非常规天然气

鄂尔多斯盆地东缘大吉地区山2 3亚段海陆过渡相页岩气富集控制因素

  • 罗力元 , 1, 2, 3 ,
  • 李勇 , 1, 2, 3, 4 ,
  • 李树新 5 ,
  • 何清波 4, 6, 7 ,
  • 陈世加 1, 2, 3 ,
  • 李翔 5 ,
  • 李星涛 5 ,
  • 路俊刚 1, 2, 3 ,
  • 肖正录 1, 2, 3 ,
  • 尹相东 1, 2, 3
展开
  • 1. 油气藏地质及开发工程全国重点实验室·西南石油大学,四川 成都 610500
  • 2. 天然气地质四川省重点实验室,四川 成都 610500
  • 3. 西南石油大学地球科学与技术学院,四川 成都 610500
  • 4. 自然资源部复杂构造区非常规天然气评价与开发重点实验室,贵州 贵阳 550000
  • 5. 中石油煤层气有限责任公司,北京 100028
  • 6. 贵州省油气勘查开发工程研究院,贵州 贵阳 550000
  • 7. 贵州省能源智能开发与高效利用实验室,贵州 贵阳 550000
李勇(1993-),男,四川广元人,博士,副研究员,主要从事油气地球化学和非常规油气地质研究.E-mail:.

罗力元(2001-),男,四川攀枝花人,硕士研究生,主要从事油气地球化学和非常规油气地质研究. E-mail:.

收稿日期: 2024-01-31

  修回日期: 2024-04-06

  网络出版日期: 2024-05-11

Controlling factors of marine and continental transitional shale gas enrichment in Shan2 3 sub-member, Daji area, eastern margin of Ordos Basin

  • Liyuan LUO , 1, 2, 3 ,
  • Yong LI , 1, 2, 3, 4 ,
  • Shuxin LI 5 ,
  • Qingbo HE 4, 6, 7 ,
  • Shijia CHEN 1, 2, 3 ,
  • Xiang LI 5 ,
  • Xingtao LI 5 ,
  • Jungang LU 1, 2, 3 ,
  • Zhenglu XIAO 1, 2, 3 ,
  • Xiangdong YIN 1, 2, 3
Expand
  • 1. National Key Laboratory of Oil and Gas Reservoir Geology and Exploitation,Southwest Petroleum University,Chengdu 610500,China
  • 2. Sichuan Natural Gas Geology Key Laboratories,Chengdu 610500,China
  • 3. School of Geoscience and Technology,Southwest Petroleum University,Chengdu 610500,China
  • 4. Key Laboratory of Unconventional Natural Gas Evaluation and Development in Complex Tectonic Areas,Ministry of Natural Resources,Guiyang 550000,China
  • 5. PetroChina Coalbed Methane Co. ,Ltd. ,Beijing 100028,China
  • 6. Guizhou Engineering Research Institute of Oil & Gas Exploration and Development,Department of Natural Resources of Guizhou Province,Guiyang 550000,China
  • 7. The Laboratory of Guizhou Province of Intelligent Development and Efficient Utilization of Energy,Guiyang 550000,China

Received date: 2024-01-31

  Revised date: 2024-04-06

  Online published: 2024-05-11

Supported by

The China Petroleum-Southwest Petroleum University Innovation Consortium Science and Technology Cooperation Project(2020CX030000)

the Open Project Fund of the Key Laboratory of Unconventional Natural Gas Evaluation and Development in Complex Tectonic Areas Ministry of Natural Resources(NRNG-202401)

the CNPC Innovation Fund(2022DQ02-0105)

摘要

海陆过渡相页岩气是继海相页岩气商业开发后的又一重要战略接替资源。海陆过渡相页岩具有非均质性强、沉积相变快、岩性组合复杂的特征,海相页岩气地质理论不能完全适用于海陆过渡相,海陆过渡相页岩气富集控制因素认识不清,制约了高效勘探与开发。以鄂尔多斯盆地东缘大吉地区山2 3亚段页岩为例,通过显微组分、气体吸附、高压压汞、突破压力、扩散系数和覆压孔渗等实验分析,开展了页岩地球化学特征和储层特征研究,明确了海陆过渡相页岩气富集控制因素。结果表明:山2 3亚段海陆过渡相页岩具有高有机质丰度、过成熟度和以腐殖型为主的特征,孔隙类型主要以无机矿物孔为主,有机孔和微裂缝相对较少。明确了海陆过渡相页岩气富集主要受有机质丰度、孔径大小、岩性组合和构造演化共同控制:高有机质丰度增强了页岩的吸附能力,微孔为甲烷气体分子提供更多的吸附点位,页—煤和页—灰组合有利于页岩气滞留原位富集,平稳的构造和适当的埋深有利于页岩气保存,建立了大吉地区海陆过渡相页岩气储集能力和封闭能力的演化模式。研究可以为海陆过渡相页岩气甜点预测和先导试验区快速建产提供地质理论指导。

