Microscopic Fluorescence Spectral Characteristics of Mixing Ratio of Crude Oil Experiment

SU Ao, CHEN Hong-han, HE Cong, LEI Ming-zhu, LEI Chuan, WANG Ping

1.Key Lab of Tectonics and Petroleum Resource of Educational Ministry, China University of Geosciences, Wuhan 430074, China 2.Key Laboratory of Petroleum Resources, Gansu Province/Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Lanzhou 730000, China 3.University of Chinese Academy of Sciences, Beijing 100049, China 4.Zhanjiang Branch, CNOOC, Zhanjiang 524057, China

Microscopic Fluorescence Spectral Characteristics of Mixing Ratio of Crude Oil Experiment

SU Ao1, CHEN Hong-han1, HE Cong2,3*, LEI Ming-zhu4, LEI Chuan2, WANG Ping1

1.Key Lab of Tectonics and Petroleum Resource of Educational Ministry, China University of Geosciences, Wuhan 430074, China 2.Key Laboratory of Petroleum Resources, Gansu Province/Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Lanzhou 730000, China 3.University of Chinese Academy of Sciences, Beijing 100049, China 4.Zhanjiang Branch, CNOOC, Zhanjiang 524057, China

The microscopic fluorescence spectroscopy has become a mature technology of fluid inclusions test and analysis system, which is used to distinguish different types of crude oil and oil inclusions.These would be the important basis to study the history of hydrocarbon accumulation of petroleum basins.The mixture of crude oil from different sources could occur in migration and accumulation process.In order to effectively identify the type of geological process, mixing ratio of crude oil experiment has been carried out.This study result shows that mixing of crude oil make fluorescence color and spectral parameters(λmax, QF535 and CIE-XY) change nonlinearly.Fluorescence spectral parameters of mixed oil is between end member oil A and B.The greater A or B ratio of mixed oil, the closer to A or B.Fluorescence color of mixed oil show nonlinear and gradual change in CIE-XY chromaticity diagram.Variation of spectral spectrum shape show that single peak is changed into double and three peaks.The relationship between QF535 and degree of mixing could calculate quantitatively relative contribution.Mixing different types of crude oil make spectral spectrum shape changes, which present characteristics of two peaks and three peaks but not unimodal peak.The main and subsidiary wavelength reserve wavelength information of end member oils.Based on variation characteristics of fluorescence spectrum, there are three different types of oil including blue, blue-green and yellow fluorescing oil filling in the bottom member of Pinghu formation in A gas field.At the same time, there also was a mixing process of blue-green fluorescing oil and yellow fluorescing oil.The degree of mixing is 47%～55%.

Fluorescence spectrum; Mixture of oil; Crude oil; Oil inclusion; Mixing ratio experiment

Introduction

Most of the geological processes on Earth are almost accompanied by fluid activity, which is one of the most important carriers that take charge of the various elements or hydrocarbon migration in diagenesis, ore forming and hydrocarbon accumulation processes.Abundant geochemical information could be provided by paleo-geofluids to prospect mineral resources or know some geological processes occurred.The paleo-geofluids are trapped in fluid inclusions, a closed system[1-2], which can avoid being damaged by structure or thermal activities in geological history period.Therefore, they become the best targets to acquire PVT-X (pressure, volume, temperature and component) parameters of paleo-geofluids[3-4].Their meaning is equivalent to “fossil”.

With the development of optical and spectroscopy analysis technology and computer technology, the geochemical information of fluids in fluid inclusions can be qualitatively or quantitatively obtained by some new formed means[5-7].In hydrocarbon fluid geology, the oil’s characteristic of emitting visible light with a certain wavelength under ultraviolet light can be used to distinguish the oil inclusions and aqueous inclusions[8].At present, the single oil inclusion is tested in-situ and losslessly by ultraviolet-visible light microscopic fluorescence spectrum and some spectral parameters such as main peak wavelength (λmax), red/green entropy, QF535 and CIE-XY, which quantitatively represent or evaluate crude oil composition and maturity, can be obtained[9-12].In general，the largerλmax, red/green entropy, QF-535 and CIE-X of fluorescence spectrum are, the lower maturity of crude oil in oil inclusions is[13].This feature can be used to distinguish different types of oil inclusions.

