Seguence Stratigraphy of Jurassic in Turpan-hami Basin

Key words: 
• seguence stratigraphy /
• seguence boundary /
• accommodation /
• trace elements /
• hydrocarbon shows

Abstract: Fourteen sequence boundaries showing onlap or erosional truncation are recognized in the seismic sections of the Juras- sic Turpan-Hami basin. The two major boundaries(at the top and bottom of the Jurassic deposits ) are regional unconformities extending hevond the basin.Three intermediate ones(between 3^re and 4^th section of Xishanvao Fm. Qikeai Fm.and Sanjianfeng Fm.and I}alaza Fm.and Qigu Fm.)are unconformities spaning most of the basin.The remaining nine sequence houndcries are local unconformities.The Jurassic sedimentary strata in Turpan-Hami basin is a basin-fill sequence(1^st order)including four tectonic sequences( 2^nd order) and thirteen depositional sequences( 3^rd order).The four tectonic sequences are bound with the four evolutionary stages( stable suhsidence, short-time contraction expansion and uplift desiccation)of the lake basin. According to classic sequence stratigraphy,there are two types of sequences and sequence boundaries in passive continental margins with a shelf and slope.The types of depositional sequences in terrestrial hasin, however are not the same as those from a passive continental margin.Four types of sequence houndaries, type A, B, C, and D ) are identified in the basin.A type A boundary is formed at the desiccation stage of the lake basin caused by strong tectonic uplift.The sequence above the type A boundary is here termed as,fluvial sequervce,in which the major systems tract is a lowstand deposit composed of alluvial fan and fluvial fades.A type B houndary where base-level falls below the previous shore-slope break and then rises rapidly occurs as the stage of basin expansion caused by rapid tectonic subsidence.The corresponding sequence without the lowstand systems tract is here termed as submerged sequence.A type C houndary where base-level also falls below the previous shore-slope break and fluvial incision occurs is similar to the type 1 boundary identified by Vail et alb 1977).The lowstand systems tract with deposits of basin-floor fan and incised valley overlies type C boundary.Therefore type C sequence is called as similar type 1 sequence.A type D houndary where base-level falls but shore-slope is not exposed is similar to type 2 houndarv identified by Vail et a1( 1977).The corresponding sequence without the lowstand systems tract is here termed as similar type 2sequeuce.The difference between these boundary types is the nature of transgression above the boundary for type B and D.which both lack a lowstand systems tract.There is a gradual transgression in type D compared with a rapid and abrupt one in type B.In other words.the sedimentary fades above the type B boundary are distinct from those below the boundary and record a rapid change in base level compared with a gradual one in type D. Four types of sequence models in this paper have demonstrated the difference between marine fades sequences and non-marine fades sequences.The type A boundary&fluvial sequence and type B boundary&submerged sequence are indicative of two extreme conditions of desiccation and rapid subsidence of the lake hasin, and are totally different from marine fades sequences.We suggest several reasons for the variety of non-marine fades sequences of the lake basin: a smaller scale multiple sediment input provenances, relative short distance from sediment source.and differences in local tectonic achmtv. There is a relationship between the sequence stratigraphy and the hvdrocarhon shows in well TC2 of Turpan-Hami basin. Hvdrocarhon shows are often adjacent to sequence boundaries in Kalaza Fm. and Qigu Fm.because both formations lack oil source rocks or/and source rocks are not at the depth of the oil window. Hvdrocarhons originating from underlying formations thas migrate through fractures occur along the sequence boundaries.which usually have higher pcrmcahilitv.However. hvdrocathon shows in the Qiketai, Sanjianfeng, and Xishanvao formations are different from the overlying formations hecause they contain good source rocks buried to the oil window. In some fore-delta sandstone bodies adjacent to the source rocks there are pools formed by stratigraphic traps and structural-stratigraphic combination traps in addition to the sequence bounilaries.