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. -
0. 引言
类比模型，尤其是野外露头实例，对于认识地下储层的构型具有极其重要的作用（Bryant and Flint，1992；Enge et al.，2007；Howell et al.，2014）。露头能直观地揭示储集体的规模、几何形态、内部结构和潜在连通性等重要储层特征，因此在始于20世纪60年代的储层地质建模研究中一直备受重视（Zeito，1956；Weber，1978；Pringle et al.，2004；Fischer et al.，2007；Lapponi et al.，2011；Onyenanu et al.，2018）。随着认识的不断深入，研究发现储层含油非均质性，即储层中原油充注的非均匀性，是影响储层质量的重要因素，直接制约着油藏描述的精度和采收率（Weber，1986；吴朝东等，2003；陈欢庆等，2017；Worden et al.，2018）。与勘探尺度相对宏观的储层非均质性不同，研究砂体内部受控于不同构型界面的含油非均质性在方法上倚重于典型露头的解剖（Dutton et al.，2002；Pranter et al.，2006；Fitch et al.，2015）。然而，能直接对应邻区地下真实储层且有大规模油气充注的野外露头相对罕见。因此，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（Luo et al.，2015；马立元等，2020）。鄂尔多斯盆地作为中国陆上第二大沉积盆地，盆内发育的延长组不仅是中国北方三叠系中研究最早、出露最好的陆相地层（王多云等，2014），同时也是整个盆地内重要的含油层系之一。其油气勘探始于1905年，并于1907年在延长县城西打下“中国陆上第一口井”。目前，延长组作为国内典型的致密砂岩油气储层和鄂尔多斯盆地中生界油气勘探的主要目标层位（朱筱敏等，2018），油气勘探难度日益增大。与北美致密油主体分布于海相地层不同，陆相延长组表现出更强的储层非均质性（崔景伟等，2013；Zhou et al.，2016；马立元等，2020；张晓辉等，2023），储层含油非均质性表现特殊，油气分布规律更为复杂（罗晓容等，2016a；陈军军等，2023）。
延长组安沟油砂露头直接对应包括邻区延长油田、七里村采油厂在内的诸多中生界油田的主力层位，出露的巨厚层致密砂岩中含油非均质性特征明显且多级构型界面界限清晰，是目前国内沉积连续、保存完整的储层露头实例，同时也是认识储层含油非均质性的珍贵素材。文章通过对安沟油砂露头的三维数字露头建模和露头区详细的沉积−层序−成岩解剖，揭示露头含油非均质性特征及其与不同尺度构型界面的关系，建立了以安沟油砂露头为例的多级构型界面控制下的曲流河复合砂体储层含油非均质性的模型。
1. 露头概况和地层归属
石油勘探部门早期将延长组划分为5个岩性段（T3y1—T3y5），奠定了延长组内部的岩性划分方案和野外填图单元（崔景伟等，2013）。目前广泛使用的划分方案将延长组划分为10个油层组，即从底部的长10油层组至顶部的长1油层组。其中鄂尔多斯盆地内广泛分布的张家滩页岩对应长7油层组，是延长组在整个盆地内最重要的烃源岩层以及进行地层对比的标志层（王多云等，2014）。长期以来，延长组被认为属于上三叠统（付金华等，2005；江德昕等，2006；屈红军等，2011）。最近对长7油层组内孢粉和植物化石的重新梳理和凝灰岩夹层中锆石的U-Pb定年结果显示中—上三叠统的界限可能位于长7油层组和长6油层组之间（邓胜徽等，2018）。
