Study on the hydrocarbon accumulation models of the upper Es3 in the gentle slope belt of Chexi Depression

Based on the analysis of hydrocarbon source, reservoir forming period, composition and classification of transportation system, and the reasons of failure well in Chexi Depression of Bohai Bay Basin，Two types of hydrocarbon accumulation models in gentle slope belt of Chexi area are established and the main controlling factors of hydrocarbon accumulation are defined. There are three sets of source rocks（Es1、middle and lover submember of Es3、Es4）in Chexi area, the different strata of source rocks have great differences in the Pr/Ph and the content of gammacerane. It has been found that the crude oil of Es3 has a good geochemical correspondence with the middle and lower of Es3 source rocks, and has the characteristics of near source accumulation. The hydrocarbon accumulation in the study area exists in the sedimentary period of the Dongying formation and the sedimentary period of the Guantao formation to the present two stages, which is dominated by late filling. There are two stages of oil and gas filling in the inner and middle belts, and only late stage hydrocarbon filling in the outer slope belt. The hydrocarbon transportation system is mainly composed of faults and sand bodies. The effective source rocks in the middle and lover submember of Es3 are connected with the upper reservoir of Es3 in a small area, which can be directly migrated to the upper sandstone reservoir of Es3 to form lithologic oil and gas reservoir. However, most of the oil and gas in the upper Es3 reservoir need to be vertically migrated by means of oil source fault, and then through the contact of sand bodies such as main channel and fan body, the main oil and gas reservoir will gradually move up with the distance from the source rock. The area with direct contact source reservoir configuration relationship is a "sand body lateral migration" reservoir formation mode, and the main controlling factors of reservoir formation are sand body connectivity and reservoir porosity and permeability. The source reservoir configuration area with fault connection type is a "fault sand combination T-type migration" reservoir forming mode, and the main controlling factors of reservoir forming are migration convergence facies (structural ridge and cross-section ridge).The area of passive reservoir contact is "fault sand combination step migration" reservoir forming mode, and the main controlling factors of reservoir forming are migration convergence facies (structural ridge) and lateral sealing of faults in preservation conditions.

Key words: Chexi Depression; Source of hydrocarbon; Accumulation period; Fault sand transport combination; Reservoir forming mode