Source rock maturity study by capillary tube interferometer

Yang Ailing∗a,c, Li Wendonga, Dong Junyub , Zhang Jinliang c

a Department of physics, Ocean University of China, 238 Songling Road, Qingdao 266100,China; b Department of Computer Science, Ocean University of China, Qingdao 266100,China;

c College of Geo-science, Ocean University of China, Qingdao 266100, China

ABSTRACT

The mechanism of capillary tube interferometer is expected to be two-beam interference by ray tracing. A computer program to simulate the interference fringe pattern was established. By comparing the simulated fringe pattern and experimental fringe pattern, the refractive index of the liquid can be given when the two fringes coincide best. The results of this method are very near the Abbe refratometer.

In the first time, the refractive indices of the low transparent extraction of source rocks were measured by capillary tube interferometer. A curve between the refractive indices of the diluted extraction of source rocks and corresponding vitrinite reflectance Ro was established. When Ro is in the range of 0.36-1.25, the refractive indices of the extraction increase with Ro, or the maturity of the source rocks. Good correlation was observed between the refractive indices and vitrinite reflectance Ro. The refractive index of the extraction of source rock is valuable for determining the degree of maturity of source rock. This technique is promising to measure the refractive indices of low transparent liquids and could be used to estimate the maturity of source rock.

Keywords: refractive indices; capillary tube interferometer; source rock; extraction; maturity; vitrinite reflectance

1. INTRODUCTION Organic matter(OM) in sediments can generate oil and gas over geologic time when geothermal heat slowly changes the carbon structure of OM. Maturity is a very important parameter of source rocks because it evaluates source rocks’ potential to generate oil and gas1. OM in source rock can be soluble or insoluble. The soluble OM can be extracted by Soxhlet2 or microwave-assisted methods3 by using CH2Cl2 or CHCl3 as solvent. The soluble OM includes saturated hydrocarbons, aromatic hydrocarbons, resins and asphaltenes3. Source rocks with different maturity have different soluble OM. The insoluble OM is also called kerogen4. Vitrinite reflectance Ro of kerogen is the traditional and most common method to evaluate thermal maturity of source rock. Now it has been widely used to determine the rank and properties of coals and also been applied to determine the thermal maturity of petroleum source rock1,5-7. But for those source rocks scarcity or absence of vitrinite in some marine sedimentary environments, and in the fact that vitrinite reflectance can be suppressed by certain process during biodegradation, early digenesis or maturation, vitrinite is not a single maceral, but a group of macerals. Reflectance among individual macerals within the vitrinite group can vary for the same maturity level4. In these cases, reflectance is not a good indicator to evaluate the maturity of source rocks. In spite of Ro, there are many methods to evaluate the maturity of source rocks, such as vitrinite-like maceral reflectance8, solid hydrocarbon reflectance9-11, FTIR absorption indices4, 13C CP/MAS NMR12, biomarker maturity indices13-16, fluorescence alteration17-19, transmittance color index of amorphous organic matter20 and so on.

Capillary tube interferometer is a simple experimental scheme(see Fig. 1), but it can be used to measure the refractive indices of low transparent liquids21.This method was also used to measure the refractive index of fiber by H. El Ghandoor22, W. A. A.A. Hamza23 et al. and Ramadan24. Ramadan called this method “lens-fiber interference method”.

In this study, ray tracing of capillary tube interferometer is presented, which revealed that the mechanism of the interference pattern is expected to be two-beam interference and a computer program to simulate the interference fringe pattern was established. By comparing the simulated fringe pattern and experimental fringe pattern, the refractive index of liquid can be given when the two fringes coincide best. The refractive indices of some transparent samples were mea-sured by using this method and Abbe refratometer.

