Abstract
The quantitative grain fluorescence and quantitative grain fluorescence on extract have become effective approaches in the analysis of hydrocarbon evolution in clastic reservoirs recently. The cutoff threshold for differentiating current/paleo-oil and water zones is crucial to reconstruct accurately hydrocarbon accumulation history. However, the absence of theoretical study on the cutoff threshold in the carbonate reservoir has precluded their application and development. In this paper, we attempted to investigate the cutoff threshold by analyzing the quantitative grain fluorescence and quantitative grain fluorescence on extract parameters and spectra of the cores and natural carbonate outcrop samples in known current/paleo-oil and water zones revealed by the frequency of oil inclusions, formation test, logging analysis, etc. Based on this, the hydrocarbon charging history of the Suqiao Buried-hill Zone, Bohai Bay Basin, eastern China was reconstructed using the gotten threshold. Results show that the carbonate minerals fluorescing will lead to a higher cutoff threshold of quantitative grain fluorescence index value in the carbonate reservoir, while the threshold of quantitative grain fluorescence on extract intensity value is coincident with the corresponding value in the clastic reservoir. The quantitative grain fluorescence and quantitative grain fluorescence on extract data have unraveled a complicated hydrocarbon accumulation history in Suqiao Buried-hill Zone including oil charging before gas and paleo-oil loss due to the tectonism. The ascertained cutoff threshold in this study is of great significance for reconstructing accurately and effectively the complicated hydrocarbon charging history in the carbonate reservoir, which provides significant models for future petroleum exploration.
Keywords
Introduction
The fluid inclusions capsuled by mineral diagenesis can preserve the original characteristics of hydrocarbon fluids and the diagenetic environment at the time of trapping and thus are not affected by later alterations such as thermal maturation, hydrocarbon mixing, and biodegradation (Head et al., 2003; Parnell, 2010; Watson and Brenan, 1987). Therefore, hydrocarbons in fluid inclusions can provide valuable information for reconstructing paleo-temperature and pressure of the reservoirs and for determining the charge time of multiple-stage fluids, the compositions of geo-fluid, and the hydrocarbon evolution (Aplin et al., 1999; Bourdet et al., 2010, 2014; Burley et al., 1989; Burruss, 1989; Ferket et al., 2011; Guo et al., 2016; Jiang et al., 2015; Liu et al., 2003; Swarbrick, 1994; Tseng and Pottorf, 2002; Xiang et al., 2015). In addition, fluorescence has been used widely in petroleum exploration for a long time (Bourdet et al., 2012, 2014; Hagemann and Hollerbach, 1986; Riecker, 1962; Su et al., 2015; Zhang et al., 2012; Zierfuss and Coumou, 1956). Conventionally, the fluid inclusions are detected by microscopic and spectroscopic techniques (Burruss, 1989; Liu et al., 2013; Ryder et al., 2004; Tsui, 1990). Then the abundance indices of oil inclusions such as the grains containing oil inclusions (GOI) and the frequency of oil inclusions (FOI) in the reservoir rock have been used as a proxy to differentiate paleo-oil and water zones and unravel the hydrocarbon charge history (Bourdet et al., 2012, 2014; Eadington et al., 1996, 2000; Lisk et al., 2002; Liu et al., 2013; Oxtoby et al., 1995; Su et al., 2015). However, these conventional methods have many drawbacks such as labor intensive, inaccurate, and require professional knowledge to carry out identifying and analysis (Liu and Eadington, 2005; Liu et al., 2007, 2014, 2016). With the development of spectroscopy and organic geochemistry, quantitative grain fluorescence technique (QGF) and quantitative grain fluorescence on extract (QGF-E) have become effective approaches in the analysis of hydrocarbon evolution process recently. Not only is the technique nondestructive, rapid, and cost effective, but also can consecutively collect samples at different depths in several wells for the system spectral analysis. QGF and QGF-E data have been used to identify oil-bearing reservoir, hydrocarbon migration path, and the current and paleo-water–oil contact (Liu et al., 2003, 2007, 2014, 2016; Liu and Eadington, 2003, 2005; Lu et al., 2012; Pang et al., 2012).
