| Quantitatively constraining key temporal aspect of petroleum evolution,for example the timing of oil and gas generation,migration and the accumulation,are significant to our understanding of hydrocarbon evolution,as well as enhancing the success of exploration programs.This is particularly the case for Proterozoic – Palaeozoic petroleum systems affected by complex tectonic events.Various methods,e.g.,geological analysis,basin modelling coupled with hydrocarbon fluid inclusion analysis and isotope analysis,which range from being qualitative to quantitative,macro to micro and indirect to direct,have tried to resolve the issue of absolute chronology within petroleum systemss.However,the problem on how to precisely constrain the timing of hydrocarbon(oil and gas)evolution is still a fundamental research question to both academia and industry.Basins from both from South China Block and Northwest China are petroliferous,hosting estimated hydrocarbon researves equal to several billion barrels of oil.In the past decade,many new giant oil and gas fields,e.g.,Puguang and Anyue gas fields in the Sichuan Basin,Halahatang and Fuyuan oilfileds from Tarim Baisn have been discovered,which further confirmed the great hydrocarbon exploration potential in China.However,compared with petroleum systems worldwide,the reservoirs both from South and Northwest China have experienced multiple overprinting tectonic events since the Paleozoic.The series of Mesozoic-Cenozoic tectonic events(Indosinian,Yanshan and Himalayan events)controlled by the Pacific and Indian plate were strong enough to alter or even destroy the Paleozoic hydrocarbon reservoirs.As stated above,both the long duration of petroleum evolution coupled with the multiple overprinting complex tectonic events in China has hampered hydrocarbon exploration.In order to address the chronology of petroleum evolution process and the relationship between hydrocarbon evolution and tectonism,this thesis select one newly found Halahatang oilfield in Tarim Basin and three typical reservoirs(Kuangshanliang,Majiang-Wanshan and Micang Shan reservoir)from South China Block as the research targets.Based on traditional widely accepted acknowledge of the petroleum evolution,this thesis presents oil and bitumen Re-Os analysis,oil and bitumen organic geochemistry analysis,oil-bearing reservoir illite K-Ar isotope dating,fluid inclusion analysis,Apatite Fission Track(AFT)analysis on the reservoir related structural belts,as well organic geochemisty(eg: Bitumen reflectance,BRo,Bitumen fluorescence,oil physical properties analysis).The synopsis of the data and implications are briefly summarized below:Chapter 2: Combining organic geochemistry(GC-MS),reservoir illite Potassium-Argon(K-Ar)dating,fluid inclusion data and oil Rhenium-Osmium(Re-Os)isotope analysis,the petroleum evolution of the newly discovered Halahatang oilfield,northern Tarim Baisn,China was quantitatively constrained.Similar biomarker characterisitics of the oils indicate they belong to same oil family(derived from anoxic marine environment)and are of low maturity and biodegraded.The isotope analysis of the oil Re-Os(ca.285 Ma)and reservoir illite K-Ar dating(ca.280-230 Ma)which coincide well with traditional basin models coupled with fluid inclusion data indicate that oil generation occurred during the Early Permian and hydrocarbon migration and accumulation occurred during the Late Permian to Early Triassic.Chapter 3: Integrating traditional organic geochemisty analysis(GC-MS)and bitumen rhenium-osmium(Re-Os)geochronology,multiple hydrocarbon generation events under complex tectonic settings of South China were identified.Biomarker analysis of the bitumen and oil seeps from Kuangshanliang areas show all the bitumen either from dykes or faults/fractures possess similar features and propose a Precambrian-Early Cambrian source.In contrast,present day seeps show greatly different characteristics and suggest a Permain source.The Re-Os data of the bitumen define two trends,which define a Re-Os date of ca.486 Ma(Dyke 1 and 3 bitumen)and ca.165 Ma(Dyke 2 and fault/fracture bitumen),respectively.The Re-Os data of the oil seeps are too simlar to yield a meaningful age.The older Re-Os bitumen age agrees well with previous basin burial and source rock maturation models,which show that the Neoproterozoic – Early Cambrian source units originally entered the oil window during the Ordovician;the younger Re-Os bitumen concides well with timing of the Yanshan Orogeny when Neoproterozoic – Early Cambrian shales re-entered into the oil window during the Jurassic.In summary,two key phases of oil generation during the Ordovician and Jurassic were identified which quantitatively constrain the petroleum evolution in the Kuangshangliang bitumen reservoir,Northern Longmen Shan Thrust Belt.Chapter 4.Combining Re-Os isotope analysis on both low and high maturity bitumen and Apatite Fission Track(AFT)analysis on sandstones in the Xuefeng Uplift,the entire oil and gas evolution,as well as their relationship with tectonism was constrained.Timing of initial oil generation is recorded by low maturity bitumen,which yields a Re-Os date of ca.430 Ma and consistent with previous basin models and ca.405 Ma bitumen Rb-Sr date.The AFT data yield considerably younger dates that constrain the tectonic evolution of the Yanshan Orogeny from the northwest(ca.150 Ma)to southeast(ca.70 Ma)in the Xuefeng Uplift.The youngest AFT date coincides with Re-Os dates(ca.70 Ma)of the widely distributed high maturity bitumen in Majiang-Wanshan Reservoir.This Late Cretaceous date agrees with basin modeling,fluid inclusion,and oil composition numerical models that indicate the cessation timing of dry gas generation.In this study,it is shown that for the first time the Re-Os dating on different types bitumen may yield different geological age information,with low maturity bitumen constraining the timing of the initial oil generation age,and highly mature bitumen(pyrobitumen)define the timing of thermal cracking and gas generation.Chapter 5.To evaluate hydrocarbon evolution and its relationship with tectonic events in the Micang Shan Uplift,Northernmost Sichuan Basin,bitumen geochemistry(vitrinite reflectance and fluorescence)and rhenium-osmium(Re-Os)isotope analyses,plus apatite fission track(AFT)analysis were used.High BRo value(3.25-4.08),High Tmax ~540 oC,no fluorescence as well as insolubility in chloroform indicates that the solid bitumen in Micang Shan area is high maturity pyrobitumen that formed by thermal cracking together with dry gas generation.The Re-Os isotope analysis of the pyrobitumen yield a ca.184 Ma age which coincides well with previous basin modeling and high gas inclusion homogenization temperatures(>160oC)during Jurassic.However,the AFT thermal history modelling results indicate major tectonic uplift occurred between ca.140 and ca.60 Ma after the thermal cracking,which caused erosion of the gas reservoirs and surface exposure of bitumen-bearing Neoproterozoic strata in the Micang Shan Uplift.The pyrobitumen Re-Os dating and AFT analysis in this work quantitatively constrain the process of gas reservoir formation and followed erosion in the Micang Shan Uplift,Northern Sichuan Basin.In summary,through comprehensive analysis of Re-Os analysis on different types of hydrocarbons(crude oil,bitumen and pyrobitumen)formed at distinct stages of petroleum evolution,this thesis not only further confirmed previous research outcomes that Re-Os dating on the oil and low maturity bitumen record the initial timing of oil generation,but also for the first time put forward that Re-Os dating on the high maturity bitumen(pyrobitumen)present timing related to dry gas generation.As bitumen and pyrobitumen occurs in basins worldwide(eg: Alberta Basin(Canada),Dahoney basin(Nigeria)and Basque-Cantabrian Basin(Spain)),Re-Os geochronology coupled with other organic geochemistry,AFT,K-Ar chronology and fluid inclusion analysis show great value to understand both the temporal and spatial evolution of hydrocarbon systems worldwide. |
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