Fluid Evolution And Hydrocarbon Accumulation Processes Of The Sinian Dengying Formation In The Central And Eastern Sichuan Basin

The Sinian in the Sichuan Basin has been explored for more than 50 years.At present,the largest marine carbonate monolithic gas field in China has been discovered in the Gaoshiti-Moxi area of the central Sichuan Basin with the main gas-producing layers including the Sinian Dengying Formation（Z₂dn）,while no industrial gas reservoir is detected in the Z₂dn reservoir of the eastern Sichuan Basin.It is of great significance for hydrocarbon exploration and development to study the differences in fluid evolution and hydrocarbon accumulation process in the Z₂dn reservoirs of the central and eastern Sichuan Basin.In this thesis,the solid bitumen and diagenetic minerals in the Z₂dn reservoirs of the central and eastern Sichuan Basin were studied,the difference of hydrocarbon generation history of Cambrian Qiongzhusi source rocks in the central and eastern Sichuan Basin was investigated,and the differences in diagenetic mineral evolution history,hydrocarbon charging history,and pressure evolution history of the Z₂dn reservoirs of the central and eastern Sichuan Basin were characterized.This thesis analyzes the differences in controlling factors of hydrocarbon accumulation,hydrocarbon generation and charging history,diagenetic fluid evolution on reservoir development and gydrocarbon accumulation,overpressure evolution on hydrocarbon accumulation and preservation,and hydrocarbon accumulation and alteration models of the central and eastern Sichuan Basin,and reveals the difference of hydrocarbon accumulation processes in the Z₂dn reservoirs of the central and eastern Sichuan Basin.Solid bitumen was found within intercrystalline pores,intercrystalline dissolution pores,karst caves,fractures,and stylolites in the central and eastern Sichuan Basin.Solid bitumen debris in the samples from the central Sichuan Basin mainly occupied intercrystalline dissolution pores,karst caves,and fractures,whereas solid bitumen debris in the samples from the eastern Sichuan Basin mainly occupied intercrystalline pores and intercrystalline dissolution pores.The loss of aliphatic CH₂ and CH₃ groups and C=O group,and enrichment of aromatic C=C and C-H groups were observed in the FTIR spectrums of solid bitumen,indicating that the vitrinite reflectance equivalent(VR_eq)values of solid bitumen in the Sichuan Basin exceed at least 1.3%.The calculated VR_eqvalues of solid bitumen inferred by Raman spectroscopy analysis are 3.51%-3.77%and3.8%-4.09%in the central and eastern Sichuan Basin,respectively.The thermal evolution history of the Qiongzhusi source rocks can be divided into two heating-cooling stages.At the end of the second heating stage,which is the Late Cretaceous,the Qiongzhusi source rocks reached the highest temperature of 225°C and 250°C in the central and eastern Sichuan Basin,respectively.The different burial and thermal history of the Qiongzhusi source rocks controls their key periods of hydrocarbon generation in the central and eastern Sichuan Basin.The intense oil generation of the Qiongzhusi source rocks occurred at 240-185 Ma and the oil cracking phase was from 185 to 90 Ma in the central Sichuan Basin.Two episodes of intense oil generation of the Qiongzhusi source rocks were determined at 440-380 Ma and 290-260 Ma,and one phase of gas generation from oil cracking was from 260 to 85 Ma in the eastern Sichuan Basin.There are multiple phases of fluid charge in the pores,caves,and fractures in the Z₂dn reservoirs of the Sichuan Basin.The Z₂dn reservoir of the central Sichuan Basin is filled with quartz I,calcite,fluorite,dolomite,bitumen,and quartz II successively,and the Z₂dn reservoirs of the eastern Sichuan Basin is filled with dolomite I,bitumen I,dolomite II,bitumen II,and quartz successively.Quartz I,calcite,fluorite,dolomite,and quartz II in the Z₂dn reservoir of the central Sichuan Basin were formed at 500 Ma,242Ma,237 Ma,222 Ma,and 178 Ma,respectively.The crystallization timings of dolomite I,dolomite II,and quartz in the Z₂dn reservoir of the eastern Sichuan Basin were 460±25 Ma,273 Ma,and 255 Ma,respectively.The formation of calcite and dolomite in the central Sichuan Basin is related to lithosphere stretching and Emei Mountain mantle plume activity from the early Permian to the Middle Triassic.The diagenetic fluid of calcite and dolomite is deep brine,which is one type of hydrothermal fluids.The continuous precipitation of calcite,fluorite,and dolomite from deep brine occupies the storage space of reservoir,reduces the porosity and permeability of reservoir,and plays