淮南潘谢矿区深部煤系烃源岩地球化学特征与成烃潜力

张文永1,2,窦新钊1,2,3,刘桂建3,孙 贵1,2,赵志义1,2

(1.安徽省煤田地质局勘查研究院,安徽 合肥 230088; 2.安徽省非常规天然气工程技术研究中心,安徽 合肥 230088; 3.中国科学技术大学 地球和空间科学学院,安徽 合肥 230088)

摘 要:为了查明淮南煤田潘谢矿区深部煤系的烃源岩生烃潜力,在对煤系烃源岩系统采样的基础上,应用总有机碳含量、氯仿沥青“A”、岩石热解、干酪根碳同位素、镜质组反射率等有机地球化学测试方法,结合深部煤炭勘查成果,深入探讨了淮南潘谢矿区深部煤系烃源岩地球化学特征、热演化及生烃组合特征。地球化学测试结果表明:潘谢矿区深部煤系有机质干酪根主要来源于陆生高等植物,类型属于Ⅱ2-Ⅲ型,局部存在少量的Ⅱ1型,具有良好的天然气生成潜力;上石盒子组、下石盒子组和山西组泥页岩TOC含量均达到了中等烃源岩标准,氯仿沥青“A”也均达到了好的烃源岩标准,上石盒子组和山西组泥页岩S1+S2达到了中等烃源岩标准,下石盒子组泥页岩S1+S2达到了好的烃源岩标准,因此泥页岩有机质丰度各项指标均达到了中等或者好的烃源岩标准,加上煤层总有机碳含量均在60%以上,表明潘谢矿区深部煤系有机质具有较高的原始生烃能力。在潘气1井煤系烃源岩有机地化和气测录井特征分析的基础上,结合深部煤炭勘查成果,以全区稳定发育的厚层煤层和泥页岩为目标层位,在潘谢矿区深部划分了3套稳定发育的生烃组合。组合1位于上石盒子组上段中-下部,厚40~60 m,是良好的页岩气-煤层气共生气藏;组合2位于下石盒子组上段,厚60~150 m,以煤层气为主,页岩气为辅;组合3位于山西组底部,厚20~35 m,以煤层气为主,页岩气为辅。

关键词:煤系烃源岩;地球化学;成烃潜力;深部;潘谢矿区;淮南煤田

国内学术界根据目前采煤技术发展现状和安全开采要求,提出深部的概念是700~1 000 m[1],深部开采面临更严重的煤矿安全隐患,是我国煤炭安全生产和持续发展的重要制约因素。淮南煤田潘谢矿区深部含煤地层普遍埋深在800~2 500 m,煤炭资源储量丰度高,是淮南煤田重要的接替资源。由于高瓦斯、高地温、高地压、高水压等地质条件的限制,短时间内深部煤炭资源开采并不现实。然而,随着煤层气勘探开发向深部进一步拓展[2-4],煤系非常规天然气共探共采理论和技术的日益完善[5-12],该区煤系非常规天然气综合勘查与开发具有现实意义。

浅部煤炭及非常规天然气勘探和生产资料表明,淮南煤田煤系呈现出煤层和泥页岩频繁交互出现的特点,煤系非常规天然气资源潜力大[13-18]。但是,潘谢矿区深部煤系非常规天然气研究工作薄弱[19-25],而且主要依托深部煤炭勘查工程,缺少针对性的深部煤系非常规天然气研究。为此,安徽省最近在淮南煤田深部实施了煤系非常规天然气综合评价,并施工了1口多气综合勘探井——潘气1井。笔者依托“潘气1井”测试成果,分析淮南煤田深部煤系有机质地球化学特征及其生烃潜力,结合深部煤炭详查成果,进一步剖析生烃组合特征,以期为淮南煤田深部煤系气勘探开发提供依据。

1 地质背景

淮南潘谢矿区位于华北陆块南缘的徐淮地块(图1)。矿区北以刘府深断裂与蚌埠隆起相邻,南以颍上—定远断裂与合肥盆地相接,东起郯庐断裂带,西部通过陈桥断裂、大兴集断裂与新集矿区相连。

受周边区域构造带不同期次活动影响,矿区整体上呈现为NWW向展布的对冲式断褶构造带,南部为阜凤逆冲推覆构造带,北部为重力滑覆构造带,中部为复向斜构造带,具有“南北分带,东西分块”的特征(图1)。从EW向剖面图可以看出(图1,剖面A),淮南复向斜东段是潘谢矿区的主体构造,由谢桥—古沟向斜、潘集背斜、尚塘—耿村向斜、唐集—朱集背斜和陈桥背斜组成,这些褶皱NWW向平行分布,平面上略有弯曲,褶皱轴部均向东倾伏,在西部抬起,具有明显的统一性(图1,剖面B)。

