Advanced Search

Indexed by SCI、CA、РЖ、PA、CSA、ZR、etc .

Volume 31 Issue 6
Dec.  2020
Turn off MathJax
Article Contents

Xiuyan Liu, Honghan Chen, Xuewei Xiao, Hongan Zhang, Tianwu Xu. Mixing Characteristics of Oil Inclusions with Different Thermal Maturities in the Wenliu Uplift, Dongpu Depression, Bohai Bay Basin, North China. Journal of Earth Science, 2020, 31(6): 1251-1258. doi: 10.1007/s12583-020-1356-0
Citation: Xiuyan Liu, Honghan Chen, Xuewei Xiao, Hongan Zhang, Tianwu Xu. Mixing Characteristics of Oil Inclusions with Different Thermal Maturities in the Wenliu Uplift, Dongpu Depression, Bohai Bay Basin, North China. Journal of Earth Science, 2020, 31(6): 1251-1258. doi: 10.1007/s12583-020-1356-0

Mixing Characteristics of Oil Inclusions with Different Thermal Maturities in the Wenliu Uplift, Dongpu Depression, Bohai Bay Basin, North China

doi: 10.1007/s12583-020-1356-0
More Information
  • Multiple types of oil inclusions with different fluorescence colors have been detected in the northern Dongpu depression. Recent evidences show that these inclusions may have been trapped simultaneously or during a very short period. Therefore, whether these oils were mixed before trapping is unknown. In this study, we analyzed the petrography and fluorescence spectral characteristics of oil inclusions in the Wenliu uplift in the northern Dongpu depression, and assessed the data with the oil mixing ratio curve obtained in the previous experiment. The results show that there are three types of oil inclusions (type Ⅰ, type Ⅱ and type Ⅲ) with yellow, green and blue fluorescence colors, corresponding to low-mature, medium-mature and high-mature oil, respectively. The "pure" oil inclusions, do exist in type Ⅱ and type Ⅲ group of oil inclusions, showing the medium-mature oil was generated from the source rock rather than being formed by the mixture of high-mature and low-mature end oils. Most of the oil inclusions are mixtures of high-mature and medium-mature end oils from the sub-sags to the Wenliu uplift, the mixing degree increases to close to 50%. The oil between the Qianliyuan sub-sag and the Wenliu uplift was mainly mixed by medium mature oils, whereas the oil between the Liutun sub-sag and the Wenliu uplift was mainly mixed by high mature oil.
  • 加载中
  • Feng, Z. D., Chen, X. S., Fu, X. L., et al., 2014. Neotectonic Movements in the Dongpu Depression and Their Controls on Hydrocarbon Accumulation in Shallow Reservoirs. Sedimentary Geology and Tethyan Geology, 34(1):8-13 (in Chinese with English Abstract) http://en.cnki.com.cn/Article_en/CJFDTOTAL-TTSD201401002.htm
    Goldstein, R. H., Reynolds, T. J., 1994. Systematics of Fluid Inclusions in Diagenetic Minerals. Society for Sedimentary Geology Short Course, 31:199 http://ci.nii.ac.jp/naid/10010291997
    Guo, X. W., Liu, K. Y., He, S., et al., 2012. Petroleum Generation and Charge History of the Northern Dongying Depression, Bohai Bay Basin, China:Insight from Integrated Fluid Inclusion Analysis and Basin Modelling. Marine and Petroleum Geology, 32(1):21-35. https://doi.org/10.1016/j.marpetgeo.2011.12.007 doi:  10.1016/j.marpetgeo.2011.12.007
    Jiang, Y. L., Fang, L., Liu, J. D., et al., 2016. Hydrocarbon Charge History of the Paleogene Reservoir in the Northern Dongpu Depression, Bohai Bay Basin, China. Petroleum Science, 13(4):625-641. https://doi.org/10.1007/s12182-016-0130-5 doi:  10.1007/s12182-016-0130-5
    Liu, J. D., Jiang, Y. L., 2013. Thermal Evolution Characteristics of Paleogene Source Rocks and Their Main Controlling Factors in Northern Part of Dongpu Depression. Geology in China, 40(2):498-507 (in Chinese with English Abstract) http://www.cqvip.com/QK/90050X/20132/45861656.html
    Liu, X. Y., Chen, H. H., Zhang, H. A., et al., 2020. Characteristics of Oil Reservoiring and Its Relationship with Pressure Evolution of the Shahejie Formation in Pucheng Area. Earth Science, 45(6):2210-2220. https://doi.org/10.3799/dqk.2019.222 (in Chinese with English Abstract) doi:  10.3799/dqk.2019.222
    Luo, Y., Liu, H. P., Zhao, Y. C., et al., 2016. Reevaluation of the Origin of Overpressure in the Inter-Salt Shale-Oil Reservoir in Liutun Sag, Dongpu Depression, China. Journal of Petroleum Science and Engineering, 146:1092-1100. https://doi.org/10.1016/j.petrol.2016.08.011 doi:  10.1016/j.petrol.2016.08.011
