Discovery and Geological Significance of Seismites of Paleogene in Jinxian Sag, Jizhong Depression

Key words: 
• seismites /
• vertical sequence /
• deep fracture /
• reservoir permeability /
• Jinxian sag

Abstract: This paper mainly discussed the sedimentary characteristics, vertical sequence and geological significance of seismites of Palaeogene in Jinxian sag, Jizhong depression. In the Bohai Bay Basin, the Jinxian sag was a typical small fault basin of Cenozoic. And a lot of soft sediment deformation structures developed in the layers in the 4th Member of Shahejie Formation and the 1st Member of Kongdian Formation, including liquefied sand-mudstone veins, mud stocks, water-escape channels, load cast and flame structures, ball-and-pillow structures, vibrational liquefaction deformation structures, beads-shaped structures, micro synsedimentary faults and ground fissures. The study indicated that deformation structures above were typical characteristics of seismites. Based on the study, vertical sequence of seismites was established, including underlying unshocked layer, seismic fall layer, shattered layer, seismic corrugated layer, ball-and-pillow layer, liquefied homogeneous layer, and overlying unshocked layer. Seismites, evaporates and tuff which were all related to the activity of deep fracture, developed in the layers in the 4th Member of Shahejie Formation and the 1st Member of Kongdian Formation. The study indicated that activity of deep fracture induced multi-period earthquakes, and lead to further the formation of seismites. At the same time, as channel of magma upwelling and deep salt sources, the deep fracture provided material basis for the formation of evaporates and tuff. It indicated that the maximum tectonic intensity of Jinxian sag happened in that period, from sedimentary perspective. Seismic deposition can improve the permeability of reservoirs through the connection of original pores by micro fissures.