石油在二次移棲過程中黏土礦物的作用對其成分變化的影響

Abstract

石油在二次移棲的過程中會導致成份的改變，一般相信地層中黏土礦物對石油中極性化合物的吸附是造成成份改變的因素之一，但是卻少有實驗研究證實黏土礦物在二次移棲過程中所扮演的角色。本研究利用伊朗的原油(API 33.5°)分別與高嶺土、蒙脫石以及伊萊石作用來探討黏土礦物對石油中飽和烴(SAT)、芳香烴(ARO)以及極性化合物(NSO)的影響。並利用高壓高溫的裝置，改變其反應時的溫度與壓力，來探討溫壓條件以及天然氣對吸附能力的影響。經過八天持續攪拌作用的石油經離心後，我們取出上層的石油進入氣相層析質譜儀(GC-MS)分析，並且將其中一小部份的石油，經過矽膠礬土淨化管柱的分離作用後，分別得到飽和烴(SAT)、芳香烴(ARO)、和極性化合物(NSO)的部份。濃縮稱重後的各樣品加入適量的二氯甲烷溶解後，則繼續進行GC-MS的分析。沉澱在瓶壁的黏土與剩下的油樣則繼續靜置使其作用。 從質譜分析的結果，我們發現就算連極性很小的飽和烴成份都會因黏土礦物的吸附而有些微的成份差異，因此我們利用幾個普遍且常用的生物指標m/z =191、217、231、253來檢驗黏土礦物對生物指標分佈的影響。發現大部份常用的成熟度參數並不會隨著黏土礦物吸附效應而失真，唯有Tricyclics/C30(17-α)hopane會隨著吸附的時間長短而呈顯著的變化，因此在應用此成熟度的參數時必須將黏土礦物的可能含量也計算進去以減小誤判的可能。而在芳香烴的化合物中我們的確看到了預期的比飽和烴顯著的吸附效應，而且不同的黏土礦物也影響了各個化合物中同分異構物(isomers)的分化係數。此外，溫度、壓力以及鹽水的存在都會讓黏土礦物對芳香烴的吸附產生複雜的成份變化。 長時間的持續靜置後，高嶺土、蒙脫石以及伊萊石的吸附都使Tricyclics/C30(17α)hopane持續下降，而且石油中飽和烴和芳香烴的吸附都還沒達到反應平衡的狀態。未來我們希望能模擬自然界的情況，去分析真正吸附在黏土礦物中的石油，藉由這些被強力吸附的化合物間分化係數的比例以及關係，以期增加未來在應用判斷二次移棲過程中的芳香烴和極性化合物(NSO)參數的精確性，並且希冀能利用分化效應去協助判斷石油可能的移棲路徑和距離。

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.