    Luo, Y., Zhao, Y. C., Chen, H. H., et al., 2015. Fracture Characteristics under the Coupling Effect of Tectonic Stress and Fluid Pressure:A Case Study of the Fractured Shale Oil Reservoir in Liutun Subsag, Dongpu Sag, Bohai Bay Basin, Eastern China. Petroleum Exploration and Development, 42(2):196-205. https://doi.org/10.1016/s1876-3804(15)30006-9 doi:  10.1016/s1876-3804(15)30006-9
    Munz, I. A., 2001. Petroleum Inclusions in Sedimentary Basins:Systematics, Analytical Methods and Applications. Lithos, 55(1/2/3/4):195-212. https://doi.org/10.1016/s0024-4937(00)00045-1 doi:  10.1016/s0024-4937(00)00045-1
    Pironon, J., Pradier, B., 1992. Ultraviolet-Fluorescence Alteration of Hydrocarbon Fluid Inclusions. Organic Geochemistry, 18(4):501-509. https://doi.org/10.1016/0146-6380(92)90113-c doi:  10.1016/0146-6380(92)90113-c
    Shao, X. H., Pang, X. Q., Li, H., et al., 2018. Pore Network Characteristics of Lacustrine Shales in the Dongpu Depression, Bohai Bay Basin, China, with Implications for Oil Retention. Marine and Petroleum Geology, 96:457-473. https://doi.org/10.1016/j.marpetgeo.2018.06.015 doi:  10.1016/j.marpetgeo.2018.06.015
    Si, S. H., Chen, H. H., Xiong, W. L., et al., 2017. Fluorescence Characteristics of Oil Inclusions of Yingshan Formation in Ma'nan Structural Belt of Maigaiti Slope. Xinjiang Petroleum Geology, 38(2):161-164 (in Chinese with English Abstract) http://en.cnki.com.cn/Article_en/CJFDTotal-XJSD201702008.htm
    Stasiuk, L. D., Snowdon, L. R., 1997. Fluorescence Micro-Spectrometry of Synthetic and Natural Hydrocarbon Fluid Inclusions:Crude Oil Chemistry, Density and Application to Petroleum Migration. Applied Geochemistry, 12(3):229-241. https://doi.org/10.1016/s0883-2927(96)00047-9 doi:  10.1016/s0883-2927(96)00047-9
    Su, A., Chen, H. H., He, C., et al., 2016. Microscopic Fluorescence Spectral Characteristics of Mixing Ratio of Crude Oil Experiment. Spectroscopy and Spectral Analysis, 36(9):3039-3046. https://doi.org/10.3964/j.issn. 1000-0593(2016)09-3039-08 doi:  10.3964/j.issn.1000-0593(2016)09-3039-08
    Su, A., Chen, H. H., Lei, M. Z., et al., 2019. Paleo-Pressure Evolution and Its Origin in the Pinghu Slope Belt of the Xihu Depression, East China Sea Basin. Marine and Petroleum Geology, 107:198-213. https://doi.org/10.1016/j.marpetgeo.2019.05.017 doi:  10.1016/j.marpetgeo.2019.05.017
    Wang, Q. R., Chen, H. H., Zhao, Y. T., et al., 2018. Differences of Hydrocarbon Accumulation Periods in Silurian of Tazhong Northern Slope, Tarim Basin. Earth Science, 43(2):577-593. https://doi.org/10.3799/dqkx.2018.026 (in Chinese with English Abstract) doi:  10.3799/dqkx.2018.026
    Wang, R. F., Shen, P. P., Zhao, L. J., 2011. Diagenesis of Deep Sandstone Reservoirs and a Quantitative Model of Porosity Evolution:Taking the Third Member of Shahejie Formation in the Wendong Oilfield, Dongpu Sag, as an Example. Petroleum Exploration and Development, 38(5):552-559. https://doi.org/10.1016/s1876-3804(11)60055-4 doi:  10.1016/s1876-3804(11)60055-4
    Xu, T. W., Zhang, H. A., Li, J. D., et al., 2019. Characters of Hydrocarbon Generation and Accumulation of Salt-Lake Facies in Dongpu Sag, Bohai Bay Basin. Oil & Gas Geology, 40(2):248-261 (in Chinese with English Abstract) http://www.researchgate.net/publication/332172701_Characters_of_hydrocarbon_generation_and_accumulation_of_salt-lake_facies_in_Dongpu_Sag_Bohai_Bay_Basin
    Zhang, X., Chen, H. H., Kong, L. T., et al., 2019. The Coupling Relationship between Paleofluid Pressure Evolution and Hydrocarbon-Charging Events in the Deep of Biyang Depression, Central China. Earth Science, 45(5):1769-1781. https://doi.org/10.3799/dqkx.2019.187 (in Chinese with English Abstract) doi:  10.3799/dqkx.2019.187
    Zhong, J. G., Peng, J., Cao, J., et al., 2017. Study on Sedimentary Microfacies of Shahejie Formation in W96 Gas Storage of Dongpu Depression. Petroleum Science and Technology, 35(14):1457-1467. https://doi.org/10.1080/10916466.2017.1344707 doi:  10.1080/10916466.2017.1344707
  • 加载中
通讯作者: 陈斌, bchen63@163.com
  • 1. 

