Geochemical Characteristics And Origins Of Oils In Yinggehai And Qiongdongnan Basins

Significant oil and gas fields such as YA13-1,DF1-1 and so on have been discovered in Yinggehai and Qiongdongnan Basins,very important Cenozoic petroleum-bearing basins in South China Sea.However,the basins have such huge potential that further exploration is still needed.After a careful study of the characteristics of light hydrocarbons,biomarkers and carbon isotopes of oils,the oils from Dongfang,Ledong and Yacheng gas fields of Yinggehai and Qiongdongnan Basins are distinguished based on their origins of organic matters and the depositional environment.Futhermore,oils in every single gas field are classified seperately.Combined with the results of GC/MS analysis of saturated fractions and carbon isotopic analysis of rock samples from different formations from Oligocene,Miocene and Pliocene,a detailed comparison between oils and rocks is carried out.At last,the source rocks for the oils in these three gas fields are comfirmed.Results indicate that oils from Dongfang and Ledong gas fields are similar in characteristics of light hydrocarbons,biomarkers and carbon isotopes which are significantly different from oils in Yacheng gas field.The oils from Dongfang and Ledong gas fields are isotopically heavy(-23.6‰?-21.8‰).They show a small value of K1,about 1.08.They are high in heptane and dimethyl-cyclopentane while low in methyl-cyclohexane and toluene in C7 light hydrocarbons.The results of the GC/MS analysis indicate a predominance of pristane in isoprenoids(Pr/Ph>3)and low content of C19 tricyclic terpane(C19/C23<3).The concentrations of biomarkers indicating angiosperm origins like oleanane and its degradations,bicadinane and taraxastane are low,but the concentrations of gammacerane and 4-methyl-sterane are high which indicate aquatic organic matter origins and sub-oxic swallow marine depositional environment.The oil samples from Well DF13-2-1,Well DF1-1-14 and Well LD20-1-2 differentiate largely from the majority from Dongfang and Ledong gas fields.The results indicate Pr/Ph ratios from 3.1 to 3.4,C19/C23 tricyclic terpane ratios<1,high concentrations of oleanane,de-A-oleanane and bicadinane and low concentration of 4-methyl-sterane.This kind of oils are derived from sub-oxic coastal depositional environment with main input of terrigenous organic matters and a small portion of aquatic organic matters input.The oils from Yacheng gas field are isotopically lighter(-26.9‰?-24.9‰).A larger K1 value(1.24)helps to distinguish them from the oils in Dongfang and Ledong gas fields.They are high in methyl-cyclohexane and toluene while low in heptane and dimethyl-cyclopentane in C7 light hydrocarbons.The results of the GC/MS analysis of the oils indicate a predominance of pristane in isoprenoids(Pr/Ph>5)and high concentration of C19 tricyclic terpane(C19/C23>6).The concentrations of biomarkers indicating angiosperm origins like oleanane and its degradations,bicadinane and taraxastane are high,but they barely contain gammacerane and 4-methyl-sterane.The results suggest the contributions of terrigenous organic matters and the oxic coastal swamp depositional environment.Oils from Dongfang and Ledong gas fields could be classified into three categories and oils from Yacheng gas field could be classified into two categories based on the characteristics of biomarkers.Miocene source rocks from Yinggehai Basin are isotopically heavy,ranging from ?27.7‰ to-20.9‰,which are closely related to the oils from Dongfang and Ledong gas fields.Mudstones from Huangliu and Meishan Formations show low concentrations of oleanane and bicadinane with higher concentration of 4-methyl-sterane which indicate a good correlation with oils from Dongfang and Ledong gas fields.The results of carbon isotopes and biomarkers of oils and rocks suggest that the majority of the oils in Dongfang and Ledong gas fields are derived from the source rocks from Huangliu and Meishan Formations.Oligocene source rocks from Qiongdongnan Basin are isotopically lighter,ranging from-27.3‰ to-23.4‰,which are closely related to the oils from Yacheng gas field.But only coals and carbonaceous mudstones show a large value of C19/C23(3.5?7.6)which correlates well with oils from Yacheng gas field.However,they only contain a low concentration of oleanane and bicadinane which disaccord with the oils.Only mudstones are rich in oleanane and bicadinane which correlate well with the oils.As a result,the oils from Yacheng gas field are believed to be a mixture of oils from different facies of Oligocene source rocks.