玛湖凹陷二叠系风城组深层陆相页岩储层天然裂缝及其有效性

Natural Fractures and Their Effectiveness in Deep Continental Shale Reservoirs of Permian Fengcheng Formation in Mahu Sag

摘要

准噶尔盆地玛湖凹陷二叠系风城组陆相页岩最大埋深超过5 000 m，油气资源丰富，其中广泛发育的天然裂缝对烃类聚集和储层勘探开发具有关键作用.通过岩心、成像测井、薄片和扫描电镜观察，建立了基于地质成因和裂缝产状的深层陆相页岩天然裂缝分类方案，阐明了不同类型天然裂缝的发育特征和有效性，并讨论了深层陆相页岩天然裂缝的非均质性及其对储层的贡献.深层陆相页岩天然裂缝依据地质成因分为构造裂缝、成岩裂缝和异常高压相关裂缝.按照裂缝产状构造裂缝细分为穿层和顺层剪切裂缝、及层内张裂缝，成岩裂缝划分为层理缝、缝合线和收缩裂缝.构造裂缝规模相对较大，组系特征明显，主要以高角度和近直立为主.成岩裂缝主要为近水平发育，缝面弯曲、易分叉.层内张裂缝、层理缝和缝合线是深层陆相页岩储层的优势裂缝类型.天然裂缝可被方解石和含有机质的细粒混合物等矿物不同程度充填，其中构造裂缝和层理缝的充填程度较低，缝合线更易被充填.微观构造裂缝的开度较小，而成岩裂缝的开度通常较大.推测认为构造裂缝主要为储层中流体渗流提供了有效通道，层理缝发育程度更高，不仅是储层流体的渗流通道，也是其有效储集空间的重要组成部分.研究成果对于完善深层陆相页岩天然裂缝分类方案及深入认识这类储层天然裂缝分布规律具有重要的借鉴意义.

Abstract

The continental shale in the Permian Fengcheng Formation of the Mahu Sag in the Junggar Basin has a maximum burial depth exceeding 5 000 m, rich in oil and gas resources. The widely developed natural fracture plays a crucial role in the accumulation and exploration of hydrocarbons in these reservoirs. A classification scheme is established for natural fractures in deep continental shale based on their geological genesis and occurrence by observing cores, image logs, thin sections, and SEM samples. Moreover, the development characteristics and effectiveness of different types of natural fractures Were analyzed, and the natural fracture heterogeneity and their contributions to reservoirs are discussed. Accordingly, natural fractures in deep continental shale are divided into tectonic, diagenetic, and abnormal high-pressure related fractures based on their geological genesis. Tectonic fractures can be subdivided into translayer shear, bed-parallel shear, and intralayer open fractures based on their occurrence, while diagenetic fractures are divided into bedding, stylolite, and shrinkage fractures. Tectonic fractures have relatively large scales, obvious groups, and high dipping and nearly vertical angles. Diagenetic fractures mainly develop horizontally, with curved surfaces and easily branched extensions. Intralayer open, bedding, and stylolite fractures are the dominant types of fractures in deep continental shale reservoirs. Natural fractures can be filled in varying degrees by minerals such as calcite and fine-grained mixtures containing organic matter, among which tectonic and bedding fractures are less filled, and stylolites are easier to be filled. Microscopic tectonic fractures have smaller apertures, while diagenetic fractures usually have larger apertures. This study speculates that tectonic fractures mainly provide effective pathways for fluid flow in reservoirs, while bedding fractures are more developed, which are not only seepage channels for reservoir fluid but also an important part of the effective storage space. The research results provide an important reference for improving the classification scheme of natural fractures in deep continental shale and for better understanding of the natural fracture distribution in such reservoirs.

关键词

天然裂缝 / 裂缝类型 / 裂缝有效性 / 深层陆相页岩 / 玛湖凹陷风城组 / 石油地质.

Key words

natural fracture / fracture type / fracture effectiveness / deep continental shale / Fengcheng Formation in Mahu Sag / petroleum geology

0 引言

随着油气资源增长的需求和勘探开发理论方法的深入，陆相页岩储层日益成为获取稳定石油和天然气产量的重要领域（金之钧等，2019；邹才能等，2019；赵文智等，2020）.陆相页岩储层通常岩性组合多样、储集空间分布非均质性强，并且由于以细粒沉积物为主，储层物性相对较差（吴松涛等，2015；靳军等，2018；黎茂稳等，2020）.研究发现，在复杂的构造运动和成岩作用下，这类储层中普遍发育多种类型的天然裂缝（张云钊等，2018；鞠玮等，2020；金之钧等，2021；李彦录等，2022）.大量资料表明，天然裂缝发育时岩石的渗透率比基质渗透率高2~3个数量级，同时广泛发育的微观裂缝为页岩油储层流体提供了有效的储集空间（Zhang et al.，2020；曾联波等，2023）.此外，页岩储层水力压裂改造过程中，天然裂缝的发育影响了人工压裂缝的形成和扩展（Zou et al.，2016；周彤等，2020）.因此，陆相页岩中不同类型天然裂缝的分布规律对于优质储层的评价及油气开发方案的制定具有重要的参考意义.

