煤系碳酸盐岩天然气勘探前景初探
——以华北太原组为例

王瑞瑞1,2,3,傅雪海1,2,张宝鑫1,2,冯嘉楠4,李伟松5

(1.中国矿业大学 煤层气资源与成藏过程教育部重点实验室,江苏 徐州 221008;2.中国矿业大学 资源与地球科学学院,江苏 徐州 221116;3.中国地质科学院地质研究所 自然资源部深地动力学重点实验室,北京 100037;4.四川省煤田地质局一四一队,四川 德阳 618000;5.陕西地矿集团有限公司,陕西 西安 710054)

摘 要:煤系碳酸盐岩天然气的勘探前景是值得探究的重要问题,以华北太原组为例进行初步研究分析。太原组是华北最有利的生烃岩系之一,煤层之上发育多套灰岩。近年来,丰富的太原组碳酸盐岩天然气资源在山西霍西煤田被发现,该发现可为今后煤系碳酸盐岩天然气的勘探提供启示。首先剖析了霍西煤田太原组碳酸盐岩气藏实例,其次对太原组灰岩的空间展布规律、太原组烃源岩和热演化程度、灰岩储层发育、盖层、圈闭、运移和保存条件进行了总结。研究认为,太原组煤层-灰岩沉积组合与其上的多套泥岩和煤层可以组成生储盖组合,而储层的近烃源岩优势使得煤系气易于在太原组灰岩中聚集。从太原组煤和灰岩厚度分布特点来看,北部煤层厚而灰岩薄,南部煤层薄而灰岩厚。从烃源岩角度来看,太原组煤层和暗色泥岩是优质的烃源岩,而中等变质程度的煤有利于游离的煤成气的形成。从储集空间来看,华北许多地区太原组灰岩普遍具有较好的岩溶和裂隙储集空间发育。从封盖条件看,太原组灰岩层上覆的泥岩可作为良好的盖层。因此,太原组具有煤系碳酸盐岩气藏形成所需的物质基础。此外,华北太原组煤系碳酸盐岩气藏的形成需要多个地质要素在时间与空间上相互匹配。煤系碳酸盐岩储集空间和圈闭的发育是气藏得以形成的前提,晚期成藏是气藏能否形成的关键。华北主要含煤盆地均广泛发育圈闭,而盆地新生代总体伸展背景下的局部隆升以及现今煤矿采动导致的储层压力降低都可能有利于煤系气向煤系碳酸盐岩储集空间中运移和聚集。

关键词:煤系碳酸盐岩;天然气;太原组;霍西煤田;华北板块

中图分类号:TE122

文献标志码:A

文章编号:0253-2336(2021)12-0138-12

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王瑞瑞,傅雪海,张宝鑫,等.煤系碳酸盐岩天然气勘探前景初探——以华北太原组为例[J].煤炭科学技术,2021,49(12):138-149.doi:10.13199/j.cnki.cst.2021.12.017

WANG Ruirui,FU Xuehai,ZHANG Baoxin,et al.Preliminary study on exploration prospect of carbonate gas in coal measures:a case study of Taiyuan Formation in North China[J].Coal Science and Technology,2021,49(12):138-149.doi:10.13199/j.cnki.cst.2021.12.017

收稿日期:2021-04-21

责任编辑:常 琛

基金项目:江苏省自然科学基金青年基金资助项目(BK20180 665);中国博士后科学基金资助项目(2019M652014);自然资源部深地动力学重点实验室开放基金

作者简介:王瑞瑞(1984—),男,江苏盐城人,副教授,硕士生导师,博士。Tel: 0516-83591000,E-mail: wrr@cumt.edu.cn

Preliminary study on exploration prospect of carbonate gas in coal measures: a case study of Taiyuan Formation in North China