    沈阳化工大学材料科学与工程学院 沈阳 110142

  1. 本站搜索
  2. 百度学术搜索
  3. 万方数据库搜索
  4. CNKI搜索

Figures(6)  / Tables(2)

Article Metrics

Article views(105) PDF downloads(9) Cited by()

Related
Proportional views

Mixing Characteristics of Oil Inclusions with Different Thermal Maturities in the Wenliu Uplift, Dongpu Depression, Bohai Bay Basin, North China

doi: 10.1007/s12583-020-1356-0

Abstract: Multiple types of oil inclusions with different fluorescence colors have been detected in the northern Dongpu depression. Recent evidences show that these inclusions may have been trapped simultaneously or during a very short period. Therefore, whether these oils were mixed before trapping is unknown. In this study, we analyzed the petrography and fluorescence spectral characteristics of oil inclusions in the Wenliu uplift in the northern Dongpu depression, and assessed the data with the oil mixing ratio curve obtained in the previous experiment. The results show that there are three types of oil inclusions (type Ⅰ, type Ⅱ and type Ⅲ) with yellow, green and blue fluorescence colors, corresponding to low-mature, medium-mature and high-mature oil, respectively. The "pure" oil inclusions, do exist in type Ⅱ and type Ⅲ group of oil inclusions, showing the medium-mature oil was generated from the source rock rather than being formed by the mixture of high-mature and low-mature end oils. Most of the oil inclusions are mixtures of high-mature and medium-mature end oils from the sub-sags to the Wenliu uplift, the mixing degree increases to close to 50%. The oil between the Qianliyuan sub-sag and the Wenliu uplift was mainly mixed by medium mature oils, whereas the oil between the Liutun sub-sag and the Wenliu uplift was mainly mixed by high mature oil.