本文引用格式

罗力元 , 李勇 , 李树新 , 何清波 , 陈世加 , 李翔 , 李星涛 , 路俊刚 , 肖正录 , 尹相东 . 鄂尔多斯盆地东缘大吉地区山2 3亚段海陆过渡相页岩气富集控制因素[J]. 天然气地球科学, 2025 , 36(3) : 554 -566 . DOI: 10.11764/j.issn.1672-1926.2024.04.025

Abstract

Marine-terrestrial transitional shale gas is another important strategic replacement resource after the commercial development of marine shale gas. Marine-terrestrial transitional shale has the characteristics of strong heterogeneity, rapid depositional phase change, and complex lithology combination. Geological theories of marine shale gas can not be fully applied to marine-continental transitional facies, and the controlling factors of shale gas enrichment in the marine and continental transition facies are not well understood, which restricts efficient exploration and development. Taking the Shan2 3 sub-member shale in the Daqi area of the eastern margin of the Ordos Basin as an example, the geochemical characteristics and reservoir characteristics of shale were investigated through experiments such as microscopic analysis, gas adsorption, high-pressure mercury intrusion, breakthrough pressure, diffusion coefficient, and overburden pore permeability. This research elucidated the controlling factors of shale gas accumulation in marine-continental transitional facies. The research results indicate that the Shan2 3 sub-member shale in the marine-continental transitional facies exhibits high organic matter abundance, high maturity, and predominantly humic-type characteristics. The pore types are mainly dominated by inorganic mineral pores, with relatively fewer organic pores and microfractures. The conclusion suggests that the enrichment of marine-continental transitional facies shale gas is primarily controlled by a combination of organic matter abundance, pore size, lithological composition, and structural evolution. High organic matter abundance enhances the adsorption capacity of shale, providing more adsorption sites for methane gas molecules in micropores. The combination of shale-coal and shale-ash favors the in-situ enrichment of shale gas. Stable tectonics and appropriate burial depth facilitate the preservation of shale gas. Furthermore, an evolutionary model for the storage and sealing capacity of marine-continental transitional facies shale gas in the Dagang area has been established. The above findings can provide geological theoretical guidance for sweet spot prediction and rapid development of pilot test areas for marine-continental transitional facies shale gas.

