Paleogeomorphological restoration and its control on gravity flow sand bodies: A case study of the Chang 73 submember of the Triassic Yanchang Formation in the Ordos Basin

Abstract

The bottom morphology of the lake basin controlled the sedimentary system, genetic types, and distribution characteristics of sand bodies in the Chang 73 submember of the Triassic Yanchang Formation in the Ordos Basin. The types, genesis, and spatial distribution of sand bodies in different regions differ. In this study, according to cores, drilling, logging, and other data, the paleogeomorphology of the Chang 73 submember was accurately restored by the impression method and sedimentology method, and the micro paleogeomorphology units were finely characterized. On this basis, the authors analyze the paleogeomorphology control on the sedimentary system, genetic types, and distribution characteristics of sand bodies in the Chang 73 submember. The result shows that the paleogeomorphology of the Chang 73 period is an asymmetrical depression in the east and a steep depression in the west. Seven micro paleogeomorphic units, including the highland, the slope break zone, the lakebed deep depression, the lakebed plain, the lakebed paleouplift, the ancient lakebed ridge, and the ancient channel, are identified in the study area. The paleogeomorphology obviously controlled the gravity flow deposition system of the Chang 73 submember in the study area. From the margin to the center of the lake basin, delta front deposition, channel-type gravity flow deposition, and semideep-deep lake deposition successively developed. From the slope break zone to the center of the lake basin, the gravity flow deposition type gradually changed from sandy debris flow deposition to turbidity flow deposition. The paleogeomorphology controls the overall distribution of the gravity flow sand body in submember 73 in the study area, while the micro paleogeomorphology units such as the highland, the slope break zone, and the ancient channel further control the provenance direction, genetic type, transportation direction, lateral connectivity and distribution pattern of the gravity flow sand bodies.

Keywords

• paleogeomorphology
• gravity flow deposition
• sand body distribution characteristics
• chang 73 submember of the yanchang formation
• ordos basin