Sedimentary characteristics and depositional model of hyperpycnites in the middle of the third member of Paleogene Shahejie Formation in the east slope of Dongying Sag

Key words:  hyperpycnal flow  gravity flow  sedimentary characteristics  distribution pattern  depositional model  lacustrine rift basin

Abstract:

The sedimentary characteristics, distribution pattern, and depositional model of hyperpycnites in the middle of the third member of the Paleogene Shahejie Formation in the east slope of Dongying Sag were studied, by combining three-dimensional seismic, logging data, core analysis, grain size analysis, thin section analysis and flume simulation experiments analysis. The results show that hyperpycnites in the study area are dominated by mud conglomerate, fine-grained sandstone and siltstone. The average content of matrix is 12.6%, and the maturity of the composition is low. The probability curve of particle size exhibit an upward arc with wide gentle shape, and has a low-inclination two-stage type, which are indication of gravity flow deposition. In the research area, typical sedimentary structures of hyperpycnites include erosion and filling structures, parallel bedding, inverse then normal grading sequence, ripple, wavy bedding, massive bedding and carbonaceous clastic layers. The Wang 59 well in proximal is mainly characterized by the vertical stacking of muddy debris flow deposits and high-density bed-load dominated hyperpycnal flow deposits. The Niu 876 well in distal is mainly characterized by the vertical stacking of low-density suspended load dominated hyperpycnal flow deposits and lofting deposits. There is an elongated shape in the plane distribution of debris flow deposits in proximal. With the increase of the transport distance, the debris flow is transformed into hyperpycnal flow. The transfer and evolution processes of hyperpycnal flow in the lacustrine basin undergo a fluid plunging zone, a fluid stabilization thickness zone and a fluid expansion zone. The fluid plunging zone determines the extent and scope of erosion, and the fluid stabilization thickness zone is the bed-load dominated hyperpycnal flow deposition zone, the fluid expansion zone is the suspended load dominated hyperpycnal flow and lofting sediment deposition zone. Channels are not obvious in hyperpycnites in lacustrine rift basins, and the grain size of the sediments is developed from coarse to fine. The distribution of sand bodies is controlled by the paleomorphology and can present a strip or fan-like distribution.