安沟露头位于1：20万宜川幅T3y3填图单元区域内，对应长7油层组、长6油层组和长4+5油层组（邓秀芹等，2009；图1）。露头紧邻延长组的正层型剖面（延河剖面），具体出露于陕北地区延长县董家河村南约2 km处（图2a；GPS位置为36°31ʹ02.6ʺN，110°07ʹ04.9ʺE），沿201省道过河进沟即可抵达（图2b）。安沟露头区包括A区、B区和C区3个出露面。油砂出露面位于露头B区（图2b），高约12 m，长约55 m。遗憾的是该区未能出露巨厚层含油砂岩的底面（图2c）。含油砂岩底面出露于A区，表现为对下伏地层有明显下切的大型侵蚀面。但A区和B区出露的地层厚度有限，没有明确的标志层用于厘定其准确层位。野外追层和基于无人机多点位航拍的露头区点云数字建模（图2b）显示该下切面与露头C区巨厚层砂岩底面为同一地层界面。
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)区域上延长组地层向北西向平缓倾斜，倾角一般小于1°。命名于张家滩镇西侧（图2a）的长7油层组张家滩页岩是整个盆地内最重要的烃源岩和地层对比的标志层（王多云等，2014），其沿205省道和延河河床一直出露于露头C区河道中。在延河剖面中，长6油层组主体出露于谭家河村至王家川之间道路两侧，长7油层组和长6油层组的界限出露于谭家河村附近（图2a），表现为长6油层组底部的透镜状巨厚层砂岩对下伏长7油层组席状地层的侵蚀截切。野外沿河床长距离的追层，并结合两地侵蚀面上下相同的相变方式（从三角州相相变为曲流河相，下文详述）及高程对比（基于手持GPS和气压式海拔高度计实测），表明谭家河村长6油层组底部的侵蚀截切和安沟露头C区出露的大型侵蚀面的层位相同。因此，安沟露头B区出露的巨厚层含油砂岩准确层位应为长6油层组底部。
2. 研究方法和主要工作
相关研究已经揭示了储层含油非均质性的重要性，并且已深刻认识到沉积作用和差异化的成岩作用是影响有效储层形成的重要因素。但目前方法上仍依赖于数值模拟（Luo et al.，2015；罗晓容等，2016b）和对各种物性参数测试结果的地质统计（Rashid et al.，2012；Fitch et al.，2015；罗静兰等，2016）。特别是对砂体层内的储层非均质性，主要通过计算渗透率变异系数、突进系数和级差以及基于测井解释的渗透率来进行。安沟油砂露头的发现无疑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。因此，此次研究的重点集中于刻画油砂露头含油非均质性的具体特征，揭示巨厚层油砂的沉积背景和不同构型界面可能的级别，在此基础上定性认识砂体内部发育含油非均质性的原因。
2.1 三维数字露头建模
数字露头模型（digital outcrop models，DOMs）是由空间中具有X、Y和Z坐标点标定的三维模型，优势体现在其能克服普通照片可能产生的影像缺失和成像畸变，提高露头的可视化程度，同时完整和详实地永久保存露头（不同视角）的特征和细节。随着消费级无人机和点云技术的进步，基于无人机多点位航拍的三维数字露头建模已逐渐成为野外露头解剖的重要手段（Enge et al.，2007；Hodgetts，2013；Nesbit et al.，2020）。由于安沟露头B区油砂出露面积较大（约为660 m2；图3a），为了精确刻画安沟露头具体的含油特征和不同构型界面的几何形态，首先利用无人机多点位航飞和点云数据处理完成了油砂出露面B区的厘米级高分辨率三维数字露头建模。通过大疆Phantom 4 pro无人机对B区出露面的多点位航拍，确保3张照片中可见相同位置特征；然后依据无人机记录的相机姿态和GPS信息，在Pix 4D软件中通过尺度不变特征变换算法检测无人机拍摄的1215张有效图像中由不同尺度的局部像素方差确定的唯一特征点，并进行点云重建；通过对相机参数的三维结构进行优化，并通过多视角立体算法添加可靠的特征匹配点和网格插值执行点云加密，当加密点云被产值到一个连续的三维网格曲面中时，便实现了二维数据的三维建模，最终获得了露头B区的高精度三维点云数字模型（7783545个三维加密点云数，分辨率为厘米级）。在露头三维点云模型中导出的正射影像（图3a）基础上，根据现场观察和写实描述，刻画出露头B区的含油非均质性的具体表现形式和不同构型界面几何特征（图3b）。
