吐哈盆地侏罗系层序地层学研究

吐哈盆地侏罗系是一个盆地充填层序 (一级层序 ),包括 4个构造层序 (二级层序 ),分别与侏罗纪湖盆的 4个演化阶段 (稳定沉降阶段、短期收缩阶段、扩张阶段和萎缩阶段 )相对应,其中又分为 13个 (三级 )层序。与海相地层不同的是,本区识别出了 4种层序边界类型 (A型、B型、C型和D型 ),其对应的层序本文分别称之为冲积层序、淹没层序、类Ⅰ型层序和类Ⅱ型层序。冲积层序形成于构造剧烈抬升时的盆地萎缩阶段,淹没层序形成于构造沉降迅速时的盆地扩张阶段.

Sequence Stratigraphy of Jurassic in Turpan-Hami Basin

Fourteen sequence boundaries showing onlap or erosional truncation are recognize d in the seismic sections of the Jurassic Turpan-Hami basin.The two major bound a ries (at the top and bottom of the Jurassic deposits)are regional unconformities extending beyond the basin.Three intermediate ones (between 3re and 4 th section of Xishanyao Fm.Qikeai Fm.and Sanjianfeng Fm.and Kalaza Fm.and Qigu Fm.) are unconformities spaning most of the basin.The remaining nine sequence boundaries are local unconformities.The Jurassic sedimentary strata in Turpan-Hami basin i s a basin-fill sequence (1st order)including four tectonic sequences(2n d order) and thirteen depositional sequences(3rd order).The four tectonic sequences are bound with the four evolutionary stages(stable subsidence,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 ty pes of depositional sequences in terrestrial basin,however,are not the same as t hose from a passive continental margin.Four types of sequence boundaries(type A, B,C,and D)are identified in the basin.A type A boundary is formed at the desicca t ion stage of the lake basin caused by strong tectonic uplift.The sequence above the type A boundary is here termed as fluvial sequence,in which the major systems tract is a lowstand deposit composed of alluvial fan and fluvial facies. A type B boundary,where base-level falls below the previous shore-slope break an d then rises rapidly,occurs as the stage of basin expansion caused by rapid tect onic subsidence.The corresponding sequence without the lowstand systems tract is here termed as submerged sequence.A type C boundary,where base-level also fall s below the previous shore-slope break and fluvial incision occurs,is similar t o the type 1 boundary identified by Vail et al(1977).The lowstand systems tract w ith deposits of basin-floor fan and incised valley overlies type C boundary.The r efore,type C sequence is called as similar type 1sequence .A type D boundary ,wh ere base-level falls but shore-slope is not exposed,is similar to type 2 bound ar y identified by Vail et al(1977).The corresponding sequence without the lowstand systems tract is here termed as similar type 2sequence.The difference betwe en these boundary types is the nature of transgression above the boundary for ty pe B and D.which both lack a lowstand systems tract.There is a gradual transgres sion in type D compared with a rapid and abrupt one in type B.In other words.the sedimentary facies above the type B boundary are distinct from those below the b oundary and record a rapid change in base level compared with a gradual one in t ype D.　　Four types of sequence models in this paper have demonstrated the difference bet ween marine facies sequences and non-marine facies sequences.The type A boundar y & fluvial sequence and type B boundary & submerged sequence are indicative of t wo extreme conditions of desiccation and rapid subsidence of the lake basin,and are totally different from marine facies sequences.We suggest several reasons fo r the variety of non-marine facies sequences of the lake basin: a smaller scale ,m ultiple sediment input provenances,relative short distance from sediment source. and differences in local tectonic activity.　　There is a relationship between the sequence stratigraphy and the hydrocarbon sh ows in well TC2 of Turpan-Hami basin.Hydrocarbon shows are often adjacent to se q uence boundaries in Kalaza Fm.and Qigu Fm.because both formations lack oil sourc e rocks or/and source rocks are not at the depth of the oil window.Hydrocarbons originating from underlying formations thas migrate through fractures occur alo ng the sequence boundaries.which usually have higher permeability.However.hydroc a tbon shows in the Qiketai,Sanjianfeng,and Xishanyao formations are different fr om the overlying formations because they contain good source rocks buried to the oil window.In some fore-delta sandstone bodies adjacent to the source rocks,th e re are pools formed by stratigraphic traps and structural-stratigraphic combina tion traps in addition to the sequence boundaries.