潘谢矿区地层自下而上发育太古代五河群/霍邱群、中新元古代青白口系、震旦系,古生代寒武系、中下奥陶统,下石炭统、二叠系、下三叠统,中生代上侏罗统、白垩系及新生代古近系和大面积覆盖的新近系—第四系。含煤地层自下而上为石炭—二叠系的太原组、山西组、下石盒子组、上石盒子组,总厚约1 000 m。潘谢矿区深部区含煤20~30层,煤层总厚45 m左右,其中1,4,8,11-2,13-1等煤层为主要可采煤层。除煤层外,暗色泥页岩同样广泛发育,在垂向上与煤层、砂岩呈互层式分展布,泥页岩累计厚度最大可达600 m以上,具有良好的煤系天然气生成的物质基础(表1)。

图1 淮南煤田构造纲要及剖面图
Fig.1 Structural outline and sectional drawing of Huainan mining area

潘谢矿区自石炭—二叠纪成煤期以来经历了4个构造演化阶段[26-30]:稳定沉降阶段(C2—T2)、构造形变阶段(T3—J3)、伸展隆升阶段(K—E)和坳陷沉积阶段(N—Q)。印支期NS向挤压作用奠定了矿区现今构造格局,矿区深部总体表现为压性构造特征,对煤系天然气资源保存有利。燕山晚期和喜马拉雅期的伸展作用,使得矿区深部小型正断层发育,这在一定程度上改善了煤系储层的物性。初步分析认为矿区深部构造稳定,地层相对平缓,煤系发育完整,煤系天然气埋深适中,向斜控气特征明显,是潘谢矿区最为有利的煤系非常规天然气勘查开发区域。

表1 潘气1井地层
Table 1 Stratigraphic chart of well PQ-1

地层新生界N-Q代号底界深度/m地质特征168.60未固结的沙、砾、泥中生界TT1刘家沟组T1l754.55紫红色砂岩,泥页岩上古生界PP3P2P1孙家沟组P3sh968.55紫红色砂岩,泥页岩上石盒子组 P3ss1 709.65粉砂岩,砂岩,泥页岩,含煤20~25层下石盒子组 P2xs1 823.95粉砂岩,泥页岩与砂岩互层,含煤9~12层山西组P1s1 898.30砂岩,粉砂岩,泥页岩,含煤1~3层,含植物化石太原组P1-2t2 003.00(未穿)灰岩和砂页岩、煤线互层,含蜓类化石

2 样品采集与测试

潘气1井钻至太原组(未打穿),终孔井深2 003 m,是两淮煤田最深的1口勘探井,本文样品主要采自该井,并结合深部煤炭勘查13-2井的样品。有机地球化学测试样品共338件,其中煤样44件,泥页岩及砂岩样294件。

样品分布在上石盒子组、下石盒子组和山西组,重点层段适当加密,涵盖了主要煤层及泥页岩层段,以泥页岩为主,煤层为辅。孙家沟组和太原组非目标层位,测试未涉及。完成的测试项目及样品数量见表2,主要含煤地层柱状及相应测试结果如图2所示。

表2 样品采集情况
Table 2 Sample collection

测试项目样品数量/件潘气1井13-2孔采样密度/(m·件-1)TOC含量306320.5~3.0 镜质体反射率Ro602010~20 岩石热解502510~20 干酪根碳同位素822510~20 氯仿沥青“A”502510~20

3 有机地球化学特征

3.1 有机质类型

研究区煤系泥页岩有机质氢指数IH相对较高,重点层位山西组和下石盒子组泥页岩样品氢指数变化范围更大,大部分样品属于Ⅱ2-Ⅲ型,少部分为Ⅱ1型(图3)。

泥页岩样品干酪根δ13C均大于-25.5‰,结合TISSOT等划分标准[31],有机质类型全部在Ⅱ2-Ⅲ型之间,上石盒子组、下石盒子组和山西组泥页岩Ⅱ2型干酪根数量略占优势(图4)。干酪根类型的碳同位素判识结果,与烃源岩热解氢指数划分结果基本一致。

3.2 有机质丰度

3.2.1 有机碳含量

潘气1井煤的总有机碳(TOC)含量均在60%以上。其中,上石盒子组煤样品21件,厚度加权平均为66.34%,下石盒子组煤样品22件,厚度加权平均为67.02%,山西组煤样品只取1件,TOC含量为69.69%。

