致密砂岩含油非均质性储层的野外实例三维数字模型和精细解剖——以鄂尔多斯盆地三叠系延长组安沟油砂露头为例

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

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。

[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.