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SHI ZhenSheng, QIU Zhen. Main Bedding Types of Marine Fine-Grained Sediments and their Significance for Oil and Gas Exploration and Development[J]. Acta Sedimentologica Sinica, 2021, 39(1): 181-196. doi: 10.14027/j.issn.1000-0550.2020.097
Citation: SHI ZhenSheng, QIU Zhen. Main Bedding Types of Marine Fine-Grained Sediments and their Significance for Oil and Gas Exploration and Development[J]. Acta Sedimentologica Sinica, 2021, 39(1): 181-196. doi: 10.14027/j.issn.1000-0550.2020.097

Main Bedding Types of Marine Fine-Grained Sediments and their Significance for Oil and Gas Exploration and Development

doi: 10.14027/j.issn.1000-0550.2020.097
Funds:

National Natural Science Foundation of China 41572079

National Science and Technology Major Project 2017ZX05035-001

  • Received Date: 2020-07-06
  • Publish Date: 2021-02-10
  • Bedding type of black marine fine-grained sediments is an important indicator for reconstructing the paleo-sedimentary environment, since it affects the quality and fracturability of shale in oil and gas reservoirs. After fine-grained materials are formed by biogenic, chemical or biochemical debris genesis, they are carried into the ocean in the form of single particles, flocs, muddy intraclasts, lithics, organo-mineralic aggregates, and fecal pellets by wind, low-density flow, gravity, and bottom currents. The fine-grained sediment is deposited via suspension settling and/or advective sediment transport processes close to the sediment-water interface. After the fine-grained materials settle, two types of laminae (clayey and silty), two types of laminaset (clayey and silty) and two types of bedding (graded and homogeneous beds) develop, forming five major bedding types (massive, graded, rhythmic and varve, horizontal, and cross-bedding). Of these, the massive bedding may be divided into bioturbation-formed type and homogeneous type. In addition, horizontal bedding occurs in four types: graded (claystone), siltstone-bearing, graded (siltstone to claystone), and interlaminated siltstone and claystone. In black marine fine-grained sediments, the clayey and silty laminae differ in material composition, pore type and pore structure, plane porosity, pore size distribution and microfracture type and density, resulting in different shale reservoir quality. In addition, bedding types vary in geomechanical properties and crack-propagation behavior due to the different development of the stratum (thickness, thickness difference, continuity, morphology and geometric relationship).
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Main Bedding Types of Marine Fine-Grained Sediments and their Significance for Oil and Gas Exploration and Development

doi: 10.14027/j.issn.1000-0550.2020.097
Funds:

National Natural Science Foundation of China 41572079

National Science and Technology Major Project 2017ZX05035-001

Abstract: Bedding type of black marine fine-grained sediments is an important indicator for reconstructing the paleo-sedimentary environment, since it affects the quality and fracturability of shale in oil and gas reservoirs. After fine-grained materials are formed by biogenic, chemical or biochemical debris genesis, they are carried into the ocean in the form of single particles, flocs, muddy intraclasts, lithics, organo-mineralic aggregates, and fecal pellets by wind, low-density flow, gravity, and bottom currents. The fine-grained sediment is deposited via suspension settling and/or advective sediment transport processes close to the sediment-water interface. After the fine-grained materials settle, two types of laminae (clayey and silty), two types of laminaset (clayey and silty) and two types of bedding (graded and homogeneous beds) develop, forming five major bedding types (massive, graded, rhythmic and varve, horizontal, and cross-bedding). Of these, the massive bedding may be divided into bioturbation-formed type and homogeneous type. In addition, horizontal bedding occurs in four types: graded (claystone), siltstone-bearing, graded (siltstone to claystone), and interlaminated siltstone and claystone. In black marine fine-grained sediments, the clayey and silty laminae differ in material composition, pore type and pore structure, plane porosity, pore size distribution and microfracture type and density, resulting in different shale reservoir quality. In addition, bedding types vary in geomechanical properties and crack-propagation behavior due to the different development of the stratum (thickness, thickness difference, continuity, morphology and geometric relationship).

SHI ZhenSheng, QIU Zhen. Main Bedding Types of Marine Fine-Grained Sediments and their Significance for Oil and Gas Exploration and Development[J]. Acta Sedimentologica Sinica, 2021, 39(1): 181-196. doi: 10.14027/j.issn.1000-0550.2020.097
Citation: SHI ZhenSheng, QIU Zhen. Main Bedding Types of Marine Fine-Grained Sediments and their Significance for Oil and Gas Exploration and Development[J]. Acta Sedimentologica Sinica, 2021, 39(1): 181-196. doi: 10.14027/j.issn.1000-0550.2020.097
  • 细粒沉积是指粒径小于62.5 µm的细粒沉积物和沉积岩[1-2]。文献中,常见的术语有泥质板岩(argillite)、黏土(clay)、黏土岩(claystone)、泥(mud)、泥岩(mudrock,mudstone)、泥质岩(pelite)、粉砂(silt)、粉砂岩(siltstone)、板岩(slate)和瓦克岩(wacke)等等。细粒沉积地表分布广泛,约占沉积岩分布面积的2/3[3],它是一种特殊的语言[4-5],记录了大量地球历史信息,是恢复古构造、古气候及古水体性质的关键。其是全球最重要的“碳汇”[6],影响和控制着全球的碳埋藏和碳循环,进而影响全球气候变化和海洋循环。黑色页岩中蕴藏着大量石油、天然气、金属矿产及非金属矿产,构成这些矿产的源岩、储集层或盖层[7-8],决定并改变着全球能源格局[9-12]

