Sequence Stratigraphy of Baoshi Structure, East China Sea Shelf Basin
Qingsheng Zhang and Dag Nummedal
Colorado School of Mines, Golden, CO
Analyzing stratigraphic systems in the East China Sea Shelf Basin had been a problem due to complex tectonic and sedimentary history and different interpretations from paleontology, well log and seismic data. This study integrates microfossils, 3D seismic data, core, well logs and mudlogs to construct a sequence stratigraphic framework and depositional environments in the Baoshi Structure of the East China Sea Shelf Basin, and to provide a correlation standard for stratigraphic study in the East China Sea.
Four regional unconformities are recognized on the seismic profiles. These unconformities, caused by tectonics in the region, divide the Cenozoic formations into four second-order sequences: Paleocene Sequence, Eocene Sequence, Oligocene & Miocene Sequence, and Pliocene & Quaternary Sequences. Well Baoshi-1 only penetrated the Eocene Sequence. The lithologies encountered in Baoshi-1 consist of interbedded sandstones and shales. Study of the assemblage of pollens and spores, well logs, mudlogs and cores further divides the Eocene Sequence into two third-order sequences and the Oligocene & Miocene Sequence into four third-order sequences.
Most pollens and spores in the Baoshi Structure indicate dominant continental environments. The existence of marine fossils in the lower part of the well indicates a marine-influenced environment during the early stage. The integrated research shows that the depositional environments changed from transitional marine and continental to completely continental environments. From bottom to top, the depositional environments ranged from bay, lagoon, lacustrine-delta, fluvial-delta, to fluvial.