Research On The Relationship Between Abnormal Pressure And Petroleum Accumulation In Yanchang Formation, South Of Ordos Basin

The previous exploration results show that the Triassic system in South Ordos Basin has rich oil and gas resources. The previous researches are more concentrated on the research of basic conditions for the accumulation and regularity, however, there is less research about relationship between abnormal pressure characters and hydrocarbon accumulation. Based on previous research, using the latest theory and analysis method such as petroleum geology, accumulation dynamics theory, pressure compartment and petroleum system. Through analyzing the characteristic of abnormal pressure of development(structure) and its major factors, to explore the relationship with hydrocarbon accumulation. The main results achieved and understanding are as follows:(1)There are three types of abnormal pressures structure characters. The abnormal pressure structure controls the main fluid discharging direction. Based on the premise that Chang7 member is the main source rock origin and hydrocarbon migration direction is primarily vertical, the major exploration prospects indifferent types of abnormal pressure structure areas are identified.TypeⅠ: It is represented by Zhenjing and Binchang areas. There are two pressure cycles. Under compaction begins to occur in Zhiluo formation. In Zhenjing area, the under compaction reaches the maximum in bottom of Yan’an formation(in Binchang area, it’s in bottom of Chang 4+5 formation and middle-upper part of Chang 6),and the nit goes down downward. The second cycle reachesthemaximum in Chang7 bottom, and decrease upward to Chang6 member, and goes down dramatically downward to Chang8 member. Oil can migrate upward and downward, as the overpressure gradient is greater downward, the migration to Chang8 member is more easier. TypeⅠbelongs to the target stratum for Chang8 member.Type Ⅱ: It is represented by the southeast corner of Zhenjing areas. The difference from TypeⅠis that it is affected by Indosinian movement from which the top of Chang6-Chang7 member was denuded and it resulted in the disappearance of maximum overpressure in the Chang 7 bottom of the second cycle. The overpressure from Chang7 member to Chang 9 member increase continuously. As the overpressure in Chang 8 member is higher than Chang 7, the oil generated from Chang 7 source rock is difficult to migrate into Chang 8 member, and the main migration direction is from Chang 7 member and its upper formation. TypeⅡbelongs to the target stratum for Chang 6 member and Yan’an formation.Type Ⅲ:It is represented by Xunyi area. In this area, there is only one pressure cycle(the second one) existed. The under compaction begins to occur in the bottom of Chang4+5 member. and the maximum of overpressure occurs in the bottom of Chang7 member(some are in the top of Chang8 horizons).The pressure gradient decreases downward and the hydrocarbon primarily migrates upward to the shallow formation, especially the horizons above Chang4+5 member. TypeⅢ belongs to the target stratum for Chang3 member in normal pressure zone.(2)The abnormal pressure in Mesozoic group in South Ordos Basin are controlled by tectonic uplift and sedimentation. These deposits(sedimentary rate and ratio between sandstone and stratum thickness) controls the formation of abnormal pressure and tectonic uplift can be regarded as the result of the pressure upthrusted. The results in the lager structural uplifts, the output becomes greater(the initial horizon of uncompaction is moving down), which is favorable for oil migration towards shallow horizons. As Binchang area and south Xunyi area in Weibei uplift have the conditions for pressure sealing, the exploration potential still exists. There are not much faults in research area and the displacement of fault is minor and the decompression is not obvious, which have little effect on the abnormal pressure.(3)The distribution features of abnormal pressure show that the main migration direction is vertical and the high abnormal pressure(source-reservoir pressure differential) is the main accumulation dynamics in Mesozoic group in South Ordos Basin.(4)Through the study on the relationship between the abnormal pressure structure and configuration of sand and shale, and according to the principle of abnormal pressure compartment, the migration and accumulation mode of compartment in Zhenjing and Binchang area and the migration and accumulation mode in normal pressure formation of the edge of compartment in Xunyi area are built.(5)The main factor that controls the accumulation in Chang3 member in Xunyi area is the tectonics uplift. The uplift results in the release of vertical abnormal pressure and it is favorable for the oil migration towards the shallow formation. The occurrence of Chang7 source rock in Xunyi area and the uplift towards east and south are beneficial for oil accumulation in Chang3 member. Although the formations towards south and east shallow, and there is no sealed for abnormal pressure(the vertical pressure release results in the normal pressure), the flat formations, the short oil column and the limited buoyancy make the petrophysical seal feasible(high pressure sealing is not required).(6)The oil enrichment regularity of Yanchang formation in South Ordos Basin is mainly controlled by source rock characteristics, the inside structure of the sealed box, regional sealing conditions, migration dynamics and migration pathway system. The source rock controls the characteristics of the reservoir distribution scale; the regional seal rock controls that the oil and gas can only be in sealed box; the inside structure of pressure compartment affect the oi-produing sections and oil content; migrate dynamics affect the oil enrichment; and migration pathway system control the petroleum accumulation. According to the main controling factors. The favorable area is predicted, and oil and gas accumulation mode of pressure compartment in Yanchang formation in South Ordos Basin is built.