    与粗碎屑岩和碳酸盐岩相似,细粒沉积中发育大量层理,岩石表现出矿物成分、结构、颜色等纵向差异[13]。细粒沉积由于形成环境复杂,因而层理类型多样[14-18]。层理的形成与物源性质、搬运营力[11,19]、水体性质及古生物活动[20]等因素有关,任一因素变化均可留下相应记录[21-22],从而造成储层各向异性,并影响细粒岩体积裂缝扩展规律及体积压裂效果。

    近年来,随着非常规油气资源工业化勘探开发的快速发展,非常规油气地质学理论体系逐步建立[23],“非常规油气沉积学”的概念也得以提出[24]。非常规油气沉积学研究对象是细粒沉积(物)岩,它通过分析其物质组分、沉积构造等,构建沉积岩形成过程、成因模式及事件沉积等等。层理作为细粒沉积(物)岩中的一种重要沉积构造类型,是再现其形成过程和形成环境的重要依据。本文通过详细综述海相细粒沉积的层理组成、类型及成因,试图揭示不同层理的形成过程及形成条件,并探讨页岩油气勘探开发意义,以期促进非常规油气沉积学的研究与发展。

  • 海洋环境中,生物成因沉积物主要有2种来源,一是生活于水体中或沉积物—水界面处生物的残骸,二是透光带中初级生产力生产的有机碳。生物活动主要有浮游植物季节性生长、细菌作用、底栖生物活动及游泳生物活动。海洋环境中,浮游藻类的生长常呈季节性变化,甚至在某一季节勃发[21,25-26]。藻类勃发期,其它属种生物生长由于光线、营养物质等缺乏而受到严重抑制。藻类勃发期可以形成大量有机质,从而形成富有机质层。细粒物质形成之后,底栖生物可以在此殖居,对其进行改造和破坏,其扰动强度与水体含氧量、沉积物沉积速率、沉积物有机质含量等密切相关[27]

  • 海洋环境中,生物化学成因物质主要是指由底栖微生物群落通过捕获与粘结碎屑沉积物,或经与微生物活动相关的无机或有机诱导矿化作用在原地形成的沉积物和/或沉积岩[28]。其物质成份可由碳酸盐岩、磷块岩、硅质岩、铁岩、锰岩和有机质页岩等组成,也可由硫化物、黏土岩和各种碎屑岩组成。其中,微生物碳酸盐岩最为发育。

    微生物是所有形体微小的单细胞或个体结构较为简单的多细胞、甚至无细胞结构低等生物的总称。自地球历史早期,微生物便广泛存在于沉积物表面和内部,广泛参与沉积物的生产、沉积及成岩。微生物类型多样,包括光合原核生物(蓝细菌)、真核微体藻类(如褐藻、红藻、硅藻等)、化学自养或异养微生物(如硫细菌等),以及一些后生生物(如介形虫及甲壳类等)[29]。细菌对细粒物质的形成和改造也会起一定的作用,甚至可以形成深水微生物席,构成纹层[16,30]。另外,细菌的活动能够在沉积物表面聚集多种金属,在沉积速率很低的情况下,可形成富金属尤其是富铁纹层。

  • 细粒物质除了生物成因、生物化学成因外,还有碎屑成因。碎屑成因沉积物主要来源于土壤的物理和化学风化产物,少量来源于火山灰和陆源有机质[31]。前人研究表明,晚古生代及更年轻土壤层的风化作用产物主要是黏土矿物、石英及少量长石和岩屑。硅质碎屑组份中,黏土级颗粒(<2 µm)矿物主要为黏土矿物,其多来源于化学风化作用,而粉砂级颗粒(2~62.5 µm)矿物主要为石英,其多来源于物理风化作用[20]

    碎屑成因沉积物通过水系输入海洋,在水体分层的情况下,随入盆水流输入的碎屑物质可以沿着温跃层或密跃层呈平流或层间流的方式运移至整个深水区。在一定的水体环境条件下,这些平流物质克服自身的内聚力和水体摩擦力沉积下来。内聚力和摩擦力对正常的平流悬移状态起保护作用,免受气候驱动力影响,直到处于某种决定性的机械沉积临界值为止。

