3D digital modelling and detailed anatomy of tight sandstone reservoir outcrop with oil-bearing heterogeneity: A case study of Angou outcrop of Triassic Yanchang Formation in Ordos Basin
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摘要: 由于缺少典型油砂露头，客观认识砂体内部受控于构型界面的储层含油非均质性受到制约，有效素材的缺乏一直阻碍着表征砂体内部含油非均质性及其与构型界面具体关系的认识。位于鄂尔多斯盆地东南部的安沟露头中致密砂岩含油非均质性表现明显，是储层含油非均质性和表征其与不同构型界面具体关系的有效素材。利用无人机多点位航拍对安沟油砂露头进行了三维数字露头建模，并对其三维数字模型进行了沉积−层序−成岩解剖，结果发现露头中原油充注仅分布于单层砂体内部，而砂体顶、底部并不含油。对实测剖面进行沉积相及层序地层分析，结果表明含油致密砂岩的沉积环境为曲流河水道，底面对应延长组7（长7）油层组和延长组6（长6）油层组界限的三级层序界面。砂体内含油非均质性与水道叠置、砂坝垂向加积和底形（交错层）构型界面关系密切。手持切割机对长7和长6油层组露头进行连续取样和岩石薄片镜下观察，研究发现单砂层顶、底和内部截然不同的结构、物性和成岩特征是造成安沟露头差异化含油的根本原因。安沟油砂露头的发现为客观认识砂体内部受控于构型界面的储层含油非均质性提供了难得的野外实例。该露头的详细解剖为致密砂岩储层含油非均质性明显受控于沉积作用和差异化的成岩作用提供了直接的地质证据。Abstract:
Objective Our understanding of architecture-controlled oil-bearing heterogeneity shown in tight sandstone reservoirs is hindered by scarcity of large-scale oil-bearing outcrops. Triassic lacustrine delta and fluvial succession exposed in a quarry near Angou village （Yanchang county, northern Shannxi province） is an analog for buried oil-bearing tight sandstones in the Ordos basin. Methods In this study, 3D digital outcrop modeling was carried out on the oil-bearing sandstone outcrop of Angou by using Unmanned Aerial Vehicle（UAV） multi-point aerial photography, and then the depositional sequence diagenetic anatomy and field anatomy were carried out on the 3D digital model of Angou oil-bearing sandstone outcrop. Based on field observations, drone-based measurement and digital outcrop modelling, continuous sampling using Husquvarna power cutter and petrographic and diagenesis analysis under section, a 2D architectural heterogeneity model incorporating spatial configuration of effective reservoir was created. Results The UAV 3D digital outcrop modeling and field dissection revealed that the oil charging was only distributed within the interior, but not at the top or bottom of sand body. The configuration and nature of bounding surface underlying this succession was reconstructed with reference to lateral tracing for distinctive markers and a detailed measured profile with facies and sequence stratigraphic analysis. The results show that the sedimentary environment of oil-bearing tight sandstone is curved river channel. In the quarry, fluvial sandstone succession is underlain by a regional surface interpreted as a third-order sequence boundaries on the basis of abrupt landward facies change and locally developed incised valleys <20 m deep. Architectural heterogeneity within the amalgamated sandbody is expressed by multiple fifth-order storey surfaces, sixth-order