The Effect Of Gas Invasion On The Molecular Composition Of Crude Oil In Lungu Area Of The Tarim Basin

With the development of exploration and development to deep and ultra-deep layers,it is very difficult to determine complex fluid phase state,fluid composition,degree of secondary alteration of deep reservoir.There are many types of fluid state in the Lungu area of the Tarim basin,such as condensate gas,transitional volatile oil and normal oil r eservoirs from east to west.Through the study of multiphase reservoirs in Lungu area,a series of effective indexes can be established to evaluate the facies and secondary altera tion degree of ultra-deep reservoirs,so as to solve the key problems of exploration and development of ultra-deep reservoirs.The organic geochemical analysis of different types of reservoirs has been carried out by means of comprehensive two-dimensional gas chro matography/time-of-flight mass spectrometry?GC×GC-TOFMS?.The changes of crude oil in different state of fluid have been studied,and the transformation process of gas in vasion on reservoir phases has been reconstructed.The study found that Ordovician reservoir in Lungu area has experienced three stages charging.The first stage occurred in the late Caledonian period,which was destroyed soon after the hydrocarbons charged into the reservoirs.The second stage occurred in the late Hercynian,which resulted in a large number of normal oil reservoirs in Lungu area.The third stage occurred in the late Himalayan period?12² Ma?,in which a large-scale gas invasion occurred in Lungu area from east to west,and the gas altered the reservoirs formed in the late Hercynian period,forming condensate?strong gas invasion?,volatile oil?weak gas invasion?and normal oil reservoir?no gas invasion?.Along the direction of gas invasion,the adamantanes content in crude oil decreased,while the biomarkers such as steranes and terpanes,increased and the loss of n-alkanes decreased.Based on the systematic analysis of the changes in the molecular composition of crude oil in different phase reservoirs,a chart for predicting the phase state of ultra-deep reservoirs is established according to the changes in the content of adamantane and ethaneadamantane compounds.By analyzing the differences of molecular composition of crude oil with different maturity,a new parameter EI?EI=EA-3/?EA-2+EA-3+EA-4??was proposed to evaluate the maturity of crude oil,and it is confirmed that there is a good correlation between EI and the original maturity parameters.By analyzing the differences of molecular composition of crude oil with different gas invasion intensities,the light/weight carbon ratio(?nC_21-/?nC₂₂₊)was applied to evaluate the loss of n-alkanes,and parameter dA?dA=total adamantanes of crude oil-regional adamantanes value?/regional adamantanes value)to evaluate adamananes increase,and parameter GI=dA/(?nC_21-/?nC₂₂₊)to evaluate gas invasion intensities.The establishment of these parameters provides a scientific basis for the exploration and resource prediction of ultra-deep resources in Lungu area.