However, some spectral parameters are excessively depended on by some domestic scholars, while some geological processes are ignored.Some secondary alterations (such as thermal cracking, water-washing, gas-washing, biodegradation and mixture oil) can give rise to fluorescence parameters change[14].For instance, some petroliferous sedimentary basins may develop many sets of effective hydrocarbon source rocks, so some crude oils generated and expulsed by different source rocks may mix in different scales, which makes microscopic fluorescent color and parameters of some crude oil change correspondingly with its geochemical component alteration.Hence, they cannot be simply used as a basis of dividing hydrocarbon infilling stage.Nevertheless, it has barely been reported about the affecting of mixed crude oil on fluorescence spectrum characteristics in China.The concrete impact of mixed oil made on fluorescence spectrum characteristics will be studied through experiment of artificially mixing different ratios of crude oil in this paper.In turn, the results can not only be used to identify whether different types of crude oil are mixed up ,but also can avoid making a hasty conclusion of only one oil-gas filling stage and calculate compositions of mixed oil.Besides, this paper will also study spectrum parameters (spectral spectrum shape and subsidiary peak wavelength) which have been hardly concerned by some researchers.In the meantime, the relationship between them and mixed crude oil is explored.

1 Experiment and Test

Two types of crude oil with different maturity or density are chosen as elements to conduct an experiment of mixing them with different ratios (mass percent).And them the fluorescence spectra and spectral parameters such asλmax, QF-535 and CIE-XY of mixing oils are respectively tested by ultraviolet-visible light microscopic fluorescence spectrum.QF-535 is a ratio of the limited area between wavelength of 720 and 535 nm to the limited area between wavelength of 535 and 420 nm[15].CIE-XY is chromaticity coordinate of standard three-primary colors.

The followings are experimental apparatus and processes: at constant room temperature (26 ℃), two types of end member oil with different mass ratio are respectively weighed by high precision electronic scale.The low maturity crude oil with high viscosity and flowing difficultly must be taken into consideration, therefore 30 mLn-hexane are taken as mother liquors to dissolve crude oil thoroughly after samples are weighed.Then these two types of oil with different weight are mixed up and stired fully to a uniform liquid by glass rod.A small amount of solution is extracted by straw, and then placed on glass and covered with another glass.Finally, the samples are tested by Nikon 80I dual channel fluorescence microscopy (Ultraviolet excitation wavelength is 380 nm, fluorescent security filter is 400 nm).The fluorescence spectrum of samples are collected by Maya 2000 Pro spectrograph.The spectroscopic data is handled by Yuanao microscopic spectrum analysis system.Eleven artificial mixing experiments of these two types of crude oil are performed.Their mixing mass proportion are respectively 100∶0, 90∶10, 80∶20, 70∶30, 60∶40, 50∶50, 40∶60, 30∶70, 20∶80, 10∶90 and 100∶0.

2 Results and Discussion

Microscopic fluorescence spectrum test results of these experiments are as follows: Table 1.

Table 1 Microscopic fluorescence spectral parameters of mixed crude oil with different ratios

Fig.1 CIE chromaticity diagram of microscopic fluorescence spectral of mixed crude oil with different ratios

The components will definitely change after two different types of crude oil are mixed up.The microscopic fluorescence spectrums which contain important fingerprint information of crude oils or oil inclusions such as composition and maturity will also change correspondingly[16].From the test results (Table 1), the microscopic fluorescence spectrum parameters and their spectrum shapes of sample are altered after mixed with various ratios.Theλmax, QF535, CIE-X and CIE-Y of end member oil A is respectively 462 nm, 0.45, 0.189 and 0.191.Theλmax, QF535, CIE-X and CIE-Y of end member oil B is respectively 541 nm, 1.39, 0.368 and 0.429.Theλmax, QF535, CIE-X and CIE-Y of mixing oil samples are between those of A end member and those of B end member, and they will close to those whose member oil possesses a larger proportion.The CIE chromaticity diagram indicates that the spectrum colors of oil samples with different mixing ratios differ in (Fig.1).The CIE-X and CIE-Y of mixing oil samples vary gradually with increasing mixing degree, reflecting that fluorescence color of mixing oil changes between those of the two types of end member oil.As the mixing ratio range from 100∶0, 90∶10 to 10∶90, 100∶0, fluorescence color of crude oil vary from blue, white to yellow.Besides, the change trend is a gradient nonlinearity (Fig.1).

