Palaeogeomorphological control on the depositional architecture of lacustrine gravity-flow deposits in a depression lacustrine basin: a case study of the Triassic Yanchang Formation, southern Ordos Basin, China

Lacustrine gravity-flow deposits have enormous hydrocarbon potential. However, their depositional architecture has long been considered very complicated and is controlled by various factors, making petroleum exploration and development highly challenging. The influence of palaeogeomorphology on the extent and formation mechanisms of lacustrine gravity-flow deposits remains poorly understood. This study uses 3D seismic data, well-log data, and core data to characterize the palaeogeomorphology and depositional architecture of the lacustrine gravity-flow deposit of the Yanchang Formation in the Ordos Basin, while analyzing the influence of palaeogeomorphology on the architecture, distribution, and evolution of lacustrine gravity-flow deposit systems. The results showed that three palaeogeomorphologic units—shelf, slope, and basin floor—developed in the basin. Gravity-flow deposits can be further divided into channel and lobe systems. Gravity-flow channel systems have developed in the upper slope area, which include three types of architectural elements: confined channels, unconfined channels, and levee-overbank. As the transport distance of gravity flow increases, the erosion ability of gravity flow weakens, resulting in the terrain formed by gravity flow erosion gradually weakening the constraint on sediment transport. Consequently, gravity-driven flow pathways shift from linear, confined channels to curved, unconfined channels, finally leading to the development of numerous distributary channels. In the lower slope area, gravity flow sediments distribute rapidly and accumulate to form lobes after encountering slope breaks, due to the decrease of dynamics, including three types of architectural elements: distributary channels, lobe axis and lobe fringe. These lobes are tongue-shaped and fan-shaped on the plane. A series of lobes are superimposed and form a lobe complex set. These lobe complex sets are distributed in a branch-like manner. There are many branch-shaped low-bend amplitude channels inside the lobe complex set. Palaeogeomorphology plays an important role in controlling the spatial distribution of depositional architecture. As the slope gradient increases, the influence of slope break on sediment gravity flow weakens, leading to a delay of the sediment gravity flow slumping to the basin bottom. Additionally, the extension length of the channel system on the slope increases. Furthermore, the extension length of the lobes toward the center of the basin also increases. This work improves the fundamental understanding of the depositional architecture of lacustrine gravity-flow deposits and may help enhance sand prediction for the same type deposit reservoirs.