In spite of this, the two methods only are widely applied to investigate hydrocarbon inclusions and adsorbed oil in clastic reservoir rocks that contain primarily quartz and feldspar so far. In general, the QGF data reflect the fluorescence from both inclusion oils capsuled in reservoir grains and hydrocarbons tightly adsorbed on the reservoir grain surface, whereas QGF-E mainly reflects solvent extractable hydrocarbons adsorbed on grain surfaces (Liu and Eadington, 2005; Liu et al., 2007, 2014, 2016). The carbonates minerals such as calcite and dolomite fluoresce when excited by the UV light (Figure 1(a) to (c)), and the fluorescence intensity is larger than that of the detrital minerals (Figure 1(d)). Therefore, a deviation would be caused if the paleo/current-oil–water zones in carbonate reservoir are defined by the cutoff threshold which is mainly used in the clastic reservoir (Liu et al., 2007). Furthermore, to date, there is little theoretical study on differentiating current/paleo-oil and water zones in carbonate reservoir with the two new fluorescence techniques.
(a) Calcite vein and oil inclusions showing colorless under transmitted light, (b) calcite vein showing weak fluorescence and oil inclusions showing obvious fluorescence under UV light, (c) dolomite crystals showing weak yellow fluorescence under UV light, and (d) fluorescence spectra of the dolomite crystals and modern beach quartz sands. The spectrum of quartz sands is from Liu and Eadington (2005).
In this paper, we attempted to ascertain the cutoff threshold by analyzing the QGF and QGF-E parameters and spectra of the cores and natural carbonate outcrop samples in known current/paleo-oil and water zones revealed by the FOI, formation test, logging analysis, etc. Based on this, we tried to unravel the charge history by differentiating the paleo/current-oil and water zones within carbonate reservoirs in the Suqiao Buried-hill Zone using the gotten cutoff threshold.
Principles and methods
The quantitative fluorescence techniques for reservoir analysis mainly include QGF and QGF-E. For the theoretical studies of the methods, some scholars have done many related researches (Liu et al., 2003, 2007, 2016; Liu and Eadington, 2005). Usually, QGF measures fluorescence emission spectra from cleaned reservoir grains between 300 and 600 nm excited by UV light. It represents the fluorescence characteristics of hydrocarbon inclusions inside the grains and the amount of hydrocarbons adsorbed tightly on reservoir grain surfaces, which indicates the paleo-oil saturation in the reservoir. The parameters for characterizing the QGF spectrum include QGF Intensity, QGF Index, λmax, and Δλ (Liu et al., 2003, 2007; Liu and Eadington, 2005). QGF intensity is defined as the average spectral intensity between 375 and 475 nm (Figure 2). QGF index is the ratio between the QGF intensity and the spectral intensity at 300 nm. Δλ is the width of the spectrum at half the maximum intensity. In general, paleo-oil zone samples from clastic reservoir have distinct QGF spectral peaks between 375 and 475 nm and the QGF index values usually exceed 4. Whereas, QGF index values for paleo-water zone samples rarely exceed 6 and are usually characterized by flat spectra (Liu et al., 2007, 2016). λmax can provide information about hydrocarbon maturity of the paleo-oils. Usually, the QGF λmax shift progressively to long wavelength with decreasing API gravity. Light oil is usually characterized by a low λmax, whereas the heavy oil is antipodal. Similarly, the Δλ values also increase with decreasing API gravities (Liu and Eadington, 2005; Liu et al., 2007, 2014, 2016).
A typical fluorescence emission spectrum of crude oil showing various parameters. Imax = maximum spectra intensity, λmax = wavelength corresponding to Imax, I300 = spectral intensity corresponding to the wavelength at 300 nm, Δλ is the width of the spectrum half the maximum intensity (modified by Liu et al. (2014)).