WANG Ruirui1,2,3,FU Xuehai1,2,ZHANG Baoxin1,2,FENG Jia’nan4,LI Weisong5

(1.Key Laboratory of Coalbed Methane Resources and Reservoir Formation ProcessMinistry of Education of ChinaXuzhou221008,China; 2.School of Resources and GeosciencesChina University of Mining and TechnologyXuzhou 221116,China;3.Key Laboratory of Deep-Earth Dynamics of Ministry of Natural ResourcesInstitute of GeologyChinese Academy of Geological SciencesBeijing 100037,China; 4.Sichuan Province Coalfield Geological Bureau One Hundred Forty-one TeamDeyang 618000, China; 5.Shaanxi Geology and Mining Group Co.Ltd.Xian 710054,China)

Abstract:The exploration potential of carbonate gas in coal measures is still rarely studied.A case study of Taiyuan Formation is presented here.The Taiyuan Formation is one of the most favorable hydrocarbon-generating deposits in North China,and several limestone layers above the coals are well-developed in this formation.In recent years,Taiyuan Formation limestone gas is discovered in Huoxi Coalfield in the North China Block.This discovery can provide us in the future with significant inspiration in the exploration of carbonate gas in the coal measures.In this study,the case of Taiyuan Formation limestone gas was initially analyzed.Then,the distribution of limestone layers of Taiyuan Formation,source rocks and their thermal evolution,reservoir space of the limestone,cap rocks,trap development,migration and preservation conditions are summarized.The preliminary understandings are as follows.The coals,limestones in Taiyuan Formation and overlying mudstones can make up source-reservoir-cap assemblages.As the limestones are close to the coals,gas in the coal measures can be easily accumulated in the limestone layers.As for the thickness of coals and limestones in Taiyuan Formation,the northern part of North China is characterized by thick coals and overlying thin limestones,while the south part by thick limestone and underlying thin coals.The coals and dark mudstones in Taiyuan Formation are premium source rocks,and coals of moderate metamorphism are in favor of the formation of free gas.Karst and fissures acting as the reservoir space are well developed in the limestone layers in Taiyuan Formation.The overlying mudstones can work as good cap rocks.Thus,the Taiyuan Formation has the material fundament for the formation of carbonate gas reservoir in coal measures.Besides,the formation of the carbonate gas pool in coal measures needs good matching of geological factors.The development of reservoir space and trap widely occurring in the coal-bearing basins in North China is the prerequisite,while the late-stage gas accumulations are the key.The local uplift in a regional extensional background in the basins,as well as the mining activities accompanied by decreasing reservoir pressure,are helpful to the migration and accumulation of the coal measures gas to the carbonate reservoir.

Key words:carbonatite in coal measures; gas; Taiyuan Formation; Huoxi Coalfield;North China

0 引 言

煤系又称含煤岩系或含煤地层或含煤建造,是一套含有煤层或煤线并在成因上有共生关系的沉积体系,主要形成于海陆交互相或陆相环境[1]。由整个煤系中的烃源岩母质在生物化学及物理化学煤化作用过程中演化生成的全部天然气被称为煤系气[2]。煤系气常以吸附气、溶解气和游离气的形式赋存于煤系之中[3]。由于煤系中的生、储、盖具有近源优势,因此,我国有近2/3的大型和特大型煤成气田是以煤系内的生储盖组合为特征[4]。煤系气按源-储组合关系可分为源-储一体型和源-储紧邻型[5-6]。源-储紧邻型又可根据煤岩与围岩的组分为煤岩-泥/页岩型、煤岩-砂岩型和煤岩-灰岩型,其分别对应于煤系页岩气、致密砂岩气和碳酸盐岩天然气[7]。近年来,我国的煤系“三气”(煤层气、煤系页岩气和致密砂岩气)勘探都相继取得了巨大的进展[8-10]。同时,我国华北和华南上古生界煤系普遍发育碳酸盐岩,例如:华北的太原组和华南的龙潭组等。煤系生烃潜力巨大,煤系碳酸盐岩分布广阔,其构造裂缝或溶蚀孔洞的储集空间可观,因而煤系碳酸盐岩天然气的勘探前景是值得探究的重要问题[1, 11]。此外,煤系碳酸盐岩天然气的研究对煤矿瓦斯防治也具有重要意义。