∗ *ailingy@ouc.edu.cn (Yang Ailing); phone 86-532-85901204; fax: 86-532-85901204

27th International Congress on High-Speed Photography and Photonics, edited by Xun Hou, Wei Zhao, Baoli Yao, Proc. of SPIE Vol. 6279,

62794W, (2007) · 0277-786X/07/$15 · doi: 10.1117/12.725566

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The results showed that the results by the two methods are very near. For low transparent liquids, although the intensity of transmission rays is weak, the difference of optical path length(OPL)of the two interference rays is very little, that means the intensity of the two rays is very near, i.e., the contrast of the fringe is good. So this technique can be used to measure the refractive indices of the low transparent liquids. In this work, soluble organic matter in the source rocks were extracted by Soxhlet method. The extractions are low transparent and includes saturated hydrocarbon, aromatic hydrocarbon, resin and bitumen. After diluted by CHCl3 as mass ratio 1:6, the refractive indices(n) were measured by capillary tube interferometer. In order to quantitatively relate n changes to maturity，we rely on vitrinite reflectance Ro as the thermal maturity in rocks containing OM. As we know, it is the first time the curve of Ro and n was established. It is a promising indicator to evaluate the maturity of source rock.

2. THEORY Fig. 1 is the experimental setup to study the refractive indices of liquids. The capillary tube was located in front of a cylindrical lens. It’s axis is parallel to z-axis.

For a capillary tube located in a fixed position at a definite distance to the cylindrical lens, the ray position on the screen will be a function of y, where y is the distance between the incident ray on the cylindrical lens and the optical axis(see Fig. 2).

The optical path of a ray with y value can be calculated according to Fig. 2. From the tangent plane of the curved surface of the cylindrical lens to the screen, the optical path length (OPL) of the ray can be calculated as following:

2a cy a g aOPL MAn ABn BCn CDn DEn FGn= + + + + +

（1） Where na is the refractive index of the air, ncy is the refractive index of the cylindrical lens, ng, n are the refractive indices of the tube wall and the investigated liquid. According to the geometry of Fig. 2 and Snell’s law, Each OPL in Eq.(1) can be obtained gradually. The detailed calculation is in Ref. 25.

Based on the theoretical analysis, ray-tracing pattern was calculated and showed in Fig.3. From this figure, one can see that two rays with different y values (or different incident angles) will reach the same point on the screen. They could interfere if the interference condition matches. One also can see that the rays from the upper part of the cylindrical lens will reach the lower screen, so the fringes on the upper screen are formed by the incident rays on the lower part of the cylindrical lens. This is proved by our experiment. If the upper rays on the cylindrical lens were blocked, the fringes on the lower screen will disappear.

Fig. 4 shows the ray position on the cylindrical lens and corresponding OPL. One can see that, for the part of AB or CD, each OPL has two y values, and this coincides with Fig.2. Fig.5 shows the ray position on the cylindrical lens and corresponding position on the screen. One can see that for partial screen, one s value has two y values. That is to say two rays on the cylindrical lens with different positions will reach same point on the screen. When the interference condition

xy

z

Fig. 1. The experimental setup to produce interference pattern

Diaphragm

Collimating Lens

Microscope objective

Laser Source

Cylindrical Lens

Capillary Tube

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OP

LIcm

(n

(n

(n

(n

P

P

P

P

n J

- o

co

(n

51cm

)

L C

L

C

ReyS

Reyl

Rey2Screen

Cyrindricar 'ens

Fig.4. OPL as a function of the ray position on the cylindrical lens.

Fig. 5. The ray position on the screen as a function of the position on the cylindrical lens.

Fig. 3. Ray-tracing diagram from cylindrical lens to the screen. Ray 1, Ray 2 and Ray 3 are three typical rays.

Fig. 2. A schematic diagram represents the path of the incident beam as passing through the capillary tube.

xX

Os

Screen

G

s

2l

Fϕ

2R

2r

y

gnnan

CψCβ C

Cϕ

E

DθD

F

H

Cα

O

Capillary tube

Cθ

1l dcy

y

BϕB

cyn

A Bθ Aϕ

Aθ

M

Cylindrical lens

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matches, the two rays will interfere.