QGF-E measures fluorescence emission spectra of the solvent extract from reservoir grains between 300 and 600 nm using excitation wavelengths of 260 nm. It represents the fluorescence characteristics of the dichloromethane (DCM) extractable hydrocarbons from reservoir grains, which indicates the current and residual oil saturation in the reservoir (Liu et al., 2007). Usually, the parameters for characterizing the QGF-E spectrum include QGF-E intensity and λmax (Liu and Eadington, 2005; Liu et al., 2007, 2014, 2016). QGF-E intensity is the maximum spectral intensities normalized to 1 g of quartz sand in 20 ml DCM solvent. λmax of the QGF-E refers to the wavelength corresponding to the maximum spectral intensity. Generally, samples in current known hydrocarbon zone have relatively high QGF-E intensity values which usually exceed 40 photometer counts (pc) and have distinct QGF-E spectra with spectral peaks typically at around 370 nm. However, QGF-E intensity values are low in the water zone (generally less than 20 pc) and the spectra are flat (Liu and Eadington, 2005; Liu et al., 2007, 2014, 2016). Therefore, consecutively collecting samples at different depths in petroleum wells within a reservoir for the systemic QGF and QGF-E analysis, combining with well logging data, paleo-oil-water contacts (POWC), current oil-water contacts (OWC), and hydrocarbon migration pathways can be detected for the reconstruction of hydrocarbon accumulation history (Liu et al., 2003, 2007, 2014; Liu and Eadington, 2003, 2005).
The FOI technique measures the abundance of oil inclusions in carbonate reservoirs (Liu et al., 2013). Square fields with 0.5 mm × 0.5 mm, which are selected randomly in a partition of the sample under the 20× objective, should contain oil inclusions particularly in cement. The FOI values are the proportion of oil inclusions in the samples by measuring on the square fields. The values are related to the petrography of the rocks to evaluate the significance of these data for paleo-oil migration and accumulation. In general, the FOI value of the paleo-oil zone, migration pathways, and paleo-water zone is greater than 5%, 1–5%, and less than 1%, respectively (Bourdet et al., 2012, 2014; George et al., 2004; Liu et al., 2013).
Samples and experiments
The FOI results of the carbonate core samples from well S1–4, S1–5, S4–6 in the Suqiao Buried-hill Zone.
FOI: frequency of oil inclusion; POL: paleo-oil layer; PWL: paleo-water layer.
The QGF and QGF-E parameters of the outcrop samples and pure dolomite crystals investigated.
CV: high content of calcite veins; QGF: quantitative grain fluorescence technique; QGF-E: quantitative grain fluorescence on extract.
The QGF and QGF-E parameters of the core samples from the petroleum wells S1–4, S1–5, and S1–6 in Suqiao Buried-hill Zone investigated.
CV: high content of calcite veins; QGF: quantitative grain fluorescence technique; QGF-E: quantitative grain fluorescence on extract.
Eighty-three of all cores, the all 22 outcrop samples and dolomite crystals mentioned above were investigated in detail using the QGF and QGF-E techniques after a standard cleaning procedure as shown in Figure 3. First, samples were disaggregated and sieved to grains with a 60–80 mesh size distribution. Approximately 2 g of grains and 20 ml HPLC grade dichloromethane (DCM) were mixed within a 50 ml beaker, and then put the beaker into an ultrasonic oscillator to bath for 10 min. Then dumped the solvent and dried the grains at room temperature. Second, the grains were digested at room temperature for 60 min in 40 ml 10% H2O2 including 20 min of ultrasound bathing at the beginning and the end of the digestion. Then the grains were washed three times with distilled water and dried in an 80℃ constant temperature drying oven for about 2 h. Finally, the remaining grains were washed again in 20 ml HPLC grade DCM in an ultrasound bathing for 10 min and then the solvent was preserved for QGF-E analysis. Meanwhile, the separated grains were dried at room temperature for QGF analysis.
QGF and QGF-E cleaning procedure (modified by Liu et al., 2003). The solvent extracts using DCM at the sixth step is preserved for QGF-E analysis and the dried grains are prepared for QGF analysis. QGF: quantitative grain fluorescence technique; QGF-E: quantitative grain fluorescence on extract.
