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. 2a–2d), 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, ⅲ) 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
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.