Xiuyan Liu, Honghan Chen, Xuewei Xiao, Hongan Zhang, Tianwu Xu. Mixing Characteristics of Oil Inclusions with Different Thermal Maturities in the Wenliu Uplift, Dongpu Depression, Bohai Bay Basin, North China. Journal of Earth Science, 2020, 31(6): 1251-1258. doi: 10.1007/s12583-020-1356-0
Citation: Xiuyan Liu, Honghan Chen, Xuewei Xiao, Hongan Zhang, Tianwu Xu. Mixing Characteristics of Oil Inclusions with Different Thermal Maturities in the Wenliu Uplift, Dongpu Depression, Bohai Bay Basin, North China. Journal of Earth Science, 2020, 31(6): 1251-1258. doi: 10.1007/s12583-020-1356-0
  • The Dongpu depression is an important hydrocarbon-bearing depression in the Bohai Bay Basin of China. Based on its tectonic evolution, the Dongpu depression can be divided into two parts, the southern part and the northern part, by a blind strike-slip fault (Xu et al., 2019). Since the breakthrough discovery of Well PC 1 in 1975, the cumulative proven resources in the Dongpu depression are 5.9×108 t and natural gas is 1 387×108 m3. Nearly 90% of the proven oil and gas reserves are distributed in the northern part (Xu et al., 2019), so this has been a hotspot for many petroleum exploration companies (Shao et al., 2018; Zhong et al., 2017; Jiang et al., 2016; Luo et al., 2016, 2015; Liu and Jiang, 2013; Wang et al., 2011). Previous fluid inclusion studies have shown that there are multiple fluorescent oil inclusions in the same oil charging phase (Jiang et al., 2016), which means that crude oils of different maturities were generated simultaneously or during a short period by multiple sets of source rocks, as the fluorescence color is considered an index of oil maturity (Stasiuk In view of this problem, oil fluorescence spectra analysis technology provides us with a good means to determine thermal maturity and mixing features of oil inclusions. Once a fluorescence spectrum of a single oil inclusion is obtained, several parameters, including λmax, QF535, and Q, can be used to describe the maturity features of an oil inclusion (Munz, 2001). λmax and QF535, have been successfully applied to distinguish oil inclusions of different thermal maturities (Su et al., 2019; Zhang et al., 2019; Wang et al., 2018; Si et al., 2017; Guo et al., 2012). However, the potential oil mixing properties, which could also be identified from the fluorescence spectra (Su et al., 2016), have been ignored.

    In this study, we applied petrology studies, including thin section observation under transmitted light (TR), ultraviolet light (UV) and fluorescence spectra analysis to determine oil mixing properties in the Wenliu uplift in the Dongpu depression.

  • The Dongpu depression lies in the southwestern margin of the Bohai Bay Basin (Fig. 1a), which is the largest petroleum producing basin in China (Shao et al., 2018). The depression is an NNE trending Cenozoic basin, ~18 km wide in the north and ~62 km wide in the south covering an area of ~5 300 km2. It is separated from the Luxi uplift by the Lanliao fault in the east, and overlaps with the Neihuang uplift in the west. To the south, it is bounded from the Kaifeng depression by the Lankao uplift and to the north it is connected with the Xinxian depression by the Maling fault. From west towards east, the Dongpu depression can be divided into four sub units: the western slope belt, the western sag belt, the central uplift belt and the eastern sag belt (Fig. 1b). Tectonically, it underwent two regional movements. The first one is the second phase of the Himalayan movement, causing the unconformity between the Dongying and Guantao formations. The second one is the third phase of the Himalayan movement, leading to the unconformity between the Minghuazhen Formation and the Quaternary (Feng et al., 2014). The stratigraphy of the Dongpu depression is comprised of the Paleogene Shahejie Formation (Es) and Dongying Formation (Ed), the Neogene Guantao Formation (Ng) and Minghuazhen Formation (Nm), and the Quaternary Pingyuan Formation (Qp). The Shahejie Formation is comprised of four members, abbreviated as Es4, Es3, Es2, Es1 in ascending order. The third member of the Shahejie Formation is subdivided into four sections: Es34, Es33, Es32, and Es31, whereas the second and the fourth members of the Shahejie Formation are subdivided into two sections: Es2U, Es2L, Es4U and Es4L (Fig. 1c).

