VIRTUAL ANALOGUE FOR ARCHITECTURAL HETEROGENEITY OF TIGHT SANDSTONE RESERVOIR: THE ANGOU OUTCROP OF TRIASSIC YANCHANG FORMATION, ORDOS BASIN

Architectural heterogeneity is a very common geological feature shown by nearly all (tight) sandstone reservoirs. Given providing important parameters connectivity, analogues, especially outcrop analogues, have been pivotal in the development of subsurface reservoir model. Triassic lacustrine delta and fluvial succession exposed in a quarry near Angou village is analogue for buried oil-bearing tight sandstones in the Ordos basin. The configuration and nature of bounding surface underlying this succession was reconstructed with reference to lateral tracing distinctive markers and a detailed measured profile with facies and sequence stratigraphic analysis. Based on field observations, drone-based measurement and digital outcrop modelling, continuous sampling using petrographic and diagenesis analysis under section, a 2D architectural heterogeneity model incorporating spatial configuration of effective reservoir was created. 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, while the overlying bounding surface is a fourth-order parasequence boundary. Amalgamated sandstone approximately 8 m thick are constituted by a number of channel bodies involving down-stream accreted sand bars. Architectural heterogeneity within the amalgamated sandbody is expressed by multiple fifth-order storey surfaces, sixth-order barform and seventh-order bedform. In contrast to oil-bearing fine-grained tight sandstone, the very fine- and middle-grained sandstone, spatially overlapped with fifth- and sixth-order stratigraphic surfaces and reflecting very different diagenesis histories, act as flow baffles. This outcrop-derived, high resolution model may contribute to a better understanding of the subsurface architecture and reservoir properties of tight sandstone in the Ordos basin. Of importance, the discovery and detailed anatomy of Angou outcrop provide direct geological evidence supporting that sedimentation and diagenesis exert a strong control on the quality and heterogeneity of most tight clastic reservoirs.