barform and seventh-order bedform. Continuous sampling and thin-section observation of outcrops show that the completely different structural properties and diagenetic characteristics of the top, bottom and interior of a single sand layer are the fundamental reasons for the different oil bearing in outcrops. The discovery of the Angou oil-bearing outcrop provides a rare field example for the objective understanding of the oil-bearing heterogeneity of the reservoir controlled by the configurational interface in the sand body. Conclusion In this study, the specific characteristics of oil-bearing heterogeneity in oil-bearing sandstone outcrop are described, the sedimentary background and possible levels of different configuration interfaces of extremely thick oil-bearing sandstone are revealed, and the causes of oil-bearing heterogeneity developing in sand bodies are qualitatively understood. [ Significance ] Of importance, the discovery and detailed anatomy of Angou outcrop provide direct geological evidence showing that sedimentation and diagenesis exert a strong control on the quality and heterogeneity of most tight clastic reservoirs. -
20世纪我国油气勘探主要集中在中、新生代陆相裂谷及前陆盆地[1~14]，21世纪油气勘探逐步向克拉通盆地海相碳酸盐岩转移。塔河大油田、苏里格大气田、普光大气田等大型油气田的相继发现，证实了海相碳酸盐岩是我国油气资源战略接替的重要领域[15~25]。但由于我国油气盆地绝大多数具有叠合盆地性质，勘探特殊，导致油气增长缓慢。从目前油气勘探程度和发展趋势来分析，一是寻找勘探新区、新领域、新层系、新类型，二是加快新区油气资源勘探前景调查与评价，运用新理论新思想突破对叠合盆地深层油气资源赋存状态的复杂地质构造的规律性认识，进行战略性油气调查和评价。
沉积盆地是油气生成和赋存的基本地质构造单元，沉积盆地中有机质向油气转化所需的温度、压力、反应时间等要素都受控于盆地形成演化的地球动力学过程。不同类型盆地动力学机制不同，其中油气生、运、聚的特点也必然不同。对于具有中国地质特色的叠合盆地而言，发生在构造变革时期的地球动力学突变过程尤为重要。因此，通过构造地质学、地球物理学、地球化学等学科的综合研究，查明叠合盆地现今的三维结构，从演化视角探讨盆－山系统形成旋回性，揭示构造活动对前期盆地的改造和对后期盆地形成的控制作用，是研究叠合盆地油气形成演化和分布规律必不可少的前提和基础[26~33]。
1. 叠合盆地及分类
目前，世界上新区油气勘查和评价，常采用四个层次:盆地分析－油气系统－成藏区带－远景评价(图 1)。世界大油气田分布与盆地形成的构造环境密切相关[34~35]，根据盆地类型与油气丰度之间的相互关系，32%大油气田与裂谷盆地有关，38%大油气田与前陆盆地有关，22%大油气田与克拉通盆地有关，走滑盆地约占7% [33]。同时，不同原型盆地类型控制了不同油气系统、成藏区带和远景圈闭。因此，原型盆地类型的划分是叠合盆地新区油气勘探和评价的第一步。
图 1 叠合盆地油气战略调查与评价基本思路Figure 1. Basic route of strategic hydrocarbon survey and evaluation in superimposed basins中国大地构造演化过程独特的多旋回性，决定了绝大多数盆地具有叠合盆地的性质。如华北板块及周缘地体，经历了多幕式裂解－漂移－拼合多次叠加改造过程，在纵向上明显地表现出伸展－收缩转化的多个巨型旋回。在板块运动体制上，各个板块之间的拼合，尤其是在华北板块运动程式上，其周缘地体裂解、拼合并不是表现出在时间上的等时性。在裂解和拼合之间的漂移期则发育稳定的克拉通盆地，而在俯冲和拼合阶段，由于地域和时间的差异则发育不同类型的原型盆地。因此，华北沉积盆地由不同演化阶段所形成的不同原型盆地在纵向上叠加、改造而形成(图 2)，纵向上多层次的油气系统的改造及叠加造就了油气区带的复杂性与多样性，所以“盆地的完整性、改造的微弱性、封盖条件的完好性”[28]应是叠合盆地油气新区初步评价、战略选区的目标所在。
图 2 渤海湾－南华北克拉通内盆地早古生代以来复杂盆地叠合改造关系① (据胡宗全等，2005)Figure 2. Superposition and reconstruction of the complex basins in Bohai bay and Southern North China craton since early Paleozoic (after Zongquan Hu et al., 2005)① 胡宗全，周新科，朱建辉，等.中石化环渤海湾地区前第三系油气资源前景.中国石化石油勘探开发研究院，2005.
