The Xihu Sag in the East China Sea Basin is rich in natural gas and condensate resources. Previous geological and geochemical studies have confirmed that large-scale gas washing and component fractionation have occurred in this area. Gas washing resulted in the development of oil inclusions with multiple fluorescences in the area. Based on the history of hydrocarbon generation and expulsion, it can be concluded that a large number of blue-green fluorescent oil inclusions contain mature oils that have not been secondarily altered. Therefore, the oil inclusions with other fluorescent colors should have recorded the oils suffering from gas washing.

The location of paleo oil-water contact was obtained using the Grains containing Oil Inclusions (GOI^TM) technique where as the well-logging curves determined the present-day oil-water contact in the Dongying Sag. It was found that the oil reservoir has been adjusted. Therefore the oil-water contact has changed and moves upward about 60 m. It means that the formation waters have entered the residual paleo oil zone, and the water-washing has also occurred. The oil inclusions in rock samples above the present oil-water interface were selected as the captured oil inclusions without being washed, and the oil inclusions in rock samples below the present oil-water interface to above the ancient oil-water interface were selected as the captured oil inclusions after being washed.

1.2.1 Micro-fluorescence spectrum

Crude oil contains unsaturated hydrocarbon compounds and therefore can emit fluorescence wavelength due to π electron transition under ultraviolet light irradiation. The fluorescence spectrum (UV-visible absorption spectrum) can be recorded by a spectrometer, which can semi-quantitatively evaluate oil components^[9].

In this study, hydrocarbon inclusions were observed using a Nikon 80I dual-channel fluorescence microscope, with an ultraviolet excitation wavelength of 330~380 nm. Maya 2000 Pro spectrometer was used to record the fluorescence spectrum. Finally, the Yuanao micro-spectral analysis system was used to process the spectral data. Some results are shown in Table 1.

Table 1

Table 1

Table 1 Fluorescence and infrared spectral parameters of single oil inclusions Region Alteration Inclusion occurrence λ max QF535 CH2/CH3 H2O/alkane Xihu sag No gas-washing HFWQG 489 0.93 1.5 0.2 HFWQG 492 0.91 1.4 1.2 HFWQG 488 0.84 1.6 2.4 HFCTQG 494 0.89 1.7 0.1 HFCTQG 496 0.68 1.5 4.5 HFWQG 490 0.75 1.8 2.9 Gas-washing HFCTQG 463 0.67 1.3 0.4 HFCTQG 462 0.53 1.6 0.1 HFCTQG 459 0.35 1.5 2.6 HFCTQG 489 1.0 1.7 3.1 HFCTQG 495 0.92 1.5 0.5 HFCTQG 493 0.86 1.3 0.9 HFCTQG 488 0.89 1.1 3.8 HFCTQG 513 1.18 1.9 2.4 HFCTQG 514 1.45 1.9 5.4 HFCTQG 515 1.65 2.0 1.5 Dongying sag No water-washing HFWQG 440 0.38 0.9 0.6 HFWQG 445 0.44 1.1 0.3 HFWQG 439 0.55 1.5 1.4 HFWQG 442 0.57 1.7 2.5 HFCTQG 442 0.41 1.3 3.5 HFCTQG 445 0.54 1.3 6.4 HFCTQG 440 0.51 1.4 0.5 Water-washing HFWQG 439 0.38 1.4 0.9 HFWQG 445 0.52 1.55 5.4 HFWQG 475 0.67 2.1 10.6 HFWQG 472 0.77 1.3 2.2 HFWQG 495 0.9 1.9 12.6 HFWQG 496 1.24 2.3 11.5 HFWQG 520 1.33 2.6 7.5 HFWQG 522 1.56 1.8 7.9 Note: HFWQG: Healed fracture within quartz grain; HFCTQG: healed fracture cutting through quartz grain

Table 1 Fluorescence and infrared spectral parameters of single oil inclusions

1.2.2 Fourier transform infrared spectrum

The relationship between fluorescence spectral parameters and thermal maturity of hydrocarbon exists controversial. Moreover, fluorescence characteristics (color, wavelength, and spectral parameters) can not be correlated with the complex chemical structures of oil. After fluorescence spectrum measurement, the fluid inclusion thin section was immersed in acetone to remove the glue.

Generally, oil maturity correlates with the spectral wavelength λ _max, QF₅₃₅ and CIE-XY of fluorescence spectrum. If the oil maturity is high, the fluorescence wavelength can shift from blue, white, yellow to orange. Both QF₅₃₅ and CIE-XY values will increase^{[10, 11]}.