From table 1, though maximum main peak wavelength(λmax) of fluorescence spectrum of the mixing oil has a change, its changing trend cannot fully reflect mixability.The change rule ofλmaxisn’t consistent with QF-535 and CIE-XY.When the mixing ratio is respectively 90∶10, 80∶20 and 70∶30, the λmax of mixing oil is still the same with that of end member A, which is 463 nm.When the mixing ratio is 60∶40, 50∶50, 40∶60 and 30∶70, theλmaxof mixing oil are 493 nm.When the mixing ratio is 20∶80 and 10∶90, theλmaxof mixing oil is the same with that of end member B, which is 540 nm.From experiment of mixing crude oil with different ratios, therefore, we can see that λmax which is a highly valued parameter only reflects priority composition and hardly characterize other components of crude oil.The crude oil which is classified just byλmaxmay expand oil infilling stages.The microscopic fluorescence spectra of mixing oil samples with different mixing ratios suggest that the spectral spectrum shape and peak shape have more obvious changes (Fig.2).The specural spectrum shape of end member A and B show “unimodal” type [Fig.2 (a) and (k)].However, the spectral spectrum shapes of mixing oil samples whose mixing ratios are 90∶10, 80∶20 and 70∶30 takes on “bimodal” type of a main peak wavelength (λmax) and a subsidiary peak wavelength (λsub) of 495 nm, suggesting that spectrum shape is affected by another end member oil [Fig.2(b)—(e)].With the increasing mixing ratio (50∶50, 40∶60, 30∶70 and 20∶80), the spectral spectrum shape presents three peak modes of two subsidiary peaks except main peak, whose wavelength are 463, 495 and 540 nm, suggesting that the components are characterized by a change of appearing a new peak wavelength and remaining two peaks of end member A and B.Compared to the two types of end member oil, the wavelengths of mixing oil samples have respectively red shift and blueshift [Fig.2(l)].Therefore, on the basis of the change tendency of the spectral shape and subsidiary peak information after mixing, the spectral spectrum shapes of mixing oil are altered but wavelength information of end member A and B is remained.

Compared to other fluorescence parameters, components and types of crude oil can be characterized by QF535 more quantitatively and comprehensively.In the Fig.3, the value of QF535 of eleven mixed oil samples are better fitted with their mixing ratios as a whole(R=0.96).If two types of crude oil respectively from two different sources are revealed in study area, the fitted relationship between the mixing ratio and QF535 can be established by experiment of mixing these two types of crude oil with different ratios to recognize their mixing contribution quickly and easily.

Fig.2 Microscopic fluorescence spectral of mixing ratio of crude oil experiment

Fig.3 The relationship between different mixing degree of crude oil and QF-535

In conclusion, the mixing of different types of crude oil can give rise to alteration of microscopic fluorescent color, spectral parameters (QF535 and CIE-XY) and spectral spectrum shape from the experiment of mixing different ratios of crude oil.According to the changing characteristics of microscopic fluorescence spectrum, spectral shape and subsidiary peak can not only be applied to determine whether mixing effect occurs, but also can obtain main peak wavelengths to identify two types of end member oil.Eventually, the relation between mixing ratio and QF535 is established to determine oil mixing degree.

3 Sample application of mixture identification and degree

Taken the bottom member of Pinghu formation in A gas field in Xihu sag, East China Sea Basins as example, mixing of crude oil is identified and mixing ratio is quantitatively calculated based on above results through experiment in the study area.

3.1 Geological background in the study area

The East China Sea Basin, located in the eastern China continental margin, is the largest offshore oil-gas-bearing basin, and distributes towards northeast-southwest as a narrow strip.The Xihu sag, one of the largest rich hydrocarbon sag in East China Sea Basin, is abundant of oil and gas resources.A gas field located in north-central of Pinghu structural belt in Xihu sag has been found.Oil inclusions of ten rock samples and seven crude oil samples in the bottom member of Pinghu formation are collected by author to analyze their microscopic fluorescence spectrum.Firstly, the physical properties of the crude oil samples are tested, indicating that the main types of crude oil can be divided into two categories, respectively gas condensate and light oil with low density which takes the large amount, and heavy oil with high density which is little.Density and viscosity of the former are respectively 0.78～0.81 and 1.01～1.07, the latter are respectively 0.84～0.87 and 1.55～1.99.

