Development characteristics of Ordovician ancient subterranean river system in thrust anticline area of Tahe Oilfield, Tarim Basin

Abstract

The study of the ancient subterranean river system in the Tahe Oilfield in the Tarim Basin is still in its initial stage. Subterranean river caves are mainly classified based on the characteristics of the depth distribution and structural patterns of the subterranean rivers. However, there is a lack of comprehensive analysis regarding the spatial development patterns of complex subterranean river systems from geological perspectives such as structure, fault, paleogeomorphology, and water table. This hinders the understanding of primary and secondary relationships of subterranean rivers, spatial superposition patterns, and original connectivity relationships, thereby impeding comprehensive management research in the later stages of Tahe Oilfield development. To address this gap, an investigation was carried out in the S67 well block to clarify the developmental characteristics of the Ordovician ancient subterranean river system in the thrust anticline area of the main Tahe Oilfield. The study utilized methods like structural fault analysis, paleogeomorphology restoration, seismic attribute characterization, and vertical section interpretation to identify river types, classify systems, and examine geological origins. Noteworthy findings include the identification and analysis of a phreatic loop river for the first time. Results show that the S67 well block is located in a low-lying region at the southern edge of the karst platform within the main Tahe Oilfield area. This region features peak cluster depressions and karst hill depressions with minimal amplitude differences and shallow incision depths of surface water systems. The low-angle thrust structural style of the thrust anticline, combined with a network of faults, provides favorable dissolution channels for the multi-layered subterranean river system. In the study area, the Ordovician period is characterized by the emergence of distinct water-table subterranean river systems and phreatic loop river systems, all of which display a dendritic structure. The water-table subterranean rivers are divided into main, branch, and abandoned types, while the phreatic loop rivers are classified as ascending or symmetrical. The formation of ancient subterranean rivers in the thrust anticline area is primarily influenced by factors such as paleogeomorphology, water table levels, thrust anticline structure, and secondary fault networks.

Keywords

• strike-slip fault
• thrust fault
• paleogeomorphology
• subterranean river
• phreatic loop
• water table
• ordovician
• tahe oilfield