Velocity of seismic waves in the basement of oil-bearing basins in the Pearl River mouth region of the South China Sea

Abstract

Analyses of the velocities of multichannel seismic reflection profiles in the Pearl River mouth region indicate that there are two main types of seismic sequences below the basement interfaces: stratified and unstratified. For the stratified sequence a normal method, like the Dix formula or a modified Shah formula, can be used to calculate the interval velocity of the basement layers. In the unstratified basement, however, within a certain thickness range; generally about 200 ~ 300 msec, the distribution of stacking velocity points varies linearly with increasing depth, so the instantaneous velocity method can be used to calculate the velocity of the average interval of basement. The results show that the basement velocity in the three basement-fault basins are 5.0–6.0 km/sec in this area. Rocks with velocities of more than 5.0 km/sec and deeper than 4 km circle the edges of the main parts of the three basement-fault basins. Thus the velocities have about the same values at the same depths, and the composition of the main parts of the three basins may be the same. Basement rocks in the western part of the eastern basement-fault basin are mainly Mesozoic granites, with acoustic velocities of 5.3–5.7 km/sec and the main parts of the three basins might be mainly Mesozoic granites. The lowest velocities of about 3.5–4.5 km/sec are found in the eastern part of the eastern basement-fault basin. There are very clear stratified reflectors below the basement surface, which might be Mesozoic sedimentary rocks. The velocities of the basement fault blocks have median values, and characteristics of the seismic waves are different from the Mesozoic granites and Mesozoic sedimentary rocks. They might be Paleozoic metamorphic rocks or locally even Paleozoic sedimentary rocks. The thickness of the basement layer is only 1.16–2.37 km. Below this is about 6.25–6.95 km/sec with an unknown thickness; this layer might be basalt. A 60-km-long refraction line stretches from the Wanshan fault block north of the basement-fault basins. The thickness of the sialma in the northern basement fault block of the eastern basement-fault basins is about 6.2 km. Below this layer there is a layer of high velocity-7.15 km/sec—probably a basalt. We infer that the three basement-fault basins off the Pearl River mouth are rift basins.