3.2 Microscopic fluorescence spectrum analysis of crude oils and oil inclusions

3.2.1 The fluorescent color of crude oil and oil inclusions

According to the observation results of fluid inclusions developed in the study area by transmitted and fluorescent light, there exists abundant oil inclusions with multiple fluorescence colors including blue, blue-green, yellow-green and yellow and so on [Fig.4(a)], suggesting a complex oil-gas filling history in the study area.Related to directly observe the fluorescent color of oil inclusions with naked eye, fluorescent spectroscopy can not only avoid human subjective errors, but also acquire spectral parameters to quantitatively or semi-quantitatively evaluate crude oil compositions and maturity for better distinguishing different types of oil inclusions.Oil inclusions of ten rock samples in A gas field in Xihu sag have been analyzed by fluorescence spectra.CIE-XY chromaticity diagrams of oil inclusions are applied to identify fluorescence colors.From Fig.4(b), the oil inclusions in study area have many fluorescence colors.The CIE-XY parameters of oil inclusions can be divided into two categories: one is pale blue and bright blue, the other has wide distribution and gradual change from near blue-green to yellow.The CIE-XY parameters of crude oil has a good corresponding distribution as oil inclusions.The crude oil with more quantity, low density and low viscosity has bright blue fluorescence color, which is similar to the former type of oil inclusions; while crude oil with small quantity, relatively high density and viscosity has yellow fluorescence color, which is close to the latter one.

Fig.4 Micro-beam fluorescent color of oil inclusions and crude oil in the study area

3.2.2 The fluorescence spectrum spectral shape of crude oil and oil inclusions

From the perspective of microscopic fluorescence spectrum shape, oil with obvious unimodal type is tested in some oil inclusions, including bright blue fluorescing oil inclusions withλmaxabout 460 nm, blue-green fluorescing oil inclusions withλmaxabout 490 nm and yellow fluorescing oil inclusions withλmaxabout 540 nm.These oil inclusions have obvious unimodal advantages and uniform fluorescence colors which are consistent with those observed from naked eye, suggesting that compositions of crude oil in oil inclusions are nearly consistent with each other.Besides, there exists some oil inclusions whose fluorescence spectrum have wide peaks of mostly two peaks or three peaks, suggesting the complex components of crude oils in oil inclusions.In Fig.5(d), the fluorescence spectrum shape have obvious three-peaks: wavelength (λmax) of the biggest main peak is about 515 nm, its left and right subsidiary peak (λsub) are respectively 490 and 540 nm.In Fig.5(e), fluorescence spectrum shape have two peaks:λmaxandλsubare two of 490, 515 and 540 nm or other value between 490 and 540 nm.Based on fluorescence spectrum and their shape characteristics in Fig.5(d) and (e), combined with CIE-XY chromaticity diagram [Fig.4(b)], we can find that fluorescence colors of oil inclusions range from near blue to near yellow.Therefore, mixing of different types of crude oil has occurred in A gas field with a certain scale according to combination of fluorescence spectrum and the shape of crude oil and oil inclusions above.That is to say, the blue-green fluorescing oil withλmaxabout 490 nm and yellow fluorescing oil withλmaxabout 540 nm are mixed together, and theirλmaxhave respectively red-shfit and blue-shfit [Fig.5(f)].The fluorescence spectrum wavelength with wide peak do not contain 460 nm, so the crude oil withλmaxof about 460 nm was not involved in mixture.In addition, the CIE-XY of blue fluorescing oil inclusions appear gradually variation, so there may also exist other secondary effects.However, they are not the main focus of this paper and won’t be discussed deeply [Fig.4(b)].

Fig.5 Micro-beam fluorescent spectrums of oil inclusions and crude oil of A gas field in the Xihu Sag

Fig.6 Quantitative calculation of crude oil mixing ratio of A gas field in the Xihu Sag

3.2.3 Mixing degree calculation

There are three different types of oil of blue, blue-green and yellow fluorescing oil filling in the bottom member of Pinghu formation in A gas field.At the same time, there also was a mixing process between blue-green fluorescing oil and yellow fluorescing oil.Based on fluorescence spectral characteristics of the produced crude oil, most oil come from the contribution of pale blue and bright blue fluorescing oil charge, only few stems from blue-green fluorescing oil and yellow fluorescing oil.The fitting relationship between QF535 and mixing degree has been established by experiment of mixing different ratios of blue-green and yellow fluorescing oil.Although mixed oil samples are not found currently, mixed oils are trapped in the oil inclusions.The QF535 of these oil inclusions (20 points), tested by fluorescence spectrum, is between 0.98 and 1.16.The mixing ratio is calculated as 47%～55% by the fitting formula above (Fig.6).That is, the contribution of blue-green fluorescing oil is 47%～55% while that of yellow fluorescing oil is 53%～45%.

