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鄂尔多斯盆地东缘临兴地区海陆过渡相页岩气地质特征及成藏潜力

2022-01-04朱光辉柳雪青李洋冰胡维强刘再振费景亮

煤田地质与勘探 2021年6期
关键词:本溪盆地鄂尔多斯

吴 鹏,曹 地,朱光辉,柳雪青,李 勇,李洋冰,胡维强,刘再振,孔 为,费景亮

鄂尔多斯盆地东缘临兴地区海陆过渡相页岩气地质特征及成藏潜力

吴 鹏1,曹 地2,3,朱光辉1,柳雪青2,3,李 勇4,李洋冰2,3,胡维强2,3,刘再振2,3,孔 为2,费景亮2,3

(1. 中联煤层气有限责任公司,北京 100011;2. 中海油能源发展股份有限公司工程技术分公司,天津 300452;3. 中海油能源发展股份有限公司非常规勘探开发重点实验室,天津 300452;4. 中国矿业大学(北京) 地球科学与测绘工程学院,北京 100083)

海陆过渡相页岩气是我国页岩气增储上产的重要接替领域,基于鄂尔多斯盆地东缘临兴地区海陆过渡相页岩实验和研究资料,选取山西组、太原组和本溪组页岩层系为研究对象,从沉积环境、页岩展布、有机地球化学特征、矿物学特征、物性特征和含气性特征等方面,系统总结解剖研究区海陆过渡相页岩气地质特征,分析其富集成藏潜力。结果表明:鄂尔多斯盆地在晚石炭世–早二叠世受区域内部构造活动影响,水体环境变化频繁,形成多期次的滨浅海–三角洲前缘–滨浅湖组合的沉积旋回,沉积多套海陆过渡相富有机质页岩;临兴地区位于鄂尔多斯盆地东部晋西挠折带的中北地区,区内沉积环境稳定,海陆过渡相富有机质页岩广泛分布;岩性主要为灰白色–浅灰色细粒砂岩和暗色泥页岩互层发育,垂向上页岩累计厚度大,为60~180 m;页岩有机质类型为Ⅱ2–Ⅲ型干酪根,总有机质碳含量较高,平均TOC质量分数为3.07%,处于成熟生气阶段;页岩矿物成分以石英和黏土类矿物为主,长石和碳酸盐岩类矿物含量较少;宏观上主要孔隙类型为无机孔隙和有机质孔隙,裂缝不发育,微观上孔隙受黏土矿物控制,孔隙形态多为开放狭缝状的微孔和介孔。研究区海陆过渡相页岩具有低孔低渗的物性特征,但平均含气量为1.15 m3/t,具有较好的含气特征,确定研究区内山西组北部、太原组东北部、本溪组东部和北部为页岩气潜力区。研究认识为该区后期页岩气勘探开发提供理论据。

晚石炭–早二叠世;页岩气;海陆过渡相;鄂尔多斯盆地;资源潜力

2000年以来全球天然气需求量逐年攀升,作为战略资源,天然气在化石能源的地位不断升高,而页岩气勘探开发是天然气资源的重要补充[1-4]。目前全球页岩气勘探和开发主要放在海相页岩层系,在中国南方长宁区块龙马溪组海相页岩的成功开发更是促进了这一趋势[3-4]。中国页岩气资源储量丰富、类型多样,海陆过渡相和陆相页岩气同样具有资源潜力,其中海陆过渡相页岩气资源量约为19.8万亿m3,占中国页岩气资源总量的四分之一,鄂尔多斯盆地和四川盆地海陆过渡相页岩气地质资源量合计为13.5万亿m3,占全国地质资源量的68%,是中国海陆过渡相页岩气资源的主体[5-10]。南方海相页岩气的中深层勘探开发和开采技术已经成熟,勘探深度已由中深层转向深层。北方海陆过渡相页岩埋深为1 500~2 500 m,远小于海相页岩埋深,勘探和开发难度低于深层海相页岩[11-13]。近年来在全国多地投入了大量钻探试井开展对海陆过渡相页岩气的试采和研究,鄂尔多斯盆地西北部的鄂页1井和东南部大吉地区的5口直井在二叠系获得工业气流,沁水盆地寿阳Y01、涟源盆地湘页1井和四川盆地川东2口页岩井等都在二叠系取得良好的页岩气显示[13-20]。钻探结果揭示了中国海陆过渡相页岩具有显著的勘探开发潜力,鄂尔多斯盆地最有望实现海陆过渡相页岩气勘探开发的突破,形成规模化产能,进而为全国海陆过渡相页岩气勘探开发提供理论实践依据,增加我国天然气资源的战略储备[21-24]。虽然南方海相页岩气的成功开发已经提供了大量的理论和技术参考,但北方海陆过渡相页岩气在构造演化、沉积环境、页岩气成因、岩石矿物学特征、地球化学特征、储层特征和页岩气成藏模式等与海相页岩存在较大差异,海相页岩气的理论和技术不能完全适用,当前急需建立海陆过渡相页岩气地质理论和评价体系,明确页岩气有利层段和“甜点”区,进而开发高效适用的钻井和储层改造技术,实现海陆过渡相页岩气规模化的开发和开采。

