Permian Magmatism And Remagnetization Of The Hydrocarbon-Bearing Strata In Northwestern Sichuan

The northwestern Sichuan is one of the key areas of western oil and gas exploration,and the Middle Permian carbonate rock is an important exploration target.To confirm the emplacement age and tectonic setting of the NE-striking diabase dykes intruding into the Middle Permian and its lower strata in the northwest Sichuan region,as well as to reveal whether or not the diabases are associated with the Emeishan mantle plume activity,this paper tested and analyzed U-Pb baddeleyite age,major element contents and trace element abundance of the diabases sample collected from this area.On the other hand,the studying of remagnetization of the Middle Permian oil-bearing strata in northwestern Sichuan was carried out with the paleomagnetic and rack magnetic methods,in attempt to know about the remagnetization event in the area and the effect of the magmatic event on the oil-bearing strata,and to test a hypothesized connection between oil and gas accumulation and remagnetization of the Middle Permian oil-bearing strata in the northwest of Sichuan.In this paper,the samples of diabase in the area of Maliucun,Changlin and Cheijiaba in the northern segment of Longmen Shan in northwestern Sichuan were collected to carry out the chronological and geochemical analysis.And the results showed that most of the zircons from the sample in Changlin and Cheijiaba areas belongs to captured zircons instead of primary zircons,which hardly give a statistically significant age.Fortunately,however,the baddeleyites,the ideal datable mineral for basic rock,were found from the diabase sample(MLC-4)of Maluku.With the analysis of LA-ICP-MS U-Pb isotopic data,the diabase's yielded a baddeleyite concordant U-Pb age of 261.1±1.8Ma.And the major and trace element analyses revealed that the diabases in Maliucun,Changlin and Cheijiaba areas have similar geochemical characteristics,indicating that they are products from the same period magma activity.Specifically,they belong to the high-Ti(Ti/Y>500)tholeiitic series,exhibiting high Al(13.39?14.02%),high Fe(13.88?14.02%),low Mg(4.43?4.56%),enriched in the large iron lithophile elements and light rare earth elements,coupled with Nb/La(0.84?0.85),low Th/Ta(2.40?2.45),Ta/Hf>0.25,which are similar to the geochemical characters of ocean island basalt(OIB)while the slightly negative anomaly of Nb-Ta indicates that they probably derive from the enriched mantle with minor contamination by crust materials.Compared to the Emeishan basaltic rocks,both of them are consistent highly in ages and geochemical characteristics.It's considered that the diabases were the products of Emeishan mantle plume activity and the activity of Emeishan basaltic magma had spread to the northern section of Longmen Shan and the northwestern of Sichuan,as well as had an impact on the area's biotic environment and hydrocarbon accumulation.In this thesis,the Middle Permian carbonate rocks of Qixia-Maokou Formation in the Chejiaba,Zhuyuanba and Changjianggou-Jianfeng sections,in the front of a thrust belt of the Northern Longmen Shan were chose to study the remagnetization event.The result of the demagnetization showed a good consistency among the samples from the three sections,as they all have 3 magnetic components acquired after tilting,which coincides with the poles of Late Triassic(Component H1),Late Cretaceous(Component M)and Cenozoic(Component L).In addition,a low-angle high component(H2)was isolated from the Changjianggou-Jianfeng and Zhuyuanba sections,which were obtained before tilting.The component(H2)of Zhuyuanba section yielded a mean direction of Ds=226.6°,Is=9.3°,?95=12.3°,and the corresponding geomagnetic pole(32.4°N,226.2°E,?95=12.4°),while the component(H2)of Changjianggou-Jianfeng section yielded a mean direction(Ds=25.0°,Is=-18.4°,?95=9.7°)and the corresponding geomagnetic pole(4.9°N,251.3°E,?95=10.1°).Both of the poles computed from the two directions fell into the range of pole acquired from the Emeishan basalts formed in the early Late Permian,which indicated a remagnetization happened in the early Late Permian in Northwestern Sichuan.The rock magnetic data confirmed that the magnetite carry the bulk of the magnetization.Coupled with the burial-thermal history and the previous study of hydrothermal dolomite,it's believed that it is a chemical remanent magnetization in the carbonate rocks,and may be closely associated with the activity of the hydrothermal fluid related to the Emeishan Mantle plume.And combined with burial history and the blocking temperature of the magnetite which carried the magnetization,the other high component(H1),was believed to be a chemical remagnetization as well,and might have a connection with the formation and migration of oil and gas in the Middle Permian source rock in the Late Triassic.