    Figure 1.  Sketch maps showing (a) the location of the Dongpu depression relative to the Bohai Bay Basin; and (b) the structural features of the Dongpu depression, location of the study area and the sampled wells; (c) cross section from the Liutun sub-sag to the Wenliu uplift to the Qianliyuan sub-sag, whose location is marked on Fig. 1b.

    The study area is a part of the Wenliu uplift (Fig. 1b), which is located in the central uplift belt of the depression. The Wenliu uplift is a main oil field in the Dongpu depression, due to the better hydrocarbon accumulation conditions in the surrounding sub-sags. There were sufficient hydrocarbon sources and the faults served as migration channels (Fig. 1c).

  • Thirty-five sandstone samples from Es3 and Es4 were collected from six wells (W142, W23, PS18, W203-15, W211 and PS7) on two sides of the Wenliu uplift, which were considered to be migration channels in which oil inclusions were most likely to be trapped. The samples were prepared as doubly polished thin sections of approximately 100–150 μm thickness for fluid inclusion analysis

    A Nikon 80I dual-channel fluorescence microscope was used, equipped with transmitted light (TR) and ultraviolet light (UV), for oil inclusion detection. The ultraviolet excitation wavelength was 365 nm. A Maya 2000 Pro micro-beam fluorescence spectrometer was used for the measurement of fluorescence spectra of oil inclusions. The least time for each measurement was taken because the exposure of petroleum to intense UV light at the focal point of an objective lens can result in alteration of the intensity and coloring of the fluorescence emission (Pironon and Pradier, 1992). All the oil inclusions detection was based on the concept of fluid inclusion assemblage (FIA) (Goldstein and Reynolds, 1994).

  • The diagenetic evolution of the northern Dongpu depression was divided into the early diagenetic stage B, the middle diagenetic phase A and the middle diagenetic phase B (Jiang et al., 2016). The dissolution of quartz and feldspar happened during the middle diagenetic phase A. At this time, the burial depth is from 3 300 to 4 200 m, and the temperature is from 135 to 160 ºC. The modeling burial and hydrocarbon generation histories indicated the source rocks came into the mediummature stage at the burial depth of 2 800 m, and came into the high-mature stage at the burial depth of 3 900 m. The oil charging time obtained by homogenization temperature and burial history projection indicated it started at the burial depth of around 3 200 m (Jiang et al., 2016). It is inferred that the feldspar dissolution happened when the oil charging started.

    The oil inclusions detected were generally in the dissolution fractures and pores of feldspar grains (Fig. 2), which indicates an early trapping event compared to the late secondary hydrocarbon generation and charging event (Jiang et al., 2016). Most of them showed a feature that oil inclusions with multiple fluorescence colors coexist in one FIA (Figs. 2a2d), whereas some of them were consistent with former results that only one type of oil inclusion was in a FIA (Liu et al., 2020; Jiang et al., 2016). These inclusions showed fluorescence colors ranging from deep yellow (Fig. 2d-ⅰ) to yellow-green (Fig. 2h), green (Figs. 2b-ⅰ, 2d-ⅱ), cyan (Fig. 2b-ⅱ, ) and blue (Fig. 2f), representing different maturities of oil. This coexistence of yellow and blue fluorescence color oil inclusions suggests a potential mixture of low-mature and high-mature oils.

    Figure 2.  Oil inclusion photos and their fluorescence maximum wavelengths. (a) TR, W203-15, 3 438.5 m, Esh3; (b) UV, same view field as A; (c) TR, W203-15, 3 438.5 m, Esh3; (d) UV, same view field as C; (e) TR, PS18, 4 075 m, Esh3; (f) UV, same view field as E; (g) TR, PS18, 4 225.54 m, Esh3; (h) UV, same view field as G.