鄂尔多斯盆地海陆过渡相页岩主要发育在石炭–二叠系,已有部分专家学者根据鄂尔多斯盆地西北部和东南部的钻探井结果,取得了初步的地质认识,肯定了鄂尔多斯盆地二叠系海陆过渡相页岩气的资源潜力[22-25]。受限于资料数据的稀缺和区域性问题,鄂尔多斯盆地海陆过渡相页岩气研究一直未取得系统性结论。为此,笔者拟通过系统梳理鄂尔多斯盆地东缘临兴地区本溪组–太原组–山西组海陆过渡相页岩气储层地质特征,明确研究区内有利页岩气储层层段分布,以期为鄂尔多斯盆地海陆过渡相页岩气未来的勘探开发提供理论依据和借鉴。

1 地质背景

1.1 临兴地区概况

研究区位于鄂尔多斯盆地东北部,晋西挠褶带的中北地区,北至兴县,南抵临县,构造上为北东–南西向单斜,地层整体西倾[21-30]。本次研究对象为上石炭统本溪组、下二叠统太原组和山西组地层,其中,山西组的岩性为灰白–浅灰白含砾中–细砂岩、粉砂岩和粉砂质泥岩,偶见薄煤层,页岩沉积往往与细粉砂岩互层,但整体沉积厚度较大;太原组主要发育含砾中砂岩、细砂岩和粉砂岩,偶见薄煤层,部分地区页岩沉积较厚;本溪组受海相沉积影响较大,岩性特征复杂,灰色细粒砂岩、灰色灰岩和暗色页岩互层沉积,偶见薄煤层和煤线发育[24, 31-36](图1)。

图1 鄂尔多斯盆地临兴区块地理位置及岩性综合柱状图(据匡立春等[24],修改)

1.2 沉积环境和页岩展布特征

研究区内地层发育特征受控于鄂尔多斯盆地的沉积构造演化,中奥陶世区块整体隆升,上部地层发生剥蚀缺失;晚石炭世–早二叠世,鄂尔多斯盆地沉积稳定,广泛发育一套海相、海陆过渡相沉积层系,部分地区伴生有煤层。受区域内部构造活动影响,水体环境变化频繁,形成多期次的滨浅海–三角洲前缘–滨浅湖组合的沉积旋回,沉积多套海陆过渡相富有机质页岩,累计厚度60~180 m。本文研究目的地层为上石炭统本溪组、下二叠统太原组和山西组,主要岩性为暗色页岩、灰色砂岩和碳酸盐岩,其中暗色页岩为海陆过渡相富有机质页岩。研究区内本溪组时期主要为滨浅海、浅水陆棚沉积,向上到太原组,水体环境逐渐变浅,滨岸相沉积增多,到山西组时期,沉积水深持续变浅,开始以三角洲相和滨浅湖相沉积为主[31-40]。对比研究区山西组、太原组和本溪组累计页岩厚度可以发现,山西组页岩累计厚度为12.50~54.65 m,平均厚度为34.51 m,平面上呈自南向北逐渐增厚的趋势,表明山西组海陆过渡相页岩沉积发育时期,北侧水体更深;太原组页岩累计厚度为29.00~76.02 m,平均厚度为49.03 m,平面上呈南北厚中间薄的展布特征;本溪组页岩累计厚度为20.00~59.03 m,平均厚度为38.05 m,平面上呈中间厚南北薄和西薄东厚的趋势,但整体厚度差距不大(图2)。

