Study On Hydrocarbon Accumulation Of Beach Bar Reservoirs In Gubei Sag Of Zhanhua Depression

Subtle reservoirs in Jiyang Subbasin have huge explorative potential, the survey of proved reserves is over 20×10⁸t, and beach bar reservoir are one of them. There are a lot of beach bar reservoirs developed in Gubei Sag of Zhanghua Depression, which mainly distribute in Zhuangxi Oilfields and Wuhaozhuang Oilfields, and the correlative formation is Shahejie Formation, including Es₁, Es₂ and upper submember of Es₄.Three source rock intervals exist in Gubei Sag of Zhanghua Depression, which are upper submember of Es₄, lower submember of Es₃ and Es₁ from old stratum to new stratum. Both the upper submember of Es₄ and the lower submember of Es₃ are excellent source rocks with good hydrocarbon generating potential, but the former one deposited in saline-salt lake with typeⅡ₁ kerogens, while the latter one deposited subsaline lake with typeⅠkerogens. Although Es1 has good kerogen type- typeⅠand deposited in saline-subsaline environment, it is not a good source rock, because it’s in immature-low mature phase due to shallow burial depth.In this research, we got 12 oil samples in Shahejie Formation（four sapmples in each member） according to geochemical analyses of oils and source rocks, Pr/Ph, gammacerane index, 4-methylsteranes, diasteranes, stable carbon isotopic composition and a series of maturity parameters（e.g. isomer sterane ratios, bisbenzothiazole, Ts/Tm etc.） can be regarded as parameters for oil-source correlation. The results indicate that the oils in beach bar reservoirs of northern Gubei Sag are mainly derived from Es₃ source rocks and the southern counterpart are mainly derived from Es₁ source rocks, and the mixing oils exsit.According to the research of hydrocarbon accumulation factors which includes source rocks, reservoirs, cap rocks, reservoir-forming dynamic, etc. the characteristics of hydrocarbon accumulation in Shahejie Formation of Gubei Sag can be stated as follows:①the normal faults are the dominant factor of hydrocarbon accumulation of beach bar reservoirs.②the reservoir type is diversified, including fault block reservoir, fault-lithologic updip pinchout reservoir, stratigraphic overlap reservoir, fault-stratigraphic reservoir and lithologic lens reservoir.③due to bad physical properties-medium porosity and low permeability, the hydrocarbon migrate in beach bar sand body is in pattern of non-darcy flow and is held up by capillary pressure and“Jiaming effect”resistance which makes reservoirs have less demand for cap rocks. The thickness of sand body, the physical properties of reservoir, the content of mud stone and the position of sand body in sequence are the controlling factors to oilness in beach bar reservoirs of upper submember of Es₄.The analyses for beach bar reservoirs of upper submember of Es₄ in Zhuang 83 block demonstrate that the oils in this area have good physical properties and are derived from lower submember of Es3 source rocks. faults are main migration pathway, lithology, physical properties and its heterogeneity of sand body are controlling factors to oilness, and the study to formation water illuminates that beach bar reservoirs in this area have good sealing conditions.