  • λmax and QF535 were used as the main fluorescence parameters in the former studies (Zhang et al., 2019; Si et al., 2017), as they both decrease with increasing oil thermal maturity. So it is a good way to identify oil inclusions with different maturities using the λmax and QF535 cross-plot. In this study, we divided the fluid inclusions into three types (Fig. 3), their λmax and QF535 ranges are listed in Table 1. These results are very similar to those of the Pucheng area, where it was concluded that these types of oils were generated from different sets of source rocks simultaneously or during a short period, or from one set of source rocks at different geological time (Liu et al., 2020). However, the petrography study indicates that these oil inclusions in the dissolution fractures and pores of feldspar grains were trapped in the early stage, so these oils were generated from different sets of source rocks simultaneously or during a short period. Whether the mixture of oils with different maturities happened or not is hard to tell, even though they have different fluorescence colors. Fortunately, an experiment about the fluorescence spectra changes of the mixture of two types of oils provided us with experimental evidence to identify the mixture of oils (Su et al., 2016).

    Figure 3.  Relationship between λmax and QF535 of micro-beam fluorescent spectrum of single oil inclusion in the Shahejie Formation in the Wenliu uplift.

    Parameter Type Ⅰ Type Ⅱ Type Ⅲ
    λmax (nm) 570–576 514–552 458–499
    QF535 1.77–2.21 0.84–1.36 0.33–0.98

    Table 1.  Corresponding ranges of λmax and QF535 of each type of oil inclusions

    If one type of oil is "pure" (without any mixture after being generated from source rock), its fluorescence spectra should have only one peak and the corresponding wavelength is λmax. However, if it is a mixture of two "pure" oils, the spectra would show two or three peaks depending on the mixture ratio (Su et al., 2016). Based on this phenomenon, we overlaid the spectra together and we found spectra with only one peak as well as those with multiple peaks.

    In the fluid inclusions in the type Ⅲ group, we found several spectra have only one peak and their λmax are between 458 and 482 nm (Fig. 4a). Based on the experiment (Su et al., 2016), these oil inclusions are classified as "pure". We also found "pure" oil inclusions in the type Ⅱ group, whose λmax are between 522 and 554 nm (Fig. 4b). However, no "pure" oil inclusions were found in type Ⅰ (Fig. 4f). Combined with the petrography study, it shows that the "pure" oil inclusions do not coexist with other types of oil inclusions in one FIA (Fig. 2). The rest of the oil inclusions show spectra with two to several peaks, which indicates they are not "pure" but mixed with two or more oils (Figs. 4c, 4d and 4e). These kind of oil inclusions were found in all type groups with different spectral shapes. In type Ⅲ group, some oil inclusions have two or three peaks with corresponding wavelengths from 444 to 517 nm, in which the highest one is around 522 nm (Fig. 4c). These oil inclusions were inferred to be mixed by the high-mature oil and the medium or low-mature "pure" oils, in which the high mature oil occupies a large percentage. Other inclusions in type Ⅲ group show more complex peak features (Fig. 4d) with corresponding wavelengths of peaks from 459–528 nm in which the highest two peaks are around 495 and 515 nm (Fig. 4d). These oil inclusions show stronger mixture by high-mature, medium-mature and low-mature oils. The percentages of high-mature oil in these oil inclusions are not as high as those in Fig. 4c, however, they still show high-mature characteristics after the mixture according to their values of λmax and QF535. In type Ⅱ group, the mixed oil inclusions generally show similar characteristics as those in Fig. 4d (Fig. 4e), the peak wavelengths are from 463–594 nm and concentrate between 522 and 547 nm, which indicates the higher contribution by medium and low-mature "pure" oils. We did not find any "pure" oil inclusions in type Ⅰ but the mixed oil inclusions with peak wavelengths from 550–606 nm (Fig. 4f). They show a potential mixture by low-mature and medium-mature "pure" oils.

    Figure 4.  Fluorescence spectra of oil inclusions. (a) Spectra with only one peak in type Ⅲ; (b) spectra with only one peak in type Ⅱ; (c) spectra with multiple peaks in type Ⅲ; (d) spectra with multiple peaks in type Ⅲ; (e) spectra with multiple peaks in type Ⅱ; (f) spectra with multiple peaks in type Ⅰ.