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)2.2 露头区沉积和层序地层解剖
露头区详细的地层实测和沉积−层序解剖不仅有助于认识巨厚层油砂具体的沉积环境，更为露头区多种构型界面不同级别的划分奠定了框架。虽然野外追层显示露头B区巨厚层砂岩的底面为一个侧向上连续可追的大型侵蚀面，但该地层界面究竟代表的是一个沉积间断面还是区域不整合面？两者对应的层序地层意义完全不同（Catuneanu，2022），影响到该界面代表的构型界面的级别和尺度。大量的实例证明露头详实的沉积相分析是高分辨率层序地层工作的基础（Van Wagoner，1995；Posamentier and Allen，1999），同时考虑到B区油砂出露面中多种构型界面的具体内涵和露头含油非均质性的具体关系，因此有必要针对垂向上连续的露头C区开展详细的沉积学和高分辨率层序地层学工作。通过对C区露头的逐层实测、岩性描述和沉积构造观察，详细描述和解释了长7油层组中部至长6油层组底部地层共计27.7m（图4）。同时在发育相关沉积构造的层段测量古流向数据4组，共计104个（图4）。
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-flows2.3 露头连续取样和显微镜下观察
为理解造成露头含油非均质性的具体原因，利用手持切割机对油砂露头多个部位进行了累积12.2m的垂向连续取样（图3a），以配合室内显微镜下砂岩结构和成岩作用的分析。研究结果将直接揭示单层砂体内部结构的非均质性和差异化成岩作用，为探究沉积过程和成岩作用如何影响单层砂体储层非均质性提供有效的细节支撑。
3. 认识和讨论
3.1 露头含油非均质性特征
安沟露头B区油砂含油非均质性在颜色、气味上具有明显的特征。露头含油区颜色呈黄褐色，不含油区呈灰白色，颜色对比明显（图2c）。靠近露头处，含油区伴有明显的烃类芳香气味，表面呈现特征的黏腻感。现场滴水测试显示，不含油区水滴迅速渗透，而含油区水滴则稳定附着在砂岩表面，变化微小。
现场观察发现，露头中含油非均质性与砂体内部不同构型界面的关系十分密切，其中，复合砂体内部不同期次的水道垂向上相互叠置、侧向相互截切，水道内部砂坝具有加积面。在水道叠置面和砂坝加积面两类构型界面控制下的单层透镜状砂体的顶部和底部并不含油，而砂体内部普遍含油（图3b）。在B区三维数字露头模型的高分辨率正射影像基础上，通过野外露头观察和现场大范围滴水验证后发现，在复合砂体普遍含油的背景上，不含油区域与水道垂向上相互叠置面和侧向相互截切面以及水道内部砂坝的加积面高度重合（图3b），并代表了致密砂岩储集体内重要的岩性和物性界面。
3.2 露头区沉积环境和地层层序
根据露头C区地层的岩性、结构、沉积构造和层面的几何形态，将实测地层划分为3种沉积相的岩相组合，分别对应湖泊相、三角洲相和河流相（图4）。其中三角洲相可进一步分为前三角洲亚相、三角洲前缘亚相和三角洲平原亚相（图4），其解释部分依赖于不同岩相组合之间的互层关系，因此将这3种亚相的岩相组合合并描述。
3.2.1 湖泊相
该岩相组合位于实测剖面底部（图4），即狭义上的张家滩页岩。由黑—灰黑色泥岩、粉砂质泥岩构成，其层厚为薄层或极薄层，粉砂质泥岩具有水平纹层或砂纹状交错层，同时粉砂质泥岩还以不同规模的透镜体形式出现。在透镜状粉砂质纹层处借助放大镜可在局部观察到正粒序结构。
富含有机质、普遍发育水平纹层以及未见中—粗粒碎屑沉积，说明该岩相组合形成于正常浪基面之下的低能缺氧环境，成因为悬浮沉降。局部发育的正粒序结构指示该组合中粉砂质泥岩为深水—半深水环境下的低密度浊流沉积。结合已有研究认为延长组沉积时鄂尔多斯盆地为一大型的湖盆（陈全红等，2007），同时发育大量淡水至微咸水类型的绿藻类Botryococcus braunii（吉利明等，2012），因此将该岩相组合解释为（半）深湖相沉积。