The QGF and QGF-E analyses were performed on a Varian Cary-Eclipse fluorescence spectrophotometer coupled with a photomultiplier detector to measure the fluorescence intensity. An equipped xenon flash lamp can ensure a high sensitivity and signal/noise ratio, and a beam splitter is to monitor and correct for variations in excitation intensity. In addition, a narrow band interference filter was added to the excitation monochromator of the instrument to reduce the stray light level of the detection monochromator (Liu et al., 2007, 2014). The excitation wavelength of 228 and 260 nm was set for QGF and QGF-E analysis, respectively. The emission fluorescence spectra were measured from 300 to 600 nm for both QGF and QGF-E (Liu and Eadington, 2005; Liu et al., 2007). For the QGF analysis, the cleaned grains were packed and leveled in a customized micro-plate sampling stage, and the spectral measurement of each sample was replicated by measuring 16 or more aliquots. For the QGF-E analysis, the solvent extract was analyzed in a 3.5 ml quartz cuvette cell. Prior to the sample measurement, a DCM blank should be measured to confirm whether the quartz cuvette is clean (Liu and Eadington, 2005; Liu et al., 2007, 2014, 2016).
Twenty-four core samples were collected from the three wells for FOI measurement. The core samples were prepared as thick doubly polished sections of approximately 60 µm thickness. The fluid inclusion petrographic analysis was carried out using ZEISS AXIO Imager A1m microscope with transmitted white light and ultraviolet excitation light source.
Results
FOI measurements
The FOI results of the carbonate core samples from well S1–4, S1–5, S4–6 in the Suqiao Buried-hill Zone are detailed in Table 1. The FOI values, which range from 7.41 to 63.27%, unravel the paleo-oil column ranging from 4166.5 to 4360.0 m in well S1–5 roughly. Likewise, the FOI values with 6.25–46.88% show the thin paleo-oil layer ranging from 4028.0 to 4039.5 m in well S1–4 roughly. With the exception of sample SU1-4–5 (FOI value is 31.95%), the FOI values with 0–0.43% unravel that the paleo-water layer ranging from 4188.0 to 4221.0 m sandwiches a thin paleo-oil layer in well S1–4 (Table 1). Furthermore, the FOI values with 7.21–56.79% unravel the paleo-oil column ranging from 4680.5 to 4967.5 m in well S4–6 roughly.
QGF spectra characteristics of paleo-oil zone in Suqiao Buried-hill Zone
Typical QGF fluorescence spectra for core samples from known paleo-oil and water zones from the Suqiao Buried-hill Zone, Bohai Bay Basin, eastern China are displayed in Figure 4. The fluorescence spectra of paleo-oil zones, which have been revealed by FOI results, have relatively high intensities and obvious peaks between 370 and 470 nm (solid lines), resembling the spectra of condensate and tetra-aromatics (Figures 4 and 5(a)). Furthermore, the fluorescence spectra of paleo-water zones unraveled by FOI values show the characteristics of low fluorescence intensities and flat waveforms (hollow lines), which resemble the spectra of natural outcrop samples (Figures 4 and 7).
Typical QGF spectra of paleo-oil zone and paleo-water zone in Suqiao Buried-hill Zone. QGF: quantitative grain fluorescence technique. (a) Typical QGF spectra of crude oils and hydrocarbon compounds and (b) typical fluorescence spectra of hydrocarbon components in solvents (Liu and Eadington, 2005). QGF: quantitative grain fluorescence technique. Typical QGF-E spectra of current oil and water zone in the Suqiao Buried-hill Zone. QGF-E: quantitative grain fluorescence on extract. Fluorescence spectra of natural outcrop carbonate samples and modern beach quartz sands. The spectra of beach quartz sands are from Liu and Eadington (2005).
QGF-E spectra characteristics of current oil and water zone in Suqiao Buried-hill Zone
Figure 6 shows the typical QGF-E fluorescence spectra for the hydrocarbon components in solvents of the core samples from known current oil and water zones, which are unraveled by the well testing, and wireline logging analysis, in the Suqiao Buried-hill Zone. The QGF-E spectra for current oil zone samples have comparatively high intensity values and have spectra with the peaks around 370 nm (solid lines), resembling the spectra of polar compounds in solvents (Figures 5(b) and 6). Nevertheless, the fluorescence spectra of water zones show the characteristics of low fluorescence intensities and flat waveforms (hollow lines) with the peaks around 370 nm, which resemble the spectrum of the DCM solvent (Figure 6).
