Characteristics of deep source-to-sink systems and sand prediction in Qiongdongnan Basin: A case study

To clearly understand the distribution patterns and sand richness in deep Paleogene fan bodies in the Qiongdongnan Basin, the third member of Lingshui Formation in the middle part of Songnan Low Uplift was studied in detail as an example by coupling analysis on the source-to-sink system. First, the provenance system and its evolution were determined by petrologic work, zircon dating, and tectonic balance profile. Secondly, the types and distribution of transport channels were characterized by utilizing fine 3D seismic interpretation for paleogeomorphological reconstruction. Thirdly, sedimentary system types and distribution patterns were clarified combining well and seismic data. Finally, multiple geophysical methods were used to predict sand richness in the sedimentary systems. Results show that the Songnan Low Uplift is an inherited paleo-uplift, where the main rock types were granite and monzogranite, plus a few granodiorite. The basement age spans 239~250 Ma. During the sedimentation period of the third member of the Lingshui Formation, an erosion area of 186 km², being steep in the east flank and gentle in the west, was developed in the middle sector of Songnan Low Uplift. Separated by the watershed boundary line, the westward source area was about 141 km², and could be divided into three secondary catchment units, in which three large ancient faulted troughs were developed from east to west correspondingly. The average width of the troughs is 2.88~3.23 km, the average depth is 0.25~0.5 km, and the width to depth ratio is 6.4~10.3. They are good channels for sediment transport, and three deltas were recognized in each trough. The seismic profiles show a clear progradational reflection structure along the source direction, and a lenticular reflection was observed perpendicular to the source direction. In addition, based on the comprehensive prediction of seismic facies, wave impedance inversion, and deep learning, the development of sand-rich deltas in the third member of the Lingshui Formation could form good reservoirs as a good pathway for deep buried-hill oil-and-gas migration, which is of great significance for deep oil-and-gas exploration in this area.