2 有机地化特征

2.1 有机质丰度

根据研究区山西组、太原组和本溪组3套地层143个页岩样品测试数据,山西组页岩总有机碳质量分数为0.04%~37.3%,平均2.26%,主频段为0~1%和1%~2%,共占山西组样品总数的72%,主频段均值为0.85%,有28%的页岩样品总有机碳含量大于2%;太原组页岩总有机碳质量分数为0.26%~34.7%,平均3.8%,主频段为0~1%、1%~2%和2%~3%,共占太原组样品总数的80%,主频段均值为1.73%,有60%的页岩样品总有机碳质量分数大于2%;本溪组页岩总有机碳质量分数为0.09%~20.6%,平均3.17%,主频段为0~1%、1%~2%和2%~3%,共占本溪组样品总数的77.2%,主频段均值为1.38%,有41%的页岩样品总有机碳质量分数大于2%(图3)。

图2 目的层累计页岩厚度等值线

图3 页岩TOC分布频率

对比研究区山西组、太原组和本溪组3套地层页岩样品的残余氯仿沥青“A”含量可知,山西组页岩样品氯仿沥青“A”质量分数为0.006 6%~0.250 4%,平均0.093 7%;太原组页岩样品氯仿沥青“A”质量分数为0.033 5%~0.203 9%,平均0.080 0%;本溪组页岩样品氯仿沥青“A”质量分数为0.0208%~0.0506%,平均0.0385%。依据SY/T 5739—1995《陆相烃源岩地球化学评价方法》,残余氯仿沥青“A”质量分数在0.05%~0.10%时都为中等。山西组和太原组页岩样品有机质丰度都为中等,且太原组的数据更为稳定。

2.2 有机质类型

利用偏光显微镜和显微分光光度计对页岩样品进行镜下显微组分观察与特征描述。研究区山西组、太原组和本溪组页岩干酪根显微组分中壳质组最发育,其次为镜质组和惰质组,腐泥组最不发育。其中壳质组体积分数为11%~83%,平均61%,镜质组体积分数为7%~76%,平均为29%。依照四分法范式图解作研究区页岩干酪根显微组分三角图(图4),由图中可知,山西组页岩样品落在腐殖型区域,太原组和本溪组页岩样品落在混合型区域。综上研究认为研究区暗色页岩具有2种干酪根类型,山西组以Ⅲ型为主,太原组和本溪组以Ⅱ2(偏腐殖型)为主(图4)。

图4 页岩干酪根显微组分三角图

2.3 有机质成熟度

本文对选取的107个页岩样品进行成熟度分析,进行镜质体反射率测试实验。测试结果表明山西组37个页岩样品有机质成熟度ran值为0.92%~1.11%,平均1.03%;太原组34个页岩样品ran值为1.01%~1.09%,平均1.06%;本溪组36个页岩样品ran=1.06%~1.30%,平均1.15%;有机质热演化程度与埋藏深度呈正相关性,由山西组向下至本溪组,暗色页岩的有机质成熟度逐渐递增。研究区整体页岩有机质成熟度ran值为0.92%~1.30%,处于成熟热演阶段,具有可观的生烃能力。与四川盆地龙马溪组、美国Marcellus和Ohio海相页岩对比,不同于龙马溪组页岩的高成熟度特征,研究区暗色页岩的成熟度与美国的海相页岩更相近,仍具有很高的生气潜力。

3 储集层物性特征

3.1 矿物特征

对研究区184个页岩样品进行镜下薄片鉴定和X-衍射分析,镜下薄片鉴定结果表明:①研究区暗色页岩中碎屑成分主要为石英、长石、酸性喷出岩和变质岩,少量的云母等岩屑,其中,石英体积分数为1.0%~93.0%,平均40.0%。②胶结物有泥质、铁方解石、方解石、铁白云石、菱铁矿和黄铁矿等,以泥质为主,其余胶结物含量极少。③由于黄铁矿和碳酸盐矿物含量较少,选择石英和长石作为脆性矿物,对比研究区山西组、太原组和本溪组的矿物脆性指数特征。山西组暗色页岩平均脆性指数为47.08%,本溪组平均脆性指数最小,为39.37%,太原组介于二者之间,为42.28%,说明山西组页岩储层的后期可压裂性更好(图5,表1)。