  • The oil mixing experiment yielded eleven mixed oils' QF535 values and their mixing ratios, which showed a non-linear relationship between the mixing ratio and the QF535 value (Su et al., 2016). In the experiment, the end oil B is high-mature oil with λmax of 462 nm and QF535 of 0.45, and the end oil A is medium-mature oil with λmax of 541 nm and QF535 of 1.39. In this study, we identified two "pure" oils, one is in type Ⅲ with λmax ranging from 458–482 nm and QF535 from 0.33–0.61, and another is in type Ⅱ with λmax ranging from 522–554 nm and QF535 from 1.07–1.26. Based on statistics, we assume that these two end oils in the experiment exist in the Wenliu uplift and the mixed oil inclusions, whose λmax and QF535 values are in between those of the two end oils, are mixed by these two end oils; therefore, their mixing ratios can be obtained.

    In type Ⅲ, sixty-seven oil inclusions were detected, in which fifty-one were mixed oil inclusions with degree of mixing (DM) of 76.1%. And fifty mixed oil inclusions' λmax and QF535 values are between those of the two end oils, indicating the percentage of the computable mixed oil inclusions (PCMOI) is 98.0%. The QF535 range of these computable mixed oil inclusions is 0.46–0.98. According to the experiment curve, the mixing ratio of end oil A in these oil inclusions is 0.3%–56.5%. In type Ⅱ, sixty-one oil inclusions were detected, in which fifty-eight were mixed oil inclusions, so the DM is 95.1%. Fifty-four mixed oil inclusions with λmax and QF535 values between those of two end oils indicate the PCMOI is 93.1%. The QF535 range of these computable mixed oil inclusions is 0.84–1.36. According to the experiment curve, the mixing ratio of end oil A in these oil inclusions is 47.3%–90.6% (Fig. 5). The type Ⅰ oil inclusions are out of concern because their λmax and QF535 values are beyond the end oil boundary. However, their fluorescence spectra show complex mixing characteristics, indicating they were mixed by medium-mature oil and some lower-mature or immature oils (Fig. 4f).

    Figure 5.  The mixing ratio of oil A in the mixed oil inclusions in types Ⅱ and Ⅲ, assuming they were mixed by two end oils (A and B) (modified after Su et al., 2016).

  • We counted the total number of oil inclusions, the number of mixed oil inclusions, and the number of mixed oil inclusions that can be used to calculate the crude oil mixing ratio in each well (Table 2). The sample locations were marked on the cross-section (Fig. 6). The sample locations of PS18 and W142 are in the margin of the Liutun sub-sag, and they are very close to a big fault which is a favorable migration channel. W236 is located on the highest point of the Wenliu uplift, and the sampling depth is much shallower. The results in this well can reveal the characteristics of oils in reservoir. W203-15, W211 and PS7 are between the Wenliu uplift and the Qianliyuan sub-sag. The results in these wells can represent the characteristics of the oil migration channel from the Qianliyuan sub-sag to the Wenliu uplift.

    Well Total oil inclusions Mixed oil inclusions DM (%) PCMOI (%) QF535 Mixed ratio A/(A+B) (%)
    PS18 27 14 51.9 92.9 0.46–1.01 0.4–58.7
    W142 1 1 100 100 1.21 74.3
    W236 9 7 77.8 100 0.66–1.19 34.6–72.6
    W203-15 52 49 94.2 87.8 0.59–1.33 25.8–86.7
    PS7 34 33 97.1 97 0.71–1.36 39.2–90.6
    W211 9 9 100 100 0.78–1.26 43.8–79.1
    DM. Degree of mixing; PCMOI. percentage of computable mixed oil inclusions.

    Table 2.  Statistics of mixing characteristics of oil inclusions in each well

    Figure 6.  Cross section of sample and well locations.

