Deepwater fan sedimentary characteristics of Zhujiang Formation in Liwan area of Zhujiang River mouth basin

Abstract：Liwan deepwater area located in Baiyun Sag of Zhujiang River mouth basin is a potential area for oil and gas exploration. The Lower Member of Zhujiang Formation in the study area is regard as a deepwater fan based on a detailed study of petrologic and paleontological assemblage characteristics. The deepwater fan in Liwan area can be divided into two types, i.e., early basin fan deposit and late slope fan deposit. Both types of the deepwater fan have developed granular flow, sandy debris flow, low-density turbidity flow and other gravity currents along the slope, with the development of a little internal wave and contour current deposit. Sandy debris flow is the mainly sedimentary type of channel sand bodies of the two deepwater fans, and there are also reverse grading, positive grading, parallel bedding and horizontal bedding. In addition, many liquefaction and fluidization structures and biological escape trace in graded bedding as well as abundant burrow pores and bioturbation in horizontal bedding are observed. The two types of deepwater fan have different sedimentary and paleontological characteristics. There is silicious pea gravel in channel sandstone and globigerina fossil in interdistributary bay mudstone of the basin fan. Slope fan deposit contains rich coccoliths and globigerina fossil, and is characterized by carbon mudstone scraps in channel sandstone. Coccoliths and globigerina fossil in the slope fan have obvious characteristics of submerged corrosion, which means that the slope fan has deeper water than the basin fan. The two types of deepwater fan have similar distribution of gravity currents, i.e., the evolution from proximal granular flow to sandy debris flow, then to high-density turbidity flow and finally to low-density turbidity flow; in other words, from continental shelf slope turned directly to the basin.