The condensates are generally characterized by high maturity,low concentration of steranes and ter-panes biomarkers and low content of non-hydrocarbon fraction. In this case commonly used steranes,terpanes and carbazoles parameters cannot be effectively applied in the reservoir-filling tracing. The hydrogen bond formed by sulfur atom in the dibenzothiophenes (DBTs) results in molecule adsorption and fractionation during oil migration in reservoir. Like carbazoles,total DBTs content decreases with the increasing of oil migration distance. Therefore,a new parameter——total DBTs content is proposed to be used to trace the oil migration orientation and filling pathway. In present study,total DBTs con-tents of condensates and light oils are obtained by adding internal standard——eight deuterium atoms substituted DBT during Gas Chromatography-Mass Spectrometry analysis of aromatic fraction. Except for a few samples with much lower content of non-hydrocarbon fraction,the total DBTs content shows a fine positive correlation with that of carbazoles. Large errors can be caused in the process of pyrrolic nitrogen compounds separation. The application of this new parameter in the Fushan Depression of Beibu Gulf Basin,South China Sea indicates that this parameter is a reliable one to trace filling pathway in condensate reservoirs. Combined with other DBTs-related parameters,such as 4-/1-methydibenzo-thiophene and 2,4-/1,4-dimethyldibenzothiophene,oil migration orientation and filling pathway of the Fushan Depression was determined. The accumulations of Huachang oil field in the Fushan Depres-sion are mainly migrated and charged from northeast to southeast along the Huachang uplift. It can be predicated that the light oil and condensates in the Huachang oil field should be sourced from the source kitchen at the Bailian Sag. It shows that total DBTs content is an effective parameter to tracing oil migration orientation and filling pathway.
LI MeiJun1,WANG TieGuan1,LIU Ju2,ZHANG MeiZhu2,LU Hong3,MA QinLin2 & GAO LiHui2 1 State Key Laboratory of Petroleum Resources and Prospecting,Earth Sciences and Geoinformatics School,China University of Petroleum,Beijing 102249,China
The Yong'an-Meitai area is the focus of the present exploration in the Fushan Depression,Beibuwan Basin,South China Sea. All oils from this area are geochemically characterized by higher Pr/Ph ratio,higher proportion of heavy molecular weight hydrocarbons,and higher proportion of C29 regular steranes,which indicate that the organic matter of source rocks might have been deposited in an oxidizing palaeoenvironment and be dominated by higher plant organic matter input. The oil from E3w2 (the second member of Weizhou Fm. of the Oligocene) has a much higher density,relatively higher Pr/nC17 and Ph/nC18 ratios,and a "UCM-unresolved complex mixture" on gas chromatograms,which indicate that it has been slightly biodegraded. CPI and other terpane and sterane isomer ratios suggest they are all mature oils. The timing of oil charging in E3w2 and E2l1 (the first member of the Liushagang Fm. of the Eocene) determined by the homogenization temperatures of fluid inclusions and thermal evolution history are from 9-3 Ma and 8-3 Ma,respectively. Thus,the interpretation of E3w2 as a secondary reservoir is unlikely. The timing of oil charging is later than that of hydrocarbon generating and expulsion of Liushagang Fm. source rocks and trap formation,which is favorable for oil accumulation in this area. All molecular parameters that are used for tracing oil filling direction decrease with shallower burial depth,which suggests vertical oil migration. The widely occurring faults that penetrate through the source rocks of the Liushagang Fm. may serve as a fine oil charging conduit.
Li MeijunWang TieguanLiu JuZhang MeizhuLu HongMa QinglinGao Lihui