    Based on the statistics, we can tell that the mixing degree and the mixing ratio of end oil A in A+B from the Liutun sub-sag to the Wenliu uplift is increasing. Most of the "pure" oil inclusions were detected in PS18, occupying 48.1% of the total inclusions in this well. And it decreases to 22.2% in W236, indicating that the mixture of oil happened at a very early stage of the migration. As the migration distance increases, the degree of oil mixing increases. The mixing ratio of A in A+B is also increases from PS18 to W236, which indicates that the contribution of end oil A to mixed oil inclusions gradually increases. However, from the Qianliyuan sub-sag to the Wenliu uplift, the mixing degree decreases and the mixing ration of A in A+B does not significantly decrease. The highest mixing ratio of A in A+B in PS7 is 90.6%, in other words, the mixing ratio of B in A+B is 9.4%, indicating the ratio of end oil B in the mixed oil inclusions increases from the Qianliyuan sub-sag to the Wenliu uplift. As the percentage of computable mixed oil inclusions (PCMOI) in each well, except W203-15, is more than 90%, it is considered that the calculated mixing ratio can represent most of the mixed oil inclusions in the wells. The main reason of the PCMOI in W203-15 lower than 90% is that the type Ⅰ oil inclusions were detected in it, which are out of concern. However, inclusions in type Ⅰ group indicate they were mixed by some oil with maturity lower than that of the end oil A.

    Combining with the statistical analysis and the cross section, it is inferred that the closer to the sub-sags, the lower the degree of mixing of crude oil, and conversely, the closer to the uplift, the higher the degree of mixing. Near the Liutun sub-sag, the mixed oil in inclusions is mainly contributed by the high-mature oil, while near the Qianliyuan sub-sag, the mixed oil in inclusions is mainly contributed by the low-mature oil.

  • λmax and QF535 cross-plot is a good way to identify oil inclusions of different maturities, and whether these oil inclusions were mixed can be determined by the fluorescence spectra. The mixing ratio of an end oil A in oil inclusions that are mixed by end oil A and B can be obtained using the relationship between the mixing ratio and QF535 experimental curve. However, the reality is the oil inclusions' fluorescence spectra are so complex that they seem to be mixed by more than two end oils; therefore, if we want to calculate the mixing ratio of these oil inclusions, we must assume that they are mixed by only two end oils. In addition, different end oils will cause different mixed results. In this study, we detected two types of "pure" oils whose λmax and QF535 are very close to those of the end oils in the mixing experiment, which is a prerequisite for using the experimental curve. And only those mixed oil inclusions whose λmax and QF535 values are in between those of the end oils can be calculated the mixing ratio. Therefore, we listed the PCMOI, which indicates to what extent this calculated mixing ratio can represent the whole mixed inclusions in the well. As the complexity of oil in the northern Dongpu depression exists, the oil-source correlation is a tough job, however, the oil mixing experiment and the analysis of fluorescence spectra of oil inclusions seem to be a promising way.

  • Three types of oil inclusions were detected in the Wenliu uplift, northern Dongpu depression. They have yellow, green, cyan or blue fluorescence colors respectively, corresponding to low-mature, medium-mature and high-mature oils. These inclusions are in the dissolution pores and fractures of feldspar, indicating an early trapping event. Oil inclusions with different fluorescence colors coexist in one FIA is a main petrographic characteristics of this area.

    The oil in each type of oil inclusions has been significantly mixed, however, the "pure" oil inclusions in type Ⅲ and type Ⅱ have still been found, indicating that medium-mature oil is generated from source rock rather than mixed by high-mature and low-mature oils. No "pure" oil was found in type Ⅰ, but their fluorescence spectra show they must be mixed by some lower-mature oils.

    The statistical data in each well shows that the closer to the sub-sags, the lower the degree of mixing of crude oil, and conversely, the closer to the uplift, the higher the degree of mixing. Near the Liutun sub-sag, the mixed oil in inclusions is mainly contributed by the high-mature oil, while near the Qianliyuan sub-sag, the mixed oil in inclusions is mainly contributed by the low-mature oil.

  • This study was supported by the National Natural Science Foundation of China (No. 41730421), the Major Science and Technology Project of SINOPEC during the 13th Five-Year Plan period (No. ZDP1705). Comments and suggestions from Dr. Simon C. George and an anonymous reviewer considerably improved the manuscript. The final publication is available at Springer via https://doi.org/10.1007/s12583-020-1356-0.

Reference (20)

Catalog

    /

    DownLoad:  Full-Size Img  PowerPoint
    Return
    Return