0 引言

随着天然气在国家能源消耗结构中占比越来越高,加之我国海相页岩气的成功商业化开发,页岩气逐步成为我国非常规天然气勘探与开发的重要接替领域1-2。中国页岩气资源潜力巨大,地质资源量高达80×1012 m3,根据沉积环境可进一步划分为海相、海陆过渡相、陆相3种类型,其中海陆过渡相页岩气地质资源量为19.8×1012 m3,约占中国页岩气总地质资源量的25%3-5。海陆过渡相页岩主要发育在石炭系—二叠系,分布于华北及南方地区,面积约为(15~20)×104 km2,如四川盆地的龙潭组和鄂尔多斯盆地本溪组—山西组6-7。近年来,我国在鄂尔多斯盆地东缘海陆过渡相页岩取得了一定的勘探进展,并在大吉地区建立了先导试验区,有望取得进一步突破8-10。2019年,我国在大吉地区实施风险探井JP1H井,目的层段为山2 3亚段底部页岩层段,2021年5月投产,最高日产气7.97×104 m3,稳定日产量3.3×104 m3。2022年3月,大吉地区DJ-P61井投产,水平段进尺592 m,最高日产气1.84×104 m3,均展现了先导试验区良好的勘探潜力。
海相和海陆过渡相页岩气富集因素差异较大。相较于海相沉积,海陆过渡相自下而上由浅海陆棚相逐渐变化为三角洲相,岩性以碳酸盐岩、页岩、砂岩和煤层等交错变化11-15,并且海相页岩有机质类型以Ⅰ型为主,储层孔隙度平均为5.58%,而海陆过渡相页岩有机质主要为Ⅱ2型或Ⅲ型,储层孔隙度平均为3.55%16,海陆过渡相页岩有机质类型和储层物性均较差。综上所述,海相页岩气地质理论不能完全适用于非均质性强的海陆过渡相页岩气。鄂尔多斯盆地东缘山2 3亚段泥页岩有机质以陆源供给的腐殖型为主17,成熟度和TOC普遍较高18-19,纵向上发育硅质页岩相、硅质黏土质页岩相、钙质硅质页岩相及黏土质页岩相4种页岩岩相20-21,储层非均质性较强,优质储层以高TOC、高硅质/钙质页岩为主22-24。目前有关鄂尔多斯盆地海陆过渡相页岩成烃成储的地质理论较为成熟,而对于海陆过渡相页岩气富集的控制因素尚不清楚,难以预测地质甜点。基于此,本文以鄂尔多斯盆地东缘大吉地区山2 3亚段海陆过渡相页岩为例,通过页岩地球化学、储集层和构造特征研究,进而明确海陆过渡相页岩气的富集控制因素,为下一步先导试验区的快速建产提供地质理论依据。

1 地质背景

大吉地区位于鄂尔多斯盆地东缘,跨越陕西省和山西省,地处大宁县和吉县两地。从鄂尔多斯盆地构造带分布上来看,研究区主要分布在晋西挠褶带南部25图1(a)]。研究区沉积了石炭系—二叠系海陆过渡相地层,自下而上包括了上石炭统本溪组、下二叠统太原组和山西组26-27。研究区海陆过渡相地层发育特征密切受到盆地构造演化影响,自奥陶统地层剥蚀顶部开始,因海西运动影响,晚石炭世末期华北板块发生沉降,海水自北东、南东方向侵入形成广阔浅海陆棚28,造成研究区本溪组沉积环境以滨岸相和浅海陆棚相沉积环境为主,岩性主要为暗色泥岩、砂岩和煤层29。太原组沉积时期,由于地层持续下降,华北板块沉积中心逐渐南移30,研究区太原组仍然发育滨岸相和浅海陆棚相沉积,并且灰岩较为发育31。山西组沉积时,华北板块开始隆升,海水开始退出盆地,沉积环境由海相沉积变化为陆相沉积32,研究区发育湖泊相—三角洲相和滨海相沉积33,岩性以页岩、砂岩和煤层为主34图1(b)]。
图1 研究区地理位置与海陆过渡相地层发育特征(据文献[2634])

Fig.1 Geographic location of the study area and stratigraphic development characteristics of the transitional facies (according to Refs.[2634])

2 样品来源和实验方法

本文样品主要取自DJ51井、DJ3-4井和DJ17-1X5井,采集样品主要包括灰黑色页岩、黑色页岩、暗色页岩、炭质页岩、灰岩、粉砂岩、砂岩和煤岩岩样。对采集样品进行薄片磨制和鉴定,并配套开展总有机碳(TOC)、低压吸附和储层孔径等分析测试。
利用Rock-Eval VI型岩石热解仪和LECO-C230型碳硫测定仪测定TOCS 1S 2,分别依据《沉积岩中总有机碳的测定》(GB/T 19145—2003)和《岩石热解分析》(GB/T 18602—2012)检测标准进行实验测试。干酪根显微组分鉴定选择奥林巴斯CX31-P型光学显微镜按照《透射光—荧光干酪根显微组分鉴定及类型划分方法》(SY/T 5125—1996)标准进行。镜质体反射率(R O)采用显微分光光度计CRAIC508PV和显微镜DM4仪器,按照《沉积岩中镜质体反射率测定方法》(SY/T 5124—2012)标准测试。扫描电镜镜下照片使用EVO MA15型氩离子抛光扫描电子显微镜拍摄,低压N2、CO2和CH4等温吸附实验和高压压汞实验分别使用Autosorb IQ型比表面和孔径分析仪分析和Autopore Ⅳ9500、V9620型全自动压汞仪进行测试。页岩覆压孔渗分析使用PoroPDP-200型覆压孔隙度渗透率测量仪。

