Seismic recognition and quantitative characterization of siliciclastic-carbonate mixed sedimentary rocks in the Bohaiwan Basin, Northeast of China

Mixed sediments often occur in continental lake basins due to the coupling effects of the climate, hydrodynamic conditions,.tectonic processes, and material abundances of provenances, and the mixed sedimentary rock formed in this process has dual.characteristics. The response of mixed sedimentary rock to seismic waves is different from that of conventional sandstones.or carbonate rocks and easily causes errors in identification and characterization by conventional research methods. In this.paper, taking the mixed sedimentary rock in the southern part of the Laizhouwan Sag in the Bohaiwan Basin as an example,.the petrological characteristics of the mixed sedimentary rock are studied and its sedimentary model is established. On this.basis, a comparative study of the seismic waveform characteristics is carried out. Compared with sandstone, the waveform.continuity of mixed sedimentary rock is poor, the amplitudes are weak and the frequencies are low, but opposite results are.found compared with the conglomerate formed by gravity flow at the bottom. Contrast analysis of the acoustic impedance.shows that the value of the acoustic impedance of mixed sedimentary rocks is higher than that of sandstone and is generally.greater than 6.2 × 10 6 kg/m 3 m/s. Therefore, the threshold value between sandstone and mixed rock is set to 6 × 10 6 kg/m 3 m/s.in inversions to delineate the boundary between mixed sedimentary rock and sandstone. Finally, using seismic attribute.analysis and inversion technology, the scales of the horizontal and vertical distributions of the mixed sedimentary rock plat-.form are predicted. The range of the platform is 7100m in length, 3100m in width, and 15.6km 2 in area. Longitudinally,.the mixed sedimentary rock layer spans the third layer of the third member of Shahejie Formation (ES 3 3 ).

由于气候、水动力条件、构造作用以及物源物质丰度的耦合效应，陆相湖盆中常常出现混合沉积，在此过程中形成的混合沉积岩具有双重特性。混合沉积岩对地震波的响应不同于常规砂岩或碳酸盐岩，容易导致常规研究方法在识别和表征上出现错误。本文以渤海湾盆地莱州湾凹陷南部的混合沉积岩为例，研究了混合沉积岩的岩石学特征并建立了其沉积模式。在此基础上，进行了地震波形特征的对比研究。与砂岩相比，混合沉积岩波形连续性差、振幅弱且频率低，但与底部重力流形成的砾岩相比结果相反。声阻抗对比分析表明，混合沉积岩的声阻抗值高于砂岩，一般大于6.2×10⁶千克/立方米·米/秒。因此，在反演中将砂岩与混合岩之间的阈值设定为6×10⁶千克/立方米·米/秒，以划分混合沉积岩与砂岩之间的边界。最后，利用地震属性分析和反演技术，预测了混合沉积岩台地水平和垂直分布的规模。该台地长7100米，宽3100米，面积15.6平方千米。纵向上，混合沉积岩层跨越沙河街组三段三层（Es₃³）。