| The genetic mechanism and geochemical attributes of rearranged hopanes have received increasing attention as biomarkers with their application for geochemical studies of petroleum geochemistry. Taking hydrocarbon source rocks and crude oils from the Songliao Basin as research objects, combined with a series of geochemical analysis techniques, such as Gas Chromatography-Mass Spectrometer (GC-MS), total organic carbon analysis, Rock-Eval pyrolysis analysis, maceral analysis, and vitrinite reflectance measurement, the present paper reports their geochemical characteristics systematically. On the basis of Gas Chromatography-Mass Spectrometer-Mass Spectrometer (GC-MS-MS), four series of rearranged hopanes are comprehensively identified and reported. The relationships between the same and different series of rearranged hopanes are analyzed. In addition, elements, X-ray diffraction of whole rock and biomarkers show the influenced factors and formation mechanism of rearranged hopanes and explain the abnormal distribution of high-extremely high abundance of rearranged hopanes in hydrocarbon source rocks and crude oils in the Songliao Basin. This research not only will be significative for oil-source correlation and identification of primary hydrocarbon source rocks in the Songliao Basin, but also improves the understanding of rearranged hopanes and their application in petroleum geology.Systematical evaluation of hydrocarbon source rocks in the Songliao Basin (Lishu Fault Depression) has been done to study their original source, thermal maturity and sedimentary environment. The content of total organic carbon (TOC) ranges from 0.05% to 24.54% with an average of 2.17%. The hydrocarbon potential (S1+S2) values range from 0.09 mg/g to 15.58mg/g with a mean of 2.62mg/g. The bitumen "A" contents are in a range of 0.02-3.29% with a mean of 0.49%. Overall, abundance of organic matter is medium-good. The TI values of organic matter is-69.72-55.99, and degradation rate (D) and hydrogen index (HI) range from 0.11% to 45.85% and 1.08% to 542.87%, respectively. It illustrates that the type of organic matter is type Ⅱ-Ⅲ, while type Ⅱ is dominated. The highest pyrolysis peak temperature (Tmax) ranges from 411 ℃ to 536℃ (average is 466℃), and vitrinite reflectance (Ro) is in a range of 0.541-1.695%(main region is 0.7-1.3%), which shows most of hydrocarbon source rocks within the study area stay in a mature stage.Saturated hydrocarbons and aromatics from the extracts of hydrocarbon source rocks and crude oils display geochemical characteristics of hydrocarbon source rocks and crude oils in the Songliao Basin. The ratio of Pr/Ph in hydrocarbon source rocks distributes from 0.29 to 4.27 (mean is 0.91). Gammacerane index ranges from 0.02 to 0.53 with a mean of 0.20, and three fluorene series shows a pattern of F> OF> SF. Similarly, the Pr/Ph values of crude oils are 0.68-1.25 with an average of 0.94. Gammacerane index ranges from 0.18 to 0.52 (mean is 0.27), and three fluorene series shows a pattern of F> OF> SF. The results show that hydrocarbon source rocks and crude oils derived from weak reducing to weak oxidizing environment with fresh to brackish water. Steranes in hydrocarbon source rocks and crude oils distribute in an asymmetric "V" pattern, while phenanthrenes dominated in aromatics of hydrocarbon source rocks with fluoranthene, benzanthracene, olefin and pyrene were detected, and naphthalenes dominated in aromatics of crude oils with fluoranthene and pyrene were detected. It proves that organic matters within the Songliao Basin are mainly from terrigenous higher plants and aquatic algaes. The ratios of C29 aaa-20S/(20S+20R) and C29 αββ/(ααα+αββ) in hydrocarbon source rocks are 0.43-0.69 and 0.31-0.57. The C3i-22S/(22S+22R) value is 0.47-0.62, and most of Rc (conversion from methylphenanthrene ratio) are larger than 0.7%. Similar to hydrocarbon source rocks, the C29 ααα-20S/(20S+20R) and C29 αββ/(ααα+αββ) values in crude oils range from 0.43 to 0.69 and 0.31 to 0.57, respectively. The C31-22S/(22S+22R) ratio is 0.39-0.60. These