  • 细粒物质可以呈单颗粒(包括碳酸盐单晶矿物)、絮凝颗粒、泥质内碎屑、岩屑、有机—矿物集合体(“海洋雪”)及浮游动物粪球粒等形式搬运和沉积。现代泥质沉积分析表明,粒径小于10 µm的颗粒可以通过范德华力结合成絮凝颗粒进行搬运[32],絮凝作用受溶液浓度及紊流强度影响[1]。絮凝颗粒与粒径大于10 µm的单颗粒一起构成细粒沉积的主要组份。细粒沉积含有大量泥质内碎屑,其由沉积物表层泥质侵蚀而成,形状不规则到圆状,大小为几十微米到几厘米[33-34]。泥质内碎屑搬运受含水量影响,其含水量越低,搬运过程中越不易分解。而成壤集合体及再改造的冲积泥壳虽然含水量低(含水量一般为30%~40%),但并不适合长距离搬运[35-36]。来源于完全固化岩石碎片的泥岩岩屑在现代及古代细粒沉积物中也普遍存在,其能以底载荷的形式搬运几百到几千公里[11]。有机—矿物集合体主要由分散的无定形有机质、黏土级颗粒和黄铁矿组成,其也构成细粒沉积的重要组份[37-38]。在现代海洋中,当其粒径大于500 µm时称为“海洋雪”,而粒径小于500 µm时称为植物腐殖质[21,25]。这些聚合体通过浮游动物分泌的胞外多糖、颗粒间的电化学吸引和不规则颗粒之间物理嵌合结合在一起,含水量与絮凝颗粒相似。粪球粒由浮游生物的排泄物形成,有机质含量高,构成细粒沉积的重要组份[21,25,39]

  • 细粒沉积存在风力搬运、重力搬运和底流搬运3大搬运营力[11,19]。风力搬运有沙尘暴[40-42]和火山灰2种方式:沙尘暴的形成需要大面积分布的物源区和合适的信风模式;而火山灰的形成与火山喷发有关,并可在区域上形成良好标志层[43]。重力搬运有4种类型,即低密度流搬运、与河流三角洲相伴生的浊流搬运、波浪和水流引发的重力流搬运以及风暴作用引发的离岸流搬运。低密度流搬运常形成于河流入海处,搬运距离一般为几十公里[44-45],甚至可达上百公里[46]。浊流的形成常与三角洲前缘滑塌、河流的异常洪水作用及小型干旱河流产生的高密度流有关[47],地形坡度通常大于0.7°,搬运距离一般为几公里[48]。波浪和水流引发的沉积物重力流与底层泥质沉积物再活化有关[49],在重力驱动下,可沿坡度为0.03°的斜坡离岸搬运[21,25,50],并形成正粒序[51]或“三层序列”[21,25]。风暴作用引发的离岸流形成的沉积物远端主要或完全由泥质组成,其形成地形坡度为0.03°~0.7°[52]。由于受风暴浪基面的限制,以上营力搬运泥质沉积物的距离均很有限,一般小于100 km。而对于陆缘海或陆表海上千公里的细粒沉积物搬运,风力或潮汐引发的底流搬运起到关键作用,其搬运距离可达1 000 km。同时,多级别的海平面升降旋回也有一定的影响[11]

  • 细粒物质的剥蚀主要受颗粒间粘合力及泥岩固结程度控制,粒径和水流速度并不能起到主要控制作用。由于颗粒间粘合力作用,泥质剥蚀所需流速比细砂还大,甚至达到砾石级颗粒的程度。研究表明,泥岩起始剥蚀速度受多种因素影响,包括固结程度[53-54]、黏土矿物类型、空隙比、剪切力以及其经历的地质过程等。对于不同的黏土矿物,在给定的剥蚀速度下,伊—蒙混层起始流速最快,高岭石最难剥蚀。泥质的起始剥蚀速度与其沉淀时悬浮溶液浓度[55]、沉降和固结过程中形成的结构及非均质性分布状况等[56]有关。另外,生物因素及有机质也有重要影响,生物扰动强度增加,起始剥蚀速度下降;而对于无生物扰动泥岩,生物席、海草密度、硅藻种群密度及有机质含量对泥质沉积物具有稳定作用[14-16],有机质含量增加,起始剥蚀速度增大[57-58]

    细粒物质主要有2种沉降方式:第一种是在水体分层的静水环境中,细粒物质以悬浮物的形式直接从水体中连续沉降下来(图1a);第二种是在流动水体中,细粒物质以颗粒集合体的形式搬运,并以水流波纹的形式聚集下来,其聚集的最大水流流速可达35 cm/s,细粒物质层与层之间可能存在着剥蚀面[1]图1b)。洪水作用、浊流作用和高初级生产力区形成的大量细粒物质,以羽状流形式在水体中搬运[12]。在特定的水体环境中,由于水体pH值、Eh值、水体盐度等水化学环境变化或者重力作用,细粒物质可以以第一种方式沉降下来。

    Figure 1.  Two settling modes of shale deposition: (a) suspension settling; (b) advective sediment transport processes. Bedding planes are indicated by solid lines, laminae by dotted lines. The vertical scale is exaggerated relative to the horizontal scale