我国华北上石炭统-下二叠统太原组形成于海侵体系域成煤环境,发育多套优质煤层,是华北地区最有利的生烃层系之一[12]。在太原组下部煤层(山西西部的9~11号煤,山西东部的15号煤)沉积时发生的海侵事件使得先前的泥炭沼泽的发育终止并被保存下来,形成了煤层-灰岩沉积组合[13-14]。华北石炭-二叠纪地层中发育多套泥岩、页岩和煤层,可以作为很好的盖层[15-16]。所以,太原组煤岩、灰岩和其上的泥岩、页岩与煤层易于形成良好的生储盖组合[4,12]。近年来,太原组碳酸盐岩天然气资源在山西霍西煤田被发现。因此,笔者首先剖析霍西煤田太原组碳酸盐岩气藏实例,然后对华北太原组灰岩的空间展布规律、太原组烃源岩和热演化程度、灰岩储层发育、盖层、圈闭、运移和保存条件进行了总结,最后初步探讨华北太原组碳酸盐岩天然气的勘探前景。

1 霍西煤田实例及启示

太原组碳酸盐岩天然气资源此前仅在少数矿区被报道,例如:山西沁水盆地阳泉矿区[17-18]和安徽淮南煤田潘谢矿区[19]。1982年,地矿部第2石油普查勘探指挥部在沁水盆地阳泉矿区的钻孔(阳1井)揭示,浅部太原组灰岩赋存了天然气资源[20]。虽然单井日产气数千立方米,但是,产气量在1个月内迅速衰减。截至目前,丰富的太原组碳酸盐岩天然气资源仅在山西霍西煤田被发现。2012年10月至2015年6月,霍西煤田义棠矿和安益矿在100503工作面共布置钻场46个、钻孔455个,累计从埋深300~600 m的太原组K2、K3和K4灰岩中抽采高浓度甲烷气约3.65×107 m3。2015年,义棠煤矿发电公司建成一期项目,开始利用天然气发电。2017年,发电公司年利用天然气约3×107 m3,年发电量约9 900万kW·h。2018年,随着三期项目的投产,发电公司总装机容量达55.8 MW,年发电量达38 000万kW·h。到目前为止,太原组碳酸盐岩天然气产量依然稳定。

山西霍西煤田义棠矿和安益矿位于现今仍在活动的山西地堑系中灵石隆起与太原盆地过渡的构造斜坡带。煤矿主要构造为宽缓的大西庄背斜和一系列NE-SW走向的小型正断层。矿井内太原组碳酸盐岩天然气抽取层位为太原组下段。太原组煤层多为低灰分烟煤,成熟度适中,具有着良好的生气能力。埋藏作用及燕山期以来的岩浆作用使得煤系中有机质快速成熟并大规模生烃。太原组灰岩厚度大,层位稳定,主要为K4、K3和K2三层灰岩,分别为7号煤,8号煤和9号煤的顶板。太原组灰岩储层较为致密,孔隙大多为微米孔,基质孔隙率为1.59%~2.54%,渗透率约为0.001 5 m2[21]。但是,太原组灰岩中节理及溶蚀孔-洞发育。研究区主要发育北东-南西、北西-南东和近南北3个方向节理,其中,北东-南西为节理优势方向。灰岩中节理的线密度24~34条/m。K2灰岩层的岩心采取率为11%~81%(平均为57%),而K2灰岩层的钻井漏失速率为0.16~20.33 m3/h(平均为2.655 m3/h) [22]。排除断层和陷落柱的影响后,较低的岩心采取率和较大的钻井漏失速率表明灰岩层的岩溶和裂隙非常发育[22]

该区太原组煤层现今处于风化带,不但煤层气含气量很低(1.19~1.36 m3/t),而且,煤层气中甲烷的体积分数也较低(3.75%~11.60%)[23]。与之形成鲜明对比的是,太原组主采煤层之上的灰岩中不但天然气资源丰富,而且天然气中甲烷的体积分数在90%以上[23-24]。初步研究认为,新构造运动及现今煤矿采动(例如:地下水抽采降压等)可能都促进了邻区煤层气的解析,而来自煤层气富集区的地下水携带或溶解煤系气运移至大西庄背斜核部聚集形成缝洞型气藏[22,25](图1)。

