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

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.

Keywords

• tight sandstone reservoir
• oil-bearing heterogeneity
• chang 6 member of yanchang formation
• angou outcrop
• ordos basin