叠合盆地新区油气战略勘查和评价过程中，首先要对含油气叠合盆地进行系统分类，那么单型盆地或原型盆地划分是叠合盆地分类和研究的主要环节。所谓单型盆地是指在统一的地球动力学背景下，在一次大的构造事件及沉积旋回作用下形成的盆地; 而叠合盆地是指经历了多期构造变革、由多个单型盆地经多方位更迭、复合而成具有复杂结构的盆地[28]。以区域性分布的不整合面和与之可以对比的整合面为界面的巨层序常与原型盆地开始和终止有关[36]，在其不整合面上下常出现原型盆地的更迭。因此，盆地巨层序的正确识别、划分和层序格架的建立，以及沉积样式和构造形迹(或构造样式)是原型盆地研究和分类的基础。
各类原型盆地的形成发展与地球动力学环境有关，按简单、实用和惯用的分类原则，可简化为拉张盆地(裂谷盆地、裂陷盆地或伸展盆地)、挤压盆地(压陷盆地、收缩盆地、挠曲盆地或前陆盆地)、走滑盆地和克拉通盆地等[28, 37]，分别对应于地质力学观点中张性、压性、剪性、垂直升降为主的地球动力学环境[26, 30~35, 37]。不同原型盆地类型其动力学机制不同，其油气形成的地质要素、油气生－运－聚等成藏作用特点也必然不同，它控制了不同油气系统、成藏区带和远景圈闭的形成和分布[31~32]。
油气系统是从源岩到圈闭的天然系统，是一个相对独立的含油气地质单元。它包括着油气生成－运移－聚集所必须的地质要素和地质作用以及它们在时空上的有效配置。最初称为石油系统(oil system)，其基础是从原油－源岩对比研究开始，而后演化为含油气系统(Petroleum system)和复合油气系统、总油气系统(Total petroleum system) [38~39]。油气系统为区域油气勘探提供了一个综合分析的框架和方法，把油气勘探规限在油气系统范围内进行，降低勘探风险，因此它成为油气勘探有效工具之一。油气系统研究内容包括两类: ①地质要素:烃源岩、储集层、封盖层及上叠层; ②成藏要素:油气生成－运移－聚集－圈闭形成关键时刻，持续时间及保存时间。
目前世界上流行的油气系统分类主要有两种方案: ①以地球动力学环境为依据进行分类，在不同地球动力学环境下可形成裂谷型油气系统、台地型油气系统和造山型油气系统等3种主要类型[38]，分别与中国东部、中部和西部大型盆地的油气系统相适应，不同单型盆地叠合时可以形成复合油气系统。②成因分类，即考虑油气生成－运移－圈闭过程[40]。但烃源岩、运移方式和圈闭封盖条件都与区域构造和地层框架有关，所以在油气系统分类中一般首先考虑地球动力学因素; 而在油气区带划分中考虑到成因分类中有关因素。
中国型沉积盆地，即经历了多期构造变革、由多个单型(单旋回)盆地经过多方位叠加、复合而形成的复杂结构的盆地[28]。叠合盆地多期构造演化(或不同构造旋回)造成了多套烃源岩叠置，并构成复合油气系统(或油气成藏体系)，发育不同型式的复合型油气成藏区带。如华北板块东部沉积盆地的成生发展从大陆克拉通开始，构造变革主要发生在中生代以来，因此，盆地纵向叠置主要表现为前中生代克拉通盆地、中生代挤压盆地和新生代伸展盆地的叠合序列，相应地形成台地型、造山型和裂谷型3种含油气系统，以及相互关联的复合油气系统(或油气成藏体系)。
不同动力学机制形成的原型盆地内层序充填序列(或沉积样式)不同。无论是Vail等提出层序形成的驱动机制是全球海平面变化[36]，还是Cross等认为控制层序的主要因素是基准面变化[41]，都分别与全球板块动力学和区域构造动力学有关，因此，层序形成和充填特征反过来可以进一步演译为地球动力学机制。层序地层展现了原型盆地充填总貌和层序界面空间和时间关系，可以预测生、储、盖组合和空间分布，进而有机地结合盆地构造变形样式，预测有利成藏区带。所以，从地球动力学出发，可以将油气系统和层序地层作为有机整体来统一考虑。
克拉通盆地沉积层序以碳酸盐岩－蒸发岩旋回为主旋律[32]，主要发育在海侵体系域和高水位体系域，形成旋回式油气系统; 横向上受碳酸盐岩台地和深水盆地之间的坡折带控制，形成相变式油气系统。因此，根据克拉通盆地的沉积旋回和沉积相带、古隆起和不整合面暴露面，以及断裂构造的研究，可以预测碳酸盐岩成藏区带[32]。国内外的油气勘探表明:克拉通盆地油气系统受构造沉积旋回与坡折带沉积环境所控制，构成完整的源岩－储集－渗滤－封盖系统; 克拉通盆地内古隆起圈闭与沉积同期形成，孔隙－溶洞－裂缝带发育，有利于早期成藏; 上叠前陆盆地褶皱－冲断带发育，促使克拉通盆地内油气的再调整富集，有利于晚期成藏。在中国大型油气盆地中已取得了辉煌战绩(如塔里木盆地古生界油气田群)。
