Secondary biogenic coalbed gas in some coal fields of China

The secondary biogenic coalbed gas, a new genetic and energy source type of coalbed gas in China, has been found in Xinji, Liyazhuang and Enhong areas. The essential characteristics of this type of gas are: (i) the major component of the gas is methane, with C1/C1-5 value higher than 0.99, indicating that the gas is part of dry gas; (ii) theδ13C1 value is in the range of -61.7‰to -47.9‰, mostly lower than -55‰, which is much lower than the estimatedδ13C1 value of thermogenic methane according to the thermal evolution degree of the coal rocks (with R0 value from 0.87% to 1.43%), showing the characteristics of the secondary biogenic gas; (iii) theδ5D value of methane ranges from -244‰to -196‰; (iv)δ13C 2 value ranges from -26.7‰to -15.9‰andδ13C 3 value ranges from -10.8‰to -25.3‰, indicating that the heavier hydrocarbons have a thermogenic origin; (v) the content of CO2 is very low, andδ13CCO2 value changes greatly, reflecting a characteristic of secondary change; (vi)δ15N2 value ranges mainly from -1‰to +1‰, indicating N2 derived significantly from air. The negative linear correlation between the contents of N2 and CH4 reflects the activity of bacteria bearing surface water infiltrating into coal beds. The comprehensive tracing indices show that the coalbed gas in the studied areas is the mixed gas of primarily secondary biogenic gas and a part of remnant thermogenic gas. The uplift of coal beds and the development of faults in the studied areas create favorable conditions for the formation of the secondary biogenic gas.     

鄂尔多斯盆地神府地区含煤层气系统特征

为指导神府地区上古生界煤系天然气共探合采,综合应用岩心、测井及生产动态等资料,对煤层气及煤系砂岩气成藏地质条件展开系统研究,划分含煤层气系统并分析其成藏模式。结果表明:神府地区稳定的构造背景和优越的沉积环境为煤系天然气成藏奠定了良好的源-储-盖条件。受区域性低渗透泥质岩层阻隔,区内上古生界煤系地层可划分为8+9~#煤-本1段、太2段砂岩下含煤层气系统和4+5~#煤-太1段、山2段砂岩上含煤层气系统2套叠置含煤层气系统,西部深陷带中南部是含煤层气系统内煤层气与煤系砂岩气的合采有利区。     

新疆准南煤田乌鲁木齐矿区煤层气储层特征

准南煤田乌鲁木齐矿区早一中侏罗纪富煤带，件、储层特征、资源量、煤层气保存条件的专门研究，通过实际打样测试，对该区煤储层的含气性、吸附性、探开发的储层条件。聚集着丰富的煤层气资源，笔者通过对该区煤层气赋存的地质条评价出了有利区块，为下一步煤层气勘探提供了依据。本文主要孔隙结构、渗透性等进行了系统研究，探讨本区煤层气有利于勘     

柴达木盆地三湖坳陷浅层生物气成藏模式分析

柴达木盆地三湖地区第四系浅层生物气一直被认为完全是由第四系腐殖型有机质生成的.最新研究结果表明:(1)第四系生物气藏探明储量与源岩条件之间不匹配;(2)高分辨率地震剖面特征表明三湖坳陷气田区域第四系发育了大量的气烟囱;(3)生物气组分特征表明有热成因气混入的可能性;(4)乙烷等重烃气碳同位素特征表明三湖坳陷第四系气藏有腐泥型生物气混入;(5)三湖坳陷上第三系狮子沟组发育偏腐泥型有机质.笔者认为第四系浅层生物气藏的形成机理比较复杂,包括“下生上储”和“自生自储”两种模式,第四系的浅生物气由上第三系狮子沟组生物气与第四系生物气混合而成.     

沁水盆地南部地区砂体与煤层空间叠置特征及其在煤层气和致密气合采开发中的意义

以沁水盆地南部煤层气和致密气共探合采研究区为例,详细解剖煤系地层砂体发育特征及其与煤层空间叠置关系,并探讨其对煤层气与致密气合采开发的影响。研究结果表明,煤系地层砂体成因类型丰富,砂体规模和形态特征各异。砂体结构特征主要受沉积相演化控制,形成五种主要空间叠置类型：孤立分散型、水平搭接型、垂向叠置型、错位叠置型和水平-错位复合叠置型。在沉积作用控制下,不同成因类型砂体与煤层可以形成多种空间叠置组合模式。砂体含气饱和度和压力及煤层含气量、物性和压力均受到两者空间叠置关系的显著影响。加强该方面研究,将对两气合采可行性评估及合采产气量预测具有重要意义。     

The influence of tectonic evolution on the accumulation and enrichment of coalbed methane (CBM)

The current accumulation and enrichment of CBM reservoir is a result of the preservation and destruction of former CBM, after the superposing evolution of reversing-and-uplifting and the subsequent evolution within the coal-bearing basin. The critical moment of the CBM reservoir formation is the time when the burying depth of the overlying net thickness amounts to the least in geological history after the gas generation of coal beds. Except the coal-bearing basins of lower metamorphism, most basins suffered the evolution stage of reversion and uplift. The formation of the CBM reservoir is controlled by the beginning and lasting time, and the intensity of reversing and uplifting. The tectonic evolution after reversing and uplifting also affects the accumulation of CBM in coal-bearing basin. The CBM constantly dissipates in the area of chronically uplifting and de-nudating. The area developed overlying sedimentation is advantageous to the preservation of CBM, but also can lead to the reduction of CBM saturation.     

