Soft-sediment deformation structures and sand body architecture in the chang 6 oil member of the upper triassic Yanchang formation, southwestern Ordos basin, China

Abstract

Soft-sediment deformation (SSD) structures of the Upper Triassic Yanchang Formation are laterally widespread in the Ordos Basin. In the Huachi-Qingyang (H-Q) area of the Ordos Basin, the Chang 6 oil member of the Upper Triassic Yanchang Formation is among the most significant Mesozoic oil-bearing strata. It is characterized by the development of reservoir sand bodies. During the depositional evolution of the Chang 6 oil member, SSD structures induced by paleo-seismic events developed in the H-Q area in the middle of the basin. The SSD structures developed in the sand bodies of the Chang 6 oil member are mainly ball-and-pillow structures, fold structures, sand dikes, irregular convolute stratifications, and synsedimentary faults. The architecture of the sand bodies resulted from paleo-seismic events and gravity slumping and mainly include two types of structures: i) SSD structures driven by paleo-seismic events with delta front sand body (SD-SSD) and ii) SSD structures driven by paleo-seismic events with turbidites (formed by delta-front slumping and re-distribution due to seismic action) (ST-SSD). As a consequence, genetic models of the sand bodies formed by different sedimentation processes are established.