| Summary: | 碩士 === 臺灣大學 === 地質科學研究所 === 95 === The study investigates the compositional fractionation of oil during secondary migration, especially under the interactions between oil and clay minerals. Iran crude oil (API 33.5°) reacted with kaolinite, montmorillonite, and illite at 26℃ and 80℃ respectively, under 1atm and 3atm; saline water (20,000 ppm) was also added into the 26℃, 1atm system. All the samples were stirringly mixed for eight days then centrifuged to separate oil from clays. The rest of oil samples and clay minerals are setting in the sealed vials for continuing reaction. Liquid chromatography was used to separate crude oil into saturate, aromatic, and polar (NSO) fraction before analysis by GC-MS.
The Pr/Ph ratios of starting sample and 15 experimental aliquots are consistent with Type II carbonaceous source rock, suggesting that the ratio can still indicate its environment after interaction with clays during migration. Our observation indicates that slightly selective adsorptions did occur in all detected compounds relative to n-alkane in saturate fraction but commonly applied biomarker parameters such as Ts/(Ts+Tm), TA/(TA+MA), ββ/(αα+ββ), and 20S/(20S+20R) C29 steranes are no significant change after interacting with excessive amount of clay minerals. Only the tricyclics/17α (H)-hopanes ratios in m/z=191 show considerable fractionation and therefore, the use of this previously proposed maturity indicator should be cautious. In aromatic compounds, especially naphthalene, fluorene, dibenzothiophene, and phenanthrene, the result shows the co-precipitation of asphaltenes, the presence of saline water, and the adsorption of different clay minerals, may cause complex behavior of fractionation for isomers. For long-term experiments, we found that adsorption of saturate and aromatic compounds on the studied clays were still far from equilibrium. In contrast to the previous observation that clay minerals would adsorb large amount of heterocyclic (NSO) compounds at fast rate, the present study reveals a slight adsorption of some aromatic and saturate compounds at slower rate, which result in fractionation of some biomarkers. Continuing kinetic variations in saturate and aromatic fractions in longer time are expected.
Our future work will focus on finding specific NSO compounds sensitive to adsorption to help model the migration distances of petroleum and correlate the petroleum with their source rock.
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