前陆盆地通常叠置在大陆边缘或克拉通盆地之上，侧向与冲断带过渡，在空间上将造山褶皱－冲断带与前陆盆地构造样式作为一个整体。在时间上前陆盆地沉积层序可分三期:前冲断作用层序(烃源岩发育)、同冲断作用层序(储层发育)和后冲断层序(盖层发育)。发育以构造圈闭为主的褶皱－冲断带、以构造－地层圈闭为主的克拉通和造山带之间的枢纽带、与克拉通古隆起叠合的前缘隆起带等主要油气成藏区带[32~33, 42]。
裂谷盆地及大陆伸展盆地沉积层序分为三期:前裂谷层序(由裂谷前基岩或克拉通层序)、同裂谷层序(断陷期，属于艾里均衡快速沉降，主要由正断层控制的沉积)和后裂谷期层序(坳陷期，主要为挠曲作用控制的热沉降)。后裂谷期层序向同裂谷层序往往强烈上超，剖面上形成“牛头”模式。裂谷盆地油气系统以深陷湖相或海相沉积为生烃中心，断层系统形成垂向油气运移网络和多源混合，构成旋回式和侧变式生储盖组合; 广泛发育各种成藏区(层)带，如掀斜断块带、滚动背斜带和三角洲体系、坡折带和礁滩层序、调节带与浊积扇体系。
总而言之，成藏区(层)带的油气评价是以油气系统分析为基础，而油气系统又与层序地层格架有关，以此来预测源岩、储层和盖层组合; 层序地层的时空展布则受控于原型盆地的发育和演化; 盆地的形成取决于地球动力学环境中地块运动和地幔对流。因此，盆地分析－油气系统－成藏区带－远景评价构成认识和预测叠合盆地新区油气资源勘查和成藏潜力评价的主旋律。
2. 国内外油气勘探发展趋势
全球油气勘探经验告诉我们，油气勘探主要从前陆盆地或裂谷盆地开始，进而向稳定克拉通盆地发展。如中东扎格罗斯山前带油田的发现，使波斯湾盆地占世界石油探明储量的66%左右[33]。进入21世纪，中国油气储量的增长无疑主要来源于古生代海相碳酸盐岩。目前大家公认，塔里木盆地、鄂尔多斯盆地和四川盆地等海相碳酸盐岩大型油气田的相继发现，证实了克拉通碳酸盐岩盆地具有很好油气远景。因此，油气勘探领域向深层新层系、碳酸盐岩储层发展成为必然，重视并加强碳酸盐岩地区石油地质调查、构造改造与油气保存研究与可采油气资源评价，成为我国今后一段时期内油气资源勘探的重要方向。
我国油气资源丰富，但勘探难度越来越大，根本问题在于中国大地构造演化过程独特的多旋回性，决定了绝大多数盆地具有叠合盆地的性质，造成了油气区带的复杂性与多样性。叠合盆地赋存了我国陆上油气资源的80%，是当今油气勘探主要对象。但由于长期演化中盆地经历多期次构造变动、烃源岩种类多，油气经历多次运移、聚集成藏和再破坏、再成藏过程，油气分布十分复杂。这类盆地油气增长缓慢，油气地质理论滞后于勘探实践。因此，叠合盆地油气成藏条件、成藏规律和成藏机理成为急需解决的重大科学问题。系统解剖我国大型叠合油气盆地成盆－成烃－成藏动力学机制和过程，建立我国油气盆地原型演化序列及其改造类型，深入揭示成盆过程与油气资源富集之间的时空关系尤为重要。
总之，全球油气勘探工作将朝着两个方向发展:一是积极寻找油气勘探的新区、新领域、新层系和新类型，二是加快对主要油气盆地油气系统和油气资源勘探潜力的综合评价工作。这不仅需要地球物理、深层钻探等高新探测技术为支撑，更重要的是要从观念认识上更新，运用新理论突破对油气资源赋存状态复杂地质构造的规律性认识，从战略高度指导油气勘探工作。
我国油气勘探新领域: (1)老区新领域、新层系。松辽盆地深层、渤海湾盆地上古生界、鄂尔多斯盆地下古生界、四川盆地深层等。(2)新区新领域(盆缘区带)。(3)板块边缘造山带，如天山－阴山山前推覆带，秦岭－大别山山前推覆带，龙门山山前推覆带，雪峰山西南缘推覆带; (4)扬子和滇黔桂板内变形带、阿拉善地块等。我国非常规油气:煤层气、页岩气、油页岩、油砂等。我国潜在烃源岩:如河西走廊石炭系; 东北石炭系; 柴达木、青藏和南方碳酸盐岩等。
3. 我国油气勘探方向与研究领域
3.1 主要勘探方向
3.1.1 中国南方海相油气勘探方向
南方碳酸盐岩地区受控于特提斯构造域和太平洋构造域之间的相互作用。到目前为止，中国南方海相中、古生界已发现的油气田(如威远震旦系气田、川南二叠系气田、川东石炭系气田、鄂西渝东区建南气田、川东北三叠系气田等)基本都位于四川盆地上古－中生界组合内(除威远气田属下组合外)。因此，从油气勘探领域来说，下组合理应成为一个重要的探索领域。
3.1.2 中国东部油气勘探方向
新华夏构造体系，即李四光先生提出的第二、第三沉降带。盆地主要包括松辽盆地和渤海湾－南华北盆地以及鄂尔多斯盆地、四川盆地以及二连、海拉尔盆地等，加强新地区、新领域、新类型和新层位的勘探，以求大发现。
