Reworking of gravity flow sandbody by bottom-current from Huangliu Formation in Dongfang area of Yinggehai Basin, northwestern South China Sea

Abstract

The sandbody in upper Miocene Huangliu Formation from Dongfang area of Yinggehai Basin are shallow-marine gravity flow sediments. However, there are some controversies on the sedimentary process and genetic mechanism of gravity flow sandstone (GFS) due to the quite complex hydrodynamic energy and direction in shallow-marine environment, especially a big debate on that whether there is reworking of GFS by bottom-current in study area. Therefore, this paper indentified the reworking of GFS by bottom-current using seismic data, core, casting sections, measured data and whole-rock geochemistry data, and the process of reworking by bottom-current was also discussed. The evidences for the reworking of GFS by bottom-current are shown in the following aspects: (1) the bottom-current reworked sandstone (BCRS) has a more wide distribution perpendicular to the gravity flow direction; the core is characterized by typical traction-current sedimentary structures such as ripple bedding, flaser bedding, S-shaped cross bedding. Hummocky pelitic strip can also be found. The fine-grained sandstone show sharp upper contacts, internal weak erosional truncation surface, and suspension mud is locally developed. (2) In terms of rock components, grain size characteristics and physical properties, the leaping components increase markedly, while the mud content of fine-grained sandstone is reduced with grain size increasing from the BCRS due to the reworking of bottom current. After reworking of bottom current, the sandstones have the average porosity of 19.2%, average permeability of 106.9 mD, and average pore-throat radius of 8.34 μm, indicating that they have significant better physical properties than GFS. Meanwhile, foraminifera shells of the BCRS are commonly damaged, but those of GFS are mostly preserved well. (3) In terms of elemental geochemistry in sandstone, the ratios of B/Al, Cr/V, Co/Ni and rare earth element (REE) index suggest the occurrence of reworking by oxygen-rich water mass. According to the regional evolution of Yinggehai basin, it can be found that there are basic conditions for the development of internal wave and internal tide in study area. The bottom-current mainly reworked the upper part of thick GFS shortly after the deposition, winnowing away of fine compositions such as original argillaceous component, resulting in larger pore throat radius and better physical properties. As a kind of high-quality reservoir, the BCRS have an important significance on development.