图5 页岩矿物含量三角图

表1 X-全岩衍射实验矿物质量分数及脆性指数特征

注:脆性指数=(石英+长石)/(石英+长石+碳酸盐岩类矿物+黏土矿物)×100%

3.2 物性特征

研究区页岩样品岩心物性(图6)分析表明,山西组66.7%的页岩样品渗透率小于0.05×10–3μm2,孔隙率为1.15%~3.03%,平均2.14%;太原组75%的页岩样品渗透率小于0.10×10–3μm2,孔隙率为0.79%~ 3.62%,平均1.97%;本溪组66.7%的页岩样品渗透率小于0.01×10–3μm2,孔隙率全部小于1.0%,平均0.53%。研究区海陆过渡相暗色页岩整体孔隙率和渗透率较低,最高孔隙率为3.62%,平均2.06%。初步表明研究区整体上海陆过渡相页岩物性较差,且山西组和太原组差距不大。

3.3 孔隙结构类型

采用氩离子抛光–扫描电镜定性观察页岩孔隙类型,采用低温氮气吸附–脱附实验定量化表征页岩孔隙结构。研究区内选取了68个页岩样品进行氩离子抛光–扫描电镜实验,162个页岩样品进行低温氮气吸附–脱附实验[41-43]。

3.3.1 孔隙类型定性表征

氩离子抛光–扫描电镜实验结果表明,研究区海陆过渡相页岩发育微纳米级孔隙,主要由无机孔隙和有机质孔隙两类共同构成,其中无机孔隙较为发育,有机质孔隙发育程度较低。无机孔隙主要包括黏土矿物片间孔、长石溶蚀孔和矿物边缘孔。页岩中含有的少量长石和碳酸盐矿物遭受侵蚀后,容易发生矿物溶解,产生矿物溶蚀孔隙,但由于矿物本身含量较少,溶蚀孔隙发育规模也不大,孔径一般在几十至几百纳米,呈圆形、椭圆形和不规则形,主要发育在矿物颗粒的粒间和粒内。黏土矿物片间孔发育在丝缕状伊利石或书页状高岭石中,形态呈扁平狭缝,沿黏土矿物层理近平行排列,孔径为微纳米级。还有部分发育在两种矿物颗粒或矿物颗粒和有机质之间的矿物边缘孔,孔径呈纳米级,孔隙形态为缝状、条带状或不规则状,但在研究区页岩内极少发育(图7)。

受页岩热演化阶段的影响,研究区海陆过渡相页岩有机质孔隙发育程度较低,主要为原始有机质结构孔隙和有机质内部生烃孔隙,部分与黄铁矿伴生。孔隙形态呈圆形、椭圆形、三角形或不规则形,孔径为几至几百纳米,连通性较好(图7)。

3.3.2 孔隙结构定量表征

根据国际应用化学联合会(IUPAC)对孔隙的分类,孔径小于2 nm的为微孔,2~50 nm的为介孔,大于50 nm的为宏孔。选用低温氮气吸附–脱附实验来定量表征研究区页岩孔隙结构特征,结果表明,研究区页岩孔隙主要为介孔,其中山西组页岩孔隙孔径为6.35~24.47 nm,平均孔径为12.69 nm;太原组页岩孔隙孔径为9.37~16.94 nm,平均孔径为13.87 nm;本溪组页岩孔隙孔径为11.32~25.84 nm,平均孔径最大,为16.29 nm。

图6 孔隙率与渗透率分布频率直方图

图7 页岩孔隙类型

低温氮气吸附–脱附曲线的回滞环曲线形态一定程度上反映了孔隙的形态特征。参照IUPAC的氮气吸附–脱附曲线分类方案,研究区海陆过渡相页岩样品回滞曲线为H3和H4两类。

H3型回滞曲线在低压段吸附和脱附曲线基本重合,表明微孔径范围内为一段封闭的不透气气孔;中压段出现明显拐点,迟滞环变宽大,说明孔隙结构复杂,类型多样;高压段曲线斜率增大,说明大孔径范围存在缝状开放型孔隙,可能与黏土矿物片间孔有关(图8)。