    静水并非细粒物质沉降的必要条件[3-4,59]。在流速为15~30 cm/s的水体中,陆源碎屑泥[12,60]、不同类型黏土以及碳酸盐泥[10]等在蒸馏水、淡水以及海水中均可发生絮凝沉降,沿水槽底部迁移并形成波纹[1]。随着时间迁移,波纹迁移可产生侧向堆积,并形成下超、削截和上超等沉积构造。细粒物的絮凝受流体浓度、层剪切力、沉降速度以及紊流作用强度等因素控制。随着时间推移,絮凝颗粒逐渐增大,并达到最大平衡直径[3]。当絮凝颗粒强度足够抗拒层面剪切力时,颗粒便发生沉降。研究表明,在给定的水体浓度和盐度下,不同黏土矿物关键沉降速率非常相似[3]。随着水流速度和层面剪切力下降,絮凝颗粒粒径增大。当泥质悬浮物达到关键沉积速率时,悬浮沉积物浓度持续降低,越来越多的絮凝颗粒降到水底,并以底载荷方式移动。随着流速进一步降低,底部絮凝颗粒移动越来越慢,更多沉积物沉降并以底载荷形式搬运并最终堆积并形成波纹。

  • 黑色细粒沉积按其成因及规模,可细分为纹层(lamina)、纹层组(laminaset)和层(bed)[61-62] 多个纹层构成纹层组,单个或多个纹层组构成层。

    黑色细粒沉积主要由粒径小于62.5 μm的颗粒组成,根据颗粒粒径可细分为粗粉砂(62.5~31.2 μm)、细粉砂(31.2~3.9 μm)和细粒泥(小于3.9 μm)。鉴于此,本文将粒径小于3.9 μm的颗粒统称为泥质,粒径为3.9~62.5 μm的颗粒统称为粉砂质。黑色细粒岩纹层按其组成颗粒粒径,可划分为泥纹层和粉砂纹层2种类型(表1图2)。泥纹层中粒径< 3.9 μm颗粒含量>50%,偏光显微镜下颜色较深,常称暗纹层[63-64]。粉砂纹层中粒径>3.9 μm颗粒含量>50%,偏光显微镜下颜色较浅,常称亮纹层。

    基本单元 组成 粒序
    纹层 泥纹层 粒径<3.9 μm的颗粒含量大于50%
    粉砂纹层 粒径>3.9 μm的颗粒含量大于50%
    纹层组 泥纹层组 泥纹层 均质状
    粉砂纹层组 粉砂纹层 正粒序/反粒序
    递变层 正递变型 泥纹层组 正粒序
    反递变型 泥纹层组 反粒序
    砂泥正递变型 粉砂纹层组和泥纹层组 正粒序
    砂泥反递变型 泥纹层组和粉砂纹层组 反粒序
    均质层 粉砂层 粉砂纹层或粉砂纹层组 均质状
    泥质层 泥纹层或泥纹层组 均质状

    Table 1.  Lamina, laminaset, and beds of marine black shale

    Figure 2.  Clayey (white arrows) and silty (red arrows) black shale laminae

    泥纹层和粉砂纹层的矿物组成常存在明显差异。以川南五峰组—龙马溪组黑色页岩为例,其泥纹层主要由黏土级硅质、黏土矿物及有机质组成,粉砂纹层主要由细粉砂级碳酸盐矿物和石英颗粒组成。其中,硅质中常伴生大量硅质海绵和放射虫[63],指示其为生物成因[64]。碳酸盐矿物主要为方解石和白云石,富碳酸盐页岩中常见分散状或密集状钙质生物碎屑。

  • 纹层组(laminaset)是由纹层组界面限定的一组成因相关的纹层组合而成。通常,在单一层内,同一纹层组的物质组成、结构和几何关系均相似且相互整合。黑色细粒岩纹层组按其组成颗粒粒径,可分为泥纹层组和粉砂纹层组2大类。泥纹层组由多个泥纹层构成,中间常夹薄层断续状或透镜状粉砂纹层(图3)。粉砂纹层组由多个粉砂纹层构成,中间常夹有薄层透镜状或断续状泥纹层。

    Figure 3.  In black shale, silty laminaset comprises series of silty laminae (red arrows) with normal grading. Muddy laminaset comprises series of clayey laminae (white arrows) intercalated with strip⁃like silty lamina

  • 层(bed)是由一组相对整合且成因相关的纹层或纹层组构成,其顶、底界面为剥蚀面、停积面或相对整合面。黑色细粒沉积中,泥纹层和粉砂纹层可构成递变层和均质层2大类。递变层进一步细分为正递变层、反递变层、砂泥正递变层和砂泥反递变层。均质层进一步细分为粉砂层和泥质层(表1)。