3.2.2 三角洲相
三角洲沉积环境中发育的3类岩相组合整体上覆于深湖相沉积，构成了实测剖面的主体（图4）。其中前三角洲亚相组合仅发育在实测剖面下段一个部位（图4），以灰黑色薄层泥岩、粉砂质泥岩夹薄层粉砂岩和细砂岩为特征，其中粉砂岩和细砂岩中发育水平层理和流水波痕。剖面中三角洲沉积的主体以三角洲前缘亚相和三角洲平原亚相的互层或交替出现为特征（图4）。三角洲前缘亚相以发育中层—巨厚层灰白色石英岩屑砂岩岩相为主，以内部发育丘状交错层为特征（图4a）。此外，沉积构造还包括底部重荷模（图4b）、内部泄水构造（图4b）、包卷层理（软沉积变形；图4c）和低角度交错层等。三角洲平原环境发育的岩相组合以薄—中层劣质煤层（图4e）和灰色—灰绿色泥岩、粉砂质泥岩（图4d）夹少量中—薄层透镜状细砂岩为主，同时大量发育植物茎干和叶片（图4e），局部保存良好的芦木化石（图4d）。此外，三角洲前缘亚相砂岩内部发育大量垂直虫孔（图4f）。
丘状交错层和垂直虫孔指示中层—巨厚层砂岩形成于风暴浪基面之上的透光带。重荷模、泄水构造和包卷层理等软沉积变形指示沉积物快速的卸载和沉积。结合流水波痕和大量陆相植物化石的发育，这些特征均指示河流携带陆源沉积物注入相对平静水体后的快速沉积，符合三角洲前缘沉积的特征（Dalrymple et al.，1994）。局部层段发育的丘状交错层同时指示风暴对该河控三角洲沉积进行了一定程度的改造。由于互层和共生关系，将深灰色的细粒碎屑岩组合解释为前三角洲亚相沉积，而煤层、灰绿色的（粉砂质）泥岩和薄层透镜状细砂岩组合解释为三角洲平原亚相沉积。
3.2.3 河流相
该岩相位于实测剖面顶部（图4），即为B区出露的含油砂岩，其颜色以灰黄色为主并以多层砂岩叠置形成的巨厚层结合型长石砂岩为特征。结合型砂体顶、底界面明显，底面起伏变化较大，其厚度在下切最深处最厚，向两侧逐渐减薄直至尖灭（图4g）。内部的单层砂体同样呈透镜状，发育槽状交错层理（图4h）并可见多期次侧向或垂向加积（图4）。单层砂体内发育正粒序结构，底面局部发育由泥质碎片和砾石组成的滞留沉积。
大量发育单向且方位集中的槽状交错层及波痕构造，指示砂体成因为牵引流。未见生物扰动和浪成波痕、单层砂体底部显示出了不连续的粗粒滞留沉积，同时该砂体与巨厚层结合砂岩互层出现（图4a）显示出陆相曲流河沉积的特征。但单层砂体内部的多期次水道叠加和水道内大量砂坝的垂向加积说明沉积时水道的曲度较低。该岩相的底面表现为三角洲相沉积的侵蚀截切，且下切深度远大于单期水道沉积的厚度，这些特征说明其为下切河谷沉积（Van Wagoner et al.，1990），河流的侧向迁移受到下切范围的约束。
3.2.4 构型界面的级别
层序界面的厘定是层序地层学分析的基础和核心。野外识别的标志可总结为：①对下伏地层明显的侵蚀截切；②跳相（上覆地层向陆方向的突然相变）；③暴露地表的各类标志（古土壤、淋滤和风化等）。上述3类标志均能独立说明不整合界面的存在，因此出现其中任何一种即可标定为层序边界。含油砂岩的层位标定和露头区沉积相分析结果显示地层的侵蚀截切和上下跳相这2种标志均在巨厚层含油致密砂岩的底面发育，结合上下地层截然不同的古流向（图4），说明区域上长7和长6油层组的界限是一个侧向上连续可追的层序界面。赵靖舟等（2006））曾认为长6油层组区域上厚度差异的原因是由局部发育低幅度的鼻状隆起构造造成的，但该层序界面在不同地区表现出下切深度不同，说明长6油层组在区域上的厚度差异表明了该层序界面形成时空间上发生了不同程度的剥蚀下切。
