Geochemical Study On Hydrocarbon Accumulation Of The Cambrian Formation In The Middle Cambrian Of The Middle Sichuan

The paleo-uplift in central SichuanBasin is an inherited large paleo-uplift suffered erosion during Caledonian period.The high position at its eastern part is favorable for natural gas enrichment,and the Gaoshiti-Moxi structure is just situated in this part.As of the end of 2013,the Cambrian Longwangmiao Formation in the Gaoshiti-Moxi structure had proved natural gas geological reserves of 4404×10⁸ m³.The Longwangmiao Formation mainly consists of crystalline dolomite,residual dolarenite,and residual oolitic dolomite etc,andhas intergranular and intercrystal pores,also including some intergranular and intercrystal dissolved pores,intragranular dissolved pores,moldic pores,biological burrows/boring pores and dissolved fractures as reservoir space.According to observation of cores and thin slices,it is concluded that the generation relationship of the filling minerals in pores,caves and fractures in the Longwangmiao Formation in the Gaoshiti-Moxi structure is: crystalline dolomite ? asphalt ? quartz.The strontium,carbon and oxygen isotopes in the surrounding rock,and in the dolomite filling pores and caves in the Longwangmiao Formation have been analyzed,and the differences between them have been compared.It is found that the ⁸⁷Sr/⁸⁶Sr ratio of the dolomite filling pores and the surrounding rock are both much higher than that of the synchronous Cambrian sea water,while the ⁸⁷Sr/⁸⁶Sr ratio of dolomite filling pores is similar to that of the surrounding rock,indicating strontium-rich fluid has apparently reformed the surrounding rock,and caused precipitation of crystalline dolomite in pores and caves.The isotopes of ?¹³C and ?¹⁸O in the dolomite filling the geodes are similar to that in the surrounding rock,but the ?¹⁸O in the dolomite filling the geodes is more negative than that in the surrounding rock,which means that the dolomitefilling geodes might be deposited in relatively higher temperature.The silicon isotope,oxygen isotope and hydrogen isotope in inclusions in the quartz filling pores and caves were also analyzed.According to the study results,the fluid forming the quartz filling pores and caves in the Longwangmiao reservoir of the Gaoshiti-Moxi structure was the diagenetic liquid,having nothing to do with magmatic water and metamorphic water.⁸⁷Sr/⁸⁶Sr ratios of the quartz minerals show that the silicic formation water giving rise to quartz is allochthonous strontium-rich fluid;combining with the features of hydrogen-oxygen isotopes,it is concluded that such strontium-rich silicic fluid might be the formation water evolved from initial meteoric water or the mixture with sea water.Taking the mineral sequence filling the pores and caves as the main line,by analyzing petrography,laser Raman and homogenization temperature etc.of the inclusions,it is considered that the Longwangmiao Formation in the Gaoshiti-Moxi structure in central SichuanBasin had at least four periods of hydrocarbon charging.The features of the fluid inclusions in various periods reveal the evolution process of the reservoir fluid from liquid hydrocarbon?ancient oil reservoir?to liquid hydrocarbon + gaseous hydrocarbon?ancient oil and gas reservoir?to natural gas cracked from oil?ancient oil and gas reservoir?and to gaseous hydrocarbon?gas reservoir?.The oil and gas charging periods have been confirmed according to homogenization temperatures of the inclusions and the burial-thermal evolution history of the Longwangmiao Formation.On the basis of dating analysis of ⁴⁰Ar-³⁹Ar in quartz inclusions,the natural gas accumulation time of the Longwangmiao Formation has beenidentifiedat 125.80±8.20 Ma.By using the geochemical data of strontium isotope and current formation pressure data,the fluid system in the Longwangmiao Formation has been established,which shows that the fluid presentedsigns of cross-formation flowing at the end of natural gas accumulation in the Longwangmiao Formation,and the gas reservoir has superior preservation conditions.