    递变层中,正递变层由多个泥纹层组合而成,颗粒正粒序排列,偏光显微镜下底部颜色较浅、上部颜色较深(图4a),物质组成表现为由下至上细粉砂级矿物颗粒减少。正递变层的底界面多呈连续、波状,突变接触。反递变层也由多个泥纹层构成,颗粒反粒序排列,偏光显微镜下底部颜色较深、上部颜色较浅(图4b),物质组成表现为由下至上细粉砂级矿物颗粒增加。反递变层顶界面多呈连续、波状,突变接触。砂泥正递变层由粉砂纹层组和泥纹层组叠置而成,下部为粉砂纹层组,上部为泥纹层组。由下至上,粉砂纹层组逐渐过渡为泥纹层组,从而构成正粒序(图4c)。砂泥正递变层底部常呈突变接触,几何形态呈连续、波状、平行。砂泥反递变层由泥纹层组和粉砂纹层组叠置而成,下部发育泥纹层组,上部发育粉砂纹层组。由下至上,泥纹层组逐渐过渡为粉砂纹层组,从而构成反粒序(图4c)。砂泥反递变层顶部突变接触,几何形态连续、波状、平行。均质层中,粉砂层由多个粉砂纹层构成(图4d),整体呈均质状,其顶、底界均为突变接触,界面呈现连续、波状、平行。泥质层主要由泥纹层组成(图4d),整体呈均质状,其顶、底界均为突变接触,界面呈现连续、波状、平行。

    Figure 4.  Composition units and their characteristics of the Longmaxi shale in Sichuan Basin

    描述黑色细粒沉积层理,纹层、纹层组和层的连续性、形状和几何关系是关键(图5)。在单个层内部,纹层连续性可分为连续和断续,形态分为板状、波状和弯曲状,几何关系分为平行或非平行。

    Figure 5.  Descriptive terms of continuity, shape, and geometry for composition units of black shale These terms are useful for all levels of stratification[61]

  • 块状层理按其成因可分为生物扰动型块状层理和均质型块状层理。生物扰动型块状层理主要由泥质层组成(图6a,c,d),生物扰动构造发育,局部区域见有生物潜穴。生物扰动型块状层理层界面多为生物殖居面,局部发育侵蚀面,侵蚀面上下存在明显的地层尖灭。均质型块状层理由厚层粉砂层构成(图6b),细粒岩内呈现均质。层理内部常见有大量介壳类生物碎屑,生物碎屑局部成层分布。均质层理细粒岩层界面多为侵蚀面,存在明显的地层尖灭。

    Figure 6.  Massive bedding of black shale and its characteristics

    川南五峰组—龙马溪组黑色页岩中,生物扰动型块状层理和均质型块状层理发育层位、形成环境及成因机制均明显差异。生物扰动型块状层理主要发育于五峰组最底部,其下发育宝塔组瘤状灰岩,而均质型块状层理主要发育于五峰组顶部的观音桥层,其顶界为龙马溪组黑色页岩(图7)。生物扰动型块状层理形成时期,盆地水体处于低能富氧的状态[66],沉积物沉积速率极低,大量生物因此在此长时期殖居,从而形成强烈的生物扰动[28]。均质型块状层理形成时期,由于全球气候变凉,水体中含氧量增高,水动力增强,介壳等生物大量生长[67]。动荡富氧的水体环境对底层沉积物强烈改造,从而形成均质型块状层理。

    Figure 7.  Bedding types and vertical distribution in the Wufeng⁃Longmaxi shale

  • 递变层理又称粒序层理,主要由粉砂层和泥质层互层组成(图8),由下至上,泥质颗粒含量逐渐增加,粉砂质颗粒含量逐渐减少,从而构成正递变。递变层理细粒岩底界面多为侵蚀面,界面之上存在明显的冲刷—充填构造、削截等现象,并发育较厚层的粉砂质滞积层。递变层理细粒岩内部,泥质层与粉砂层界面多为连续、波状、平行。

    Figure 8.  Graded bedding of black shale and characteristics

    递变层理常形成于水深只有几十米的潮下环境[68],风暴作用定期性发生,或存在于低速底流的浅水海洋环境[69-70]。底流活动强烈时期,较强的水体流动对下伏泥岩冲刷,并形成侵蚀面和滞积层。底流活动较弱时期,水流能量的脉动形成多期泥质层和粉砂层,随着水体能量的减弱,粉砂层含量逐渐降低。底流活动平静期,泥级颗粒逐渐堆积,从而形成厚层的泥质层。

  • 韵律层理由层与层间平行或近于平行的、等厚或不等厚的、两种或两种以上的岩性层的互层重复出现所组成(图9a)。海相沉积中,韵律层理的成因很多,可以由潮汐环境中潮汐流的周期变化形成潮汐韵律层理,也可由浊流沉积形成复理石韵律层理等[71]

    Figure 9.  Rhythmic bedding and varve of black shale and characteristics

    海相黑色细粒沉积中,年纹层最为常见。年纹层由粉砂纹层与泥纹层互层组成,外表呈现为浅色层与深色层的成对互层,纹层与纹层之间平行或近于平行(图9b)。海相年纹层由气候季节性变化形成,形成于海洋或与全球海相相连的咸水环境,底层水体缺氧是年纹层形成和保存的先决条件[72]。前人研究表明,海相年纹层常形成于以下区域:1)水体发育缺氧的中深带;2)水体存在密度分层;3)水体存在富营养化;4)受限的潟胡和海湾;5)水体分层崩坍及“冬季倾泻”。