A computer program has been constructed to theoretically simulate the interference pattern. From equation (1), the OPL has been calculated for both rays; we then calculate the optical path difference ( ∆ ), which is easily transformed into the phase difference( δ ) by using the relation

2πδλ

∆＝ (2)

where λ is the wavelength of the light used. The intensity distribution can be calculated by the equation below:

2 24 cos ( )2

I A δ= (3)

where I is the intensity of the interference fringes and A the amplitude of each pair of interfering rays. Because the distance given by equation (1) is a function of liquid refractive index, the position of the fringe on the screen is also a function of the refractive index of the liquid. To obtain the intensity distribution as an estimated images, we make the maximum intensity( δ=0 ) equal to 256 which corresponds to the brightness grey level of the computer monitor used. The brightest grey level will be at δ equal to 0; all the other grey values will depend on the cosine value. Starting from the maximum position on the cylindrical lens to the minimum point of it, the intensity distribution was calculated using equation (1), (2) and (3), which gives a line of pixels describing the intensity distribution on the screen. Repeating that line of pixels using the same grey level we can obtain a two-dimensional array which represents the estimated image of the theoretical interferogram.

3. EXPERIMENTAL SETUP Fig. 1 shows the experimental set-up. A He-Ne laser of wavelength 632.8nm is used as a light source. After a micro-objective mirror, a spatial filter and a collimating lens, a parallel beam with nearly 10cm diameter is obtained. The cylindrical lens made of K-9 glass of refractive index 1.5163 is used , whose focal length is about 31.2cm. The inner and outer diameters of the capillary tube are 3mm and 4mm respectively. The width of the focused beam is less than 1mm, which is very small compared with the diameter of the capillary tube. The capillary tube is parallel to and within the focal lines of the cylindrical lens.

The rays passing through the capillary tube form an interference pattern on the screen. At the same time, we also observed the interference pattern formed by the reflected light of the two first walls of the capillary tube. This fringe pattern is reflected by the cylindrical lens and superimposes on the fringe pattern formed by the transmission rays. This will influence the contrast of the fringe pattern. So a diaphragm was put on the left of the capillary tube to filter this fringe.

A high performance CCD camera records the interference pattern, which is stored in a computer for later digital processing. The desired part of the fringe pattern can be selected and processed with an image-enhancing computer program. The enhanced pattern is compared with a theoretically estimated one. When the two sets of fringes coincide with each other, the refractive index of the simulated pattern is that of the investigated liquid.

4. MEASUREMENT AND RESULTS In the experimental process, we found the heights of the fringes (s value) are very sensitive to the distance l1. But it is very difficult to measure this distance with high accuracy. We obtained this distance by fitting to two known index as the trial liquids. This distance is 31.018cm by this method. Once this distance is determined, all the elements in the experiment setup keep stationary for later samples. Here l2 is 67.5cm.

Fig. 6 (a) is a selected part of the interference pattern of mixture of ethanol and carbon bisulfide（CS2）. The middle part of the interferogram was selected and expanded and showed in Figure6 (b). Figure6 (c) is the theoretical estimated pattern. For accurate contrast, a curve of intensity with s value is established. In Figure6 (d), the experimental and simulated curves are put togeather, they can be enlarged and made a detailed contrast. The intensities in the two curves are normalized. When the two curves coincide with each other, the refractive index of the simulated pattern is that of the

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1.3

1.2

1.1

0.0

0.8

0.7

0.5

0.5

0.4

0.31.44 1.45 1.45 1.47 1.48 1.40 1.5 1.51 1.52

——

—

—

=—

=—

—

—

s/cm

investigated liquid.

For checking the precision of the measurement, we measured the indices of five kinds of liquids by using Abbe refractometer and capillary tube interferometer. The results are showed in Table 1. From this table, one can see that the results are very approximate.

Further more, we measured the refractive indices of eleven diluted extraction of source rocks. All the source rocks were selected from four oil wells in Shengli Oil Field(China). The Sample locations, depths, lithology and vitrinite reflectance Ro are showed in Table 2. Ro is in the range of 0.36-1.25, which corresponds non-maturity and maturity.

Fig. 7 shows the Ro-n curve. One can see that n increases with Ro. When Ro is in the range of 0.36-1.25, the two parameters have good correlation. This shows that n could be as an indicator of maturity of source rock.

For test this technique, we measured the refractive index of diluted extraction of sample 12 by capillary tube interferometer, n=1.5000. From Ro-n curve, we read that Ro is 1.12. Then we used traditional method to measure Ro of this sample, the result is 1.2. The two results are slight different. The main reason is this sample is scarcity of vitrinate maceral, the measured points are relative fewer, so the result of traditional method is larger than the real.