According to the results (Table 1 and Fig.1), gas-liquid two-phase oil inclusions with various fluorescence colors (bright blue, blue-green and yellow-white) occur in the reservoirs of the Xihu Sag, East China Sea Basin. The main peak wavelength λ max in the corresponding fluorescence spectrum is about 460, 490 and 515 nm, and the QF₅₃₅ has a wide distribution ranging from 0.3 to 1.78. In addition to the maximum main peak, suggesting the heterogeneity of incmusion oil components. Human observation error can be eliminated by CIE-XY chromatogram, which therefore depicts accurate fluorescence color of oil inclusions. As shown in Fig.1, it can be found that from that the fluorescence color of oil inclusions displays a dispersive distribution, ranging from blue, white, to yellow white, while both bubble size and color of oil inclusions do not covary.

Fig.1

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	Fig.1 Fluorescence spectra and CIE chromaticity of oil inclusions in reservoirs altered by gas-washing

Table 1 and Fig.2 show that the wide fluorescence color range of oil inclusions is also observed in the reservoirs of the Dongying Depression. There are four types of maximum main peak wavelength (λ _max) in the fluorescence spectrum, including 440, 475, 490 and 520 nm. The corresponding CIE-XY chromaticity diagram also shows that the fluorescence color of oil inclusions vary from blue white, white to yellow. The yellow fluorescent oil inclusions have small bubbles, with a wide QF₅₃₅ ranging from 0.22 to 1.8.

Fig.2

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	Fig.2 Fluorescence spectra and CIE chromaticity of oil inclusions in reservoirs altered by water-washing

Overall, from the perspective of fluorescence color alone, both water-washing and gas-washing can lead to various fluorescence colors of oil inclusions. Hence it is difficult to distinguish the two types of secondary alteration processes.

FTIR spectra of hydrocarbon inclusions show an obvious stretching vibration in the absorption peak position of 2 800~3 000 cm^-1 that indicates the aliphatic structure of the organic compounds in the inclusions^[9]. This peak segment generally includes four types: CH₃ and CH₂ asymmetric stretching vibrations and CH₃ and CH₂ symmetric stretching vibrations. The intensity ratio of CH₂/CH₃ absorption peak can represent the chaio length of alkanes and thus is used to evaluate the thermal maturity of organic matter. Generally, a negative correlation between CH₂/CH₃ ratio and thermal maturity occurs. In addition, the absorption peak position of 3 700~3 200 cm^-1 represents the existence of H₂O, hence H₂O/Alkanes (water/oil ratio) can be used to evaluate the relative abundance of water and oil within the inclusions.

As shown in Fig.3(a), the infrared spectrum pattern of trapped oils suffering from gas washing shows an obvious alteration of the relative peak intensity of CH₃ and CH₂, whereas the absorption peak intensity of water has not changed significantly. The infrared spectra of trapped oils that underwent water-washing are also different [Fig.3(b)]. Besides, the relative peak intensity of CH₃ and CH₂, the absorption peak intensity of water increases obviously. Overall, the CH₂/CH₃ and H₂O/Alkanes of the oil inclusions changed before and after gas-washing and water-washing.

Fig.3

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	Fig.3 (a) Fourier infrared spectra of inclusion trapping oil altered by gas-washing; (b) Fourier infrared spectra of inclusion trapping oil altered by water-washing

Gas-washing refers to the geological process in which external natural gas or condensate enters the oil reservoirs and then interacts with oils, leading to oil compositions^[12]. The fluorescence characteristics of oil inclusions can be related to gas-washing fractionation in the Xihu Sag of the East China Sea Basin. The original oils have been recorded by the blue-white fluorescent oil inclusions, with the main peak wavelength of 490 nm. Due to the effect of later gas-washing, the short-chain alkanes in the original oils were gasified into the natural gas while the residual oils are rich in heavy components because of the loss of light components. As a result, the fluorescence λ _max of residual oils is gradually shifted to about 515 nm (Fig.1). The light hydrocarbon components are mixed with natural gas, forming gas condensate. It leads to a blue shift of fluorescence color relative to the original oils. The corresponding λ _max is altered into about 460 nm. Therefore, the fluorescent color of oil inclusions in the CIE chromaticity diagram appears a gradual distribution.

The fluorescence spectral parameters of oil inclusions also change concerning the oils before gas-washing. Based on the histograms of fluorescence and infrared spectral parameters (Fig.4), it can be found that the QF₅₃₅ range of oils altered by gas-washing expand toward decrease and increase directions, reflecting a component fractionation related to gas-washing. In addition, the histogram distribution of CH₂/CH₃ ratio of oils that suffered from gas-washing has been altered to some extent. The distribution range of CH₂/CH₃ was not significantly expanded while the difference of histogram peaks becomes small.

Fig.4

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	Fig.4 Histogram of spectral parameters QF-535, CH2/CH3, and H2O/Alkanes for inclusions trapping oil altered by gas-washing and original oil

Water-washing is a process in which reservoir oils are washed by formation waters and thus causes the change of oil composition. In fact, water washing is a common phenomenon and mainly occurs near the oil-water contact. Because of physical adjustment or chemical modification of the oil reservoirs, the oil-water contact surface often changes. The present oil-water contact location can be obtained by well-logging, but the paleo oil-water contact is not easy to identify. When the fluid inclusion technology is used to analyze the reservoir filling stages, it is easy to ignore this type of secondary alteration. Therefore, various fluorescent oil inclusions may be considered as the result of multi-stage charge.

