APPLICATION OF SEISMIC VELOCITY TO THE AREA OF NORTHEASTERN NANSHA WATERS

The Eastern Nansha Waters are rich in resources such as oil and gas, gas hydrate and polymetallic nodules. However, geological survey is only limited to 2D seismics, gravity, magnetism, surface sampling due to technical limitations and other reasons. The understanding of the structural geology and stratigraphy of the sedimentary basins in the Nansha is almost blank. In this regard, seismic velocity data has become the only mean to explore the regional pattern of the seismic velocity field, to establish the relationship between speed and lithology and to reconstruct palaeofacies. Based on 2D seismic data, we made seismic stratigraphic calibration and structural interpretation, velocity spectrum interpretation and velocity calculations, time-depth conversion, the envelope template of sand and shale, and compilation of maps of sand distribution. Results show that, depth and lithology are the main factors controlling the difference in velocity, the envelope template of sand and shale can be used to identify the distribution pattern of lithology and the evolution of depositional provenance. It is recognized that the Cenozoic sediments of the region could be grouped into 3 super sequences, i.e. the pre-rifting, drifting and foreland sequences. As to the provenance, sediments are mainly from the Huanan paleoland, endogenic deposition and Philippine arc. It is also found that difference in understanding of energy group velocity formula has lead to different results of velocity calculation, and attentions should be paid to every step of the performance. The above work will help clarifying the structural and stratigraphic pattern of the area and be effective for further geological survey in the Nansha Waters.