Paleoenvironment and Depositional Analysis of a Paleogene Formation: Terminal Fan Sedimentation Approach for Reservoir Quality Study in the Lower Shahejie Fourth Member, Bonan sag, China

The lower fourth member of the Paleogene Shahejie Formation (Es4^L) in Bonan sag, Bohai Bay Basin, East China was taken as an important target for oil and gas exploration in Shengli oilfield. Bonan sag is one of several sag in the Bohai Bay Basin, located in north-east China. Paleogene deposits are characterized by red rock layers, formed under a hot-warm and arid climate. Four depositional systems were developed in the Bonan sag. Among them, the fan delta developed in the central part of the study area, consisting of gray-white fine glutenite with poor sorting and rounding. The braided river delta is developed in the southern part with large lithofacies. The lithology shows some light gray conglomerate, pebbly sandstone, sandstone and small amount of mudstone. Floodplain facies is mainly developed in the inundated plain that in some weather conditions have the characteristics of shallow lake facies. However, a number of questions remain unanswered about the disparate presence of evaporites, the diverse colors of the mudstones and the varying levels of total organic carbon (TOC) in the area. These disparities can be seen in the many oscillations in TOC content, from the source of the sediments in the upper relief to the flood lacustrine plain. Consequently, given to this conditions, the depositional system which occurs in the area remains unclear from previous works. However, looking at the distribution of the sediments in an apparently short distance of less than 50km from the south to the North of sag, suggests that they are more likely to be terminal fan deposits. Terminal fans are an architectural system for draining and depositional process from high relief to flood plain, with gradual discharge of water through infiltration and evaporation. Although some researchers have conducted studies on terminal fans around the world, the concept remains unfamiliar and deserves to be more elucidated. This approach postulates to be useful to better understanding the sedimentary deposits in the Bonan sag and must be reviewed for similar hydrocarbon explorations elsewhere.