关于油气储层中天然裂缝的研究，已有的研究成果主要集中在砂岩和碳酸盐岩等常规岩性，形成了一系列相关理论认识和预测方法（Laubach et al.，2004；Gale et al.，2010；Zeng et al.，2012；巩磊等，2015；Aghli et al.，2020）.然而，针对页岩储层中天然裂缝的研究仍然相对较少，尤其是深层陆相页岩天然裂缝的研究更加缺乏.相较于常规岩性，陆相页岩的沉积环境稳定、纹层交替频繁、岩性结构多样，并且有机质生烃作用使得其成岩演化也更加复杂（丁文龙等，2011；Cao et al.，2016；Yang et al.，2019；田鹤等，2020；Liu et al.，2023）.因此，陆相页岩层系中天然裂缝的发育具有明显特殊性，而相关科学问题的研究依然处于探索阶段，比如深层陆相页岩天然裂缝的分类方案还有待完善，优势裂缝类型的发育特征和有效性不够明确等.这些问题制约了储层中天然裂缝分布规律的深入认识，影响了陆相页岩油的勘探开发.

准噶尔盆地玛湖凹陷风城组发育一套厚层的细粒碱湖相地层，属于典型的深层陆相页岩储层，勘探开发潜力巨大（支东明等，2021；金之钧等，2022）.本次研究以玛湖凹陷玛北地区风城组为研究对象，通过岩心、成像测井、薄片和扫描电镜观察，建立了基于地质成因及其产状的深层陆相页岩天然裂缝分类方案，并在此基础上分析了不同类型裂缝的发育特征和有效性，讨论了深层陆相页岩天然裂缝的非均质性及其对储层物性的影响.研究成果对于完善深层陆相页岩层系中天然裂缝的分类方案以及深入认识陆相页岩层系中天然裂缝的分布规律具有重要的借鉴意义.

1 地质背景

玛湖凹陷位于准噶尔盆地中央坳陷西北缘，西部和西北部受克百断裂带和乌夏断裂带控制，西南部和东南部紧邻中拐凸起与达巴松凸起，东部到东北部依次与夏盐凸起、英西凹陷和石英滩凸起相邻（黄玉越等，2022）（图1）.凹陷呈椭圆状展布，长轴方向为NE-SW向，构造单元面积约为5 200 km²（支东明等，2019）.凹陷形成于前石炭纪褶皱基底之上，先后经历了中晚海西、印支、燕山以及喜马拉雅等多期构造运动（匡立春等，2021）.凹陷内断层广泛发育，主要为NE-SW向和近E-W向展布.

玛湖凹陷发育从石炭系到新生代的全套地层，其中早二叠统风城组沉积分布向凹陷内倾斜，呈现西北厚、东南薄的楔状特征（匡立春等，2012）（图2）.风城组属于前陆盆地构造背景上发育的一套以碱湖相细粒沉积为主的地层，是准噶尔盆地西北缘多套油气储层形成的重要物质基础.风城组自下向上可以分为风1段、风2段和风3段，沉积厚度为200~1 800 m，最大埋深超过5 000 m（王剑等，2022）.地层中陆源碎屑组分、碳酸盐组分和火山碎屑组分并存，岩性以富有机质页岩、云质泥岩、云质粉砂岩、粉砂质泥岩、粉砂岩和火山碎屑岩为主，纵向上组合复杂、变化较大，属于典型的陆相混积页岩层系（何文军等，2021）.储层储集空间主要为基质溶孔和晶间孔，此外还有少量微裂缝和残余粒间孔，孔隙度大多小于3.0%，渗透率通常小于0.03 mD，物性较差（雷海艳等，2022）.目前，风城组多口重点井已获得工业油流，具有良好的勘探开发前景.

