Application Of Molecular Geochemical Proxies To Monitor The Production Effect Of Heavy Oil In-situ Combustion In The Western Depression, Liaohe Basin

Block Du66 belongs to Liaohe Oilfield,which is located in the west slope of Western Depression,Liaohe Basin.The production horizon Dujiatai oil layer is the fourth member of Shahejie formation?Es₄?,and produced oil is mainly heavy oil.At present,the main oil recovery method is in situ combustion?ISC?.In this paper,a series of samples from a core well are systematicly analyzed,and the vertical variation characteristics of compounds are described.Oil sand residues in different zones?burned zone,combustion front,coking zone,condensation zone,oil wall and cold oil zone?from 1-D tube physical simulation experiment and oxidized oil samples at 200?and 400?are collected to comprehensively analyze the characteristics during ISC process at the level of molecular geochemistry,and several geochemical proxies are proposed to monitor ISC process.Finally,the core samples from ISC test well group and the crude oil produced by the production well at different times are analyzed to observe the underground ISC combustion state and the ISC effect on crude oils by geochemical proxies.Based on the geochemical analysis of the core extracts from Es4 reservoir of a coring well,it can be concluded that these samples have similar source input and thermal maturity and have undergone light to medium degree of biodegradation.The level of the biodegradation decreased from PM 5?almost all the n-alkanes and isoprenoid alkanes are degraded?at the top of Es4 to PM2?n-alkanes are degraded?at the bottom of Es4 with increasing depth.There are several barrels and baffles developed in the Es4 reservoir.The overall reversed compositional pseudo-gradient is controlled by oil-water contacts and lithologic structure.From the cold oil zone to combustion front,the relative abundances and concentrations of n-alkanes,isoprenoids,none and less alkyls substituted alkylnaphthalene and alkylphenanthrene gradually increased.However,most of the compounds are dissapeared in the burned zone.Compared with the original oil,the hydrocarbons and NSO compounds characteristics of low temperature oxidized oil at 200?shows little change but obvious oxygen addition trend,while that of higher temperature oxidized oil at 400?indicates thermal cracking.The content of low molecular weighted n-alkanes,isoprenoids,naphthalenes,phenanthrenes and their low alkyl substituted homologues increased significantly.The results of simulated experiment suggests that the molecular composition characteristics and reactions between them are affected by reaction temperature and during time,and some geochemical proxies are proposed temporarily.On the basis of analysis of samples from ISC test group,it is recognized that the core samples located at 978-980 m are thermally upgraded,however,they are still in the downstream of combustion front and have not yet experienced high temperature oxidation.The upper sample is more upgraded than the lower one,which suggests that the occurrence of the updip overlap of fire line.The produced oils show no linear change rule with production time,however,five of eight samples are clearly affected by thermal recovery of ISC together with cyclic steam stimulation than three others,which shows increased n-alkanes and isoprenoids concentration,C0-C2N and C0-C1P,O2 DBE1,and decreased steranes,terpanes and aromatic steroids.Anyway,all of the produced oil is a mixture of initial crude oil and upgraded oil.According to the results of samples from both experiment simulation and ISC test group,the molecular geochemical charateristics of upgraded samples are the relative content of saturated hydrocarbons increases and the content of resins and asphatenes decreases,the content of low molecular n-alkanes,C0-C2N,C0-C1P increases and that of steranes,terpanes and steroid aromatics decreases,meanwhile,the generation of anthracene series are detected.Molecular ratios such as n-C17/Pr,n-C18/Ph,?n-C20-/?n-C21+,n-alkane/PT,N/MN,MN/C3N,P/MP,An/P,MAn/MP,BAn/Ch and O1 DBE1/DBE4,O2 DBE1/DBE3 and S1 C10-20/C30-40 can be used as geochemical proxies to monitor the ISC effect.