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

Abstract

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 × 106 kg/m3 m/s. Therefore, the threshold value between sandstone and mixed rock is set to 6 × 106 kg/m3 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 platform are predicted. The range of the platform is 7100 m in length, 3100 m in width, and 15.6 km2 in area. Longitudinally, the mixed sedimentary rock layer spans the third layer of the third member of Shahejie Formation (ES33).