    年纹层根据其形成过程和组分特征,可分为3大类,即碎屑年纹层、生物化学成因年纹层(如硅藻年纹层等)和化学成因年纹层(如方解石年纹层、菱铁矿年纹层、黄铁矿年纹层、蒸发盐年纹层等)[73]。海相年纹层的形成受控于特殊的环境和沉积条件,如足够高的沉积速率、底层水体严重缺氧、沉积物供给季节性变化等。

  • 水平层理的特点是纹层呈直线状互相平行,并且平行于层面。一般认为这种层理是在比较稳定的水动力条件下,物质从悬浮物或溶液中沉淀而成。黑色细粒沉积中,水平层理可细分为4种类型:递变型水平层理、条带状粉砂型水平层理、砂泥递变型水平层理、砂泥互层型水平层理。

    递变型水平层理由多个正递变层和(或)反递变层构成(图10),层界面上下颗粒粒径及颜色略有差异,层界面多呈连续、板状、平行或连续、波状、平行。露头剖面和岩芯上,不同层的颜色常呈现出微弱深浅差异,层界面一般在光学显微镜下也能识别。递变型水平层理细粒沉积内部,正递变层单层厚0.8~12 mm,平均值5 mm;反递变层单层厚2~9.7 mm,平均值5.3 mm。川南五峰组—龙马溪组递变型水平层理页岩单个层组厚度26~129 cm,平均值52 cm,层组与层组之间常发育0.3~4 cm的斑脱岩,层组界面之下颗粒粒度较粗,界面之上颗粒粒度较细。

    Figure 10.  Parallel bedding (grading type) of black shale and characteristics

    条带状粉砂型水平层理由粉砂纹层和泥纹层组合构成(图11),多个泥纹层构成泥质层。条带状粉砂型水平层理粉砂纹层多呈条带状、弥散状或断续状,局部可见透镜状,泥质层/粉砂纹层厚度比>3。泥质层与粉砂纹层顶底均呈突变接触,界面多为断续、板状、平行,偶见连续、板状、平行。川南龙马溪组黑色细粒岩中,粉砂纹层单层厚度0.05~0.75 mm,平均0.26 mm,泥质层厚度0.1~6.6 mm,平均1.1 mm。条带状粉砂型水平层理单个层组厚度33~83 cm,层组界面之下颗粒粒径粗,界面之上粒径细。露头和岩芯上,条带状粉砂型水平层理可见浅色层与深色层相间排列,浅色层呈条带状分布。

    Figure 11.  Parallel bedding (siltstone⁃bearing type) of black shale and characteristics

    砂泥递变型水平层理由砂泥正递变层和/或砂泥反递变层构成,中间夹有少量泥纹层(图12)。层界面多呈连续、板状、平行或连续、波状、平行,其底界面突变接触,顶界面渐变接触。川南龙马溪组黑色细粒沉积中,砂泥正递变层单层厚1~2.85 mm,平均值1.87 mm;泥纹层厚0.45~0.75 mm,平均值0.56 mm。砂泥反递变层厚1.8~2.1 mm,平均值1.95 mm。砂泥递变型水平层理细粒岩单个层组厚24~53 cm,平均值42 cm,层组界面之下颗粒粒径粗,界面之上粒径细。露头和岩芯上,砂泥递变型水平层理肉眼可见浅色层与深色层相间排列,间夹条带状方解石浅色层。

    Figure 12.  Parallel bedding (siltstone-to-claystone graded type) of black shale and characteristics

    砂泥互层型水平层理细分为两种类型(图13):第一种为粉砂纹层与泥质层互层,第二种为粉砂层与泥质层互层。第一种砂泥互层型水平层理中,粉砂纹层多呈长条带状,单层厚0.05~2.4 mm,平均值0.35 mm;泥质层厚0.1~1.7 mm,平均值0.58 mm。粉砂纹层与泥质层突变接触,多为连续、板状、平行,少数为断续、板状、平行。第二种砂泥互层型水平层理中,粉砂层厚0.35~4.5 mm,平均值1.57 mm,泥质层厚0.6~3.1 mm,平均值1.35 mm。层顶底界面均为突变接触,多呈连续、板状、平行,断续、板状、平行,断续、波状、平行三种。川南龙马溪组露头和岩芯上,砂泥互层型水平层理细粒岩单个层组厚22~97 cm,平均值34.7 cm,层组界面之下颗粒粒径粗,界面之上粒径细,肉眼可见浅色层与深色层相间排列,浅色层厚度明显增大。

    Figure 13.  Parallel bedding (siltstone and claystone interlaminated type) and characteristics

    川南五峰组—龙马溪组不同类型水平层理纵向分布明显差异(图7)。递变型水平层理主要分布于五峰组中上部,层位相当于笔石带D.complexusP.pacificus。条带状粉砂型水平层理多数发育于龙马溪组底部,层位相当于笔石带P.persculptus,页岩中常发育大量顺层缝和非顺层缝,相互交织构成网状。砂泥递变型水平层理发育于龙马溪组下部,层位相当于笔石带A.ascensus,页岩中顺层缝密度相对较大,非顺层缝密度相对较低。砂泥互层型水平层理发育于龙马溪组中部及上部,层位相对于笔石带P.acuminatus-S.sedgwickii,页岩裂缝密度进一步减少,只发育少量顺层缝。

