Structural Evolution Of Southeast Uplift In Songliao Basin And Its Control On Natural Gas Pool Formation

Southeast uplift, located in the southeast of Songliao Basin, is an extremely special and important structure unit. According to the evolution of petroleum resources and comprehensive research, Southeast uplift has abundant hydrocarbon reserves. The reserve of petroleum is 5.76×l08t,natural gas is 3956×l08m3.However, the degree of prospecting in Southeast uplift is at lower level .This area has more potential hydrocarbon to explore. Therefore, Southeast uplift is an important petroleum producing area of Jilin oil-field in future. According to the present formation conditions and distribution rule of natural gas pool, its distribution and formation process are obviously controlled by structural evolution. Southeast uplift is one of key controlling factors for natural gas pool accumulation in this area. Therefore, it is undoubtedly of great significance to exploration of natural gas that to carry out study of structural evolution and its control on natural gas pool formation.This paper, by means of collecting predecessors' research data comprehensively, carried out the summary study on regional tectonic of Southeast uplift under the background of regional tectonic analysis of Songliao basin. Southeast uplift and its structural evolution is controlled strictly by the basement fractures and multiple phase tectonic movement. Basement fracture system is well developed in this area, with many structural types in the tensional, transtensional and transpressional stress field, etc., provided important conditions for natural gas migration and accumulation.Cretaceous is mainly developed stratigraphy in Southeast uplift. Because of the denudation made by regional uplift in the end Nenjiang Formation, there are great difference about preservation and remaining thickness of Qingshankou Formation(Klqn), Yaojia Formation ( Kly ) and Nenjiang Formation(Kln)in different place. These formations are denuded seriously in the south of Southeast uplift, and becoming better with thickness and stability toward the north. Upper Crataceous, Paleogene and Neocene do not develop in this area. Quaternary distributes extensively, but the thickness is thin.The study of Southeast uplift structural evolution is the emphasis of this paper. Firstly, the study of structural type is carried out in the study area. The structural types are divided into rift period structural type, depression period structural type and reversion period structural type. Faults of controlling basin are mainly developing during the rift period, with"plough"structural type, isodirectional hemihorst-hemigraben and off-type opposite directional hemihorst-hemigraben as its main combination structural type. Small-scale positive faults are main structural type during the depression period. Positive reversed structure is mainly developing during the reversion period, with upper"reversed"structure-lower"normal"structure as its fault-profile characteristic.Secondly, denuded-thickness of Yingcheng Formation and Nenjiang Formation has been calculated according to the principles and methods of basin modelling in study area. That of Yingcheng is among 350-550 metres and Nenjiang 400-740 meters. The result demonstrates that structural movement at the end of the deposition of Nenjiang Formation is stronger than that of Yingcheng Formation, and the former plays main control action on natural gas pool in the study area. The study area was seriously denuded as the result of regional uplift at the end of the deposition of Yingcheng Formation. Denuded-thickness of He-5 borefild is about 546 meters, Deshen-5 about 353 meters and Wan-22 about 487 meters. The result demonstrates that Southeast uplift is strong in the south, weak in north and worst in the centre of rift-subsidence.At last, the area of interest is regarded as the result of six evolution periods, including beginning chasmic period, rift period, sag period, depression period, reversion period and atrophic structure period. According to the analysis of sedimentary deposit, Huoshiling Formation is deposited with a set of basic volcanic rock and pyroclastic rock, belonging to the beginning chasmic period. From Late Crataceous to Neogene, Songliao Basin uplifted in the east and subsided in the west entirely, with lake basin and depocencer moving to the west. As a result, the majority of the study area loses strata of Late Crataceous to Neogene. At Early Quaternary, the entire low-amplitude subsidence of the basin cause the study area deposited with thin Quaternary strata.This paper, based on the study of structural feature, with natural gas pool formation geologic conditions and its distribution rule in the study area and test data of fluid inclusion homogenization temperature, carried out the study of structural evolution of Southeast uplift and its control on natural gas pool formation.Dark mudstone in the deep formation of Shahezi Formation and Yingcheng Formation is the main hydrocarbon rock of Southeast uplift. Mudstone of each interval is the main cap rock in this area .Mudstone of Qingshankou Formation distributes stably on plane. It can be a good regional cap formation.Deep rift basin controls natural gas distribution on plane in the study area, natural gas pools that have already been found are all in the rift basin. Longitudinally, there are many sets of hydrocarbon combination. Their distribution intervals are fourth section of Quantou Formation, third section of Quantou Formation, second section of Quantou Formation, first section of Quantou Formation, Denglouku Formation, Yingcheng Formation, Shahezi Formation and basement rock. Among them, hydrocarbon combination of first section of Quantou Formation is main combination in the study area. It exits in the all conformations except Changchunling structural zone, presenting regional hydrocarbon bearing feature.According to the study of feature of structural evolution and the major controlling factors for natural gas formation, and with experimental data of fluid inclusion, natural gas in the study area shows three periods of migration and accumulation. And Structural movement is the key factor for generation, accumulation and preservation of natural gas. The first period is from the end of the deposition of Denglouku Formation to the early of the deposition of Quantou Formation. The second period is from the end of the deposition of Quantou Formation to the end of the deposition of Qingshankou Formation. The third period is the deposition of Nenjiang Formation. The second and the third periods are main ones of generation and accumulation of natural gas.