Division and formation mechanism of coalbed methane reservoir in Huainan Coalfield,Anhui Province,China

The coalbed methane (CBM) reservoir is the basic geological unit of CBM storing and CBM resource developing. The forming conditions of the CBM reservoir include coal thickness, coal rank, seam buried depth, caprock, and geological structure. The division of a CBM reservoir in the Huainan coalfield is mainly based on the geological structure form and seam buried depth. According to the Fufeng nappe and the secondary structures of Huainan syn-clinorium, seven CBM reservoirs are divided in the Huainan Coalfield, such as Caijiagang, and the dynamic mechanism of CBM forming is analyzed. The structural position where the CBM reservoir is located has the important controlling role on the features of CBM reservoir.     

Discussion of gas enrichment mechanism and natural gas origin in marine sedimentary basin,China

There are abundant natural gas resources in Chinese marine sedimentary basin. The exploration hot shots of natural gas are the Palaeozoic marine strata here in recent years, and several large scale gas fields have been discovered. Chinese Palaeozoic high-post matured and coal measure hydrocarbon source rocks are mainly prone to gas generation in the present. This research considered that gas source rocks and TSR are the key cause of gas enrichment of marine strata. High-quality argillaceous and coal measure hydrocarbon rocks are distributed widely in the Palaeozoic marine strata, which have been in highly matured phase in the present. The argillaceous source rock generally contains various sulfates that could accelerate crude oil cracking to gas for TSR occurrence, and coal measure source rock mainly generates gas, so Chinese marine basin gives priority to accumulating gas. Marine strata have not founded oil reservoirs in the Sichuan Basin and Ordos Basin, and they consist mainly of dry gas. Marine natural gases are the mixed gases of oil cracking gas and coal-formed gas in a general way, oil cracking gases contain usually some H2S and CO2. Hydrocarbon carbon isotopes are very complicated, and methane and ethane isotopic values bear apparent reversal caused by thermal evolution and mixing among different genetic types of natural gas. Coal-formed gases are the main component of Chinese marine natural gas. The Upper Permian of the Sichuan Basin and the Carboniferous-Permian of the Ordos Basin coal measure hydrocarbon source rock present large hydrocarbon generation potential, which are the prospecting highlight of marine natural gas hereafter. Oil cracking gas exploration will be paid much attention to in the Tarim Basin because of the lack of coal measure hydrocarbon source rock.     

川西中段山前带雷口坡组气藏形成条件及过程

利用探井和地球物理资料,系统分析川西中段彭州气田钻探成果和中三叠统雷口坡组天然气成藏特征。川西中段深层有优质的下寒武统烃源岩,雷口坡组微生物型储集层沿山前带呈北西向展布,晚期发育的沟源断层形成的输导体系,早于断层形成的圈闭体系,具备形成大气田的基本条件。川西中段雷口坡组气藏为次生气藏,其天然气来源于下伏古气藏及烃源岩,深层的天然气通过断层向上运移聚集。川西中段雷口坡组的成藏特点为:(1)深层古油气藏发育;(2)微生物储层发育;(3)沟源断层晚期发育;(4)地震促使断层活化;(5)天然气跨层运移成藏。除雷口坡组外,深层应该还存在残余的古气藏,可能赋存的层位为二叠系和震旦系。     

Comprehensive study on genetic pathways and parent materialsof secondary biogenic gas in coalbeds

The types of substances in coal rock used by microbes,the specific ways in which microbes produce secondary biogenic gas(SBG)and whether substances exist in the coal seam for the formation of a large amount of SBG are important basic scientific issues.This paper conducts a systematic and comprehensive research study on the above issues using methods such as the isotopic tracing of gas,the analysis of coal organic geochemistry,and gas-producing simulation experiments of coal.Results show that the formation of SBG is by the microbial reduction of CO2and the SBG-producing coal seam undergoes microbial degradation.The thermogenic heavy gaseous hydrocarbons have also been degraded by microorganisms and possibly transformed into microbialoriginated CO2.A large amount of CO2,a relatively large amount of H2and a certain amount of heavy gaseous hydrocarbons may form in the thermal evolution of coal.These substances and the microbial-originated CO2and coal seam water can finally become parent materials of SBG.These components are rich in coal seams of medium–low thermal evolution,which should be the main coal seams for SBG formation and exploration.