图1 山西霍西煤田太原组碳酸盐岩天然气成藏模式[25]

Fig.1 Accumulation model of Taiyuan Formation carbonate gas in Huoxi Coalfield in Shanxi Province[25]

通过上述分析可以看出,太原组碳酸盐岩气藏可能为不连续型气藏,与煤系其他类型气藏(煤层气、致密砂岩气和页岩气)具有显著的差别[1,11]。同时,煤系碳酸盐岩天然气也具有常规天然气所不具备的近烃源岩优势。

2 太原组空间展布规律

我国华北板块的石炭-二叠纪煤系形成于华北聚煤盆地,其南北分别为秦岭-大别造山带和阴山燕山造山带所夹持。该聚煤盆地主要形成于海陆交互相,包括冲积扇、河流、湖泊、三角洲、潟湖-潮坪、碳酸盐台地等沉积环境,自下而上沉积形成了本溪组、太原组、山西组、下石盒子组、上石盒子组和石千峰组地层[15-16]

邵龙义等[16]将华北石炭-二叠系划分为2个2级构造层序和7个3级层序,其中,3级层序SQ1的底界面为本溪组底,顶界面为太原西山9号煤的底(在南华北盆地为一1煤的底、在山东地区为17煤的底),而SQ1的顶界面至太原组的顶部之间的地层构成了3级层序SQ2(图2a)。在这2个3级层序中,北华北地区的沉积体系主要为陆相,中华北地区和南华北地区的沉积体系主要为海陆过渡相,而碳酸盐台地沉积主要在中华北和南华北地区的本溪组和太原组(图2b和2c)。SQ1时期,华北盆地聚煤作用较差(图2d);而SQ2时期,聚煤作用较强,全区煤层总厚度0~38 m,平均7.6 m(图2e)。

图2 华北板块晚石炭世-早二叠世层序划分及沉积相图[16]

Fig.2 Subdivision of the third-order sequences and lithofacies paleogeograpy map of the Late Carboniferous to Early Permian in North China[16]

太原组下部煤层(山西西部的9~11号煤,山西东部的15号煤)沉积时,华北板块整体格局由“南升北降”翘变为“北升南降”,物源区从南向北转换,同时海侵方向由北变为南[26]。海侵事件使得先前的泥炭沼泽的发育终止并被保存,形成了煤层-灰岩沉积组合[14]。受南东方向海侵的影响,煤层厚度总体具有由南东向北西逐渐增厚趋势,而灰岩的厚度总体具有由南东向北西逐渐减薄的趋势(图3a和3b)[12,14-15]。其中,灰岩在华北东南部的宿南、蚌埠一带最厚,累计达76 m[27]

图3 华北太原组南北方向剖面[14-15]

Fig.3 North-south trending cross sections of the Taiyuan Formation in North China[14-15]

3 太原组烃源岩和热演化程度

太原组的煤岩、暗色泥岩和灰岩是主要的潜在烃源岩[27-28]。由于华北不同盆地经历了截然不同的构造-热演化历史,因而下文对华北主要的含煤盆地(鄂尔多斯盆地、沁水盆地、南华北盆地和渤海湾盆地)的烃源岩及其热演化程度分别进行分析(图4)。

图4 华北主要盆地及东西方向剖面[29-32]

Fig.4 The distribution of the main coal-bearing basins and east-west trending cross section of North China[29-32]