2 天然裂缝类型

通过岩心、薄片、扫描电镜以及成像测井资料的观测分析，本次研究基于天然裂缝的地质成因和产状建立了准噶尔盆地玛湖凹陷风城组深层陆相页岩的天然裂缝分类方案（表1）.首先，根据天然裂缝的地质成因将其分为构造裂缝、成岩裂缝和异常高压相关裂缝三种类型.构造裂缝是在构造应力作用下形成的，按照其产状可细分为穿层剪切裂缝、顺层剪切裂缝和层内张裂缝.陆相页岩中的成岩裂缝是指储层演化过程中在成岩作用下形成的裂缝，依据裂缝的产状可进一步划分为层理缝、缝合线和收缩裂缝.异常高压相关裂缝是在储层异常压力作用下，由于最小主应力为张应力而形成的一种产状不稳定的拉张裂缝.

风城组陆相页岩构造裂缝普遍发育，其中穿层剪切裂缝在纵向上贯穿一个或多个岩层界面，裂缝面通常较为平直，倾角较高（图3a、3e、4a）.顺层剪切裂缝倾角相对较低，通常与岩层界面近平行.这类裂缝缝面常有镜面特征，划痕和阶步发育（图3c）.层内张裂缝发育在岩层内部，裂缝在纵向上的延伸受岩层界面的限制明显，与岩层界面呈高角度相交（图3b、3d、4b）.风城组陆相页岩构造裂缝中层内张裂缝最为发育，穿层剪切裂缝和顺层剪切裂缝发育相对较少.

成岩裂缝是指岩石在成岩演化过程中由于压实、压溶和收缩等成岩作用形成的一类天然裂缝（Zeng et al.，2016；Liu et al.，2021）.玛湖凹陷风城组深层陆相页岩中发育的成岩裂缝主要包括层理缝、缝合线和收缩裂缝.其中，层理缝的发育程度较高，主要沿页岩纹层发育，裂缝面较弯曲、易分叉（图5a、5c）.层理缝的横向连续性较差，裂缝长度变化大，延伸过程中通常不会切穿矿物颗粒而是沿着颗粒边缘分布.缝合线通常呈不规则波浪状或锯齿状，多数与岩层界面近平行或小角度相交（图5b、5d）.此外，陆相页岩中还发育少量与岩层界面斜交或近垂直的缝合线，这类缝合线是构造和成岩综合作用的结果.收缩裂缝是粘土矿物、硬石膏和有机质等塑性矿物在成岩过程中脱水或生烃所形成，横向延伸较短，无方向性（图5e）.在风城组深层陆相页岩的成岩裂缝中，层理缝和缝合线发育程度较高，而收缩裂缝相对发育较少.

异常高压相关裂缝是指一些被认为是在地层超压作用下形成的天然裂缝（曾联波等，2023）.构造挤压应力、有机质演化生烃以及厚层泥页岩的封闭作用等是形成陆相页岩地层超压的主要原因.异常高压相关裂缝通常呈两端窄中间宽的纺锤形或透镜状，其特殊几何形状与超压流体形成过程中的应力条件有关，表现为拉张裂缝的特征.这类裂缝延伸短，规模相对较小，开度较大（图6）.异常高压相关裂缝缝面形态不规则，不成组系，没有稳定的产状.因此，陆相页岩中这类裂缝分布不稳定，规律性较差.风城组陆相页岩中，异常高压相关裂缝发育相对较少，易被矿物充填.

3 天然裂缝发育特征

二叠系沉积以来，玛湖凹陷经历了晚海西期、印支期、燕山期和喜马拉雅期等多期次的构造运动（匡立春等，2021）.风城组陆相页岩在多期构造运动的叠加作用下，发育了多个组系、较为复杂的构造裂缝.成像测井解释表明，玛湖凹陷风城组陆相页岩构造裂缝的优势走向为NW-SE向、近N-S向、以及NNE-SSW向三组裂缝（图7）.其中，NW-SE向和近N-S向构造裂缝在凹陷内普遍发育，属于区域裂缝.而NNE-SSW向构造裂缝与玛湖凹陷的主要断层走向一致，在断层附近更为发育，远离断层发育程度较低，属于断层伴生裂缝.

风城组构造裂缝倾角较大，高角度（>60°）和近直立的构造裂缝占比超过70%，主要是层内张裂缝和少量穿层剪切裂缝，而10°~60°倾角的构造裂缝发育相对较少（图8）.然而，近水平（<10°）的构造裂缝发育程度相对较高，主要为顺层剪切裂缝.层理缝和缝合线通常近水平发育，裂缝倾角较小.风城组岩心裂缝的统计数据显示构造裂缝高度集中分布在25 cm以下，顺层剪切裂缝的规模多小于5 cm，而规模大于40 cm的构造裂缝主要是穿层剪切裂缝（图9）.刘敬寿等（2023）分析认为，陆相页岩纹层发育，岩石力学层厚度较小，因此限制了发育在岩石力学层内部的层内张裂缝的纵向延伸.