H4型回滞曲线在低压和中压段吸附曲线和脱附曲线基本重合,高压段吸附曲线和脱附曲线斜率明显增大,说明孔隙类型主要为开放型圆筒状孔隙,含狭窄裂隙孔(图8)。

3.4 含气性特征

针对研究区4口井79块不同层位的页岩样品现场解吸数据,过去采用的直线拟合法计算损失气,计算结果低于实际数值。结合现场解吸数据,考虑到页岩取心周期长,气体损失时间增加和气体损失速率的动态变化,本文采用多项式法计算总含气量。研究区山西组总含气量为0.60~2.49 m3/t,平均1.04 m3/t;太原组总含气量为0.48~2.46 m3/t,平均含气量为1.30 m3/t;本溪组总含气量为0.61~1.74 m3/t,平均1.11 m3/t。研究区海陆过渡相页岩表现良好的含气性特征,且太原组优于山西组和本溪组(图9)。

图8 研究区页岩氮气吸附–脱附特征曲线

图9 页岩含气量分布直方图

4 综合潜力评价

不同类型页岩气的基本地质条件,如埋深、有效页岩厚度、有机质类型、有机质成熟度、有机质丰度、物性、孔隙结构、含气性、压力系数等参数存在明显差异,尤其在四川盆地海相页岩气勘探开发取得重大成功,相应综合潜力评价方法和标准也陆续建立,而海陆过渡相页岩气尽管表现出良好的勘探前景,但近年来始终未取得革命性突破,相应评价方法和标准也没有建立。与四川盆地龙马溪组海相页岩相比,临兴地区海陆过渡相页岩在有机质丰度、有机质成熟度、孔隙率、脆性指数等多项地质参数都处于劣势,这极大地影响了页岩气的勘探潜力。但海陆过渡相页岩同样有优势性地质特征,Ⅱ2、Ⅲ型干酪根意味着海陆过渡相页岩不需要页岩有机质热演化程度达到过熟就能大量产气,有效页岩厚度大和埋深浅同样意味着海陆过渡相页岩不仅有较好的储集性能,也更方便未来的勘探和开发。

本文依据鄂尔多斯盆地东缘临兴地区海陆过渡相页岩基本地质特征,参考海相页岩评价方法和GB/T 31483—2015《页岩气地质评价方法》,从烃源岩、储集性和产能3个方面出发,选取TOC、有机质成熟度ran、有机质类型、矿物脆性指数、孔隙率、含气量和有效页岩厚度等多个参数[41-45],初步提出相对有利区划分指标,指示研究区海陆过渡相页岩相对有利区分布特征(表2)。临兴地区海陆过渡相页岩有利区主要位于研究区的北部、东部和东北部,其中山西组北部、太原组东北部、本溪组东部和北部是主要的相对有利区。整体而言,说明研究区北部片区页岩气综合潜力较大,可以作为未来主要勘探区域(图10)。

表2 海陆过渡相页岩评价参数及指标

图10 临兴地区海陆过渡相页岩相对有利区分布

5 结论

a. 鄂尔多斯盆地东缘临兴地区山西组、太原组和本溪组以三角洲、潮坪等海陆过渡相沉积为主,地层稳定,发育页岩与砂岩韵律互层,夹薄煤层,页岩单层厚度小,但累计厚度大;有机质类型为Ⅱ2、Ⅲ型,页岩ran值为0.92%~1.30%,处于成熟热演阶段,具有较强的生气能力;是页岩孔隙类型主要为黏土矿物片间孔、长石溶蚀孔、矿物边缘孔和有机质孔,孔径以微孔和介孔为主,是页岩气主要的储集场所和运移通道;页岩矿物以石英和黏土矿物为主,碳酸盐矿物和长石次之,山西组的矿物脆性指数最大,具有良好的可压裂性。

b. 与四川盆地龙马溪组海相页岩相比,临兴地区海陆过渡相页岩在有机质丰度、有机质成熟度、孔隙率、脆性指数等多项地质参数都处于劣势,但其Ⅱ2、Ⅲ型的有机质类型、有效页岩厚度大和埋深浅意味着海陆过渡相页岩同样具有较好的储集性能和生烃潜力。

c. 依据鄂尔多斯盆地东缘临兴地区海陆过渡相页岩基本地质特征,参考南方海相页岩评价方法和GB/T 31483—2015《页岩气地质评价方法》,从烃源岩、储集性和产能3个方面出发,选取TOC、有机质成熟度ran、有机质类型、矿物脆性指数、孔隙率、含气量和有效页岩厚度等多个参数,初步尝试划分临兴地区目的层海陆过渡相页岩相对有利区分布特征。结果表明,山西组北部、太原组东北部、本溪组东部和北部是主要有利分布区。整体而言,研究区北部片区页岩气综合潜力较大,可以作为未来主要勘探区域。