3 海陆过渡相页岩地球化学与储集层特征

3.1 海陆过渡相页岩地球化学特征

研究区山2 3亚段海陆过渡相页岩和炭质页岩有机质丰度(TOC)分布在0.84%~27.5%之间,平均值为4.52%,高于四川盆地典型海相龙马溪组页岩的平均有机质丰度2.67%。从TOC频率分布图来看[图2(a)],山2 3亚段页岩TOC<0.75%、0.75%~1.0%、1.0%~2%、2.0%~4.0%和>4%的样品分别占9.38%、15.22%、20.16%、34.15%和21.09%,其中TOC>2%的富有机质页岩占比超过55.24%。山2 3亚段海陆过渡相页岩生烃潜力(S 1+S 2)值分布在0.1~7.77 mg/g之间,平均值为0.42 mg/g。从生烃潜力频率分布图[图2(b)]来看,山2 3亚段S 1+S 2<0.05 mg/g、0.05~0.1 mg/g、0.1~0.2 mg/g和>0.2 mg/g的占比分别为4.42%、7.08%、39.82%和48.68%。
图2 研究区海陆过渡相页岩有机质丰度和生烃潜力分布直方图

Fig.2 Histogram of the distribution of organic matter abundance and hydrocarbon potential of transitional facies shales in the study area

干酪根镜下显微组分鉴定结果表明,研究区山2 3亚段海陆过渡相页岩主要以腐殖无定形体和镜质组为主,并含有一定量的腐泥无定形体,表明其不仅具有生气能力,还具有一定的生油能力[图3(a)]。镜质体反射率(R O)分析结果表明,山2 3亚段海陆过渡相页岩R O值介于2.21%~3.12%之间,平均值为2.60%,整体上处于过成熟生干气阶段[图3(b)]。
图3 研究区海陆过渡相页岩有机质类型和成熟度判识图版

Fig.3 Plates of organic matter types and maturity determination of transitional facies shales in the study area

3.2 海陆过渡相页岩储集层特征

研究区山2 3亚段海陆过渡相页岩孔隙类型主要以无机矿物孔为主,有机孔和微裂缝相对较少(图4)。无机矿物孔形态差异较大,以狭缝状、不规则多边形和椭圆形为主,大多数为微米级孔,包括粒内孔、粒间孔、黏土矿物层间孔和黄铁矿晶间孔等。有机质孔不发育,仅仅部分样品中可见零星分布的少量有机质孔,形态主要以圆形、椭圆形和不规则多边形为主。页岩储层可见微裂缝,但分布密度不大,依据成因可划分为有机质收缩缝、成岩收缩缝和构造应力破裂缝3类。
图4 研究区海陆过渡相储集层扫描电镜镜下特征

(a)DJ51井,2 271.86 m,山2 3亚段,黑色页岩,黏土矿物层间孔;(b)DJ51井,2 293.25 m,山2 3亚段,黑色页岩,黄铁矿晶间孔,碎屑黏土矿物充填;(c)DJ3-4井,2 137.56 m,山2 3亚段,灰黑色页岩,有机质孔;(d)DJ3-4井,2 137.56 m,山2 3亚段,灰黑色页岩,有机质孔,存在大孔套小孔的特征,连通性较好;(e)DJ3-4井,2 143.05 m,山2 3亚段,黑色页岩,有机质收缩边缘缝;(f)DJ51井,2 294.9 m,山2 3亚段,黑色页岩,微裂缝

Fig.4 Scanning electron microscope microscopic characteristics of the transitional facies reservoirs in the study area

页岩储层的孔径可以采用密度泛函理论(DFT)、Barrett-Joyner-Halenda(BJH)和高压压汞的计算方法取得35-37。本文研究采用低压CO2吸附、低压N2吸附和高压压汞联合表征页岩储层孔径,其中小于2 nm的孔径采用DFT理论方法计算低压CO2吸附实验数据获得,2~10 nm的孔径采用DFT方法计算低压N2吸附实验数据获得,10~50 nm的孔径采用BJH方法计算低压N2吸附实验数据获得,大于50 nm的孔径采用高压压汞实验数据获得。研究结果表明(图5),研究区海陆过渡相页岩总孔体积分布在0.007 3~0.067 8 cm3/g之间,平均值为0.024 8 cm3/g。其中,微孔体积分布在0.001 6~0.045 0 cm3/g之间,平均值为0.009 5 cm3/g,占总孔体积的12.57%~66.35%,平均值为35.19%;介孔体积分布在0.000 7~0.012 6 cm3/g之间,平均值为0.006 1 cm3/g,占总孔体积的1.05%~54.83%,平均值为29.92%;宏孔体积分布在0.002 8~0.029 5 cm3/g之间,平均值为0.009 3 cm3/g,占总孔体积的16.26%~69.45%,平均值为34.89%。
图5 研究区海陆过渡相页岩储层孔体积和占比分布

