GEOLOGIC SETTINGS AND POOL FORMING MODELS OF SUBTLE PETROLEUM ACCUMULATIONS IN MIDDLE-SMALL BASINS——A CASE STUDY OF THE NAPU SAG OF THE BOHAI BAY BASIN

Uplifts connected with sags,syndepositional faults and synsedimentary folds and more unconformities have made middle-small basins more favorable for subtle oil pool formation in tectonic and geologic settings.There are kinds of slope break belts,i.e.the fault,the warp and the sedimentary ones in the Nanpu Sag of the Bohai Bay Basin,controlling types of subtle traps/pools,i.e.the thin out,the lenticular body,the fault-lithologic,the stratigraphic overlap,the unconformable barrier and the physical sealing ones.Subtle traps provide room for subtle pool formation.Migration systems connect oil sources and subtle traps.Hydrocarbon source rocks give birth to mature oil and gas.The above-mentioned 3 elements cooperate in time and space and result in the formation of subtle pools.The formation has experienced 3 stages,i.e.faults controlling slope break belts,slope break belts controlling subtle traps and source rock faults routing hydrocarbon into subtle traps to form subtle pools.