5. CONCLUSION In this work, a curve between the refractive indices of diluted extraction of source rocks and Ro was established. The results show that n increases with Ro or the maturity of source rocks. When Ro is in the range of 0.36-1.25, the two

Figure 7. Correlation between Ro and refractive indices of diluted extraction of mudstone samples.

Fig.6. Experimental and theoretical estimated interference pattern. (a) experimental pattern, (b) experimental pattern after digital processing, (c) estimated pattern, (d) intensity changes with the experimental and simulated fringe position on the screen.

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parameters have good correlation. The refractive index of soluble OM of source rock could be an indicator of maturity. Because the measurement range of capillary tube interferometer is wide and can successfully measure the refractive indices of low transparent liquids, it is a promising method to evaluate the maturity of source rock, especially those source rocks scarcity or absence of vitrinite in some marine sedimentary environments, or the vitrinite reflectance suppressed by certain process during biodegradation, early digenesis or maturation.

ACKNOWLEDGEMENTS This work was sponsored by National 863 High Technique Plan of China( granted No. 2004AA616230). The authors are very appreciated for the Shengli Oil Field for the applying for the source rock samples.

Table 1. The refractive indices of Abbe refractometor and capillary tube interferometer methods

Table 2. Sample locations, depths, lithology and vitrinite reflectance Ro.

REFERENCES

1． Rui Lin, “An interlaboratory comparison of virtrinate reflecatance measurement, ” Org. Geochem. 22, 1-9(1995). 2. Eric Mueller, and R. Paul Philp, “Extraction of high molecular weight hydrocarbons from source rocks: an example

from the Green River Formation,” Unita Basin, Utah. Org. Geochem. 28, 625-631 (1998). 3. M. Letellier, H. Budzinski, J. Bellocq, and J. Connan, “Focused microwave-assisted extraction of polycyclic

aromatic hydrocarbons and alkanes from sediments and source rocks,” Organic Geochemistry 30, 1353-1365 (1999).

4. Grzegorz P. Lis, Maria Mastalerz, Arndt Schimmelmann, Michael D. Lewan, and B. Artur Stankiewicz, “FTIR absorption indices for thermal maturity in comparison from Devonian black shales,” Organic Geochemistry 36, 1533-1552 (2005).

Sample name n1: Abbe refractometor n2: capillary tube n1-n2 ethanol 1.3510 1.3540 -0.0030 gasoline 1.4394 1.4395 -0.0001 carbon tetrachloride 1.4460 1.4463 -0.0003 diesel oil 1.4604 1.4601 0.0003 mixture of ethanol and CS2 1.5724 1.5724 -0.0000

Sample Well name Depth Lithology Ro 1 Laixie61 2293 light gray 0.36 2 Laixie61 2429 gray black 0.38 3 Lai64 3345 gray black 0.54 4 Lai64 3663.5 gray black 0.58 5 Lai64 3746.4 Black 0.68 6 Lai64 3791 Black 0.73 7 Lai64 3813 Black 0.75 8 Lai64 3907 Black 0.75 9 Laisheng1 4161.7 deep black 0.90 10 Laisheng1 4163 Black 1.06 11 Wang46 4203 Black 1.25 12 Bo930 3025 Black 1.20

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5. L.I.P.Dzou, R.A.Noble, and J.T.Senftle, “Maturation effects on absolute biomarker concentration in a suite of coals and associated vitrinite concentrates,” Org. Geochim. 33, 681-697 (1995).

6. Maria Mastalerz, and R.Marc Bustin, “Variation in reflectance and chemistry of vitrinite and vitrinite precursors in a series of Tertiary Coals, ” Arctic Canada. Org Geochem. 22, 921-933 (1994).

7. Christoph Spotl, David W. Houseknecht, and Robert C. Jaques, “Kerogen maturation and incipient graphitization of hydrocarbon source rocks in the Arkoma Basin,Oklahoma and Arkansas: a conbined petrographic and Raman spectrometric study,” Org Geochem. 28, 535-542 (1998).

