Characteristics And Significance Of The Fluid Inclusion Of Ordovician Reservoir In South Part Of Northern Tarim Basin

Tarim Basin is a large superimposed sedimentary basin which is developed on the stable basement of pre-Sinian continental crust after a series of complex tectonic movement. Northern uplift is one of the most oil-riched areas of Tarim Basin and marine carbonate reservoirs mainly developed in the south part. From west to east, tectonic units are Yingmaili low-uplift, Halahatang depression and Lunnan low-uplift respectively. Yijianfang formation in middle Ordovician and Yingshan formation in lower-middle Ordovician are the main hydrocarbon reservoir in this study area.25 wells in Ordovician of 5 well fields are involved, and they are Yingmai-2, Yingmai-1, Xinken, Halahatang and Repu well field from west to east. Based on direct core observation of intercalated relationship among holes, seams and veins, through microscopic identification of mineral growth relationship in thin section, combined with fluorescence characteristics of hydrocarbon inclusions, it can be concluded that there are three stages of hydrocarbon inclusions in study area. The first stage of hydrocarbon inclusions are light to dark brown gas-liquid two-phase and gas or liquid phase which exhibit brown fluorescence under the UV-light. The second stage of hydrocarbon inclusions are light brown to colorless gas-liquid two-phase with strong yellow fluorescence under the UV-light. The third stage of hydrocarbon inclusions are colorless which are characterized by strong blue or blue white fluorescence under the UV-light. Temperature and salinity analysis of the associated brine inclusions show that three stages of hydrocarbon inclusions formed in the environment that salinity decreased with temperature increased.According to fluorescence characteristics analysis of hydrocarbon inclusions, the result indicates that three stages of hydrocarbon inclusions represent three stages of hydrocarbon accumulation. The first stage is highly matured heavy oil, which is mainly accumulated in north part of Yingmai-2, Yingmai-1 and Xinken well field. The second stage is matured normal oil widely distributed throughout the study area. The third stage is highly matured light oil and gas, which is mainly accumulated in south part of Yingmai-2 and Repu well field.Analysis on age of veins contained hydrocarbon inclusions and source of oil and gas proved that the first stage of hydrocarbon was from Cambrian-Lower Ordovician source rock, while the second and third were from Middle-Upper Ordovician source rock. They were accumulated during Late Caledonian-Early Hercynian, Late Hercynian and Late Yanshanian-Himalayan period respectively.In conclusion, there are three stages of hydrocarbon accumulation in south part of northern Tarim Basin. Southern part was dominated by the first stage of heavy oil which had been mostly degraded. The second stage of normal oil was accumulated widely in study area. The third stage of light oil or gas condensate was mainly accumulated in south part where is considered to be the significant exploration target in northern Tarim Basin.