陆相断陷盆地低凸起斜坡带层序划分及其沉积充填演化特征:以埕岛东斜坡古近系为例

Sequence stratigraphic classification and depositional filling analysis of low-rising slope belt in continental rift basin:A case study from the Paleogene strata in east slope of Chengdao

层序分级不一致和井震层序不统一都会导致沉积充填演化特征反映的不准确,将制约储盖组合与油气分布的预测。文中应用沉积层序的概念,利用岩心、岩性、测井与地震资料,以层序界面与内部构成特征相结合的原则,从坡下到坡上,对东斜坡古近系进行了层序划分。一级层序相当于系地层单元,与构造运动相对应,层序界面为区域性角度不整合;二级层序相当于统或组地层单元,与构造运动幕相对应,层序界面为局部不整合;三级层序相当于段或亚段地层单元,与构造幕内的次级构造作用相对应,层序界面为超覆不整合面、沉积体系转换面及重力流下切面;四级层序即体系域,相当于油组与砂组,与三级层序基准面旋回的阶段性变化相对应,层序界面为初始洪泛面、最大洪泛面,共识别出SQ1、SQ2、SQ3、SQ4、SQ5和SQ6等6个三级层序及15个体系域。井震统一的层序格架结合了钻井资料纵向高分辨率与地震资料横向好连续性的优点,从而高精度、高分辨率地揭示了层序格架充填演化特征,体现出层序旋回性与体系域及平面沉积相分布的差异性。古近纪埕岛凸起夷平,斜坡及凹陷补齐,发育超覆、披覆沉积;古地形由南低北高向南高北低转换;本地物源、邻近物源、区域物源阶段性变迁;多物源的沉积体系交叉、叠置,沉积相发生了扇三角洲—浊积扇—半深湖—滑塌浊积扇—辫状河三角洲—曲流河泛滥平原演变。埕岛低凸起斜坡带层序及沉积充填演化特征具有陆相断陷盆地的代表性,因而,此项研究对于解决层序地层学面临的工业化应用研究不足的问题以及油气储盖组合预测方面的难点可提供有益参考。

Lack of uniform sequence classification and well-to-seismic sequence integration led to inaccuracy of depositional filling characterization and restricted forecasting reservoir-caprock assemblages and oil-gas distribution. Applying the concept of sedimentary sequence,using core,lithology,log and seismic data,and under the guidance of the principle of combining sequence boundaries with internal features,the author carried out the sequence division of the Paleogene strata in the east slope of Chengdao from the botlom to the top of the slope. The first order sequence is equivalent to a system strata unit,corresponding to tectonic movement,and the sequence boundary is a regional angular unconformity; the second order sequence is equivalent to a series or formation strata unit,corresponding to a tectonic phase in the tectonic movement,and the sequence boundary is a partial unconformity; the third order sequence is equivalent to a member or sub-member strata unit,corresponding to the secondary tectonic activity during the tectonic phase,and the sequence boundary includes unconformity,overlapping unconformity,transform surfaces of depositional systems and etched surface of gravity flow; the fourth order sequence（system tract）is equivalent to an oil set or a sand set,corresponding to third order sequence base level changes,and the sequence boundary is the first flooding surface and the maximum flooding surface. Six sequences,the SQ1,SQ2,SQ3,SQ4,SQ5 and SQ6,and 15 system tracts were identified. The well-seismic sequence stratigraphic framework,which combined the high vertical resolution of drilling data with the high lateral continuity of seismic data,could reveal the filling and evolution characteristics of sequence stratigraphic framework with high-precision and high-resolution. And it could show the cyclicity of sequences and the distributional difference between the system tract and the sedimentary facies. The following events occurred in the east slope of Chengdao in the Paleogene：the planation of uplift; the padding of the slope and depression; the development of overlapping and draping formations; the change of paleo-topography from the south being low to the north being low; the change of local source,neighbor source and regional source; and the stacking of multi-source deposition systems. Along with these events,the depositional systems changed as follows：fan delta,turbidite fan,semi-deep lake,slump turbidite fans,braided river delta,and meandering river floodplain. The sequence stratigraphy and depositional filling characteristics of low-rise slope belt in Paleogene Chengdao are typical for continental rift basin; thus,this study may give some help in solving the problem of insufficient application of sequence stratigraphic classification to oil industry and in forecasting reservoir-caprock assemblages and oil distribution.