在泥页岩总有机碳含量测试中,上石盒子组泥页岩样44件,TOC含量在0.02%~19.54%,厚度加权平均为2.19%;下石盒子组泥页岩样59件,TOC含量分布在0.40%~11.85%,厚度加权平均为1.88%;山西组泥页岩样179件,TOC含量分布在0.10%~5.57%,厚度加权平均为2.23%。根据煤系烃源岩的TOC的评价标准[32],研究区煤系泥页岩TOC含量均达到了中等烃源岩标准。

为更好地分析研究区不同层位泥页岩TOC含量水平,结合实验数据情况,统计了有机碳含量分布频率,并绘制了有机碳频率分布图(图5)。由频率分布图可以看出,尽管上石盒子组平均TOC含量相对较高,但<1%的样品占62%,即有机质丰度低的层位较多,而且下部TOC含量高于上部,需要进一步筛选目标层位。下石盒子组泥页岩TOC含量总体中等,但>1%的样品数量占73%,>2%的样品数量也超过了37%,显示了该组总体上具有较好的产气潜力。山西组TOC含量 >1%的样品数量占61.36%,>2%的样品数量占34%,TOC含量厚度加权平均含量也达到了2.23%,显示山西组有机质丰度高,生气潜力大,可作为研究区首选目的层位。

3.2.2 氯仿沥青“A”

氯仿沥青“A”是评价烃源岩有机质丰度的重要指标之一。研究区煤中的氯仿沥青“A”测试样品较少,总体变化范围较大,最高可达到5.39%,厚度加权平均为3.12%。泥页岩氯仿沥青“A”在上石盒子组、下石盒子组和山西组厚度加权平均为0.29%,0.32%和0.41%。根据煤系烃源岩的氯仿沥青“A”的评价标准[32],研究区煤样品达到了中等烃源岩标准,泥页岩达到了好的烃源岩标准。

图2 潘气1井主要含煤地层柱状及相应测试成果垂向变化
Fig.2 Vertical change map of columnar and corresponding test results of coal-bearing strata for well PQ-1

3.2.3 热解生烃潜量

烃源岩生烃潜力进行定量评价时,一般用S1(可溶烃)和S2(裂解烃)的总量(S1+S2)来表示烃源岩的生烃潜力。研究区煤层的生烃潜量(S1+S2)未做系统测试,不作评价。上石盒子组泥页岩S1+S2分布在0.06~48.31 mg/g,厚度加权平均为5.58 mg/g,下石盒子组泥页岩S1+S2分布在0.03~60.73 mg/g,厚度加权平均为7.69 mg/g,山西组泥页岩S1+S2分布在0.03~7.59 mg/g,厚度加权平均为5.40 mg/g。据煤系泥岩生烃潜力评价标准[32],下石盒子组泥页岩S1+S2达到了好的烃源岩标准,上石盒子组和山西组泥页岩S1+S2达到了中等烃源岩标准。

4 生烃及组合特征

4.1 烃源岩热演化与成烃分析

图3 煤系泥岩有机质类型图解
Fig.3 Diagrams of organic matter types in coal measures

石炭—二叠纪煤系形成后,区域上经历过燕山中期的构造-热事件,淮南煤田仅在浅部的潘集地区发现有岩浆侵入(图6),煤系有机质热演化以深成变质作用为主。盆地模拟结果表明[33],在经历了印支期沉降埋藏后,该井煤系最大埋深达到3 200 m左右,有机质热演化在三叠纪末期达到最大值,进入成熟阶段,并生成大量的热成因气,随后煤系抬升,热演化作用终止。在遭受了侏罗纪—古近纪持续隆升剥蚀以及新近纪以来的坳陷沉积作用后,潘气1井区煤系现今最大埋深仍超过2 000 m(图1,6)。

图4 煤系泥岩干酪根碳同位素频率分布
Fig.4 Isotopic frequency distribution of kerogen

图5 煤系泥岩总有机碳含量频率分布
Fig.5 Frequency distribution of total organic carbon content

煤炭详查结果表明,淮南煤田深部现今煤阶以气煤为主,其次为1/3焦煤等其他煤类。潘气1井煤层及煤系泥页岩镜质组最大反射率(Ro,max)分布在0.63%~1.31%。即,深部煤系有机质热演化程度位于成熟阶段,进入了热降解气生成阶段。

图6 潘气1井石炭—二叠纪煤系埋藏-成熟史[33]
Fig.6 Burial and maturity history of C-P coal measure of well PQ-1[33]

