Meandering River Reservoir Sedimentary Characterization And Geological Modelling At The High Water Cut Oilfield

After years of water flooding, most oil fields in eastern China have entered the high water cut stage. However, the whole recovery rate is relatively low (about 30%). The distribution of remaining oil appears to be highly scattered in the whole field and relatively abundant in some localities. To further explore residual oil and increase recovery oil ratios, underground knowledge system of reservoir needs in-depth studies and fine reservoir geological models should be constructed. Take Neogene Minghuazhen Formation of Gangxi Oilfield for an example, data such as well cores, logs, seismic and production performance are used to carry out the research. The final results provide foundation for remaining oil prospecting at the later high water cut stage and a case example for fluvial reservoir sedimentary characterization by means of modern sedimentology, sequence stratigraphy, reservoir architectural element analysis and geological modeling.Based on the core theory of base level cycle and accommodation change, sequence stratigraphy is considered to be the study of statal stacking pattern and its change trend analysis within a chronostratigraphic framework. Founded on regional tectonic and depositional settings and combined with relatively preferable marker beds and spectrual decomposition cycles, sequence stratigraphic study shows that Minghuazhen Formation of Gangxi Oilfield can be subdivided into 60 super short-term base level cycles, and be further grouped into 22 short-term and 2 mid-term base level cycles. The 2 mid-term base level cycles are equal to 2 third-order sequences. Taking account of channel sandstone stacking relationships and lateral changes, sequence stratigraphic framework is constructed.Under the guidance of classic meandering river facies model, sedimentary microfacies for 60 sandbeds are depicted in detail combined with sandstone isopach map. The microfacies units include complex channel belts, abandoned channel, crevasse-splay, interchannel thin-beded sandstone and floodplain mudstones. There are two distributing modes for the channel sandstone including wide multi-stacking channel sandstone complex and borrow isolated channel distributed in the background of floodplain mudstone. The vertical evolutionary pattern of meandering channel sandstone is mainly controlled by the ratio of accommodation and sediment supply rates (A/S). The detailed research of architectural element indicates that the range of single channel is 80-400m. The calculation results show that the dip angle of lateral accretion bed within a point bar is 4-6°and the ratio of width and depth is 18-22 and the panlar width is about 50m.Synthesizing the above research results and the statistics of reservoir physical parameter, geological knowledge database for the Gangxi Oilfield is established, which include lithofacies, sedimentary models, sandstone scales, physical parameters and interbeds. At last, under the constraint of sedimentary microfacies model, preferable modeling means are selected to establish the three-dimensional geological models consisting of structure and attribute models. The reservoir models provide the foundation for future remaining oil prospecting and have practical significance.