Paleo-fluid Dynamics And Petroleum Accumulation Characteristics In Low Permeable Reserviors Of Yanchang Formation, Longdong Area, Ordos Basin

Yanchang formation is the typic reservior which is low-porosity and low-permeability in Longdong area, Orods basin. The main types of petroleum reservoir are lithologic hydrocarbon reservoirs, and it is non-homogeneous inside of hydrocarbon reservoirs which was caused by the strong diagenesis. But in the case of low-permeability, there are some high-yielding oil wells and several high-yielding water wells, even it is very different between adjacent wells in the same reservoir. And it is very different between the south and the north of Longdong area about oil and water production. Therefore, the rule of petroleum accumulation is very important when the characters of the low-permeability resevoir are known.Hydrocarbon accumulation dynamic theory is the core of the paper. Base on the rock cores observing and some testing, the critical period of oil accumulation was fixed. And equivalent depth method was used to resume overpressure in the critical period. Then the primary migration orientation of hydrocarbon which was drived by paleo-pressure was analysised. Fluid potential analysis method was used to calculate paleo-oil potential, and the relationship between paleo-oil potential and oil accumulaion was studyed. Lastly, jiont the result of oil-potential with sand carrier system and geologic condition, the regular pattern of oil in Longdong area was generalized. At the same time, its efffection to the oil distribution nowadays was probed.The main conclusions in this paper were presented as follows:(1)The critical period of oil accumulation is the Late of Early Cretaceous which is the maximum burial depth period, in Triassic yanchang formation, Longdong area. And the distribution of overpressure is higher in northeast than it is in southwest, all of the chang6, chang7, chang 8. Vertically, the pattern of expulsion from chang7 source rock to chang6 and chang 8 was classifyed to three doubleaction-expulsion, down-expulsion and upward- expulsion by the shape of overpressure curve.(2)Oil reservoirs of chang8 in the central and the south of longdong area were caused by vertical and lateral migration. And the vertical migration in situ and accumulation is mainly in the north. Oil reserviors in the central region, which is encircled by Huachi, Qingcheng, Heshui, was the result of lateral migration of chang 6 formation. And the northern is both vertical and lateral migration,but the distance of lateral migration is shorter.(3)The regions where the paleo-pressure gradient reduces intensely but pressure gradient reduces slow nowadays are more advantageous to oil accumulation, so their production of oil is higher than others. The stronger the pressure gradient reduces from hydrocarbon kitchen to reservoir is, the better the relationship between oil and water is, then the production of testing for oil is high. The overpressure tendency change small in reservoir is, the better the relationship between oil and water is.