    川南五峰组—龙马溪组砂泥互层型水平层理特征纵向存在差异性。在龙马溪组中部及上部,由下至上,砂泥互层型水平层理中粉砂纹层单层厚度逐渐增大,粉砂纹层/泥纹层比值逐渐增大。笔石带P.acuminatus-S.sedgwickii下部,砂泥互层型水平层理主要表现为砂泥薄互层,粉砂纹层/泥纹层比值为1/3~1/2。笔石带P.acuminatus-S.sedgwickii中部,砂泥互层型水平层理主要表现为砂泥等厚互层,粉砂纹层/泥纹层比值为1/2~1。笔石带P.acuminatus-S.sedgwickii上部,砂泥互层型水平层理主要表现为厚砂薄泥型,粉砂纹层/泥纹层比值为>1。

    水平层理主要形成于静水、缺氧的水体环境中,但不同类型水平层理形成的环境封闭性及物源条件可能存在差异。递变型水平层理主要形成于闭塞的潟湖环境,水体封闭性强,陆源碎屑供给严重不足,气候季节性变化形成正粒序层或反粒序层。条带状粉砂型水平层理、砂泥递变型水平层理和砂泥互层型水平层理均形成于相对开阔的海洋环境,水体以平流为主。陆源碎屑供给不足时期,多形成条带状粉砂型水平层理;陆源碎屑供给相对丰富时期,多形成砂泥递变型水平层理;陆源碎屑供给非常丰富时期,多形成砂泥互层型水平层理。随着陆源碎屑供给量的增加,砂/泥比值和砂质层单层厚度增加。

  • 黑色细粒沉积中,交错层理广泛发育。交错层理主要由粉砂纹层组和泥纹层组互层组成(图14),粉砂纹层组与泥纹层组相互交切,从而构成交错层理。与粗碎屑沉积相比,细粒沉积中纹层组与层界面的交角较小。

    Figure 14.  Cross⁃bedded black shale and characteristics

    黑色细粒交错层理的形成常与底流活动有关[70]。前人研究表明[1],细粒物质在流动水体中常呈絮状集合体形式搬运,絮凝作用随着水体盐度和粘性有机质结壳能力的增加而增加。在一定的水流速度和水体地球化学条件下,絮状集合体逐渐堆积,从而形成交错层理。在流速为15~30 cm/s的水体中,陆源碎屑泥[12,60]、不同类型黏土以及碳酸盐泥[10]等在蒸馏水、淡水以及海水中均可发生絮凝沉降。

  • 黑色页岩中,泥纹层和粉砂纹层的物质组成、孔隙类型及结构、面孔率、孔径分布、微裂缝类型及密度等均存在明显差异[64]。泥纹层和粉砂纹层差异性组合,构成不同类型层理,从而造成其孔隙组成、孔隙度、渗透率等明显差异。

    层理类型影响页岩TOC含量、孔隙组成和孔隙度。前期研究表明,泥纹层的粒度相对较小,细粒物质对有机质吸附能力相对较强,从而TOC含量相对较高。同时,页岩由于以有机孔为主,泥纹层中有机孔也自然相对发育。相对而言,粉砂纹层中TOC含量及有机孔含量均相对较低。因此,对于细粒沉积不同类型层理,泥纹层含量越高,其TOC含量、有机孔含量及孔隙度就越高。以川南五峰组—龙马溪组页岩为例,条带状粉砂型水平层理页岩由于泥纹层含量高,故其TOC含量和孔隙度最大(表2),分别为9.5%和8.1%,而生物扰动型块状层理页岩最小,分别为0.4%和1.3%。交错层理页岩和递变层理页岩的TOC含量和孔隙度也较低。涪陵地区五峰组—龙马溪组页岩测试结果也表明,砂泥互层型水平层理页岩中,随着粉砂纹层砂量增加,页岩有机质含量和有机质孔隙含量降低,无机孔含量增加[74]

    层理类型 TOC 孔隙度/% 水平渗透率/(×10-3µm2 垂直渗透率/(×10-3µm2 (水平/垂直)渗透率
    递变层理 2.7 2.3
    交错层理 2.5 3.9
    块状层理 均质型 6.0 3.2 0.000 342 0.000 419 0.82
    生物扰动型 0.4 1.3 0.001 420 0.000 066 1.82
    水平层理 递变型 4.6 1.5 0.000 931 0.000 575 1.62
    条带状粉砂型 9.5 8.1 0.184 285 0.000 655 281.35
    砂泥递变型 6.3 5.5 0.047 955 0.002 771 17.39
    砂泥互层型 4.7 4.2 0.010 954 0.002 877 3.81