在鄂尔多斯盆地,太原组煤层属腐植Ⅲ型干酪根;太原组泥岩有机碳含量一般为2%~4%,属腐植Ⅲ型干酪根,部分泥岩为Ⅱ2[33];太原组生物灰岩含有丰富的碳质沥青,属于腐泥-腐植过渡Ⅱ2型干酪根,具有较强的生烃能力[34]。鄂尔多斯盆地主要具有“W”型埋藏史特征,而盆地东缘则存在差异(图5a)。盆地东缘在晚三叠世开始隆升,直至约150 Ma时经历构造转换而达到最大埋深,随后盆地东缘持续隆升。鄂尔多斯盆地大部分地区,石炭-二叠纪煤系的Ro都在1.5%以上,在盆地南部庆阳-富县-榆6井一带有机质热演化程度最高,Ro达2.8% 以上,Ro向盆地东缘方向逐渐降低[34] (图6a)。

在沁水盆地,太原组煤层属高演化Ⅲ型干酪根,显微组分以镜质组为主,其次是惰质组;太原组泥岩以Ⅲ型干酪根为主,显微组分以镜质组为主,其次为惰质组,壳质组较少;太原组灰岩以Ⅱ2型干酪根为主[36]。沁水盆地主要具有“V”型埋藏史特征,其特点是自印支晚期以来,地层持续隆升(图5b)。榆社、临汾-洪洞等地区受新生代山西地堑系影响[37],埋藏史曲线呈现反“N”型特征。盆地内大部分地区石炭-二叠纪煤岩的Ro大于2%,Tmax大于510 ℃已进入过成熟阶段。南北两端即阳城地区和阳泉地区煤演化程度高,Ro>2.59%,为无烟煤区;东西两侧热演化程度较低,东侧Ro为1.5%~2.0%,以瘦煤为主,属湿气阶段,而西侧Ro为0.75%~1.25%,为焦煤至气煤,处于生油阶段[38](图6b)。

在渤海湾盆地,太原组烃源岩主要为Ⅱ2型,少部分为Ⅲ型[33]。在渤海湾盆地,各坳陷具有不同的埋藏历史。例如,济阳坳陷具有沉降-抬升剥蚀-再沉降特点,太原组最大埋藏深度大于4 000 m,这有利于太原组碳酸盐岩天然气的保存[40](图5c);而临清坳陷经历了早石炭世晚期-三叠纪早期的埋藏沉积、印支期缓慢的隆升剥蚀、喜马拉雅早期埋藏接受沉积、喜马拉雅早期末的隆升以及新生代晚期的埋藏[41]。渤海湾盆地C-P烃源岩成熟度在平面上分布极不均匀,总体具有盆地北部烃源岩成熟度低而南部高的特征(图6c)。部分地区烃源岩达到过成熟阶段,其Ro高达3.0%,其他地区主要处于成熟阶段。

在南华北盆地,不仅太原组煤和泥岩有机质含量高,生烃潜力较大,干酪根为Ⅲ型,而且灰岩有机质含量也较高(有机碳平均大于1.00%),也属于Ⅲ型干酪根[43]。南华北盆地总体具有反“N”型埋藏史特征(图5d)。由于盆地在印支期与秦岭-大别造山带为盆山耦合关系,因而石炭-二叠纪地层受印支运动影响较大,且隆升-剥蚀强度具有由南向北、由东向西逐渐减弱的趋势[26]。石炭-二叠系主要以复式向斜的形式残存于盆地中西部地区的中新生代凹陷中,例如,济源-开封坳陷带、豫西、太康、周口坳陷、鹿邑-倪丘集凹陷[44]。南华北盆地太原组烃源岩在盆地西北部和西南部相对较高,Ro在2.0%以上,而盆地的东部大部分地区演化程度较低,其Ro低于1.0%[44](图6d)。