3.1.3 中国西部油气勘探方向
该区前陆盆地的发育主要与特提斯域造山带有关，油气主要赋存于克拉通盆地与上叠前陆盆地岩层中。因此，加强特提斯构造域新区油气系统战略调查、研究和评价，有利于中国西部地区油气勘探与开发。
3.2 主要研究领域
3.2.1 中－西部中新生代盆山结合带油气资源潜力调查与评价
世界油气资源94%分布于盆地边缘，6%富集于克拉通盆地[33]，其中绝大部分赋存于中－新生代碎屑岩中，这一客观规律决定了勘察对象应定位于“中﹑新生代盆地的盆山结合部碎屑岩地层”。因此，中－西部盆山结合带能源资源远景调查与成藏规律研究是最为现实领域之一。
3.2.2 中国克拉通盆地油气资源远景调查与评价
从稳定与活动的关系上，把握选区海相油气藏勘查层次: (1)大面积被陆相坳陷覆盖的断块区的勘探放在首位(中国中－西部地区); (2)中、新生代陆相裂谷系覆盖的海相层发育区(中国东部地区)。
3.2.3 南方碳酸盐岩复杂构造带油气地质调查与保存评价
特提斯构造域和太平洋构造域之间的相互作用不仅控制了古生代扬子地台大陆边缘的发育，同时也导致了该构造带中－新生代时期复杂的构造变动历史。由于后期构造运动强烈，古生界碳酸盐岩台地的原形特征遭受强烈改造，油气赋存规律变得异常复杂。尽管石油公司对南方碳酸盐岩地区的几个区带做了一定的勘探，但效果甚微，至今油气勘探没有取得实质性的突破。一方面归结于复杂的石油地质条件，更重要的是需要改变传统的勘探思路，加强区域石油地质条件的评价，其中应以油气保存条件评价为重点。
3.2.4 西藏高原盆地类型及油气资源潜力调查
在青藏高原经历漫长的板块构造发展过程中，形成了不同类型的海相盆地，特别是中生代海相盆地，无论基础地质研究还是能源勘探，在我国均有极其重要的位置。这些盆地中形成的良好生油层、储集层和盖层及其多套有利的生储盖组合，特别是其生烃强度之大，在我国诸多的含油气盆地中也属罕见。羌塘盆地成为我国新一轮油气勘探的首选目标，是陆上新区油气资源勘探取得突破最现实的选择之一。
3.2.5 东部深层油气资源成藏潜力研究与新区油气战略勘查方向
包括松辽盆地(已发现徐家围子白垩系千亿方大气田)和渤海湾－南华北盆地深层(已发现石炭－二叠系为源岩气田(藏))。根据活动论构造历史观和盆地运动体制理论，从原型盆地并列叠加关系中，研究盆地原型的地质作用，研究新旧体制改变中地质作用的继承和新生关系，了解深层油气成藏机制和分布规律。以盆地分析－油气系统为主线，开展深层油气资源和成藏潜力研究。
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图 1 鄂尔多斯盆地延河剖面位置和中生界油田分布简图
Figure 1. Simplified map of the location of Yanhe section and the distribution of Mesozoic Oil fields in Ordos Basin
图 2 安沟露头野外特征
a—三叠系延长组正层型（延河）剖面和安沟露头区位置图；b—安沟露头区三维点云模型及分区解剖方案（白色虚线为图3剖面实测路线）；c—B区露头致密砂岩中含油非均质性特征（箭头所指为结合型砂岩中灰白色不含油区域，其余棕色区域均普遍含油；白圈内为比例尺）
Figure 2. Field characteristics og Angou outcrop
(a) Location map of the stratotype (Yan river) section of Triassic Yanchang Formation and the Angou outcrop; (b) Low resolution point-cloud dataset showing the extent of three studied surfaces of the Angou outcrop (black dotted round rectangle) and location of measured section (white dotted line); (c) Close-up view of B-surface showing the heterogeneity of oil charging（The most brown areas are oil-bearing, whereas others are oil-free (white arrows)；proportional scale in the white circle)
