Study For Offshore Break And Non-structural Trap Formation In Depression Basin

Western slop belt is one of the important first order petroleum exploration elements in Songliao basin. Most of reservoirs discovered in the belt are structure traps, and only few of the reserve are from non-structural reservoir. The study shows that non-structural trap has more potential in the area and is major exploration field in future exploration. With use of wells data and seismic data, paper present introduces the conception of offshore break (similar to shelf break in marine environment), and a new understand, offshore break dominated the deposition, facies distribution and reservoir forming in the western slop, is proposed based on the research results in the relation ships between offshore break distribution and its origin, offshore break and deposition, offshore break and non-structural trap. The understand lays down theoretical foundation for the research in the distribution principle of non-structural trap in the southern depression of Songliao basin, and it is the key of successful subtle reservoir exploration in the belt.Unconformity provided background setting for the formation of offshore break in the belt. The western slop belt dominated by Paleozoic basement generally was a slop dipping to east in evolved history, and there were lot of erosive mountains developed which formed an erosion mountain belt. Several stratigraphic unconformities were developed during the deposition in the slop. All mentioned provided the background for offshore break formation.The evolution of the slop belt dominated internal framework of third order sequence in the belt. The major target section of fourth member in Quantou formation to Nenjiang formation thickening toward east is characterized with on-lapping deposition pattern in each layer, and can be divided into nine third order sequences. Among them, six sequences are in the interval of fourth member of Quantou formation to Yaojia formation, and three ones are in Nenjiang formation. Six first flooding surfaces are recognized and twenty system tracks are divided in the belt. Offshore break belt dominated the distribution of low stand system track also.The offshore break belt dominated the deposition character of the western slop. The slop located in transitional zone between marginal facies and basin facies. Three kinds of belt can be divided based on the geometry of offshore break and level of lake, and they are: marginal offshore break belt, high stand offshore break belt and low stand offshore break belt. High stand offshore break belt dominated the boundary between river and deltaic plain, and low stand offshore break dominated the development of delta front and basin floor fan gravity sliding system.The offshore break belt dominated the development and distribution of stratigraphic over-lap reservoir. Study result of third order sequence shows that the over-lap line was parallel with low stand offshore break, and offshore break belt dominated the distribution of stratigraphic over-lap reservoir. Mudstone in second member of Nenjiang formation was resulted from the deposition of maximum flooding surface in the basin, and is regional seal rock in the belt; and the stable mudstones developed at top of first member of Yaojia formation and third member of Qingshankou formation respectively can be local seal rock for internal trap. The mudstones mentioned combine with sands developed in the western can form stratigraphic over-lap reservoir. Qijia-Gulong depression was kitchen area, and the expulsion hydrocarbon migrated for a longdistance upward and laterally through fault and delivery sands and accumulated in the western slop to form reservoir, and therefore stratigraphic over-lap reservoir is favorable to form in the area. Thus, stratigraphic over-lap reservoir will be the key to break through of future petroleum exploration in the area.