岩心中构造裂缝的单井平均线密度主要分布在2 m^-1附近，而在断层附近单井平均线密度可以达到8 m^-1以上.成岩裂缝的发育程度较高，单井平均线密度可以达到40 m^-1以上.岩性对天然裂缝的发育程度有重要的影响.在玄武岩和凝灰岩等厚层火山岩中，高角度构造裂缝最为发育，成岩裂缝发育较少.然而，在互层的碎屑岩和碳酸盐岩中构造裂缝主要是层内张裂缝，泥质含量高时发育顺层剪切裂缝，纹层越发育成岩裂缝发育程度越高.岩心观测统计表明，风城组天然裂缝在纵向上的发育程度也表现出明显的差异性（图10）.其中，风一段和风二段构造裂缝较少发育，风三段构造裂缝发育程度高，平均线密度超过9 m^-1.成岩裂缝在风一段发育较少，平均线密度低于25 m^-1，而在风二段和风三段集中发育，最大平均线密度超过50 m^-1.

4 天然裂缝有效性

天然裂缝的有效性是评价其对储层贡献的关键要素，充填性和开度是表征裂缝有效性的主要参数（曾联波等，2012）.在储层成岩演化过程中，天然裂缝由于各种地质作用可被矿物充填，成为储层流体渗流的屏障，使其有效性明显变差.根据天然裂缝充填程度的差异，由高到低划分为全充填、局部充填和未充填，反映了裂缝的有效性依次变好.岩心观察和薄片分析表明，玛湖凹陷风城组陆相页岩中不同尺度的天然裂缝都可能被充填，充填矿物类型多样，主要为方解石和含有基质的细粒混合物，还有一些石英、碱性矿物和黄铁矿等（图11）.此外，不同类型的天然裂缝充填矿物有显著差别.构造裂缝的充填矿物主要是方解石，其次是碱性矿物和石英.成岩裂缝的充填矿物通常是含有机质的细粒混合物，少量为方解石和黄铁矿等.

玛湖凹陷风城组陆相页岩储层中，不同类型天然裂缝的充填程度差异很大（图12）.8口取心井的岩心观察表明，43%的构造裂缝没有被矿物充填，36%的构造裂缝被矿物完全充填.层理裂缝中未被矿物充填的比例达到54%以上，而缝合线全部被矿物全充填.因此，从裂缝充填性出发，层理缝的有效性最好，其次是构造裂缝，而缝合线的有效性相对最差.此外，由于后期的构造活动和溶解作用，被矿物充填的裂缝可能重新裂开或者充填物被溶蚀而变得有效.储层中开度较大的天然裂缝对孔隙度和渗透率具有更大的贡献，因而有更好的有效性.薄片观察发现，风城组陆相页岩中天然裂缝开度存在很大的差异.微观构造裂缝的开度通常较小，集中分布在30 μm以下，大于50 μm的占比不超过16%.层理缝的开度相对较大，主要分布在10~50 μm.然而，缝合线的开度较大，通常大于 20 μm，分布范围广，大于50 μm的占比超过30%.

5 讨论

深层陆相页岩由于沉积环境、岩性结构、以及成岩演化等的特殊性，与其他岩性相比其天然裂缝的发育类型更加多样复杂.首先，陆相页岩通常形成于稳定的湖相沉积环境中，纹层结构发育且交替频繁，为层理缝的发育提供了良好的条件（Zolitschka et al.， 2015；金之钧等，2022）.在有机质热演化过程中，干酪根的热降解生烃以及烃类的热演化会产生大量的油气并形成异常高压，从而导致页岩沿层理或纹层界面发生破裂形成层理缝（Gale et al.，2014；Zhang et al.，2020）.并且，相较于海相页岩，陆相页岩发育更多的碎屑岩、碳酸盐岩及火山岩夹层，这些不同类型的夹层比泥岩有更高的脆性，相同构造应力作用下更易形成构造裂缝，尤其是层内张裂缝更加发育（Liu et al.，2023）.此外，对于埋深大于3 000 m甚至超过5 000 m的深层陆相页岩，成岩演化过程中经历了更强的压实和压溶等作用，在塑性矿物含量较高的岩石中以及脆塑性纹层的界面处，由于矿物抗压能力的差异和晶型的改变岩石发生变形，进而广泛发育缝合线（Baud et al.，2016；Toussaint et al.，2018）.因此，针对深层陆相页岩储层，天然裂缝的分类方案需要进一步补充和完善，明确层理缝及层内张裂缝的重要性，增加缝合线作为主要的天然裂缝亚类.