图 3 安沟露头致密砂岩储层结构非均质性模型
a—B区露头三维点云模型面向105°方位的正射影像（红色条带为野外手持切割机连续取样位置）；b—结合型致密砂岩储层中不同级别构型界面和含油非均质性特征示意图（其中水道叠置和砂坝加积界面为写实，交错层等底形界面仅为示意，n代表古流向测量数值，玫瑰花图显示古流向）
Figure 3. Model for architectural heterogeneity in tight sandstone reservoir exposed in the Angou outcrop
(a) Digital orthophoto of 3D point-cloud model of B-surface facing 105-degree, area of continuous sampling using Husquvarna power cutter is denoted by red bands; (b) Diagram showing different-scale architectural boundaries and heterogeneity of oil charging in amalgamated tight sandstone reservoir (For simplicity, storey and barform surfaces are depicted explicitly, whereas bedform surfaces are shown schematically, n represents the measured number of paleo-flows, and the rose diagram shows the paleo-flows)
图 4 安沟露头C区实测地层剖面柱状图及典型岩相野外照片
n代表古流向测量数据a—实测剖面底部的深湖相长7张家滩页岩及上覆三角洲前缘亚相发育槽状交错层（白色箭头所指）和丘状交错层（黑色箭头所指）的灰白色巨厚层中粒石英岩屑砂岩；b—三角洲前缘亚相砂岩底部发育的重荷模（白色箭头所指）及内部发育的泄水构造（黑色箭头所指）；c—三角洲前缘亚相砂岩内部发育的包卷层理；d—三角州平原相粉砂质泥岩中直立的芦木化石；e—三角州平原亚相劣质煤层中大量保存的植物茎干（白色箭头所指）和叶片（黑色箭头所指）；f—三角洲前缘亚相砂岩内部发育的垂直虫孔；g—长6和长7地层界限上下不同方向的槽状交错层（箭头所指）；h—长6地层底部河流相砂岩底部发育的大型下切侵蚀面
Figure 4. Columnar diagram of measured stratigraphic section and field photographs of typical petrographic facies in Area C of Angou outcrop
(a) Profundal facies Zhangjiatan shale (lower-left corner) and trough (white arrows) and hummocky (black arrows) cross-stratifications and in overlying delta front facies sandstone; (b) Load casts (white arrows) and dewatering structures (black arrows) in delta front facies sandstone; (c) Convolute lamination in delta front facies sandstone; (d) Trunk of calamite fossil preserved in delta plain facies silty mudstone; (e) Stems and leaves of unidentified plant fossils in delta plain facies coal-bearing beds; (f) Rooting or unidentified burrowing (white arrows) in delta front facies sandstone; (g) Channelized