Schlager（2004）在地层记录中识别出来的6种不同级别的海平面变化旋回，分别对应不同级别的地层序列。目前常见的层级分类将地层厚度在十至百米级别且持续时间在1～100 Ma的地层旋回定义为三级层序，而将限定厚度为几米甚至几十米且持续时间在0.1～1 Ma的沉积旋回的准层序界面定义为四级界面（Van Wagoner et al.，1990；Schlager，2004；Miall，2010；Catuneanu，2019）。在延河剖面长61油层露头的底部，相当于长6油层组上段内，陈飞等（2010）已识别出一个可靠的层序界面。考虑到长6油层组厚度在100 m左右，同时长6油层组到延长组顶部跨越整个晚三叠世（约36 Ma），因此将安沟露头含油砂岩的底面解释为一个三级层序界面。
野外发现，在安沟露头的巨厚层致密砂体内部，发育诸多更低级别的构型界面（图3b）。需要特别说明的是，在Miall（1996）早期对河流相砂体界面等级的分类方案中，将各类纹层定义为一级界面，然后依次类推，将单水道的顶底定义为五级界面。这种分类方案和目前广泛使用的地层层序级别的分类存在冲突，需要协调（Van Wagoner et al.，1990；Miall，2010；Schlager，2004；Catuneanu，2019）。在吴胜和等（2013）对碎屑岩构型分级方案的基础上，文章借鉴Owen et al.（2017）对河流相砂体中不同级别构型界面的划分方案，将结合型砂体中水道的截切面和叠置界面定义为五级构型界面，将单水道砂体内部砂坝之间的加积面和单砂坝内部的交错层或波痕分别定义为六级和七级构型界面，使不同成因的低级（五级及以上）界面的层级划分简洁明确且能与四级准层序界面衔接，便于整合统一。基于上述界面等级的划分方案，对露头B区河道复合砂体的结构非均质性进行了等级识别与划分描述，由大到小可识别出3种（五级至七级）地层界面（图3b）。其中最低级别的七级地层界面为野外肉眼能明确识别的中—小型沉积构造，即露头大量发育的槽状交错层理，代表了沉积过程中的底形界面。六级界面为水道内部砂坝的加积面，通过对槽状交错层理的测量和统计，得出古水流方向平均为186°（图3b），与砂坝加积面的倾向基本一致。因此该界面为单水道砂体内部砂坝之间的垂向加积面。而五级界面则为复合砂体中不同期次水道垂向上相互叠置和侧向相互截切所代表的地层界面。
3.3 沉积和成岩作用对储层含油非均质性的控制
为进一步揭示复合砂体内部不同级别的构型界面如何影响致密砂岩内的储层含油非均质性，通过对多个部位的单水道砂体连续取样（图5a），并结合显微镜下观察，发现露头中单层砂体内部发育典型的储层非均质性。由于露头中含油非均质性具体表现为单水道的顶部和底部通常不含油，而在五级和六级界面一定距离之外的区域均显示原油充注。因此将样品总体分为单水道底部、中部和顶部3组（图5b）。
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单水道砂体底部为典型的中粒长石砂岩，碎屑颗粒间紧密接触，胶结物总体含量相对较少，主要为少量的石英和长石自生加大和充填粒间孔隙的黄铁矿（图5c、5d）。同时，该岩相类型中岩屑含量相对较高，其中塑性岩屑普遍发生扭曲变形和假杂基化（图5d），使得大量粒间孔隙和喉道阻塞，为后期流体活动提供的有效空间十分有限。另外，多组槽状交错层理向底部的逐步收敛形成集中发育暗色细粒纹层的包络线（图5c）和大量假杂基的定向排列形成影响流体活动的隔夹层。集中发育细粒和塑性颗粒组成的隔夹纹层是造成该类岩相不含油的根本原因。
单水道砂体的主体为细粒长石砂岩，碎屑颗粒间接触较少，胶结物含量相对较高，主要为连晶式的方解石和浊沸石（图5e）。早期的连晶式胶结抑制了机械压实作用，使得该类岩相致密程度相对较低的。碳酸盐胶结物是导致这类砂岩储层低孔、低渗−致密储层的主要胶结物，但早期基底式碳酸盐胶结物的形成抵抗了压实作用的强度，同时为后期发生溶蚀作用形成次生溶孔奠定了物质基础。杜贵超等（2019）对长6油层组浊沸石胶结的系统研究也表明晚成岩期发生的浊沸石溶蚀是改善储集层孔喉结构和连通性的重要因素，为原油充注提供了有效储集空间。