4.2 生烃组合特征

在潘气1井煤系有机质生烃潜力和气测录井特征分析的基础上,结合近年来深部煤炭勘查成果,以全区稳定发育的厚层煤层和泥页岩为目标层位,认为研究区存在三套稳定发育的生烃组合(图2)。

组合1:位于上石盒子组上段中-下部,典型的煤炭勘查钻孔包括2-4孔、3-4孔、4-7孔、4-8孔、5-3孔、6-6孔、6-9孔、12-3孔、20-5孔等(图7)。该组合包括13-1煤层及其顶、底板泥页岩,13-1煤全区稳定可采,平均厚约5 m,顶板泥页岩厚15~30 m,底板泥页岩厚20~30 m,整个组合厚40~60 m。潘气1井在该组合段气测录井异常明显,13-1煤层厚4.9 m,全烃由5.54%↑54.42%,C1由3.79%↑38.03%,泥页岩气测异常有3层共4.6 m,其中1 590.00~1 591.78 m段气测异常最明显,全烃由6.62%↑29.53%,C1由6.13%↑23.46%。该组合泥页岩TOC含量平均达到2.17%,13-1煤生烃潜量(S1+S2)达到130.02 mg/g,镜质组最大反射率平均为0.96%,达到了成熟阶段,是良好的页岩气-煤层气共生气藏。

图7 组合1典型钻孔柱状
Fig.7 Typical drill columns of combination 1

组合2:位于下石盒子组上段,典型的煤炭勘查钻孔包括2-4孔、3-1孔、3-2孔、4-5孔、4-6孔、5-3孔、15-3孔、17-1孔、19-2孔、L1-2-1孔等(图8)。该组合包括3层可采煤层(8煤、7煤、6煤)以及厚层泥页岩和薄层砂岩。3层煤总厚5~20 m,整个组合厚60~150 m。潘气1井在该组合段气测录井异常明显,8煤厚2.50 m,全烃由23.92%↑38.89%,C1由20.56%↑33.05%,7煤厚1.40 m,全烃由15.70%↑32.09%,C1由13.25%↑28.95%,6煤厚4.0 m,全烃由12.30%↑38.98%,C1由10.56%↑32.75%。泥页岩气测异常有1层,位于1 719.80~1 721.00 m段,全烃由9.55%↑16.38%,C1由6.55%↑13.68%。组合内泥页岩TOC含量平均值为1.94%,有机质丰度中等,煤层层数多、厚度大。Ro,max平均为1.12%,达到成熟阶段,生成大量的热成因气,以煤层气为主,页岩气为辅。

图8 组合2典型钻孔柱状
Fig.8 Typical drill columns of combination 2

图9 组合3典型钻孔柱状
Fig.9 Typical drill columns of combination 3

组合3:位于山西组底部,典型的煤炭勘查钻孔包括3-2孔、5-2孔、5-3孔、6-7孔、8-5孔、16-4孔、18-3孔等(图9)。整个山西组有机质丰度高,砂岩气测录井异常明显,是致密砂岩气最有可能取得突破的层位。该组合包括两层稳定煤层(1煤和3煤)和泥页岩。泥页岩总厚10~30 m,两层煤总厚约6 m,整个组合厚度达到20~35 m。潘气1井的3煤层气测异常最为明显,3煤厚2.00 m,全烃由20.83%↑33.63%,C1由17.50%↑30.08%,泥页岩气测异常不明显。组合内泥页岩TOC含量平均为1.35%,有机质丰度中等,生烃潜力良好。煤层和泥页岩Ro,max平均为1.05%,达到成熟阶段。本组合以煤层气为主,页岩气为辅。

5 结 论

(1)煤系烃源岩热解氢指数及干酪根碳同位素测试分析表明,淮南煤田深部煤系有机质干酪根类型属于Ⅱ2-Ⅲ型,局部存在少量的Ⅱ1型。干酪根主要来源于陆生高等植物,具有良好的天然气生成潜力。

(2)总有机碳含量、氯仿沥青“A”和岩石热解等实验结果表明,上石盒子组泥页岩TOC含量厚度加权平均为2.19%,氯仿沥青“A”厚度加权平均为0.29%,S1+S2厚度加权平均为5.58 mg/g,下石盒子组泥页岩TOC含量厚度加权平均为1.88%,氯仿沥青“A”厚度加权平均为0.32%,S1+S2厚度加权平均为7.69 mg/g,山西组泥页岩TOC含量厚度加权平均为2.23%,氯仿沥青“A”厚度加权平均为0.41%,S1+S2厚度加权平均为5.40 mg/g,泥页岩有机质丰度各项指标均达到了中等或好的烃源岩标准。煤层总有机碳含量均在60%以上,氯仿沥青“A”厚度加权平均为3.12%,也达到了中等烃源岩标准。综合分析认为,潘谢矿区深部煤系有机质具有较高的原始生烃能力。