incision surface developed on the top of delta front facies deposits of Chang 7 member, Overlying facies are coarser fluvial facies of Chang 6 member; (h) The stratigraphic boundary between Chang 6 and 7 members underlain and overlain by delta and fluvial facies sandstone with NEE- and SSW-oriented trough cross-stratifications (white arrows) n represents the number of measured paleo-flows
图 5 局部露头含油非均质性以及单层砂岩不同部位的物性与成岩特征
a—单层砂岩含油特征露头近照及连续取样记录，采样位置（红色箭头）；b—单层砂岩含油非均质性示意图及薄片样品位置；c—单层砂岩底部样品，发育有机质纹层的中粒长石砂岩，正交偏光；d—单层砂岩底部样品，发育假杂基（黑色箭头）、长石自生加大（灰色箭头）和黄铁矿胶结（白色箭头）的中粒长石砂岩，单偏光；e—单层砂岩中部样品，发育方解石和浊沸石连晶式胶结的细粒长石砂岩，正交偏光；f—单层砂岩顶部发育亮晶（白色箭头）和泥晶（黑色箭头）方解石基底式胶结的极细粒长石砂岩，正交偏光
Figure 5. Details of oil-bearing heterogeneity in local outcrops and physical properties and diagenetic characteristics in different parts of individual sandstone layer
(a) Close-up view of oil-bearing characteristic of each sandstone bed and record of continuous sampling for detailed observation, sampling location (red arrows); (b) Schematic diagram showing heterogeneity of oil charging in single bed of tight sandstone and positions of representative samples for thin-section; (c)The bottom sample of single-layer sandstone, medium grained arkose sandstone with organic lamination, cross-polarized light, sample B-1; (d) The bottom sample of single-layer sandstone is a mediumgrained arkose with pseudoheterobasic (black arrow), autogenetic extension of feldspar (gray arrow) and pyrite cement (white arrow), plane polarized light, sample B-2; (e)The central sample of single-layer sandstone developed fine grained arkose with calcite and turbidite intergranular cementation, cross-polarized light, sample B-3; (f)The top of the singlelayer sandstone is a very finegrained arkose with sparry (white arrow) and micrite (black arrow) calcite basement cement, cross-polarized light, sample B-4
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