Delta depositional system and source rock characteristics of Enping Formation,Liwan sag in ultra deep-water area of northern South China Sea

Liwan sag is a new exploration area in the ultra deep-water zone of Pearl River Mouth Basin,northern South China Sea,where the source rocks have arisen one of the most important geological problems.Based on the analysis of seismic data,we consider that Liwan sag is characterized by typical S-shaped seismic reflections and belongs to river delta depositional system.Through a comparative analysis with the adjacent Baiyun sag and combined with seismic and drilling data,we conclude that the delta system of Enping Formation is the main development zone of source rocks in Liwan sag.The results of paleontological analysis show that Liwan sag has a similar sedimentary environment as Baiyun sag,namely,a restricted marine depositional environment.The northern uplift of Liwan sag as the provenance enters into the Liwan marine environment,resulting in the formation of large-scale delta system.This delta system experiences a similar coal-forming environment as the delta system of Enping Formation on the northern slope of Baiyun sag,which can form coal-measure source rocks.As influenced by the delta system,terrigenous marine source rocks and marine source rocks are also developed in the adjacent marine environment.The results of geochemical analysis show that coal-measure source rocks,terrigenous marine source rocks,and marine source rocks are substantially different types of source rocks,all of which have significant implications for the exploration potential in deep-water to ultra deep-water areas.