Abstract:

Sedimentary characteristics and depositional model of Yanchang Formation of Longdong area in Ordos Basin were studied based on core, logging data and gravity flow theory. The gravity flow deposits of the area can be divided into turbidite, sandy debris flow sediment, muddy debris flow sediment and slump rock. The turbidite develops positive grading. The sandy debris flow sediment is characterized by freezing and massive deposition. The muddy debris flow sediment mainly contains muddy and a small number of sandy particles and clumps in its interior. The slump rock develops liquefied tectonic structure, such as convolute bedding. The differences of the development degree of gravity flow sediments are obvious. In terms of the number of drilling wells, the turbidite and sandy debris flow sediment are the major sedimentary types, and the muddy debris flow sediment is the less one. In terms of the single deposition thickness of gravity flow, the average thickness of sandy debris flow deposition is 0.986 m, which is significantly higher than other types. The average deposition thickness of turbidite is the lowest, which is 0.414 m. The gravity flow of the area is due to the slump of delta front sediments. The sandy debris flow sediment and turbidite are the main types of gravity flow deposits, followed by the slump rock and muddy debris flow sediment. The sandy debris flow sediment developed in deepwater area in front of the northeast delta front from meandering river. The turbidite mainly developed in deepwater area in front of the braided river delta front, whose provenance came from west, southwest and south. In terms of distribution area, the muddy debrisflow sediment and sandy debris flow sediment with mud gravel are very small in plane, and their distribution is not in a regular pattern.

Key words: Ordos Basin, Yanchang Formation, gravity flow, sedimentary characteristics, depositional model