a destructive role in the development of reservoir in the central Sichuan Basin.The diagenetic fluid of dolomite I in the eastern Basin is formation brine,and the crystallization of minerals in formation brine leads to the gradual decrease of reservoir porosity and densification of the reservoir.The dolomite II in the eastern Sichuan Basin is derived from deep brine,and the deep brine has a similar effect on reservoir reconstructioin as that in the central Sichuan Basin.The charging of deep brine into the Dengying reservoir was accompanied by the charging of crude oil,and the crystallization of hydrothermal minerals（calcite,fluorite,and dolomite in the central Sichuan Basin and dolomite II in the eastern Sichuan Basin）recorded the process of oil charging.The oil charge timing of the Z₂dn reservoir in the central Sichuan Basin was 220-197 Ma.The massive charging of crude oil not only further dissolved the dolomite reservoir and increased the storage space,but also significantly inhibited the precipitation of carbonate minerals in the later period,so that the pore space of reservoir was well preserved.The oil charge timings of the Z₂dn reservoir in the eastern Sichuan Basin could be approximated as 440-380 Ma and 273-260 Ma.The amount of oil charge in the first phase was low,and the oil charge could not inhibit the precipitation and crystallization of carbonate minerals in the Z₂dn reservoir in the eastern Sichuan Basin,and had a weak effect on improving the physical properties of the reservoir.The natural gas in the Z₂dn reservoir of the Sichuan Basin is generated by in-situ cracking of oil.The gas charge timings of the Z₂dn reservoir in the central Sichuan Basin was 175-92 Ma and 191-183Ma,respectively.Pore fluid pressure evolution of the Z₂dn reservoir of the central Sichuan Basin experiences normal pressure stage（200-155 Ma）,overpressure development stage（155-90 Ma）,and overpressure release stage（90-0 Ma）.The maximum pore fluid pressure and its corresponding pressure coefficient of the Z₂dn reservoir of the central Sichuan Basin is approximately 141.4 MPa and 1.95,respectively.The overpressure development stage reflects the processes of oil cracking and gas accumulation,and the overpressure release stage reflects the dissipation of some natural gas in the Z₂dn reservoir of the central Sichuan Basin.Pore fluid pressure of the Z₂dn reservoir of Well Jiaoshi1 in the eastern Sichuan Basin maintained at normal pressure since 200 Ma,which indicates that the gas reservoir was small during oil cracking stage and natural gas was completely lost due to tectonic uplift.The Z₂dn reservoir of the central Sichuan Basin has good physical property,and the Cambrian Qiongzhusi source rocks are of high quality.The Qiongzhusi source rocks in the Mianyang-Changning extensional trough are located at the structural low,and the Z₂dn reservoir of the Gaoshiti-Moxi paleo-uplift are located at the structural high.Hydrocarbon generated by high-quality Qiongzhusi source rocks located in the Mianyang-Changning extensional trough could accumulate in the Z₂dn reservoir of the Gaoshiti-Moxi paleo-uplift through lateral migration and vertical migration.Affected by the Yanshanian and Himalayan tectonic movements from the Late Cretaceous to the present,the Jurassic strata of the central Sichuan Basin was uplifted and eroded,the oil reservoir of the Z₂dn reservoir of the central Sichuan Basin was destroyed,and approximately 37.5%of the methane in reservoir has leaked.The hydrocarbon accumulation and alteration model of the Z₂dn reservoir in the central Sichuan Basin is the model of“late accumulation and late alteration”.The physical property of the Z₂dn reservoir in the eastern Sichuan Basin is poor,and the quality of the Qiongzhusi source rocks in the eastern Sichuan Basin is generally poor.The Caledonian movement caused the strata of Well Jiaoshi1 to tilt,and hydrocarbon generated from the Qiongzhusi source rocks located at the structural low could migrate to the Z₂dn reservoir located at the structural high through lateral migration.Due to poor quality of source rocks and reservoir physical property and absence of large trap,crude oil has not been charged into the Z₂dn reservoir in the eastern Sichuan Basin on a large scale.During the Yanshanian and Himalayan tectonic movements,Triassic strata in Well Jiaoshi1 of the eastern Sichuan Basin was uplifted and being eroded,and the natural gas in the Z₂dn reservoir of the eastern Sichuan Basin was completly leaeked.The hydrocarbon accumulation and alteration model of the Z₂dn reservoir in the eastern Sichuan Basin is the model of“early accumulation and late alteration”.