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High-resolution Sequence Stratigraphy and Seismic Sedimentologic Characteristics of a Fluvial Depositional System, Guantao Formation of Lower Neogene, Wuqiang Oilfield, Jizhong Depression, Bohai Bay Basin, China

Jiang, Li; Ji, Youliang; Zhang, Yunlong; Zhou, Yuqi

The high-resolution sequence-stratigraphic framework and depositional systems of Guan Tao Formation in Wuqiang oilfield of the Bohai Bay Basin in east China were investigated by combining high-resolution sequence stratigraphic analysis, seismic-sedimentological study, and analysis of production data. Lower Neogene Guantao Formation in study area was deposited in a low-gradient fluvial depositional system. In Wuqiang Oilfield, 11 fifth-order sequences in 3 fourth-order sequences were identified in Guantao Formation interval. Higher order sequences (fourth- and fifth-order) interpreted from well data can be seismically mapped at a resolution equivalent to 10m in thickness from a 35-Hz dominant-frequency seismic data set. Thus, the seismic resolution is sufficient for establishing high-frequency sequence-stratigraphic framework in the region without well control.

Within fourth-order sequence framework interpreted from well log and seismic data, seismic stratal slices were made from three-dimensional (3-D) seismic volume to reveal high-resolution facies distribution patterns. Geomorphologic characteristics of the strata observed in seismic stratal slices reveal that the fluvial depositional setting is predominant with a spectrum from braided system with channel-belt widths of more than 5 km to high-sinuosity meandering systems with meander-belt width of a few hundred meters. These seismic strata slices reveal a series of large-scale depositional elements, including sandstone-filled meandering channels, point bars associated with lateral accretion, flood-plain, abandoned channels and oxbow lake fills. Therefore, the vertical facies evolution could be interpreted from the changes in strata slices through time.

The evolution of fluvial systems is controlled by the changes of accommodation caused by base-level changes, which is the interaction between tectonic and A/S ratio (accommodation versus sediment supply). It is proposed that the varying fluvial systems in study area was formed in response of accommodation changes: (1) Incised valley formed during lowest accommodation, (2) Braided channels stacked laterally, forming wide multistory channel belts during low accommodation, (3) high-sinuosity meandering fluvial systems were common during high accomodation. This model could be used for prediction strata pattern and facies distribution of fluvial systems in non-marine basins of the same setting.

 

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013