study results illustrate that both hydrocarbon source rocks and crude oils are mature.On the basis of previous research, combined with GC-MS-MS, relative retention time and peak sequence of rearranged hopanes, four series of rearranged hopanes in geological bodies are indentified. These four series of rearranged hopanes elute in a order of the early-eluting rearranged hopane series> the 17a(H)-diahopane series> the 18a(H)-neohopane series> the 21-methyl-28-nor-hopane series. A suite of fifty-six core samples and fifty-nine oil samples from the Songliao Basin in northeastern China were found to contain variable amounts of four series of rearranged hopanes:the 17a(H)-diahopane series (C29 diahopane and C30 diahopane), the 18α(H)-neohopane series (Ts and C29TS), the early-eluting rearranged hopane series (C30 early-eluting rearranged hopane or 30E) and the 21-methyl-28-nor-hopane series (C29 28-nor-spergulane or 29Nsp). In total, there are forty-three monomer compounds, and the 17a(H)-diahopane series and the 18α(H)-neohopane series are the most widely distributed of these four series of rearranged hopanes.The relationships between different and same series of rearranged hopanes are complicated. Generally speaking, compounds from the same series of rearranged hopanes correlate with each other very well. However, compounds from the different series of rearranged hopanes exhibit different correlations with each other. For rearranged hopanes in hydrocarbon source rocks, the 17a(H)-diahopane series shows a good correlation with the early-eluting rearranged hopane series. Both the 21-methyl-28-nor-hopane series and the early-eluting rearranged hopane series exhibit a relatively poor relationship with the 18a(H)-neohopane series, while the 17a(H)-diahopane series displays a general correlation with the 18a(H)-neohopane series. Moreover, a general correlation also exists between the 21-methyl-28-nor-hopane series and the 17a(H)-diahopane series or the early-eluting rearranged hopane series. Interestingly, crude oils show a different phenomenon. The early-eluting rearranged hopane series, the 18a(H)-neohopane series and the 21-methyl-28-nor-hopane series in crude oils show a good correlation with the 17a(H)-diahopane series. The early-eluting rearranged hopane series also correlates well with the 21-methyl-28-nor-hopane series, but the 18α(H)-neohopane series displays a general relationship with the 21-methyl-28-nor-hopane series.Based on analysis of biomarkers, TOC, Ro, X-ray diffraction of whole rock and elements, the relative content of rearranged hopanes can be influenced by diverse factors. Sedimentary environment indicators suggest that weak reducing to weak oxidizing environment with weak brackish water is the most beneficial condition to formation and enrichment of rearranged hopanes. The concentrations of clay content and clay minerals have little influence on abundance of rearranged hopanes. Gammacerane index and different elements that can reflect environmental paleosalinity and sedimentary facies show that salinity has different effects on enrichment of different series of rearranged hopanes. On the whole, contents of rearranged hopanes are controlled by both oxidation or reducing condition of sedimentary environment and water salinity. In addition, parameters of parent materials (such as C24 tetracylicditerpene/C26 tricyclic terpane and C27/C29 sterane) display that both aquatic plants and bacteria-alga and higher plants have a close relationship with rearranged hopanes. Maturity indicators (such as sterane isomerization ratios, methylphenanthrene ratios and methylnaphthalene ratios) exhibit that abundance of rearranged hopanes increase with the increasing of maturity. Meanwhile, the concentrations of rearranged hopanes stay in the highest level in the stage of mass generation of hydrocarbon source rocks. As C30 diahopane/C3o hopane= 0.2-1.0, their abundance are greatly affected by maturity. What’s more, rearranged hopanes do not covary with TOC. |
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