| Rearranged hopanes are the important composition of saturated hydrocarbon biomarkers, widely distributed in oils and rock extracts. The oils and rock extracts with abundant diahopanes have been widely observed in earlier studies, but its origin and genesis is yet unclear now. The reseach on the formation conditions, the controlling factors, genetic mechanism and the application in petroleum geology, should be significative for precise oil-source correlation and the identification of chief hydrocarbon source rocks in oil and gas exploration process of some superimpose basins.On the basis of vast literatures at home and abroad, previous research results about biomarker characteristics in the studied area and related geological data collected, 46 crude oil samples have been collectes from Kuqa depression, Tarim Basin (DaWanqi oilfield and DaBei oilfield). This paper reveals the geochemical characteristics of the crude oils systemically on the basis of GC and GC-MS quantitative analysis. Geochemical characteristics of oil were studied in detail including physical properties, group composition, biomarker composition, and abundance, types and maturity of organic matter.This paper gets the main studies as follows through sample analysis, data processing, as well as comprehensive geological - geochemical studies, and it is concluded the follow research results:1. Most of the 46 oil samples in Kuqa Depression are condensate oils, the sulfur and the wax content is low. The distribution of total hydrocarbon chromatographic from crude oil mainly shows complete distribution of alkanes, the abundances of benzenes and methyl cyclohexane with a good comparability in the compounds with low carbon number.2. Through the researching of light hydrocarbon from crude oil in Kuqa depression, the content of methylcyclohexane in composition of C7 light hydrocarbons series is relatively high. The distribution of regular steranes C27, C28 and C29 performed a "V"-shape, with an intermediate value of C27, lower than C29 and higher than C28. The naphthalene take predominant part in the aromatic compounds. Some aromatic compounds such as fluoranthene, benzo [a]anthracene and pyrene can be detected, which indicates the typical characteristics of higher plants imported. The Pr/Ph ratios of the crude oils in Kuqa depression are respectively in the range of 1.2-3.0, and the gammacerane/C3o hopane values of the crude oils in Kuqa depression vary between 0.18-0.28, which indicated the crude oils deposited in the weakly oxidation - weakly reduction fresh to brackish water environment. The research of maturity parameters is not obvious:MPI-1, MPR, F1, F2 are showed the crude oils is in the mature stage. Two kinds of crude oils were existed in this oilfield, with obviously different biomarker characteristics between them. One of them is having high abundance of gammacerane, and low abundance of rearranged steranes. The other one is having low abundance of gammacerane, and high abundance of diahopanes.3.17α(H)-diahopane series, the 18a(H)-neohopane series, and the early-eluting rearranged hopane series was mainly detected in Kuqa depression, Tarim Basin. The standard of diahopanes abundance(ratio of C3o*/C3oH) is divided into three levels:the abnormal high abundance diahopane compounds values of C3o*/C3oH is more than 1.0; the low abundance diahopane compounds values of C3o*/C3oH is less than 0.2; the values range from 0.2 to 1.0 is high abundance diahopane compounds. The relationship between diahopanes compounds in crude oil samples shows the similar carbon skeleton structure relatively good correlation, while the correlation between the diahopanes compounds which have slightly different from the carbon skeleton is weaker than former. Indicating the same series diahopanes compounds have the same evolutionary path and the same formation mechanism, and different series diahopanes compounds are not.4. The relationship between rearrangement hopane compounds and the biomarker combinations:Relation graph of rearranged hopanes respectively with Pr/Ph and G/C3ohopane showed some correlation, indicating indicating the sedimentary environment may have some influence on diahopanes compounds. The ratio of C24Te/C23TT and C24Te/C26TT were 0.26～1.79 and 0.67～4.5, respectively, and their mean value were 1.28 and 2.9. There would be some correlation among them. Beside, the relative abundance of rearranged steranes has positive correlation with diahopanes.5. The relationship between rearrangement hopane compounds and aromatic compounds:The aromatic compounds are characterized by relatively high concentrations of bicyclic and tricyclic aromatic hydrocarbons(>96%) and relatively low concentrations of tetracyclic and polycyclic aromatic hydrocarbons(<3%). which shows a origin of terrigenous higher plants in crude oils. Meanwhile, the relative percentage of dibenzofurans (OF) between 22.95% -74.69%, with an average of 55.17%; followed by dibenzothiophen (SF) averaged 23.97% in crude oil samples. The parameters of dibenzothiophene, dibenzofuran and fluorene indicate that a weak oxidizing sedimentary environment is important to the formation of rearranged hopanes compounds.6. The relationship between rearrangement hopane compounds and the secondary actions:with decreasing of multi-alkyl naphthalene series, the abundance of naphthalene or methyl naphthalene is significantly increased. It’s exactly the same as phenanthrene series compounds. Because of the small differences of the maturity and the slight degradation of microorganisms have little effect on the composition of aromatic compounds. On this distribution characteristics may be related to migrational fractionation of crude oil. Adamantane hydrocarbon is the secondary product during the evolution of hydrocarbons, and if once formed, more stable than most other hydrocarbons. Two new migration parameters,1,3-DMD/1,2,5,7-FMD, 1M-AD/1Et-AD, proved to be effective in the preliminary research, have some correlation with rearrangement hopane compounds.7. The research results seem to indicate the diahopanes compounds are mainly controlled by sedimentary environment and biological source. In weak oxidation weak reduction environment, higher oxidation and lower salinity water beneficial to form diahopanes. In this sedimentary environment, diahopanes compounds have a positive correlation with the import of the higher plants. Migration fractionation has some influence on the formation of rearrangement hopane compounds, but not the main factor. In addition, the maturity closely resembles in crude oil which shows that diahopanes compounds and maturity parameters are not well correlated correspondence. |
PDF Full Text Request