--> Reconstruction of Post-Rift Anomalous Subsidence in Qiongdongnan Basin, Northern South China Sea

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Reconstruction of Post-Rift Anomalous Subsidence in Qiongdongnan Basin, Northern South China Sea

Abstract

Abstract

The Qiongdongnan basin is an important extensional basin in the northwestern margin of the South China Sea due to its close link to hydrocarbon exploration. However, it remains unclear on the details of regional post-rift anomalous subsidence because of lacking precise thickness information of sediment layers covering the basin. In this investigation, we collect 18 sediment layers interpreted from more than 100 seismic lines, with geologic ages from early Eocene to present day. A total of 229850 virtual wells are built to reconstruct the subsidence history by using backstripping technique and strain inverse method where the formation lithology is determined by the lithological statistics of 30 wells. The statistical results of computed subsidence rate divide the post-rift tectonic evolution of Qiongdongnan basin into two stages, including: the slow subsiding stage from early Miocene to middle Miocene and the rapid subsiding stage from late Miocene to present day. The results reveal that post-rift anomalous subsidence develops widely in the Qiongdongnan basin with temporal and spatial differences. The post-rift anomalous subsidence value is generally small in the slow subsiding stage, while in the rapid subsiding stage it shows a wide distribution and great magnitude. In the early Miocene, the post-rift anomalous subsidence appears. Since the middle Miocene, the post-rift anomalous subsidence in the central south and west of the basin start to increase. Following with the further development of post-rift anomalous subsidence, a large post-rift anomalous subsidence zone in the nearly EW direction is formed. In the Pliocene, the extent of post-rift anomalous subsidence covers the total basin and on the central basin it has the amount up to 2 km. In the Quaternary, the post-rift anomalous subsidence increases rapidly and it is up to 2.5 km in the central basin and 3 km in the Changchang sag. Besides, the post-rift anomalous subsidence in the Yabei sag increases as well. The regional existence of post-rift anomalous subsidence indicates that the evolution Qiongdongnan basin experiences a complex subsiding process caused by the lithology thinning and deep faulting activity with base faults as media and boundary.