Paleogene-Neogene Cap Rocks and its Relationship with Hydrocarbon Accumulation in the Zhanhua Sag

To analyse the Zhanhua Paleogene–Neogene cap rocks and its relationship with hydrocarbon accumulation, the seal lithology, the relationship between compaction of argillite rock and its sealing capacity, and its destruction by faults and fractures were studied. The results indicate that there are four types of cap rocks: argillite rock and silty mudstone, microcrystalline carbonate, dense cemented sandstone and dense cemented carbonate. Among these cap rocks, argillite rock is the main type in the Zhanhua Sag. According to the evolutionary characteristics of the argillite rock and its destruction by fractures and faults, the argillite cap can be classified into three categories: porosity cap, fracture transformation cap and the fault transformation cap. Among their sealing capacities, the porosity cap is the best, followed by the fracture transformation cap, and the fault transformation cap is the worst. Through the analysis of the relationship between existing oil & gas reservoirs and the distribution characteristics of the Paleogene–Neogene cap rocks in the Zhanhua Sag, it was found that the cap combination which was below or above the reservoir together controlled the hydrocarbon accumulation and preservation. It means that the destruction of the cap below or down-dip the reservoir is a necessary condition for hydrocarbon accumulation, and only when the sealing capacity of the cap rock above or up-dip the reservoir is better than that of below or down-dip the reservoir, hydrocarbon could be efficiently stored in reservoirs, thus could be effectively enriched.