Multi-stage Fluid Activities And Processes Of Hydrocarbon Accumulation In Sinian And Lower Palaeozoic In Southeast Part Of Sichuan Basin

The marine strata from Sinian to Lower Paleozoic in the Sichuan Basin developed multiple sets of source-reservoir-cap assemblages,which are rich in oil and gas resources and are important areas for petroleum exploration and reserves growth.The marine carbonate reservoir of Sinian Dengying Formation and Silurian Shiniulan Formation in the lower assemblage strata of typical well in the southeastern part of Sichuan Basin were studied in this thesis.The typical core samples of marine carbonate reservoirs in the second and fourth member of the Sinian Dengying Formation and the Silurian Shiniulan Formation were systematically observed and collected.Meanwhile,the core samples of the source rocks in the Qiongzhusi Formation and Wufeng-Longmaxi Formation were also collected.A series of test techniques were used,including microscope and cathodoluminescence observation,in-situ rare earth element testing,carbon,oxygen,and strontium isotope testing,fluid inclusion analysis of minerals,thermoacoustic emission analysis of source rocks,laser Raman analysis of solid asphalt,etc.The petrographic observation and diagenetic sequence division of diagenetic minerals and veins,sources and genesis of multistage paleo-fluid,microthermometry and salinity measurement of fluid inclusions in diagenetic minerals and veins,laser Raman composition analysis of fluid inclusions,calculation of paleo-pressure for trapping single phase methane inclusions,determination of maximum paleotemperature experienced by source rocks,calculation of equivalent vitrinite reflectance of solid asphalt,simulation of burial,thermal,and hydrocarbon generation history of typical wells were carried out.In this thesis,the multi-stage fluid activity and dynamic hydrocarbon accumulation process of Sinian Dengying Formation and Silurian Shiniulan Formation carbonate reservoirs were comprehensively studied.Combined with the research results of structural geology and seismic data,the influence of Yanshanian-Himalayan tectonic movements on gas reservoir preservation conditions in typical well areas was analyzed.The main conclusions of this thesis are as follows:1.Multi-stage diagenetic minerals and veins are developed in the pores and fractures of the carbonate reservoirs of lower assemblage strata in the southeastern part of Sichuan Basin,corresponding to multi-stage paleo-fluid activities.The multi-stage fluid activities of Dengying Formation reservoir mainly occurred in the Late Caledonian and Hercynian period,and the multi-stage fluid activity of Shiniulan Formation mainly occurred in the Late Hercynian to Early and Middle Yanshanian period.The diagenetic fluid activities developed from the Late Hercynian to Early Indosinian are generally influenced by deep hydrothermal fluid,which are speculated to be related to the activity of the Emei mantle plume,and the single-phase methane inclusions and the two-phase brine inclusions containing hydrocarbon in diagenetic minerals are mainly related to the fluid activities during this period.Multi-stage of dolomite and one stage of quartz are developed in the Dengying Formation in the southeast of Sichuan Basin,while two stages of dolomite are developed in the Shiniulan Formation;three stages of dolomite are developed in Dengying Formation in the southern Sichuan Basin;two stages of dolomite and one stage of quartz are developed in Dengying Formation in Central Sichuan Basin.The REE and isotope characteristics indicate that the fluids are mainly sourced from diagenetic fluids,the late dolomite is influenced by deep hydrothermal caused by Emei mantle plume activity,and some diagenetic minerals and veins are influenced by atmospheric water and other external source fresh water fluids.2.Solid asphalt and multi-stage diagenetic minerals and veins are developed in carbonate reservoirs of lower assemblage strata in typical areas of the southeastern part and central part of the Sichuan Basin.The period and process of hydrocarbon accumulation can be studied by analyzing the diagenetic sequence of the filling minerals of reservoir pores,the relationship between the occurrence of solid asphalt and diagenetic minerals,and the fluid inclusion and fluid evolution.The simulation of burial,thermal,and hydrocarbon generation history show that the source rock of the Qiongzhusi Formation in the southeastern and southern Sichuan Basin mainly have two stages: early oil generation and late oil