Sequence Stratigraphic Delineation and Correlation of the Dongying Formation in the Nearshore and Adjacent Sea Areas, Bohai Bay Basin

Based on the analysis of well logs, seismic data, core studies, the Dongying （东营） Formation along the nearshore and adjacent sea areas, Bohai （渤海） Bay basin has been subdivided into three third-order sequences from base to top： namely, sequences SQ1, SQ2 and SQ3. The three sequences have different wireline （SP） log responses, showing triple-section characteristics with SQ1 being characterized by primarily flat baseline with intercalation of relatively low spontaneous potential, SQ2 generally exhibiting weak or moderate amplitude spontaneous potential with finger-shaped peaks, and SQ3 having relatively high spontaneous potential with funnel-shaped log curves. On the basis of the triple-section characteristics, the stratigraphic sequences can be correlated consistently throughout the entire study area. A stratigraphic and sedimentary model for sequences SQ1 to SQ3 of the Dongying Formation in the study area has been proposed. The accommodation space change in the two sides of the asymmetrical basin was examined and the asymmetrical basin has the feature of the asymmetrical accommodation space change. The asymmetrical physiography is a vital factor to influence the accommodation changes in additional to the lake level change, tectonism and sediment supply. This may have important implications to similar basins in other parts of China or elsewhere.

Role of bedding planes played in enhancing dissolution in sandstones

Diagenesis exerts an important control on porosity evolution,and research of diagenesis and diagenetic minerals provides insights into reservoir quality evaluation and CO_(2) storage.Thin section,XRD(X-ray diffraction),CT(Computed Tomography),scanning electron microscopy(SEM),and NMR(Nuclear Magnetic Resonance)tests were used to investigate composition,texture,pore spaces,and diagenesis of sandstones in Paleogene Dongying Formation in Bohai Bay Basin,China,with special aims to unravel diagentic dissolution along bedding planes.The oversized pores,remnants in feldspar-hosted pores,and kaolinite within feldspar grains indicate a high degree of dissolution the framework grains experienced during burial.The CO_(2)-rich or organic acids are responsible for the feldspar dissolution.Grain size plays the primary role in enhancing bedding dissolution process,and bedding planes in fine-medium grained sandstones with high content of feldspars are frequently enlarged by dissolution.The CT scanning image confirms dissolution pores are distributed discontinuously along the bedding planes.The dissolution pores along bedding planes have large pore size,and correspond to the right peak of the bi-modal T_(2)(transverse relaxation time)spectrum.The laminated sandstones and siltstones,or sandstones with cross beddings help improve framework grain dissolution.These new findings help improve the understanding of diagenetic models,and have implications in reservoir quality prediction and resource assessments in sandstones.