Sedimentary features and depositional model of the submarine fan of Mingyuefeng Formation in the Western Lishui Sag, East China Sea

The Mingyuefeng Formation of Paleocene in the Lishui Sag of East China Sea Shelf Basin is characterized by typical submarine fan deposits,in which industrial hydrocarbon has been discovered. In this paper, a depositional model for the Sag is summarized based upon the existing seismic-geological data under the guidance of sequence stratigraphy. Sedimentary facies features of the submarine fan observed from cores include collapse and crumpling structures, entrapment beddings, drainage structures, massive sandstone, tear debris of black mudstone, drifting gravel structures, sand injection and other structures. The C-M diagrams show a typical pattern of gravity flow deposits with moderate to poor component and textural maturities. The seismic reflection of the fan body is characterized by two-way dipping along source direction, hill-like reflection in cross sections, and typical fan-shaped seismic attributes in plane views. The Mingyuefeng Formation in the Lishui Sag is composed of lowstand system tract, transgressive system tract and highstandl system tract of a sequence, in which submarine fans occur in the lowstand system tract. Multiple incised large valleys develop in the proximal end of the fan body. Both the fault breaks and sedimentary breaks are observed and the fault breaks are always in large scale with a slope of about 7.2 degrees, while the sedimentary slope breaks are relatively smaller with a slope around 5 degrees. Most of the depositional fans occur at the mouth of the incised valleys in a pattern including valley, slope and submarine fans. The largest fan covers an area of 124 km², and the lowstand system tact is composed of six fans, showing good prospects for lithologic traps in the Lishui Sag. According to the genesis of the fan bodies, we established in this paper a ditch-slope-fan depositional model for the lowstand system tract of the Mingyuefeng Formation as a guide for exploration of lithologic traps in the sag.