generation.A large amount of oil generation period in the Hercynian-Indosinian,which is speculated to be related to the thermal influence of the Emei mantle plume activity.The intense oil generation period in the south Sichuan Basin is a little later than that in the southeast Sichuan Basin.The abundant oil generation period of the Qiongzhusi source rock in the central Sichuan Basin is Late Indosinian to Early Yanshanian.The abundant oil generation period of the Wufeng-Longmaxi Formation in the southeastern Sichuan Basin is mainly around Indosinian period.Solid asphalt and hydrocarbon gas are formed by thermal cracking of crude oil injected into the reservoir,and methane inclusions are captured by the dolomite formed in the late Paleozoic to Mesozoic under the influence of hydrothermal activity.The occurrence and distribution of solid asphalt indicate that the oil charging and gaseous hydrocarbon inclusions are mainly related to the dolomite fluid activity in the Late Paleozoic to the Late Mesozoic.3.The key stage of the preservation of natural gas reservoirs in the lower assemblage strata in the southeast part of Sichuan Basin is the tectonic movement period in the Middle and Late Yanshanian to the Himalayan period.The influence factors and degrees of the tectonic movement on the preservation and destruction of ancient gas reservoirs in different areas are distinct.The main preservation factor of the ancient gas reservoir in the structural trap near the big fault in the southeast Sichuan basin is the sealing property of the fault.The main structural preservation factors of typical paleo-gas reservoirs in the southern and central Sichuan Basin are the disappearance or reduction of anticlinal structural traps due to structural reconstruction and natural gas adjusted to higher structural positions.The preservation and adjustment of gas reservoirs in the southeast Sichuan Basin is mainly affected by the activity of Qiyueshan fault in the basin margin in Yanshan-Himalayan period,which makes the preservation conditions of anticlinal traps of Lin 1 well and Dingshan 1 well were destroyed.As a result,the natural gas escapes upward along the fault and damages the ancient oil and gas reservoirs in the southeast Sichuan Basin.The hydrocarbon accumulation and reconstruction in the Jinshi structural area of the southern Sichuan Basin is mainly affected by the position change of the structural high point.The tectonic high point in the Jinshi area shift to a lower position and is obviously lower than that in Weiyuan area after the Himalayan tectonic movement,which lead to the lateral migration of the natural gas in the ancient gas reservoir to the structural high point in Weiyuan area,and cause the gas reservoir in the Jinshi area to be adjusted and destroyed.The high point of the anticline shifted from Ziyang area to Weiyuan area during the Late Yanshanian and Himalayan period,and the Ziyang area became a low amplitude anticline in the north slope belt of Weiyuan anticline trap.The natural gas migrated to the high part of Weiyuan structure or partially dispersed,which resulted in the decrease of the scale,gas content,and gas pressure of Ziyang ancient gas reservoir.4.According to the paleo-pressure recovery results of primary and secondary single-phase methane inclusions captured by quartz mineral,the paleo-pressure coefficient of the 2nd Dengying gas reservoir in Ziyang gas-bearing area of the central Sichuan Basin is 1.35～1.51,which is in weak-medium overpressure state at about140 Ma.At about 40 Ma,the pressure coefficient of the paleo-gas reservoir decreased to1.09～1.22,and the paleo-gas reservoir evolved from overpressure to normal pressure.The paleo-pressure recovery of methane inclusion in Dengying Formation in the southeastern Sichuan Basin is normal pressure,and the preservation condition of gas reservoir is poor.The recovery paleo-pressure of Shiniulan Formation is overpressure,and the gas reservoir of Shiniulan Formation is more potential than that of Dengying Formation,and the destruction and adjustment of gas reservoir are mainly affected by faulting activity.The gas reservoir of Dengying Formation in the tension sag in southern Sichuan Basin is overpressure and well preserved.The recovery paleo-pressure of methane inclusion in Dengying Formation in Ziyang area of the central Sichuan Basin has changed from overpressure during burial period to normal pressure during uplift period,and the pressure evolution process indicates that the preservation conditions have been damaged to some extent.