| The Sinian-Cambrian oil and gas exploration in the Sichuan Basin begins at 1950 s or 1960 s,and the Sinian and Cambrian formations have been the critical exploration layers among the lower units since 50 years ago,until now,Weiyuan gas field,Anyue gas field,Ziyang gas pool,Anpingdian and Longnvsi gas-bearing structures have been found consecutively.Sichuan basin,as a typical superimposed basin,it is characterized by oil and rich gas,and it has went through multiple stage of tectonic movement,basin formation,hydrocarbon generation and oil & gas accumulation,especially for the Sinian and Cambrian formations.This thesis is based on petrology of the Sinian Dengying Formation and the Cambrian Longwangmiao Formation in Gaoshit-Moxi and Weiyuan-Ziyang area,by laser Raman analysis of fluid inclusions and Sr,C,O isotope analysis of minerals filled in the pores,vugs or fractures,combined with burial and thermal evolution history of the study area,the fluid pressure,fluid system,oil & gas filling and accumulation processes of the Sinian Dengying Formation and the Cambrian Longwangmiao Formation have been restored,also,the hydrocarbon accumulation mode of the Sinian and Cambrian formations has been built,at last,to reveal the oil & gas enrichment patterns of the Sinian and Cambrian in the central Sichuan Basin.The main achievements of this thesis is as follows:(1)Different kinds of filling minerals of the Sinian Dengying Formation and the Cambrian Longwangmiao Formation in the Gaoshiti-Moxi structures represents first stage brine water filling,first stage crude oil filling,second stage brine water filling,second stage oil & gas filling and third stage natural gas filling,respectively.And the evolution process and filling sequence of the accumulation fluid has been recorded by the sequence of filling minerals.(2)The Sr,C,O isotope analysis of the dolomite,calcite and quartz in the pore space of the Sinian Dengying Formation and the Cambrian Longwangmiao Formation indicate that in the period of the Qiongzhushi Formation oil generation window,the Sinian Dengying Formation and the Cambrian Longwangmiao Formation are in one connected and open fluid system,hydrocarbon-bearing and Sr-rich formation fluid laterally traveled from the Sinian Dengying Formation to the Cambrian Longwangmiao Formation along the paleo weathering crust between them.However,after the gas generation window,according to the isotope results,the Cambrian Longwangmiao Formation is in an isolated fluid system,the fluid inside could not travel to the underlying Sinian formations nor the overlying Permian formations.(3)By the laser Raman spectrum analysis of the gas hydrocarbon fluid inclusions in the pore space of the target strata,combined with series of geochemical methods,the evolution process of fluid pressure during different stage of source rock evolution has been recovered,specifically,pressure coefficient of the Sinian Dengying Formation is changed as the sequence of 1.0?1.1~1.2?1.2?1.2~1.3?1.0~1.1,pressure coefficient of the Cambrian Longwangmiao Formation is changed as the sequence of 1.0?2.1~2.2?1.8~1.9?1.6~1.8?1.55~1.75.Results shows that over high pressure has never occurred in the geologic history of the Sinian Dengying Formation,but overpressure kept occurred in the geologic history of the Cambrian Longwangmiao Formation.(4)Since ancient times,the oil & gas accumulation of the Sinian and Cambrian formations in the Gaoshiti-Moxi and Weiyuan-Ziyang areas has went through stage of formation uplift and erosion after the Sinian deposition;stage of formation of the Mianyang-Changning intracratonic sag after Cambrian deposition;stage of paleo oil pools formation after Triassic deposition;stage of early-stage paleo gas pools formation after Jurassic deposition;stage of paleo oil pools all transformed into gas pools after cretaceous deposition and stage of gas pools adjusting and locally rich at present.The lower Cambrian Qiongzhushi Formation is the main source rock of the Sinian Dengying Formation,together with the hydrocarbon generation from the 3rd Dengying Formation,the Sinian Dengying Formation in the Gaoshiti-Moxi area has a high degree of oil & gas accumulation,and the relationship between source rock and reservoir is called ?side generating and laterally accumulating?,oil & gas generate from the Qiongzhusi Formation laterally migrate from the Sinian Dengying Formation to the Cambrian Longwangmiao Formation along the paleo weathering crust between them.The Qiongzhushi Formation is also the main source rock of the Cambrian Longwangmiao Formation.Vertically,the Longwangmiao Formation is near the source rock,and horizontally,the Cambrian Longwangmiao Formation in the Gaoshiti-Moxi area is close to the area with high gas generation intensity.Thus,the Cambrian Longwangmiao Formation in the Gaoshiti-Moxi area has a sufficient supply of oil & gas.(5)The measured formation pressure data at present and lithology association in the central Sichuan indicate that the Sinian Dengying Formation and Cambrian Longwangmiao Formation has a effective multiple stage of overpressure and lithology sealing system,the overlying formations with poor physical properties and high pressure has become the ?double seal? formation group towards the Sinian and Cambrian gas pools in the Gaoshiti-Moxi area,thus,the great Sinian and Cambrian gas field reserved until now.However,strong tectonic uplift and erosion disabled the overlyingoverpressure and lithology sealing system,only residual gas reservoirs reserved in the Weiyuan-Ziyang area.(6)Structurally,the Sinian and Cambrian oil & gas accumulation in the Sichuan Basin mainly influenced by tectonic location of intracratonic sag and generation and evolution of paleo uplift.The shaping process of the Mianyang-Changning intracratonic sag controls the generation of the Dengying Formation karst reservoir and the Longwangmiao Formation grain shoal reservoir.Moreover,the intracratonic sag also controls the development of the Qiongzhushi Formation source rock.The generation and axis migration of Leshan-Longnvsi paleo uplift caused the damage of paleo traps and appearance of new traps,thus influences the evolution and adjusting process of the Sinian and Cambrian gas pools.While the preservation and enrichment of present oil & gas pools mainly controlled by the multiple sealing capability of overlying strata and developing degree of paleokarst and shoal facies reservoir.The multiple overpressure and lithology sealing ability determines the reserving and accumulation scale of oil & gas.Generally speaking,natural gas mainly accumulate in the karst and shoal reservoir area with multiple overpressure and lithology sealing system on both sides of the intracratonic sag. |
PDF Full Text Request