Fig.5 Pore volume and percentage distribution of shale reservoirs in the transitional facies reservoir in the study area

4 海陆过渡相页岩气富集控制因素

4.1 有机质丰度

有机质丰度(TOC)控制着页岩储层中甲烷的吸附量38-39。本文研究中海陆过渡相页岩样品的最大过剩吸附量和最大绝对吸附量均与TOC呈现出两段式的正相关关系[图6(a),图6(b)]。其中,TOC大于4%的页岩样品最大过剩吸附量和最大绝对吸附量均与TOC具有较高的相关性(R 2值分别为0.960 3和0.944 5),而TOC含量小于4%的样品相关性相对较低。以上研究结果表明,页岩TOC含量越高,有机质丰度对页岩的吸附能力贡献越大、影响越明显。Langmuir压力(P L)代表了吸附气体对页岩表面的亲和力,其中P L值越低,代表亲和力越大40。根据P LTOC的关系分析可以看出[图6(c)],P L值随着TOC含量的增大而持续变小,表明了甲烷气体对高有机质丰度的页岩具有更强的亲和力。吸附相密度也代表了页岩对甲烷的亲和力,其值越小,单位体积内吸附的甲烷气体分子越多41。分析结果表明[图6(d)],在TOC含量大于4%的页岩样品中,吸附相密度具有随TOC含量增加而增大趋势,而在低TOC含量页岩样品中,吸附相密度与TOC的相关性不明显。以上分析表明,页岩中高有机质丰度控制了吸附能力,有机质的发育为甲烷气体提供了吸附点位,增加了页岩对气体的吸附能力。随着页岩有机质含量的增加,吸附气体对页岩的亲和力增加,吸附量增多,含气性越好。
图6 研究区山西组页岩样品TOC含量与吸附能力关系

(a)最大过剩吸附量;(b)最大绝对吸附量;(c)Langmuir压力;(d)吸附相密度

Fig.6 Plot of TOC content versus adsorption capacity of shale samples from Shanxi Formation in the study area

4.2 页岩孔径大小

为了明确不同尺度孔隙对页岩甲烷吸附能力的影响,建立了最大绝对吸附量与微孔、介孔和宏孔的孔体积和比表面积关系(图7)。结果显示,最大绝对吸附量与微孔的体积和孔比面积也存在明显的两段式相关性。TOC含量大于4%的页岩样品中最大绝对吸附量与微孔体积和比表面积呈现较好的正相关性(R 2值分别为0.821 9和0.905 7),而TOC含量小于4%的页岩样品相关性较弱。另外,介孔和宏孔的孔体积和比表面积与最大绝对吸附量关系不明显。由此表明,有机质提供微孔结构对甲烷吸附能力起到了重要的控制作用,在富有机质页岩中有机质形成的微孔可明显增强页岩的甲烷吸附能力,为甲烷气体分子提供更多的吸附点位。
图7 研究区山西组页岩样品的V L与不同尺度孔隙结构参数的关系

Fig.7 Relationship between V L and pore structure parameters at different scales for shale samples from the Shanxi Formation in the study area

