Depositional Characteristics and Distribution of Lake-Floor Fan of Paleogene Lower Member 3 of Shahejie Formation in Northwestern Part of Huanghekou Sag, Bohai Bay Basin

The lake-floor fan, as an important deposition of Paleogene in Huanghekou Sag, is characterized with both good physical character of reservoir and oil-bearing property, thus it has higher values for petroleum exploration. The lower Member 3 of Shahejie Formation, rich in sandstone, can be divided into lowstand system tract, extensive system tract and highstand system tracts based on seismic reflection signature, and core and logging data clearly. The sand bodies in the study area belonged to two kinds of lake-floor fan deposition formed under semi-deep-lake to deep-lake environment: deposition of the deep-water turbidite fan corresponded to lowstand system tract; deposition of the slump turbidite fan corresponded to extensive system tract and highstand system tract, which was formed by slumping fan delta front sand. Viewed from spatial distribution, the deep-water turbidite fan was mainly developed in the second-order fault downthrown of the basin, while the slump turbidite fan was distributed in the abrupt slope which located in the south of study area. The complex of lake-floor fan consists of various sediments’gravity flow deposits, including:(1)turbidity current deposit with characteristics of Bouma sequences; (2)mud-bearing sandy debris flow deposits with characteristics of dominated sand and mixed by mud; (3)sand-bearing muddy debris flow deposits with characteristics of dominated mud and mixed by sand; (4)gritty debris flow deposits with characteristics of massive gravel accumulation;(5)sandy slump deposits with characteristics of deformed sedimentary structure. During lower water level period(LSTs3L), most of the study area were erosion or sediment pass-by areas, and terrigenous clastic matters were directly transported into deep-water area under the second-order faulted belt, and then formed the deep-water turbidite fan. During higher water level period (ESTs3L, HSTs3L), the slump tuibidite fan, which resulted from various sediments’gravity flows, mainly developed in front of fan dalta in south. The sandstones of the lake-floor fan are the main reservoirs of oil in the northwestern part of the Huanghekou sag.