kind I: located in the methane seepage flow area. Kind II: locates in methane seepage ar ea, locally in escape area or retention area; Kind III: mainly in rete ntion area. The 6 principles above must be considered comprehensively in deter mining the types of the exploration and development. However,as metha ne reserves in this area is abundant, the resources volume of unit ar ea do not vary significantly, the degree of easiness for drainage beco mes the first consideration in determining exploration and development type. Therefore, when applying the principles in classification, coa l texture, tectonic condition and buried depth should be considered fi rst,then select other index respectively. There may be discrepancies s uch as thickness of coal seam, methane contents, etc. may not match values set in the categories above.This also shows that the gas recove ry ability is the most important factor for resources evaluation in so me area. 5ª±2 Selection of CBM exploration target area 5ª±2ª±1 General principles for target area selection Besides the age, rank and tectonic event in the area, the factors should be first considered in selecting target areas are the characte ristics of microscopic coal body texture its distribution both vertica lly and latterally and the distribution of seepage areas in gently inc lined zones, etc.. Based on this, the selection of the target area fo r methane exploitation should comply following principles: (1) The coal seams should be buried at a certain depth. After com prehensively considering the technology requirement for drilling and g as extracting, the depth of weathering and oxidizing zone and the effe ct of mining level of active mine on the discharge of coalbed methane , the burial depth of coal seam should be within the range from below the weathering and oxidizition zone to 1000m deep. (2) Avoid powdered coal area, or lump and powdered area ( retentio n area) if possible. Selecting lump coal areas which suffered the leas t tectonic actions, the primary texture was well reserved; cleat and f racture well developed, so that the coal seam has good permeability. (3) Selecting thick coal seams with good continuity and high metha ne contents, so that there are abundant methane resources in unit area . (4) Selecting a coal seam which roof and floor lithologies are exc ellent in sealing ability and areas suffered the slightest structural disruption to guarantee an excellent storage condition. (5) As far apart as possible from the methane escape area. Selecti ng the mid and deep parts of the gently inclined zone and the seepage area of the second flexure zone, where the tectonic opening of coal se am is good, and the status of seepage flow is reasonable. (6) Topographically, selecting low land area or river channel are a parallel to the main tectonic directions. 5ª±2ª±2 Seletion of target area Coal seams Noª±3 and Noª±11 which were deposited widely in the are a are taken as the main target seams in target area selection. According to the above principles, the most favorable targets shou ld rely on the type ¢ñ exploration and development areas. Considering the depth of weathering and oxidizing zone of 300m and 500m respective ly for Noª±3 and Noª±11 coal seams in the area, the minimum burial dep th of more than 500m should first be selected, and powder coal and lum p plus powder coal area should be avoided. That is, areas confined to the south of S20 and 56ª¬££ boreholes and west of 185ª¬££ and 136ª¬££ boreholes in the south. The northern area is located to the west of b oreholes 63ª¬££, 78ª¬££, 88ª¬££, 110ª¬££, 115ª¬££. In the(continued on page 21) above two areas the target area can be scaled down further to determi ne the positions for drilling. In the deeper strata beyond the studied mine field, the dip angle of the coal seam is very gentle (3-5 degree), tectonic events very sli ght, except several methane escaping zones and the boundary of deep p art of Xiayukou mine area where the coal body was disrupted greatly by folds, these areas can be possible methane seepage flow zones and pos sible methane prosepctive target areas. References 1. China National Coal Corporation (1992), Compilation of the 1:20 0,000 Gas Geological Map of China, Xi¡äan Maps Publishing House 2. Zhang Xinmin et al. (1991), Coalbed Methane in China, Shaanxi S cience and Technology Publishing House 3. Zhao Qingbo et al. (1995), Characteristics of Coalbed Methane D istribution and the Current State of CBM Exploration Technology of Chi na, the 1995 Collection of Papers on CBM Exploration compiled by the M anagement Division for CBM Exploration Projects in New Areas of the Ch ina National Petroleum Corporation in April, 1996 4. Zhang Wuchai et al. (1995), the Characteristics of CBM Distribu tion in China and the Controlling Factors, the 1995 Collection of Pape rs on CBM Exploration 5. Sun Maoyuan et al. (1995), Joint Report on the Potential for CB M Development in China (Chinese Version) prepared by the China Coalbed Methane Clearinghouse and USEPA.