Abstract:

The bottom boundary(interface) of Ⅲ sequence of Jurassic in Junggar basin could be chiefly divided into following three types in structure:(1).unconformity with basal conglomerate;(2).pseudo-conformity with weathered layer;(3).conformity with apparent variation in sedimentary type etc.In the waveform seismic phase of 40ms extension upwards and downwards in the bottom boundary of Ⅲ sequence of Jurassic,high frequency of waveform seismic type Ⅰ-Ⅱ corresponds to sediments of sandstone layered with mudstone;Low frequency of type V-VI is in correspondence with large complete set of sandstone sediments while type Ⅲ-IV waveform is in between above two types.The amplitude of seismic wave is connected with rock assemblage on the bottom boundary,the medium stronger amplitude corresponds to the sand-mudstone assemblage as well as to the assemblages of conglomerate and silt rock,the weaker amplitude is in correspondence with the large complete set of sandstone assemblage while the medium intensity of amplitude is in between above two assemblages.Moreover,the low frequency seismic wave and the medium-stronger amplitude have a good correspondence with the progradational reflection of seismic.It is believed that,by a comprehensive analysis of waveform seismic phase,the amplitude and the reflection structure,the widespread of sandbodies in large area of northwestern slopes of Mahu Lake and Shinan-Mobei region in the central part,of which sandbodies of underwater fan of Che-Guai,Mahu braided delta,delta of Zhongguai meandering river and Shinan-Xiayan meandering river delta are well-developed and are favorable for forming nonstructural traps and have good potential for prospecting hydrocarbon.

Key words: Ⅲ sequence, topographic break zone, seismic phase, sandbody, prediction, evaluation