Characteristics and formation mechanisms of gravity-flow deposits in a lacustrine depression basin: Examples from the Late Triassic Chang 7 oil member of the Yanchang Formation, Ordos Basin, Central China

Abstract

Understanding the flow processes that form gravity-flow deposits is vital for modeling and predicting sandstone bodies in the subsurface, which is of great significance for conventional and unconventional oil and gas exploration and development in lacustrine basins. This study analyses gravity-flow deposits of the Late Triassic Chang 7 oil member of the Yanchang Formation in the Ordos Basin, using a combination of well-log analysis, core observations, thin-section analysis, and laboratory measurements. The sedimentary facies, formation mechanisms, distribution patterns, and depositional models of gravity-flow deposits are investigated. Thirteen facies and eight bed-types are recognized in the gravity-flow deposits. Bed types represent deposits of sandy slides, sandy slumps, debrites, high-density turbidites, hybrid event beds, transitional flow deposits, surge-like low-density turbidites, and quasi-steady low-density turbidites. Gravity-flow deposits, particularly those caused by sediment failure, are composed of slides, slumps, debrites, high-density turbidites, hybrid event beds, transitional flow deposits, and surge-like low-density turbidites. These deposits form as isolated lenticular sand bodies, with retrogradational internal stacking patterns. In contrast, gravity-flow deposits caused by flooding rivers are composed of high-density turbidites, hybrid event beds, transitional flow deposits, and quasi-steady low-density turbidites. These deposits exhibit elongate morphologies with progradation stacking patterns internally. The collision between the North China Block and the South China Block, which occurred during the closing of the Qinling Ocean, lead to frequent volcanic eruptions and earthquakes. These activities promoted the development of gravity-flow deposits associated with sediment failure from delta fronts in both the northeastern and southwestern parts of the basin. Concurrently, extremely humid climatic conditions promoted increased fluvial drainage, leading to enhanced hyperpycnal flows into the deep-lacustrine basin depocenters in the Late Triassic. This study emphasized that detailed facies analysis and distribution pattern analysis are the fundamental way to identify the formation mechanisms of gravity flow deposits in lacustrine depression basins.