4 Conclusions

(1) Based on experiment of mixing different ratios of crude oil, the result in this study shows that mixing different ratios of crude oil makes fluorescence color and spectral parameters (λmax，QF-535 and CIE-XY) change nonlinearly.Fluorescence spectral parameters of mixed oil is between that of end member oil A and B, which is related to the proportion of the end member oil.That is to say, if the mixed oil contained more end memberoil A, its fluorescence spectral parameters will be closer to that of end member oil A, and the reverse is also true.Fluorescence color of mixing oil show nonlinear and gradual change in CIE-XY chromaticity diagram.The fitted relation between the mixing ratio and QF-535 can be established by experiment of mixing different ratios of crude oil to calculate quantitatively contribution.

(2) Mixing different types of crude oil make spectral spectrum shape changes, which present characteristics of two peaks and three peaks but not unimodal peak.The main and subsidiary wavelength reserve wavelength information of end member oils.The changing characteristics of spectral can be applied to identify whether mixing of different types of crude oil happen in study area.

(3) When oil-gas infilling stage are divided by fluid inclusions, microscopic fluorescence spectrum spectral shape of wide peak should be paid particular attention to identify whether mixing effect or other secondary alterations of crude oil happen and so we can avoid dividing the oil-gas infilling stage into one stage.

(4) Based on variation characteristics of fluorescence spectrum, there are three different types of oil charging in the bottom member of Pinghu formation in A gas field, which are respectively blue, blue-green and yellow fluorescing oil filling.At the same time, there was also a mixing process between blue-green fluorescing oil and yellow fluorescing oil.The degree of mixing is 47%～55%.

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基于原油配比混合實驗的顯微熒光光譜特征研究

蘇 奧1，陳紅漢1，賀 聰2,3*，雷明珠4，雷 川2，王 萍1

1.中國地質大學構造與油氣資源教育部重點實驗室，湖北 武漢 430074 2.甘肅省油氣資源研究重點實驗室/中國科學院地質與地球物理研究所油氣資源研究重點實驗室，甘肅 蘭州 730000 3.中國科學院大學，北京 100049 4.中海石油(中國)有限公司湛江分公司，廣東 湛江 524057

顯微熒光光譜已經成為流體包裹體系統測試分析中較為成熟的一項技術，可用于區分不同類型的原油與油包裹體，從而為研究含油氣盆地的油氣成藏歷史提供重要依據。不同來源的原油在運聚的過程中可能會發生不同程度的混合作用，為了有效識別這一類地質過程，基于不同比例的原油配比混合實驗，研究原油混合后的顯微熒光光譜的具體變化特征。結果表明: 原油混源使得顯微熒光光譜參數λmax，QF-535和CIE-XY發生了非線性變化，具體表現為混源后原油的熒光光譜參數均介于兩個端元油之間，混源油中某一端元油的比例越大，其熒光光譜參數越靠近這一端元油。在CIE-XY色品圖中主要表現為非線性漸變的熒光顏色特征。光譜譜形的改變主要表現為譜形由“單峰型”變為“雙峰型”和“三峰型”，同時主峰波長和次峰波長保留了兩個端元油的信息；QF-535與混源比例可建立曲線用來定量計算兩端元油的相對貢獻度。綜合上述熒光光譜參數和譜形的變化特征，利用原油和油包裹體的顯微熒光分析，識別出東海盆地西湖凹陷A氣田有三種不同類型原油充注，中間還發生了一次原油混源作用，即藍綠色熒光原油和黃色熒光原油發生了混合，定量計算其混源程度為介于47%～55%。

顯微熒光光譜；混源；原油；油包裹體；配比實驗

2014-06-17，

2015-01-22)

Foundation item：The National Science and Technology Projects(2011ZX05023-004-010); The Open Fund of Key Lab of Tectonics and Petroleum Resource of Educational Ministry, China University of Geosciences (TPR-2015-11)

10.3964/j.issn.1000-0593(2016)09-3039-08

Received：2014-06-17; accepted：2015-01-22

Biography：SU Ao, (1989—), PhD student at China University of Geosciences (Wuhan) e-mail: suao446@163.com *Corresponding author e-mail: hecong@lzb.ac.cn