QGF and QGF-E spectra characteristics of outcrop samples
Natural carbonate mineral grains may contain various impurities such as coatings on the surface fluorescing. In addition, the mineral grain itself shows weak fluorescence under the UV light (Figure 1(b) and (c)). However, the fluorescence intensity usually has distinct characteristics compared to the spectra of oil and hydrocarbon compounds.
Figure 7 shows the typical QGF fluorescence spectra of natural carbonate samples from outcrops distributed in China mentioned above and a number of modern beach sands from around Australia. All the outcrop carbonate mineral grains were cleaned using the strict cleaning procedure and checked under the fluorescence microscope to have no oil inclusion. The QGF fluorescence spectra of natural outcrop samples have relatively fluorescence intensities and flat waveforms with the peaks around 370 nm. Compared with the quartz sands, the fluorescence intensities of the carbonate samples from outcrop are slightly larger than that of the quartz sands (Figure 7), which may be the reason of the weak fluorescence emitted by the carbonate minerals (Figure 1). Usually, the spectra of modern beach quartz sands have two peaks at around 350 and 450 nm of about equal intensities (Liu and Eadington, 2005; Liu et al., 2003, 2007), while the QGF fluorescence spectra of the most of the natural outcrop samples display the characteristics of unimodal distribution with the peaks around 370 nm (Figure 7).
Results of QGF and QGF-E parameters
The natural outcrop carbonate samples display relatively narrow variations in QGF parameters including QGF intensity, index, Δλ, and λmax (Table 2). All the samples have the QGF intensity values between 1.9 pc and 19.8 pc with intensity maxima at around 370 nm. QGF index values of the samples are between 0.2 and 19.2 and the Δλ values range from 67.5 to 220 nm. Moreover, the QGF-E intensity values range from 4.7 to 17.6 pc with the maximum also at around 375 nm. Overall, the vast majority of QGF intensity and QGF index values not exceed 5.0 pc and 7.0, respectively, suggesting that the outcrop samples have rather flat spectra with relatively low intensities.
QGF parameters and QGF-E parameters of the samples from the Suqiao Buried-hill Zone.
QGF: quantitative grain fluorescence technique; QGF-E: quantitative grain fluorescence on extract.
Determination of the cutoff threshold
QGF responds to paleo-oil saturation with the distinct emission spectra between 375 and 475 nm and elevated QGF index values for known paleo-oil zones (Liu et al., 2003, 2007, 2014, 2016; Liu and Eadington, 2005). QGF index values of the core samples from the known paleo-oil columns, which are unraveled by the FOI values with 6.25–46.88%, 7.41–63.27%, and 7.21–56.79%, respectively (Table 1), within the well S1–4, S1–5, and S4–6 in Suqiao Buried-hill Zone range from 5.0 to 65.3 and usually (but not always) exceed 7 with spectra peaks between 370 and 470 nm (Figures 4 and 8). Furthermore, QGF index values of core samples for known paleo-water columns (FOI = 0 - 0.43%) and natural outcrop samples rarely exceed 7, and the fluorescence spectra are usually flat (Figures 4, 7 and 8). It should be pointed out that the two relatively high QGF index values (QGF index are 17.1 and 19.8, respectively) within the natural outcrop samples (arrows pointed in Figure 8), which resemble the QGF index value of the tetra-aromatics (Figures 8), coincide with relatively high contents of calcite veins (Table 2). This phenomenon suggests that samples with too many calcite veins may lead to a higher QGF index value in virtue of the relatively pure calcite or dolomite fluorescing demonstrated above. Therefore, the samples with considerable calcite veins are not suitable for the QGF analysis to some extent, which is validated by the relatively high QGF index value (∼11.5) of the pure dolomite crystals (green five-pointed star in Figure 8).
The QGF index and [QGF index] × [Δλ] cross plot for crude oils, tetra-aromatic, polar compounds, and samples from known current and paleo-oil zones, water zones in clastic reservoirs, and modern beach sands from Australia (Liu et al., 2007). The cross plot of samples from known paleo-oil and water zones in carbonate reservoirs, natural outcrop samples, and dolomite crystals from China. QGF: quantitative grain fluorescence technique; SQ, Suqiao Buried-hill Zone.
