柴北缘地区层序格架下的沉积特征

中国中西部含油气沉积盆地大多具有前陆盆地层序组合特征,可以识别出前陆层序和非前陆层序。柴达木盆地柴北缘层序地层格架主要包括非前陆层序的侏罗系和古近系以及前陆层序的新近系。通过井剖面层序分析结合有限的地震剖面,可以对柴达木北缘侏罗系划分7套III级层序。古近系—第四系划分11套III级层序。侏罗系属于非前陆期沉积,新近系属于前陆期沉积。通过柴北缘地区层序框架下沉积特征的解剖,表明柴北缘地区非前陆期遗迹化石以缺氧环境化石群为主,沉积相以河流-湖沼体系为特征。前陆期遗迹化石以陆相氧化河湖相环境化石群为主,湖退体系域(RST)冲积扇-河流三角洲-滨浅湖相沉积体系非常发育。柴达木盆地的沉积层序表现为受构造强烈活动和气候变化的影响,强烈活动期潮湿气候下层序表现为山前带发育进积式准层序组,凹陷区形成退积式准层序组。干旱气候配合下的层序表现为多物源更加明显,山前带沉积物供应更加迅速,且沉积中心迁移强烈。

Sedimentary features in a sequence stratigraphic framework in the north area of Qaidam Basin

The sequence stratigraphic framework in the north area of Qaidam Basin mainly includes Jurassic,Palaeogene and Neogene strata.The Jurassic is distributed in the north area of the basin, where the Palaeogene and Neogene occur basin-wide. Based on analysis of well section sequences and several seismic interpretations, the Jurassic is divided into 7 sets of grade-III sequence, and Paleaogene-Quaternary units into 11 sets of grade-III sequence. The Jurassic and Palaeogene deposits belong to a pre-foreland period; Neogene deposits belong to the foreland period. This study of sedimentary reservoirs in a sequence stratigraphic framework shows that in pre-foreland deposits in the north area of the basin, trace fossils mainly indicate oxygen-deficient environments, and the sedimentary facies feature fluvial and limnic depositional systems. Trace fossils in foreland deposits are mainly those for oxygenated environments.The sedimentary facies feature nonmarine fluvial and lacustrine depositional systems, especially alluvial fan-river-delta-shore and shallow lakes in a regressive system tract (RST). The depositional sequence in Qaidam Basin reveals the influence of tectonic movement and climate change. The sequence developed during strong deformation under humid climate conditions is marked by a progradational parasequence set in the foreland and by a retrogradational parasequence set in the depression area. Under dry climate conditions, the sequence shows more obviously polygenic sediments, more rapid sediment supply in the foreland,and strong depocenter migration.