玛湖凹陷风城组深层陆相页岩由于其特殊的地质背景，天然裂缝的发育规律表现出较强的差异性.研究区位于盆地边缘并且目的层经历了多期强烈的构造运动，储层中大规模穿层剪切裂缝相较其他区域陆相页岩更为发育，岩心中高度大于40 cm的构造裂缝超过了17%（匡立春等，2012）.受凹陷中普遍发育的多尺度断层影响，风城组陆相页岩构造裂缝呈多组系发育且分布非均质性强，尤其在断层附近构造裂缝组系更为复杂，局部区域构造裂缝优势方位达到四组.同时，风城组岩心统计表明倾角小于10°的构造裂缝占比超过13%，这是因为页岩相较于其他类型岩石泥质含量更高，相同构造应力作用下更易发生滑脱而形成低角度的顺层剪切裂缝（Zeng et al.， 2016）.此外，不同类型天然裂缝发育强度的研究结果表明这类储层中天然裂缝主要为构造裂缝和成岩裂缝，其中层内张裂缝的单井平均线密度达到4 m^-1，穿层和顺层剪切裂缝的单井平均线密度均小于1 m^-1.而层理缝和缝合线的单井平均线密度分别超过了32 m^-1和10 m^-1，收缩裂缝发育程度低.基于上述研究认为，层内张裂缝、层理缝以及缝合线是深层陆相页岩储层的优势裂缝类型.

综合分析认为，由于玛湖凹陷风城组深层陆相页岩较大的埋深以及更强的压实和胶结作用，储层物性较差，未被矿物全充填的有效裂缝对烃类的运移和富集具有重要的影响（Gale et al.， 2014；曾联波等，2023）.构造裂缝主要为储层中流体的渗流提供了有效的通道，而成岩裂缝发育程度更高、开度更大，不仅是储层流体重要的渗流通道，也是其储集空间的重要组成部分.风城组陆相页岩孔隙类型主要为矿物粒间和粒内溶蚀孔，原生粒间孔和有机质孔发育较差（Zhang et al.， 2024）.然而，广泛发育的天然裂缝连通了储层中离散分布的孔隙，提高了陆相页岩储层储集空间的有效性.同时，荧光普扫和激光共聚焦观测发现天然裂缝发育的陆相页岩烃类含量明显较高，并且裂缝内部也表现出良好的含油性（Wang et al.， 2023）.因此，深层陆相页岩中有效天然裂缝的发育对储层的甜点优选和勘探开发具有重要的影响.

6 结论

（1）准噶尔盆地玛湖凹陷风城组深层陆相页岩天然裂缝依据地质成因可以分为构造裂缝、成岩裂缝和异常高压相关裂缝三种类型.进一步按照天然裂缝产状，构造裂缝可细分为穿层剪切裂缝、顺层剪切裂缝和层内张裂缝，成岩裂缝可划分为层理缝、缝合线和收缩裂缝.

（2）深层陆相页岩不同类型天然裂缝发育特征差异较大.构造裂缝规模相对较大，表现出明显的组系特征，主要以高角度和近直立为主，中低角度发育较少.成岩裂缝主要为近水平发育，缝面弯曲，容易分叉和尖灭.天然裂缝发育强度的研究表明，层内张裂缝、层理缝和缝合线是深层陆相页岩储层的优势裂缝类型.

（3）风城组陆相页岩天然裂缝可被矿物不同程度的充填，充填矿物以方解石和含有机质的细粒混合物为主，此外还有石英、碱性矿物和黄铁矿等.构造裂缝和层理缝的充填程度相对较低，而缝合线更易被充填.微观构造裂缝的开度较小，集中分布在30 μm以下，而成岩裂缝的开度通常较大，主要分布在10~50 μm.有效裂缝的发育对储层中流体的渗流和富集具有积极的影响，是深层陆相页岩储层评价和甜点优选的重要参考因素.

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基金资助

国家自然科学基金项目(42090025;42302148)

中国石油科技创新基金项目(2023DQ02-0103)

页岩油气富集机理与有效开发国家重点实验室开放基金项目(33550000-22-ZC0613-0336)