Provenance Analysis and Petroleum Geological Significance of Shallow-Marine Gravity Flow Sandstone for Huangliu Formation of Dongfang Area in Yinggehai Basin, the South China Sea

The reservoir in Upper Miocene Huangliu Formation of Yinggehai basin, which is located between Kuntum paleo-uplift and Hainan paleo-uplift, is shallow-marine gravity flow deposits with burial depth ranging from 2 600 to 3 500 m. Analyzing the provenance of the sandbody is significant in understanding its distribution and "source to sink system", hence for the oil-gas exploration and development in this area. The provenance of sediments in Upper Miocene Huangliu Formation was analyzed by integrating the components of the sandstones, assemblage styles of the heavy mineral, paleo-current direction and geochemistry characteristics of the sandstones. Results show follows. (1) There are two provenances, one from west and the other from east, in the study area. Western sourced sandstones form the shallow-marine gravity flow which are mainly sublitharenite with high feldspar (average is 6.1%) and lithic (average is 11.7%) contents, low shale content (average is 3.8%). However, eastern sourced sandstones, which are mainly subarkose-quartz siltstone with low feldspar and lithic contents with averages of 4.6% and 2.7%, respectively, high compositional maturity, high shale contents (average is 18.6%), form the neritic sand bar. (2) The shallow-marine gravity flow sandstones exhibit low zircon, tourmaline contents, and high magnetite, garnet contents, while the neritic sand bar sandstones exhibit high zircon, tourmaline and leucosphenite contents, and low magnetite, garnet contents. The direction of paleo-current in study area is from southeastward to eastward. (3) The similar rare earth element (REE) patterns of 28 sandstones from three western wells indicate that they were sourced from the same provenance. The diagrams of Th-Sc and Co/Th-La/Sc and elemental ratios of these sandstone samples show that they were derived from intermediate to felsic source rocks. The measured geochemistry data of these core samples fall into the fields of active continental margin or continental island arc in the diagrams of La-Th-Sc, Th-Sc-Zr/10, Ti/Zr-La/Sc and major element discrimination, indicating that the source rocks of the shallow-marine gravity flow sandstones were formed under the tectonic setting of an convergent environment. (4) The integrated method for provenance analysis indicates that the shallow-marine gravity flow sandstones were derived from the western Kuntum uplift. The shallow-marine gravity flow sandstones in western mud diaper belts are large in scale, rich in gas, better in reservoir properties and the main favorable exploration target towards the west, whereas the neritic sand bar sandstones in eastern mud diaper belts are small in scale, poor in reservoir quality.