单水道砂体的顶部发育极细粒长石杂砂岩，碎屑颗粒呈漂浮状分布于填隙物中，互不接触。胶结物含量较高，主要为基底式的方解石胶结（图5f）。与中部细粒长石砂岩中大量发育亮晶方解石胶结不同，该类岩相中泥微晶方解石胶结含量相对较高。泥微晶方解石胶结通常形成于同生及早成岩阶段，因钙离子供源充足而快速着核沉淀（Longman，1980），并快速占据原生粒间孔，导致储集体致密程度的增大。早期发育泥微晶方解石基底式胶结和较少发育浊沸石溶蚀胶结可能是单水道砂体的顶部未能有后期原油充注的重要原因。
不同尺度构型界面的形成原因、时间和空间展布特征存在具大差异（Mikes and Geel，2006；Owen et al.，2017）。时间上表现为从纹层级别的较短时间到层序界面级别的几百万年（Miall，1996；Catuneanu，2019，2022）；空间上表现为从厘米甚至毫米尺度的孔径级别到几百上千米尺度的盆地级别（Weber，1986；Enge et al.，2007；Morad et al.，2010；Catuneanu，2019）。但就砂体内部而言，影响储层非均质性的界面主要与五级、六级和七级构型界面（水道叠置、砂坝垂向加积和底形）关系密切。因此这些界面也被越来越广泛地认为是影响油气运移汇聚过程（Bekele et al.，1999；Luo，2011；Luo et al.，2015）和储层有效性（罗晓容等，2016b；Porter et al.，2018）的重要因素。基于上述分析，文章构建了安沟油砂露头储层含油非均质性模式，并详细划分了曲流河复合砂体内部的多级构型界面（图6）。在沉积作用的控制下，巨厚层砂体内部沿水道叠置和砂坝加积2类构型界面发育与其他部位截然不同的结构特征，造成了储层内部在原始物性方面存在显著差异。在原始物性的影响下，单层砂岩顶、底部发育有与内部截然不同的结构特征表明经历了不同的成岩作用，导致单层砂岩内部物性较好，而顶、底部物性较差。砂体内部物性相对较好的部位有利于后期原油的充注，从而成为含油砂岩。而沿水道叠置和砂坝加积2类构型界面发育物性较差的砂岩则成为相对致密的隔夹层，控制和影响着复合砂体内部后期原油充注的非均匀性。
图 6 安沟露头致密砂岩含油非均质性结构和沉积相模式图Figure 6. Architectural and facies model of tight sandstone reservoir with heterogeneity for the Angou outcrop4. 结论
（1）鄂尔多斯盆地三叠系延长组安沟含油砂岩露头是出露于地表的真实典型储层实例，直接揭示了致密砂体内部普遍发育的储层非均质性。巨厚层含油砂岩的底界−长7和长6油层组分界面可能代表中—上三叠统界限，为下切数至数十米且侧向上连续可追的三级层序界面。
（2）露头显示的长6油层组河流相致密砂岩储层中含油非均质性与低级构型界面（水道叠置、垂向加积和交错层等底形）的形成过程关系密切。单层砂岩中部含油，而顶、底部均不含油，并显示出不同的结构、物性和成岩特征。单层砂岩顶部主要发育泥微晶方解石基底式胶结、少量发育浊沸石溶蚀胶结，而底部发育细粒、塑性颗粒组成的隔夹纹层，这可能是造成单层砂体顶、底部位不含油的根本原因。安沟露头的发现和解剖为致密砂岩储层中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。
致谢：感谢中国科学院地质与地球物理研究所王清晨研究员、中国石油大学张立强教授、西北大学屈红军教授、延长油田七里村采油厂沈振振和高飞龙等在野外工作过程中给予的支持和帮助。
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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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