To sum up, the QGF spectra for carbonate reservoir rocks from known paleo-oil zones in Suqiao Buried-hill Zone are characterized by distinct fluorescence spectra with peaks between 370 and 470 nm, relatively high QGF index values usually exceed 7, resembling the fluorescence spectra for crude oils. The QGF spectra of water zones are characterized by low QGF index values and flat spectra with peaks around 370 nm, which are comparable to baseline fluorescence of the natural outcrop samples. Compared to the QGF spectra for clastic reservoir rocks from known paleo-oil zones in NW Shelf, Australia characterized by broad emission spectra with peak between 375 and 475 nm, elevated QGF index values generally greater than 4 (Liu et al., 2003, 2007; Liu and Eadington, 2005), the carbonate minerals fluorescing may lead to a higher cutoff threshold of QGF index value for differentiating paleo-oil and water zones in the carbonate reservoirs (Figure 8).
QGF-E responds to current oil saturation with sharp asymmetrical spectra with spectral peaks around 370 nm and relatively higher QGF-E intensity values (Liu et al., 2003, 2007, 2014, 2016; Liu and Eadington, 2005). QGF-E intensity values of the core samples from the current known oil columns, which are unraveled by the logging analysis, within the well S1–4, S1–5, and S4–6 in Suqiao Buried-hill Zone, China range from 20.5 to 4902.3 pc and usually exceed 20 pc with asymmetrical spectra peaks at around 370 nm (Figures 6 and 9), resembling the fluorescence spectra for polar compounds in DCM solvent (Figure 5(b)). Furthermore, QGF-E intensity values of core samples for water columns and natural outcrop samples rarely exceed 40 pc and are usually characterized by flat spectra, resembling the QGF-E spectra and intensity values of DCM solvent (Figures 6 and 9). This is consistent with the QGF-E spectra and intensity values for clastic reservoir rocks from current known zones in NW Shelf, Australia (Figure 9). Therefore, the cutoff threshold of QGF-E intensity values for differentiating current oil and water zones in carbonate reservoirs is the same as that of samples in clastic reservoirs.
QGF-E intensity and λmax cross plot for samples from known current oil and water zones in Suqiao Buried-hill Zone and the natural outcrop carbonate samples from China. The cross plot for samples from known oil zone below current OWC, water zone from dry wells in clastic reservoirs, NW Shelf, Australia, modern beach sands and DCM blanks from Liu et al. (2007). DCM: dichloromethane; OWC: oil-water contact; QGF-E: quantitative grain fluorescence on extract; SQ, Suqiao Buried-hill Zone.
Application of QGF and QGF-E in carbonate reservoirs
Based on the cutoff threshold investigated by the experiment results mentioned above, the petroleum producing well, namely S1–5 from the carbonate reservoir in the Suqiao Buried-hill Zone, Bohai Bay Basin, eastern China was investigated in detail using the QGF and QGF-E techniques. S1–5 was drilled in 1984 by the PetroChina Huabei Oilfield Company. The discovered hydrocarbons mainly accumulate in the deep buried Ordovician Fengfeng and Shangmajiagou formation, whose main lithology is limestone and dolomite. Furthermore, the current reservoir in which S1–5 locates is condensate gas reservoir with oil ring, and the reservoir is the mixing products of the multistage charged oil and gas (Du et al., 2002; Qin et al., 2000; Wang et al., 1989; Xiao et al., 2004).