(3)在潘气1井煤系烃源岩有机地化和气测录井特征分析的基础上,结合深部煤炭勘查成果,在潘谢矿区深部划分了三套稳定发育的生烃组合。

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Geochemical characteristics and hydrocarbon-generation potential of coal- bearing source rocks in the deep part of Panxie mining area,Huainan

ZHANG Wenyong1,2,DOU Xinzhao1,2,3,LIU Guijian3,SUN Gui1,2,ZHAO Zhiyi1,2

(1.Exploration and Research Institute,Anhui Bureau of Coal Geological Exploration,Hefei 230088,China; 2.Engineering Technology Research Center for Unconventional Gas of Anhui Province,Hefei 230088,China; 3.School of Earth and Space Sciences,University of Science and Technology of China,Hefei 230088,China)

Abstract:In order to ascertain the hydrocarbon generation potential of the deep part of Panxie mining area in Huainan coalfield,on the basis of sampling the source rocks of coal measures,some organic geochemical methods such as total organic carbon content (TOC),chloroform bitumen “A”,rock pyrolysis,kerogen carbon isotope and vitrinite reflectance were used,combined with the achievements of deep coal exploration,to investigate the geochemical characteristics,thermal evolution and hydrocarbon generation combination characteristics of coal-bearing source rocks in the deep part of Huainan mining area.The geochemical test results show that the organic kerogen in coal measures belongs to type Ⅱ2-Ⅲ,with a small amount of type Ⅱ1,and has a good potential for gas generation.The TOCs of shale in Upper Shihezi Formation,Lower Shihezi Formation and Shanxi Formation all meet the standard of medium hydrocarbon source rocks,the chloroform asphalt “A” also meets the standard of good hy-drocarbon source rocks,the S1+S2 of shale in Upper Shihezi Formation and Shanxi Formation meets the standard of medium hydrocarbon source rocks,and the S1+S2 of shale in Lower Shihezi Formation meets the standard of good hydrocarbon source rocks.Therefore,all indexes of organic matter abundance in shale have reached the standard of medium or good hydrocarbon source rocks,and the total organic carbon content in coal seam is above 60%,which indicates that the organic matter in deep coal measures in Panxie mining area has high primary hydrocarbon generation ability.Based on the analysis of organic geochemistry and gas logging characteristics of coal-bearing source rocks in PQ-1 well,combining with the borehole columnar of deep coal exploration,and taking the thick coal seam and shale as the target horizon,three sets of stable hydrocarbon-generating combinations were divided in the deep part of Panxie mining area.Combination 1 is located in the middle-lower part of the upper segment of Upper Shihezi Formation,with a thickness of about 40-60 m,and is a good shale gas-coalbed methane co-generation reservoir.Combination 2 is located in the upper part of Lower Shihezi Formation,with a thickness of about 60-150 m,and dominated by coalbed methane and supplemented by shale gas.Combination 3 is located at the bottom of Shanxi Formation,with a thickness of about 20-35 m,and dominated by coalbed methane and supplemented by shale gas.

Key words:coal-bearing source rock;geochemical characteristics;hydrocarbon-generating potential;deep part;Panxie mining area;Huainan Coalfield

移动阅读

张文永,窦新钊,刘桂建,等.淮南潘谢矿区深部煤系烃源岩地球化学特征与成烃潜力[J].煤炭学报,2020,45(2):731-739.doi:10.13225/j.cnki.jccs.2019.0245

ZHANG Wenyong,DOU Xinzhao,LIU Guijian,et al.Geochemical characteristics and hydrocarbon-generation potential of coal-bearing source rocks in the deep part of Panxie mining area,Huainan[J].Journal of China Coal Society,2020,45(2):731-739.doi:10.13225/j.cnki.jccs.2019.0245

中图分类号:P618.11

文献标志码:A

文章编号:0253-9993(2020)02-0731-09

收稿日期:2019-02-28

修回日期:2019-06-10

责任编辑:韩晋平

基金项目:安徽省公益性地质工作资助项目(2013-g-31,2015-g-29);安徽省重点研究和开发计划资助项目(1804a0802203)

作者简介:张文永(1965—),男,安徽太湖人,教授级高级工程师,硕士。E-mail:ahmtsd@126.com

通讯作者:窦新钊(1985—),男,江苏睢宁人,高级工程师,博士。E-mail:douxinzhao@163.com