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.