The QGF index values in well S1–5 range from 5.0 to 39.3 and generally exceed 7 from 4160 to 4360 m except for samples from 4170 and 4211.5 m, which have QGF index values of 6.7 and 5.0, respectively. The QGF fluorescence spectra for most of the samples have relatively high intensities and obvious double peaks at ∼375 and 470 nm, respectively (Table 3 and Figure 10), resembling the spectra of condensate and tetra-aromatics. The characteristics of the fluorescence spectra indicate that multistage hydrocarbon charging of condensate and relative heavy oils, which is consistent with the previous study results (Du et al., 2002; Jin et al., 2014; Qin et al., 2000; Wang et al., 1989; Xiao et al., 2004). The QGF parameters with the FOI analysis indicate that the current gas–oil–water layer was once a paleo-oil column ranging from 4160.0 to 4360.0 m and sandwiched two thin paleo-water layers (∼4170.0 and 4211.5 m) in well S1–5 (Figure 10). In addition, the presence of paleo-oil column indicates that oil was charged prior to gas charging. Moreover, the relatively high QGF index (almost exceed 7) at the topmost part of the reservoir indicates that there was no gas cap during the initial oil charging. The two relatively low QGF index values within the paleo-oil column at 4170.0 m (QGF index = 6.7) and 4211.5 m (QGF index = 5.0) may be caused by heterogeneity of carbonate reservoirs.
QGF index, QGF-E intensity, and OGF-E spectra depth profiles for well S1–5, Suqiao buried-hill Zone showing current gas–oil contact (GOC) and oil–water contact (OWC). GR logs are used as lithological references. QGF: quantitative grain fluorescence technique; QGF-E: quantitative grain fluorescence on extract.
In addition, the QGF-E intensity values in well S1–5 range from 12.9 to 2925.0 pc. It is easy to find out that the inflection point of QGF-E intensity values occurs around 4357.0 m. The QGF-E intensity depth profile displays distinct upward increasing trend above the inflection point, whereas displays downward decreasing trend below the point. Furthermore, the QGF-E spectra are characterized by obvious asymmetrical spectra with peaks around 370 nm above the point, while the spectra are flat with peaks around 370 nm below the point, resembling the spectra of the DCM solvent blank and natural outcrop samples (Figure 10). Therefore, the QGF-E data reveal the presence of current OWC at around 4357.0 m. In current oil zone the QGF-E intensity values are greater than 58.3 pc (Figure 10), and the λmax values of QGF-E spectra range from 369 to 419 nm with the distinct peaks around 370 nm, which resemble the fluorescence spectra of hydrocarbon components in solvents (Figures 10 and 5(b)). In addition, the enrichment intensity of hydrocarbon in the paleo-oil reservoir is similar, whereas the current gas–oil contact (GOC) and OWC occur in the current oil–gas reservoir. It illuminates that an upward adjustment has been taken place in the paleo-oil reservoir. Therefore, the deep buried Ordovician reservoir was initially charged by the early formed oil prior to gas charge, which is confirmed by FOI and QGF data. Furthermore, the reservoir was adjusted by the tectonic movement after the paleo-oil charging, which is confirmed by the QGF-E data and consistent with the uplifting and denudation of the entire Suqiao Buried-hill Zone at the end of Oligocene (Du et al., 2002; Zhao et al., 2013).
Discussion and conclusions
Discussion
The combined QGF and QGF-E data provide both current and paleo-oil saturation, which have become effective and efficient methods in understanding hydrocarbon charge history in the reservoirs (Liu et al., 2003, 2007, 2014, 2016; Liu and Eadington, 2003, 2005; Lu et al., 2012; Pang et al., 2012). This paper ascertained the threshold value of carbonate reservoir using the FOI, formation test, wireline logging analysis, etc. Therefore, the current/paleo-oil and water zones in carbonate reservoir can be discriminated rapidly and cost effectively using the gotten cutoff threshold. Then, combining with the existing fluid inclusion methods (Fourier transform infrared spectroscopy, Raman spectroscopy, etc.) and some geochemical techniques, a complicated carbonate reservoir charging history concluding gas displacement of oil, gas flushing, oil loss due to tectonism, and so on can be revealed effectively, which provides significant models for future petroleum exploration.