4.3 岩性组合

受沉积环境的影响,研究区山西组页岩层系岩性较为复杂,主要发育炭质页岩、粉砂质页岩、含灰质页岩、泥质粉砂岩,夹细砂岩、粉砂岩和煤层以及煤线,并且具有多种岩性纵向叠置的分布特征。对于富有机质页岩层系来说,不同的岩性组合关系将会影响页岩气保存和富集。已有学者分析认为大宁—吉县地区山2 3亚段主要发育障壁海岸型的潮坪沉积和潟湖沉积,并认为潟湖相和潮坪相是最有利生气的沉积相组合42。本文研究中综合考虑富有机质页岩的形成环境、生烃能力以及顶底板的岩性,建立了页—煤组合、页—灰组合、页—粉砂组合和页—砂组合4种岩性组合关系(图8)。研究区DJ51井2 296~2 297 m炭质页岩平均TOC含量为7.98%,顶板岩性为泥岩,底板为灰岩,保存条件好,含气性高,全烃含量平均为5.6%。例如大吉51井对2 294~2 298 m山2 3亚段进行压裂,试气72 h,稳定产量1.08×104 m3/d,开采效果良好3
图8 研究区山西组富有机质页岩不同岩性组合模式

Fig.8 Different lithologic assemblage patterns of organic-rich shales of Shanxi Formation in the study area

为了研究不同岩性组合顶底板的封闭能力,分别采取灰岩、粉砂岩、砂岩和煤岩开展突破压力、排驱压力、扩散系数和渗透率实验分析测试。研究结果表明(图9),灰岩具有相对较高的突破压力和排驱压力,相对较低的渗透率和扩散系数,具有较好的封闭能力。而煤岩和粉砂岩实验分析结果相当,但煤岩具有非常高的有机质丰度,自身具备烃浓度封闭条件,因此封闭能力也相对较强。而砂岩突破压力低、排驱压力低、渗透率高、扩散系数大,封闭能力最差。因此,页岩—灰岩和页岩—煤岩是海陆过渡相页岩气保存最有利的岩性组合,其次为页岩—粉砂岩,页岩—砂岩的岩性组合最不利于海陆过渡相页岩气的保存富集。蒋裕强等43根据夹层、互层单层厚度与总厚度纵向分布规律,将大吉地区山西组页岩岩性组合划分为纯页岩型、富粉砂页岩型、富砂页岩型以及富煤页岩型。并且得出纯页岩型岩性组合是研究区最具勘探前景的“黄金靶体”。该研究虽然阐述了不同岩性组合之间的差异,但没有考虑到不同岩性组合顶底板封闭能力对页岩含气性的影响。
图9 不同岩性的渗透率、突破压力、排驱压力和扩散系数分布直方图

Fig.9 Distribution histogram of permeability, breakthrough pressure, displacement pressure and diffusion coefficient of different lithology

4.4 构造演化

地层倾角是构造作用强度的直观体现,对天然气逸散具有一定的影响44-46。前人的研究成果表明,地层倾角低于小于10°时,页岩储层中含气量较高47。研究区位于鄂尔多斯盆地东缘晋西挠褶带南端,基本构造格局表现为“一隆一凹两斜坡”,其西部平缓斜坡构造简单(图10),断层不发育,地层倾角<10°,有利于页岩气的富集和保存。自山西组沉积以来,经历了海西、印支、燕山和喜马拉雅4期构造活动,遭受多次地层埋藏和构造抬升。在随着地层埋藏和构造抬升过程中,在地层温度和压力的耦合控制作用下,构造演化不仅仅控制了页岩气的赋存状态变化,还会影响顶底板的封闭能力。
图10 研究区过G7-DJ20-JS17井地震剖面

Fig.10 Seismic profile of the Wells G7-DJ20-JS17 in the study area

选取研究区山西组3个标准页岩柱塞样,依次开展3 MPa、6 MPa、9 MPa、12 MPa、15 MPa、18 MPa及21 MPa的覆压孔渗分析测试。实验结果表明[图11(a)],随着压力从3 MPa增加到21 MPa时,3个样品的孔隙度均随之下降。其中F-1号样品的孔隙度从12.65%降至4.87%,总孔隙度降低率为61.48%;F-2号样品的孔隙度从1.79%降至1.53%,总孔隙度降低率为14.62%;F-3号样品的孔隙度从1.16%降至1.01%,总孔隙度降低率为12.53%。显然,原始孔隙度越大,随着压力的增加,孔隙度的降低越明显。覆压渗透率的测试结果表明[图11(b)],随着压力从3 MPa增加到21 MPa时,3个样品的渗透率降低了1~2个数量级。以上实验表明,随着埋藏深度的增加,上覆地层压力增加,页岩的储集空间被压缩,孔喉越窄,渗透率越小,页岩自身和顶底板的封闭性也就越强。刘雯等9指出了地层压差是影响页岩气垂向分布的关键因素之一,但没有针对地层压力对页岩含气性的影响进行讨论。GP06-1H水平井位于研究区西南部,页岩气稳定日产气量为3.0×104 m3,最高日产气量达3.3×104 m3[43,JY-P01水平井位于研究区东北部,较于GP06-1H水平井埋深较浅(图1),页岩气产量不稳定,最高日产气量1.18×104 m3。从生产测试结果上来看,页岩埋深越深,稳定日产气量和最高日产气量越大。
图11 研究区山西组典型样品的孔隙度、渗透率随压力的变化关系

