A semi-quantitative method for the reconstruction of eustatic sea level history from seismic profiles and its application to the southern South China Sea

Quantitative studies on the extension and subsidence of the Wanan Basin were carried out based on available seismic and borehole data together with regional geological data. Using balanced cross-section and backstripping techniques, we reconstructed the stratigraphic deposition and tectonic evolution histories of the basin. The basin formed from the Eocene and was generally in an extensional/transtensional state except for the Late Miocene local compressoin. The major basin extension ocurred in the Oligocene and Early Miocene (before approximate to 16.3 Ma) and thereafter uniform stretch in a smaller rate. The northern and middle basin extended intensely earlier during 38.6-23.3 Ma, while the southern basin was mainly stretched during 23.3-16.3 Ma. The basin formation and development are related to alternating sinistral to dextral strike-slip motions along the Wanan Fault Zone. The dominant dynamics may be caused by the seafloor spreading of the South China Sea and the its peripheral plate interaction. The basin tectonic evolution is divided into five phases: initial rifting, main rifting, rift-drift transition, structural inversion, and thermal subsidence.

Tectonic Evolution of the Wanan Basin, Southwestern South China Sea

Based on seismic and drilling data, we calculated tectonic subsidence amounts and rates of the Wan'an Basin by backstripping. The genetic mechanism and syn-rifting process of the basin were analyzed in combination with the regional geological setting. The results reveal that the basin syn-rifted in the Eocene and early Miocene under the control of the dextral strike-slip Wan'an Fault Zone. The transtensional/extentional stresses along this fault zone may be attributed to seafloor spreading of the South China Sea (SCS) in multiple episodes. Extensive basal faults and some small initial rifts in the early Paleogene can be related to southeastward extrusion and clockwise rotation of the Indochina Block. During the Oligocene, the nearly N-S directed spreading of the SCS derived the transtensional stresses in a roughly NW-SE orientation. The basin subsided rapidly in the middle and north to form two major subsidence centers. In the early Miocene, the SCS spread again in a nearly NW-SE direction, resulting in rapid subsidence in the southern basin continuous extending until the period similar to 16.3 Ma.

Formation and syn-rifting process of the Wan’an Basin, South China Sea

In this study, high-resolution seismic profiles and well data provided a good opportunity for better understanding the reefs and carbonate platforms in the Wan’an Basin, southwest of the South China Sea, and also provided valuable information for the oil–gas exploration in the reef reservoirs. Four evolutional phases, including the initial phase, the prosperous phase, the recession phase and the submerged phase, of the reefs and carbonate platforms are proposed according to our data. In the Early Miocene, a few small, isolated carbonate platforms initiated in the center of the basin. In the Middle Miocene, they flourished and mainly formed around the Northern Uplift and Central Uplift, with two belts of carbonate platforms in the western area and eastern area that were mainly platform-edge reefs, massive reefs and a few point reefs. In the Late Miocene, the carbonate platforms began to retreat towards the high topographic position because of the rising of sea level. Meanwhile, the numbers and styles of reefs increased to include platform-edge reefs, massive reefs, atoll reefs and point reefs. Since the Pliocene, most of the carbonate platforms have been covered by detrital materials from terrestrial sources. Crustal tectonic activity provides favorable topography for reef growth and the distribution of platforms, and eustasy controlled the vertical growth and lateral migration of reefs. Since the Late Miocene, the rapidly crustal tectonic subsidence and the rising of relative sea level may lead to the drowning of the carbonate platform.

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Evolution and Controlling Factors of the Reef and Carbonate Platform in Wan’an Basin, South China Sea

In view of the characteristics of low resolution and weak signal of marine gravity data, the satellite gravity in the study area is separated from each other in multi-space, and the separated anomalies are extended downward iteratively to obtain multi-depth high-resolution gravity anomalies. Apparent density inversion, linear structure enhancement processing, and basement interface inversion of gravity anomalies are carried out. Geophysical analysis of deep structure, fault structure identification, basement structure, and structural division of sedimentary basins are also carried out. Based on the comprehensive analysis of gravity anomalies, it is considered that the basin structure is mainly controlled by early SN, NE, and NNE faults, and the later NW faults have obviously reformed the southern part of the basin. The basin is mainly composed of 4 NE tectonic zones, which have been transformed into 12 structural areas due to the later reformation. This multi-attribute analysis of gravity anomalies is based on the rationality of the structural division and the stronger identification of the secondary tectonic zones, providing a more detailed geophysical basis for further oil and gas exploration in the Wan’an Basin.

Gravity anomaly characteristics and basin geological structure in Wan’an Basin

The Wan'an basin is a typical Cenozoic shearextensional basin in the southwest of the South China Sea. Inferred from the results of tectonic subsidence modeling for the seismic profiles across the basin and from the calculation of the faulting movement velocity in the basin, it has undergone four tectonic subsidence processes since early Eogene period, and most of the faults are growth ones coming from the basement. The faults were strongly active when the basin underwent shearextension and riftdepression subsidence from late Oligocene to early Miocene. The geological factors and processes for hydrocarbon formation, migration, accumulation and preservation were controlled by the tectonic subsidence stages.

Tectonic subsidence and its control on hydrocarbon resources in the wan'an basin in the south china sea

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Tectonic subsidence and its control on hydrocarbon resources in the wan'an basin in the south china sea