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A New Approach to Effectively Identifying Deep Fan-Delta Sandbodies


Fan-delta sandbodies act as important deep reservoirs in basins offshore China. However, these deep fan-delta sandbodies are poorly imaged in seismic reflection data through conventional techniques. In this research, we present an effective workflow to identify deep fan-delta sandbodies. The workflow consists of five key techniques, including (1) paleogeomorphology reconstruction, (2) seismic forward modeling iteration, (3) three-dimension seismic reflection data filtering, (4) logistic recomputation, and (5) visualization. Paleogeomorphology reconstruction is employed to locate valleys and mark the potential areas where fan deltas may develop. Backstripping technique coupled with fault activity analysis is adopted to carry out this issue. Seismic forward modeling iteration determines two critical filter thresholds of seismic reflection amplitude within which the fan-delta sandbodies with given range of net-to-gross sand ratios can be identified. The net-to-gross sand ratios could be obtained from previous drilling around the study area. A group of filter threshold couples of seismic reflection amplitude are estimated through forward modeling iteration. Filtering of the three-dimension seismic reflection data with calculated threshold couples generate the data that represent the potential fan-delta sandbodies. These resultant data, however, may contain some background deposits that represent shale and cannot be separated from sandbodies by filtering. Therefore, a logistic model based on statistical analysis is established to remove background deposits from filtered results. Finally, a three-dimensional grid of resultant data is generated for visualization. We apply this approach to the Yanan sag of the Qiongdongnan basin, northern South China Sea, where the fan-delta sandbodies at depths more than 3.5km are difficult to identify through conventional techniques and no wells have drilled. By using this approach, a potential area is determined along the fault-bounded northern margin, in which two set of fan-delta bodies with areas of 10 km2 and 12 km2 are identified at depths of 3.8-4.2 km. The result is now adopted for the assessment of exploration targets in this area. In conclusion, the approach can be applied to identify deep fan-delta sandbodies in places with no or few drilling wells.