Fig.11 Relationships between porosity and permeability as a function of pressure for typical samples from the Shanxi Formation in the study area

显然,埋藏和构造的演化不仅会影响有机质的生烃和页岩气的赋存状态,也会影响页岩自身和顶底板的封闭性能力。因此,在供气充足的情况下,当埋藏达到一定深度后(含气临界深度),页岩自身和顶底板封闭能力开始生效,页岩储层开始储集页岩气,并随着深度的不断增加,页岩气储集能力和封闭能力也随着增加,页岩储层中含气量增大。基于以上分析,建立了研究区海陆过渡相页岩气储集能力和封闭能力的演化模式(图12)。在持续沉降阶段初期,此时A点位于含气临界深度上方,即便页岩储层具有一定的页岩气储集能力,但由于页岩自身和顶底板封闭能力差,页岩储层中几乎不含气,而B点埋深大,封闭能力和储集能力均大于A点,因此B点含有一定量的页岩气。在持续沉降阶段,此时A点的封闭能力大于临界值具有封闭页岩气的能力,而B点埋藏深度更大,在温度和压力的耦合控制作用下,具有比A点更高的页岩气储集能力和封闭能力,因此B点含气量远大于A点。然而,在局部隆升阶段,B点位于隆升区,受构造抬升影响,孔隙度和渗透率会随着深度的降低而增加,并且在抬升过程中可能会形成新的裂缝进一步增加页岩气储集空间,因此封闭能力将会随着构造抬升而降低,而页岩气储集能力在温度和压力的共同作用下,也会随着埋深变浅而降低,游离气向吸附气转化,总含气量变低。此时A点处于沉降区,随着深度的增加其页岩气储集能力和封闭能力均随之增强,A点含气量开始大于B点。
图12 页岩气储集能力和封闭能力剖面及其演化模式

Fig.12 Shale gas storage and containment capacity profiles and their evolutionary patterns

5 结论

鄂尔多斯盆地东缘大吉地区山2 3亚段海陆过渡相页岩有机质丰度较高,有机质类型以Ⅱ2型为主,处于过成熟生干气阶段。海陆过渡相页岩TOC平均值为4.52%,TOC>2%的富有机质页岩占比超过55.24%,生烃潜力平均值为0.42 mg/g,展现了良好的生烃潜力。干酪根显微组分主要以腐殖无定形体和镜质组为主,含有一定量的腐泥无定形体镜质体反射率R O介于2.21%~3.12%之间。
大吉地区山2 3亚段海陆过渡相页岩储层孔隙类型主要以无机矿物孔为主,有机孔和微裂缝相对较少。页岩储层总孔体积分布在0.007 3~0.067 8 cm3/g之间,平均值为0.024 8 cm3/g,其中微孔体积占比为35.19%,介孔体积占比为29.92%;宏孔体积占比为34.89%。
海陆过渡相页岩气富集主要受有机质丰度、微孔体积、岩性体积和适当的构造演化联合控制。页岩中高有机质丰度增强了吸附能力,页岩有机质含量增加,吸附气体对页岩的亲和力增加,吸附量增多,含气性越好。有机质提供微孔结构对甲烷吸附能力起到了重要的控制作用,微孔可以为甲烷气体分子提供更多的吸附点位。煤和灰岩突破压力高、扩散系数小,封闭能力强,而砂岩封闭性差,页—煤和页—灰是页岩气滞留原位富集最优岩性组合,其次为页—粉砂组合,页—砂组合最差。平稳的构造环境和适当的埋深演化,控制了页岩自身和顶底板的封闭能力。基于以上控制因素建立了研究区海陆过渡相页岩气储集能力和封闭能力的演化模式。
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