CHARACTERISTICS OF UNCONFORMITY STRUCTURE AND ITS CONTROL ON STRATIGRAPHIC TRAPS: A CASE STUDY FROM THE YACHENG AREA, QIONGDONGNAN BASIN, SOUTH CHINA SEA

The T60 unconformity in the Qiongdongdong Basin, as the boundary between the Paleocene and Neocene, witnessed the transition of a faulted basin into a depression basin, and the T100 unconformity, as the basement unconformity, is clearly manifested in the whole basin, and the basin began to develop gradually above it. After years of hydrocarbon exploration, the work in non-structural traps has achieved certain results in Qiongdongnan Basin, and the buried hill hydrocarbon reservoirs have gradually become the focus of offshore oil and gas exploration, but the insufficient study of the structural characteristics and distribution range of the unconformity restricts the development of the exploration. This study combined the seismic data interpretation and logging comprehensive identification technology to conduct a comprehensive study on the unconformities of T60 and 100, in order to explore the role of the unconformity structure in hydrocarbon formation. This work is crucial for guiding the evaluation of the unconformity reservoirs and the deployment of exploration. The research has four significant outcomes. (1) The T60 unconformity structure is intact, the thickness of weathered clay layer is 3-6m, the T100 unconformity weathered clay layer can reach a thickness of 8m, and the thickness of weathered clay is negatively correlated with the thickness of the leached layer. (2) The lithological configuration of the T60 unconformity is dominated by transgressive sand-weathered clay-sandstone, while the lithological configuration of the T100 unconformity is dominated by basal conglomerate-weathered clay-metamorphic rock (magmatic rock). (3) The unconformity structure plays a controlling role in hydrocarbon formation in the area. The upper and lower layers of the structure can be the storage spaces or migration channels for oil and gas, and the weathered clay layer with a thickness of over 3m can seal natural gas. (4) The reservoir formation model in the Yacheng area can be summarized as "dual source hydrocarbon supply, composite stepped transport, and structural + stratigraphic trap hydrocarbon accumulation".

Acknowledge: The research was supported by Pilot Project of the Chinese Academy of Sciences(XDA14010301) and National Natural Science Foundation of China (42272155) .