8. Xiao Xiaoming, R．W．T．Wilkins, Liu Dehan, Liu Zufa, and Fu Jiamu, “Investigation of thermal maturity of lower Palaeozoic hydrocarbon source rocks by means of vitrinite-like maceral reflectance — a Tarim Basin case study,” Organic Geochememisty 31,1041-1052 (2000).

9. Charles R．Landis and Jone R．Castano． “Maturation and bulk chemical properties of a suite of solid hydrocarbons,” Organic Geochememisty 22, 137-149 (1995).

10. Chen Jianping, Luo Ping, and Li JInchao, “Using kerogen RTIR parameters for determination of organic facies,” Chinese Science Bulletin 43, 681-684 (1998).

11. Maria Mastalerz, and R. Marc Bustin, “Application of reflectance micro-Fourier transform infrared spectrometry in studying coal macerals: comparison with other Fourier transform infrared techniques,” Fuel 74, 536-542 (1995).

12. Ulrike Werner-Zwanziger, Grzegorz Lis, Maria Mastalerz, and Arndt Schimmelmann, “Thermal maturity of type II kerogen from the New Albany Shale assessed by C CP/MAS NMR,” Solid State Nuclear Magnetic Resonance 27, 140-148 (2005).

13. Junhong Chen, Jiamo Fu, Guoying Sheng, Dehan Liu, and Jiajun Zhang, “Diamondoid hydrocarbon ratios: novel maturity indices for highly mature crude oils,” Organic Geochememisty 25, 179-190 (1996).

14. M.Radke, S.P.Vriend, and L.R.Ramanampisoa, “Alkyldibenzofurans in terrestrial rocks:Influence of organic facies and maturation,” Geochimica et Cosmochimica Acta 64, 275-286 (2000).

15. Imam B .Sosrowidjojo, Andrew P. Murray, Robert Alexander, Robert I.Kagi, and Roger E. Summons, “Bicadinanes and related compounds as maturity indicators for oils and sediments,” Org Geochem. 24, 43-55 (1996).

16. Alexander Chakhlexander Chakhmakhchev and Noriyuki Suzukf, “Aromatic sulfur compounds as maturity indicators for petroleums from the Buzuluk depression,” Russia Org. Geochem. 23, 617-625 (1995).

17. Xiao XianMing, Liu Zufa, and Shen Jiagui, “Characterization and origin types of immature and low-mature amorphous kerogen in terrestrial source rocks,” Chinese Science Bulletin 43, 241-244 (1998).

18. M.Obermajer, L.D.Stasiuk, M.G.Fowler, and K.G.Osadetz, “Application of acritarch fluorescence in themal marutiry studies,” International Journal of Coal Geology 39, 185-204 (1999).

19. Ronald W.T., Wilkins, John R.Wilmshurst, George Hladky, Michael V. Ellacott, and Carol P. Buckingham, “Shoud fluorescence alteration replace vitrinite reflectance as a major tool for thermal maturity determination in oil exploration?,” Org Geochem., 191-209 (1995).

20. C.R.Robison, P.van Gijzel, and L.M.Darnell, “The transmittance color index of amorphous organic matter:a therman maturity indicator for petroleum source rocks,” International Journal of Coal Geology 43, 83-103(2000).

21. H. El Ghandoor, E. Hegazi, Ibraheem Nasser, and G. M. Behery, “Measuring the refractive index of crude oil using a capillary tube interferometer,” Optics & Laser Technology 35, 361-367 (2003).

22. El Ghandoor，H. Nasser, I., Abdel-Hady, and A. Al-Shukri, “A comparation study of spliced optical fibers,” Optics and lasers in Engineering 41, 277-287 (2004).

23. A. A. Hamza, M. A. Mabrouk, W. A. Ramadan, and H.H. Wahba, “Core index determination of a thick fiber using lens-fiber interference(LIF) technique,” Optics and Lasers in Engineering 42, 121-130 (2004).

24. W A Ramadan, “On-line lens-fiber interference method for testing a thick fiber,” J. Opt. A: Pure Appl. Opt. 2, 234-238 (2000).

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25. Yang Ailing，Liwendong, Yuan Guang, Dong Junyu, and Zhang Jinliang, “Measing refractive indices by capillary tube interferometer,” Appli. Opt. 31, 2006(to be published)

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