图5 华北石炭-二叠纪煤系埋藏史图

Fig.5 Burial history of Carboniferous to Permian coal mearsures in North China

按煤阶可以分为低煤阶(Ro,max<0.65%)、中煤阶(0.65%<Ro,max<1.65%)和高煤阶(Ro,max>1.65%)[9]。华北石炭-二叠系煤阶在华北的分布具有下述特征(图6):①高煤阶煤主要分布于鄂尔多斯盆地的南部、沁水盆地的东部和西南部、渤海湾盆地的西南部以及南华北盆地的郑州-太康和驻马店-确山地区;②中煤级煤分布于鄂尔多斯盆地的中部偏北地区、沁水盆地的西北部、渤海湾盆地的中部和北部以及南华北盆地的中部;③低煤阶煤分布于鄂尔多斯盆地的东北部以及渤海湾盆地北部的局部地区。受岩浆岩侵入影响,北京、邯郸和徐州等地局部地区的煤变质程度较高,形成了无烟煤和天然焦。一般来说,煤阶越高,煤层含气性越好[39]。但当中-低煤阶煤层(Ro<1.5%)受构造抬升至浅部时,大气降水和其他地表水携带细菌补给煤层,可使煤层次生生物成因气得以形成[45]。此外,中等变质程度的煤在勘探开发中易降压、解吸、扩散和运移,有利于煤成气运移至储集空间中富集[46]

图6 华北石炭-二叠纪煤系镜质体反射率(Ro)等值线

Fig.6 Contour map of vitrinite reflectance of Carboniferous to Permian coal mearsures in North China

4 太原组灰岩储集空间

太原组灰岩通常比较致密,其基质孔隙度和渗透率一般非常低[47-48]。太原组灰岩与常规碳酸盐岩相比,在储集空间方面具有鲜明的特点:①太原组灰岩单层薄而层数多;②太原组灰岩通常有机质含量较高,富有机酸成岩流体有利于太原组灰岩溶蚀作用发生而形成溶蚀孔洞;③太原组直接覆盖在中奥陶统的风化剥蚀面上,而中奥陶统为富含岩溶水的巨厚灰岩,因而太原组多层薄层灰岩容易与奥灰水产生水力联系[49],有利于灰岩岩溶储集空间的形成;④华北陆块在中-新生代经历了多期构造叠加,太原组灰岩形成了许多构造裂缝,有利于地表水沿着先期形成的裂缝下渗发育不同程度的埋藏型岩溶。因此,构造裂缝及溶蚀孔-洞可为太原组碳酸盐岩天然气提供主要的储集空间。

太原组灰岩岩溶和裂缝在华北板块的西部、南部和东南部较为发育,可作为良好的储集空间,自西向东具体实例有[47,50-53]:①鄂尔多斯盆地东部;②山西霍西煤田李雅庄矿、新阳矿、西郭矿和黑龙矿;③山西沁水煤田寺河矿、新潮矿、新景矿、长平矿和固县勘查区;④山西河东煤田庞庞塔矿和聚德矿;⑤河南安鹤煤田大众煤矿和寺湾矿;⑥河南登封煤田白坪井田和国投登封教学三矿;⑦河南焦作煤田九里山煤矿、韩王矿和演马庄矿;⑧河南平顶山煤田;⑨河南汝州煤田新朝川矿;⑩河南新安煤田正村井田;河南新密煤田李粮店矿;河南偃龙煤田郭村矿;河南永夏煤田;江苏徐州矿区拾屯矿、新河煤矿、唐庄煤矿、三河尖矿、大屯矿区姚桥矿;淮北煤田临涣矿区、宿县矿区、濉肖矿区;淮南煤田;山东肥城煤田国家庄矿;山东黄河北煤田赵官井田;山东济宁煤田许厂矿;山东兖州煤田南屯矿。

5 盖层条件

盖层物性和厚度对于天然气的逸散与否起到关键作用。太原组煤层-灰岩沉积组合之上的泥岩、页岩与煤层可以作为很好的盖层[15-16]。太原组泥岩厚度普遍较高,一般在130 m以内,主要有河北德州(泥岩总厚度为130 m)和鲁西南(平均厚度101 m)2个沉积中心。太原组之上的地层也发育了多套泥岩,且泥岩的总厚度较大,例如:山西组泥岩厚度也普遍较高,为5~160 m,平均厚度43 m[16]。此外,煤系中多套生储盖叠置会产生累积封闭效应[1]。因此,太原组碳酸盐岩气藏总体来说具有良好的封盖条件。