Some scholars observed that the effect of water-washing on oil mainly lies in the dissolution of methane, small molecular n-alkanes and small molecular aromatic compounds in crude oil into waters, while medium to large molecular compounds is almost unaffected^{[13, 14]}. In other words, water-washing causes a loss of light end and an enrichment of heavy components in residual oils, resulting in the shift of fluorescence color toward the red region. The local formation waters enter into the paleo oil zone and interact with the oils. The oil components were gradually changed, resulting in oil inclusions with multiple main peak wavelengths of fluorescence spectra. Moreover, the fluorescence color in the CIE chromaticity diagram also showed a gradual change. In Fig.5, it can be seen that the distribution range of microscopic fluorescence spectrum parameter QF₅₃₅ of oil altered by water-washing expands to the increasing direction, which indicates a gradual enrichment of heavy components. It can also be observed that the two types of oils altered by water-washing and gas-washing show an different change trend of CH₂/CH₃. The distribution range of CH₂/CH₃ of oils altered by water-washing increases obviously, which may be because water-washing leads to a more strong enrichment in the large molecular chain hydrocarbons. The H₂O/alkanes ratios in the inclusions that contain oils altered by water-washing determined by infrared spectroscopy are significantly higher than those of the unaltered oils. The formation waters are displaced during hydrocarbon filling, but a small amount of water remains at the capillary ends among the grain pores. Hence, sometimes the trapped oil inclusions may contain a small number of water. If the oil is subject to large-scale water-washing, the water content of the trapped oil inclusions may increase. Also, the H₂O/alkanes ratio would be elevated significantly.

Fig.5

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	Fig.5 Histogram of spectral parameters QF-535, CH2/CH3 and H2O/Alkanes for inclusions trapping oil altered by water-washing and original oil

Regardless of water-washing or gas-washing, the fluorescence color of the oil inclusions trapped in the reservoirs is various, with a wide distribution range. Therefore, it is difficult to distinguish the two alteration types of the oils accurately. However, based on the characteristics of fluorescence and infrared spectra of single oil inclusions in the oil reservoirs that experienced gas-washing and water-washing, the two different change trends mentioned above can be used to identify them.

(1) Distribution trend of fluorescence spectral parameter QF₅₃₅

Compared with the unaltered oils, the minimum and maximum fluorescence spectral QF₅₃₅ values of those altered by gas-washing decrease and increase, respectively. Hence, they have a wider QF₅₃₅ distribution. The QF₅₃₅ values of water-washing-altered oils also have a broader range, but almost all of them change toward increased direction. However, it should be noted that due to large amounts of light hydrocarbon components in the light oil reservoirs, the fluorescence color of evaporated oils caused by gas-washing hardly has an obvious blue shift.

(2) Distribution trend of infrared spectral parameter CH₂/CH₃

The distribution range of CH₂/CH₃ histogram of oils altered by gas-washing does not expand significantly, but its peak value is averaged. However, the distribution range of CH₂/CH₃ histogram of those altered by water-washing obviously increases. For heavy reservoirs, however, the heavy oil itself contains less light hydrocarbon components. Therefore oil compositions may not change much after water-washing. It is difficult to distinguish by the CH₂/CH₃ ratio.

(3) Infrared spectral parameters H₂O/Alkanes

Due to the involvement of waters, the H₂O/Alkanes distribution of inclusion oils altered by water-washing increases significantly, while that of the oils altered by gas-washing has no obvious change.

To sum up, the variation characteristics of the spectra of oils altered by gas-washing and waters-washing can be summarized as the two charts to distinguish them (Fig.6).

Fig.6

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	Fig.6 Two spectral parameter distribution trend pattern diagrams to identify oil reservoirs subjected to water-washing and gas-washing

If the paleo-oil reservoir is a medium-heavy oil reservoir in this area, a combination of QF₅₃₅ and H₂O/Alkanes can effectively distinguish between these two secondary alterations. The QF₅₃₅ value of oil altered by gas-washing increases obviously, whereas the H₂O/Alkanes ratio exhibits little change. Thus, the datasets (QF₅₃₅ and H₂O/Alkanes) show a flat ellipsoid distribution in the chart. After water-washing, the QF₅₃₅ value of oil would expand toward the increasing area while H₂O/Alkanes ratio is significantly elevated. Their datasets would show a spherical distribution in the chart.

If the paleo-oil reservoir is a light oil reservoir in this area, the combination of spectral parameters CH₂/CH₃ and H₂O/alkanes in single oil inclusions is effective. Both CH₂/CH₃ and H₂O/alkanes ratios of the oils altered by gas-washing have no distinct changes, while those of the oils altered by water-washing increase significantly. The completely different distribution trend leads to a good identification effect for the two secondary processes.