    Table 2.  TOC content, porosity and permeability of various types of bedding

    层理类型造成页岩渗透性各向异性。黑色页岩中,由于黏土矿物片层状结构的存在,矿物颗粒和孔隙常顺层排列。前人研究同时发现[75-76],微裂缝也多平行于层理方向,从而造成黑色页岩水平渗透率常高于垂直渗透率。以川南长宁双河剖面五峰组—龙马溪组页岩为例,其条带状粉砂型水平层理页岩和砂泥递变型水平层理页岩水平渗透率远大于垂直渗透率,递变型水平层理页岩和均质型块状层理页岩基本相近(表2)。其中,条带状粉砂型水平层理页岩和砂泥递变型水平层理页岩水平与垂直渗透率比值分别为281.35和17.39,砂泥互层型和递变型水平层理页岩水平与垂直渗透率比值分别为3.81和1.62,而均质状纹理页岩水平与垂直渗透率接近。涪陵地区龙马溪组页岩水平层理广泛发育,其水平渗透率普遍高于0.01×10-3 µm2(平均值为1.33×10-3 µm2),远远高于相同深度的垂直渗透率(普遍低于0.001×10-3 µm2,平均值为0.003 2×10-3 µm2),二者相差超过3个数量级[77]。含气页岩垂向上渗透率低,有利于页岩气的保存,而水平渗透率较高则有利于水平渗流能力的提高[78]

  • 页岩的可压裂性与其脆性矿物密切相关,但纹层发育程度、纹层厚度、厚度差异性、纹层连续性、形态和几何关系等因素也对岩石力学性质以及裂缝扩展的内在因素产生重要影响。平直、连续和清晰的纹层界面,压裂过程中易造成应力集中,从而形成单一缝网。非平直、断续、不清晰的纹层界面,压裂过程应力不易集中,有利于形成复杂缝网体系[63]。同时,纹层界面的角度也影响页岩的岩石力学性质,在岩样单轴受压直到破坏过程中,随着页岩层理倾角的增大,其单轴抗压强度线性增大。

    熊周海等[79]实验研究表明,细粒沉积岩的可压裂性与纹层厚度及连续性呈负相关,但与纹层厚度方差、颗粒垂向分布方差呈正相关。纹层发育且连续性强的页岩塑性较强,压裂缝以沿纹层界面或塑性纹层(黏土纹层或有机质纹层)扩展为主,裂缝易再次闭合,从而导致岩石的可压裂性降低。纹层厚度差异性较大、颗粒垂向分布均匀度较高的页岩脆性较高,在压裂过程中易形成复杂有效的网状缝,从而提高岩石的可压裂性。此外,页岩矿物组分、颗粒结构以及成岩作用对可压裂性也具有重要影响。

    许丹等[80]和王永辉等[81]研究表明,层状页岩储层水力裂缝垂向扩展是否穿透层理面与主应力分布有关。当水平主应力差较小时,试样的主破裂面为平行于层理走向的面,一级裂缝穿过层理面,在层面处发生较大偏转,并沿着层面扩展,然后发生较大偏转并穿过层理面。当水平主应力差较大时,试样的主破裂面为垂直于层理走向的面,一级裂缝穿过纹层,发生较大偏转,然后再穿过纹层。衡帅等[82]研究表明,层理面开裂和断裂路径偏移是引起断裂韧性各向异性的主要原因。页岩层理的弱胶结作用使其断裂韧性较小,阻止裂纹失稳扩展的能力较弱,而在垂直层理方向,断裂韧性较大,阻止裂纹扩展能力较强,当水力裂缝垂直层理扩展时,在弱层理面处会发生分叉、转向,且在继续延伸的过程中会进一步沟通天然裂缝或弱层理面而形成复杂裂缝网络,达到体积压裂。孙可明等[83]研究表明,水力压裂过程中,垂直最小地应力稳定扩展的主裂缝遇层理面时,层理面与主裂缝初始扩展方向夹角越小,主裂缝越易沿着层理面方向扩展;层理面与主裂缝初始扩展方向夹角越大,主裂缝遇层理面时越易贯穿层理面沿原方向扩展;层理方位、地应力及基质抗拉强度不变,层理的抗拉强度远弱于基质抗拉强度时,主裂缝与层理面相遇后越易沿着层理面方向扩展,层理抗拉强度与基质抗拉强度越相近,主裂缝遇层理时越易贯穿层理沿原方向扩展;层理方位和强度不变,地应力及应力差越大,主裂缝遇层理后越易贯穿层理面沿原方向扩展。

  • 近年来由于页岩气工业的迅速发展,人们对细粒沉积有了更深入的认识。前人研究表明,细粒沉积物存在强非均质性,发育大量宏观和微观沉积构造。然而,细粒沉积由于形成环境多样、形成机制复杂,目前对其沉积构造类型及特征、纹层类型及特征、纹层形成条件和形成机制等方面的研究仍处于探索阶段。现阶段,可以针对不同层系、不同形成背景,选择典型露头剖面,通过大量薄片系统观察分析,开展精细沉积构造及纹层描述,建立“铁柱子”。在此基础上,通过广泛的现代地质考察,结合水槽、沉积试管、沉积水箱等实验及计算机数值模拟方法,明确各沉积构造及纹层的成因机理。

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