6 圈闭、运移和保存条件

华北多期构造活动形成了许多圈闭。鄂尔多斯盆地北部的伊蒙隆起为古生代、中生代的超覆沉积区,圈闭类型主要为超覆不整合圈闭[54];而盆地西缘构造活动强烈,在晚侏罗世受挤压形成了一系列逆断层及夹于其间的背斜、半背斜构造,具有面积小而埋藏浅的特点[55]。沁水盆地内主要圈闭为背斜,具有数量多、分布广、规模小、在平面上成排成带分布等特点,单个构造多为NNE及NW向狭长状展布,其形成受燕山期及喜山期所控制[38]。渤海湾盆地主要发育隐蔽圈闭[56],但济阳坳陷深层发育丰富的构造圈闭,包括压性、张性和复合型等圈闭类型,多形成于燕山期-喜马拉雅期[57]。南华北盆地圈闭类型以断鼻构造和背斜构造为主,具有成排、成带分布的特点,是印支-燕山运动的产物,但后期构造活动对其进行了改造,最终定型于古近纪末(约23.5 Ma),这有利于晚期生烃的聚集[58]

煤系碳酸盐岩具有邻近烃源岩的优势,因此,煤系碳酸盐岩天然气具有运移距离短的特征。盆地抬升使得太原组煤层气大规模解吸或者中-低煤级煤在浅部有利条件下生成次生生物气,并被运移至圈闭中。天然气充注与圈闭存在2种可能 :①天然气充注时间早于圈闭形成时间,那么气藏不可能形成;②天然气充注时间等于或晚于圈闭形成时间,那么气藏有可能形成。而后者又存在2种可能:①天然气充注和圈闭在时空上可以匹配,但由于充注时间较早,气藏虽能在早期形成但是天然气会持续逸散。在华北许多煤矿,太原组灰岩储层中大都被地下水所充注,也证实了早期形成的气藏后期保存效果相对较差。②圈闭先于天然气充注时间形成,而天然气在晚期充注成藏。事实上,天然气具有分子体积小,重量轻,易扩散的特点,而气藏的形成是天然气运聚和逸散之间动态平衡的结果[59]。早期成藏的气田易受晚期或新构造运动的影响而发生调整或逸散。因此,越晚成藏越有利于天然气较大规模的聚集及保存[60-61]。如果煤系长期处于隆升状态,则煤系气会大量逸散。换言之,盆地在新生代总体伸展背景下的局部隆升(例如,山西地堑系中灵石等隆起的局部隆升)以及现今煤矿采动导致的储层压力降低都可能有利于煤系气向煤系碳酸盐岩储集空间中运移和聚集。

7 结 论

1)太原组煤层-灰岩沉积组合与其上的多套泥岩和煤层可以组成生储盖组合,而储层的近烃源岩优势使得煤系气易于在太原组灰岩中聚集。从太原组煤和灰岩厚度分布特点来看,北部煤层厚而灰岩薄,南部煤层薄而灰岩厚。从烃源岩角度来看,太原组煤层和暗色泥岩是优质的烃源岩,而中等变质程度的煤系有利于游离的煤成气的形成。从储集空间来看,华北许多地区太原组灰岩都具有较好的岩溶裂隙储集空间发育。从封盖条件看,太原组灰岩层上覆的泥岩以及煤系多套生储盖叠置产生的累积封闭效应都有利于煤系气保存。总体来看,太原组具有煤系碳酸盐岩气藏形成所需的物质基础。

2)华北太原组煤系碳酸盐岩气藏的形成需要多个地质要素在时间与空间上相互匹配。煤系碳酸盐岩储集空间和圈闭的发育是气藏得以形成的前提,晚期成藏是气藏能否形成的关键。华北主要含煤盆地均广泛发育圈闭,而盆地新生代总体伸展背景下的局部隆升以及现今煤矿采动导致的储层压力降低都可能有利于煤系气向煤系碳酸盐岩储集空间中运移和聚集。

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