准噶尔盆地阜东斜坡区侏罗系层序地层格架的建立

Sequence Stratigraphic Frame of Jurassic in East Fukang Slope, Junggar Basin

综合阜东斜坡区侏罗系钻测井及地震资料 ,将侏罗系划分为六套层序。层序边界识别标志包括不整合面、古生物化石突变面、底砾岩及冲刷面等标志。体系域划分的关键是确定首次湖泛面和最大湖泛面的位置。研究区侏罗系沉积时期存在较为明显的地形坡折带 ,因此能够确定首次湖泛面的位置 ,建立起具有地形坡折带的层序地层模型。研究了侏罗系层序形成的主要控制因素 ,即构造沉降、物质供应速率、气候和基准面升降等。

This paper is concerned about the study of sequence stratigraphy of Jurasssic in East Fukang Slope, Junggar Basin. East Fukang Slope is located in Northeast Junggar basin, a dipping southwest sloping zone. The area is about 5 000 km 2.Because Jurassic was rarely deformed by the tectonic movement, there are few structural traps with obvious structural closure in this area. Therefore, it is necessary to seek lithological or stratigraphic traps guided by sequence stratigraphic theory. Jurassic in East Fukang Slope consists of Lower Jurassic ,including Badaowan Formation(J 1 b ) and Sangonghe Formation(J 1 s ), Middle Jurassic, Xishanyao Formation(J 2 x ) and Toutunhe Formation(J 2 t ), and Upper Jurassic, Qigu Formation(J 3 q )and Kalazha Formation(J 3 k ). Combining with core data, well logging data, seismic data and analysis data in lab, Jurassic could be divided into six sequences, that is: Sequence I (SI)(J 1 b 1), SII(J 1 b 2), SIII(J 1 s -J 2 x 1), SIV (J 2 x 2), SV(J 2 t )and SVI(J 3 q -J 3 k )from bottom to top. Moreover, according to synthetic seismic data, the divisions of core and well logging sequence are identical with seismic sequence. The distinguished marks of sequence boundary include: surface of unconformity, the mutation surfaces of fossil, lithology, well logging curve and sedimentary facies, basal conglomerate and surface of scour, coal formation , etc. The definition of the first flooding surface (FFS) and the maximum surface (MFS) is the significant step for the division of systems tract. In the period of Jurassic, due to the steep slope from Baijiahai uplift to Changji sag and the gentle slope from Cainan region to Kelameili Mountain, a slope break formed in the southeast of Baijiahai uplift. Therefore , the FFS can be determined, resulting in the definition of lowstand systems tract (LST), transgrassive systems tract (TST) and highstand system tract(HST). In the study area, identified marks of systems tract boundary include the followings: the first steady and continuous seismic axis across the slope break can indicate the FFS; coal formation or danby? can indicate FFS or MFS; the change of lapping style of subsequences and the change of the colour of sedimentary rocks are available as well. LST is mainly composed of prograding subsequence ,TST of aggradating and retrograding subsequences, and HST of aggradating and prograding subsequences. Combining with the paleo-topography feature, the distribution of significant coal formation and the position of FFS and MFS, the sequence stratigraphic model or frame has been established in East Fukang Slope. This model shows that the sequence stratigraphic style of Jurassic apparently was influenced by cycle of relative change of lake level, resulting in the onlaps on the steep side of lake basin regularly moving. The principal factors of sequence stratigraphic style of Jurassic include tectonic subsidence, the supplying rate of sediment, climate and the change of base level. Tectonization was the direct one ,by which the unconformity boundary, the most apparent identified mark, was formed. Tectonic subsidence controlled the generation and consumption of accommodation. The changes of sediment supplying rate and subsidence rate could influence the lapping styles of subsequence sets and systems tract. In addition, the change of base level was one of the most significant factors which resulted in the change of accommodation and relative lake level.