折射方法在南沙礼乐盆地西南海域中生界研究中的应用

钻探和拖网取样资料都表明南沙海区东北部礼乐滩及附近海域存在大套中生代海相沉积,但其西南侧是否也存在中生界不清楚。新的长排列(偏移距超过6 km)多道地震测线C2B经过南沙礼乐滩西南海域(水深1.5~1.7 km),反射剖面显示了重要的角度不整合界面,上覆地层呈平行披盖结构,下伏地层则存在明显的挤压变形;长排列炮集记录则显示了清晰的折射波。利用OBSTOOL软件进行了折射波震相的拾取,折射波速度计算结果表明不整合界面上覆层地震速度为2.5~3.0 km/s,下伏层地震速度为3.5~5.5 km/s,跨不整合界面发生速度突变(其差值约1.0~2.0 km/s),表明地层严重缺失。结合南海的构造演化过程、钻探和拖网取样结果,以及与南海北部潮汕坳陷已经确定的中、新生代地层速度的对比,将上覆层划为新生界,下伏层划为中生界。C2B测线东南端下构造层属于中生界褶皱构造,地层速度较低,约3.5 km/s,低于潮汕坳陷西南部中生界的地震折射波速度为3.5~4.2 km/s,推测为早白垩世边缘海相沉积,可能预示礼乐盆地西南海域较好的中生界油气储集。

APPLICATION OF REFRACTION VELOCITY ANALYSIS TO IDENTIFYING MESOZOIC STRATA IN THE SOUTHWEST OF LIYUE BASIN ON THE SOUTHERN MARGIN OF SOUTH CHINA SEA

Extensive drilling and geological sampling demonstrate that Mesozoic marine deposits occur in the northeast of the Nansha Waters, the southern margin of the South China Sea (SCS). It has been long debated whether the lower structural layer with compressional structures and closely spaced reflections below the strongly eroded unconformity is the Mesozoic or Cenozoic strata in the southwest of Liyue Basin. A new high resolution 2D seismic survey with long cable was carried out over the regions of the southwest of the Liyue bank in 2013. The water depth is 1.5~1.7 km there, and the maximum offset of the survey line is about 6.1 km. The multi-channel seismic data shows remarkable information of refraction wave from the erosional unconformity. In this paper, OBSTOOL is used to pick up the refraction wave from the multi-channel seismic data. A major velocity leap (1.0~2.0 km/s) is found across the prominent unconformity calculated from the refraction wave of long cable seismic data, which indicates a hiatus and an abrupt lithological change. According to velocity leap and velocity contrast between the Mesozoic and Cenozoic of Chaoshan Depression in the northern margin of the SCS, the compressional structures are interpreted as the marine Mesozoic, while the overlying strata are Cenozoic. The refraction velocity of the Mesozoic in the southeastern part of line C2B is about 3.5 km/s, generally lower than that of 3.5~4.2 km/s in the southeastern part of the Chaoshan Depression, which may imply that the reservoir properties for oil and gas accumulation are better in the southwest of Liyue bank.