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Geological characteristics and reservoir-forming potential of shale gas of transitional facies in Linxing area, eastern margin of Ordos Basin

WU Peng1, CAO Di2,3, ZHU Guanghui1, LIU Xueqing2,3, LI Yong4, LI Yangbing2,3, HU Weiqiang2,3, LIU Zaizhen2,3, KONG Wei2, FEI Jingliang2,3

(1. China United Coalbed Methane Company Limited, Beijing 100011, China; 2. CNOOC Energy Tech-Drilling & Production Co., Tianjin 300452, China; 3. Key Laboratory for Exploration & Development of Unconventional Resources, CNOOC Energy Technology & Services CO., Ltd., Tianjin 300452, China; 4. School of Geoscience and Surveying Engineering, China University of Mining and Technology(Beijing), Beijing 100083, China)

Shale gas of transitional facies is the important replacing area for increase of shale gas reserves and production. On the basis of the experiment and research data of shale of transitional facies of Linxing area in the eastern margin of Ordos Basin, by selecting the strata series of Shanxi Formation, Taiyuan Formation and Benxi Formation as the research object, from the sedimentary environments, shale distribution, organic geochemical characteristics, mineralogy, physical characteristics and gas-bearing characteristics, etc., the geology characteristics of the shale gas transitional facies in the study area were summarized and dissected systematically to analyze its enrichment and reservoir-forming potential. During the Late Carboniferous to the Early Permian, the Ordos Basin was affected by the active tectonic activities in the region, and the water environment changed frequently, resulting in the formation of multiple sedimentary cycles of littoral shallow sea, delta front and littoral shallow lake assemblages, and many sets of organic-rich shales of marine-continental transition facies were deposited. The study area is located in the north-central area of the western Shanxi flexural belt in the eastern part of the Ordos Basin, where the sedimentary environments are stable and organic-rich transitional shale is widely distributed. The rock characteristics of the shale are mainly interbedded gray-light gray fine-grained sandstone and dark mudstone, and the accumulated vertical thickness of the shale is big(60-180 m). The organic matter types of the organic-rich transitional shale in the study area are Ⅱ2-Ⅲ type kerogen, and the total organic matter carbon content is high, with TOC 3.07% which is in the mature gas generation stage. The mineral composition of the shale is mainly quartz and clay, and the content of feldspar and carbonate minerals is low. Macroscopically, the main types of pores are inorganic pores and organic pores, and the fractures are not developed. Microscopically, the pores are controlled by clay minerals, and the pore morphology is mostly open slit micropores and mesopores. The shale of the transitional facies in the study area has the physical characteristics of low porosity and low permeability, but has good gas-bearing characteristics, the average gas content is 1.15 m3/t. The comprehensive analysis of the shales’ characteristics shows that the shale gas in the study area has great potential, providing a theoretical basis for the exploration and development of shale gas in the later stage of the area.

Late Carboniferous-Early Permian; shale gas; transitional facies; Ordos Basin; resource potential

语音讲解

TP028.8

A

1001-1986(2021)06-0024-11

2021-05-20;

2021-09-28

国家自然科学基金项目(42072194);国家科技重大专项项目(2016ZX05066)

吴鹏,1988年生,男,山东泰安人,博士,高级工程师,从事非常规油气勘探与开发工作. E-mail:wupeng19@cnooc.com.cn

曹地,1993年生,男,河南濮阳人,硕士,助理工程师,从事非常规油气勘探与开发工作. E-mail:caodi19931116@163.com

吴鹏,曹地,朱光辉,等. 鄂尔多斯盆地东缘临兴地区海陆过渡相页岩气地质特征及成藏潜力[J]. 煤田地质与勘探,2021,49(6):24–34. doi: 10.3969/j.issn.1001-1986.2021.06.003

WU Peng,CAO Di,ZHU Guanghui,et al. Geological characteristics and reservoir-forming potential of shale gas of transitional facies in Linxing area, eastern margin of Ordos Basin[J]. Coal Geology & Exploration,2021,49(6):24–34. doi: 10.3969/ j.issn.1001-1986.2021.06.003

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