Usually, the cutoff threshold of QGF index values for differentiating paleo-oil and water zones in clastic reservoirs is 4 with spectral peaks between 375 and 475 nm. QGF-E intensities for current known oil and residual zones in clastic reservoirs usually exceed 40 pc with spectral peaks typically around 370 nm (Liu et al., 2003, 2007, 2014, 2016; Liu and Eadington, 2003, 2005). Based on the analysis mentioned above, however, the carbonate minerals fluorescing may cause a higher QGF index threshold value (usually exceeds 7) for differentiating paleo-oil and water zones in the carbonate reservoir, while the threshold of QGF-E intensity value for differentiating current oil and water zones is coincident with the corresponding value in clastic reservoir. Therefore, a deviation or mistaken interpretation would be caused if the paleo-oil zones in carbonate reservoir are discriminated by the cutoff threshold for the clastic reservoir. A virtual producing well in carbonate reservoir is used here to illuminate the notable question. A paleo-oil column from 2000 to 2055 m with the interpreted POWC around 2055 m can be revealed by the QGF index according to the previous cutoff threshold (about 4) of the clastic reservoir (Figure 11). Similarly, the OWC around 2162 m can be revealed by the QGF-E intensity. Therefore, the hydrocarbon charging history can be interpreted as the reservoir was charged previously by oil prior to the gas charging and the oil column was displaced by the late stage gas. Actually, there is no paleo-oil column on the basis of the interpretation results of QGF index by the cutoff threshold (about 7) of carbonate reservoir. So, the reservoir may have not experienced the early oil filling and just is a primary oil–gas pool. Therefore, the appropriate threshold value is of great significance for reconstructing accurately hydrocarbon accumulation history.
A virtual producing well in carbonate reservoir is used here to illuminate the importance of the accurate threshold. QGF Index, QGF-E Intensity depth profiles showing current oil-water contact (OWC) and paleo-oil-water contact (POWC). The GOC was determined from well testing and logging analysis. Resistivity logs are used as current hydrocarbon saturation references (modified by Liu et al. (2007)).
In addition, several aspects should be noticed when these methods are applied in the carbonate reservoir. As mentioned above, samples with too many calcite veins will lead to an inaccurate or even error result in virtue of the carbonate minerals fluorescing. Therefore, the sample containing too many calcite veins should be abandoned when sampling. Furthermore, when the OWC and POWC are delineated using the new methods, integrated evidences and methods involving the QGF intensity and QGF index, the changes in spectral characteristics, FOI data, fluid inclusion and conventional core and logging analysis methods, and so on should be used as much as possible in order to get an accurate result. In addition, it should be pointed out that there is no uniform cutoff threshold for either QGF index or QGF-E intensity data for differentiating current/paleo-oil and water zones and it may be unique for each reservoir because considerable geological variables will affect the QGF and QGF-E data (Liu and Eadington, 2005; Liu et al., 2007). Therefore, it is better to investigate an exclusive cutoff threshold for each reservoir in order to get the accurate result in accordance with the geological settings.
Conclusions
This study has provided a cutoff threshold of QGF index value (approximate 7) for differentiating paleo-oil and water zones in the Suqiao Buried-hill Zone, which is higher than the corresponding value (about 4) in clastic reservoir, by analyzing the QGF and QGF-E parameters of the cores and natural carbonate outcrop samples in known current/paleo-oil and water zones revealed by the FOI, formation test, logging analysis, etc. The carbonate minerals fluorescing when excited by the UV light may cause the higher cutoff threshold. In addition, the study has demonstrated that the cutoff threshold of QGF-E intensity value for differentiating current oil and water zones is coincident with the corresponding value in the clastic reservoir.
Based on the gotten cutoff threshold, the QGF and QGF-E data have unraveled a complicated reservoir accumulation history in the Suqiao Buried-hill Zone including oil charging before gas and pale-oil loss due to tectonism. The ascertained cutoff threshold in this study is of great significance for reconstructing accurately and effectively the complicated hydrocarbon charging history in the carbonate reservoir, which can provide significant models for future petroleum exploration.
Footnotes
Acknowledgements
The Huabei Oilfield Company and many individuals who contributed the core samples for this work are gratefully acknowledged. In addition, Prof. Tonggang Zhang in China University of Petroleum is thanked gratefully for providing the outcrop samples in this study. We are appreciative of collaboration and enthusiastic support from Chunyuan Han in the Huabei Research Institute of Petroleum Exploration and Exploitation; Xuesong Lu and Xiuli Li in the Research Institute of Petroleum Exploration and Development.
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was supported by the NSFC (No. 41125010, No. 91114202) and the PetroChina Key Project (HBYT-YJY-2015-JS-222).