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Densification and hydrocarbon accumulation of Triassic Yanchang Formation Chang 8 Member, Ordos Basin, NW China: Evidence from geochemistry and fluid inclusions
PETROLEUM EXPLORATION AND DEVELOPMENT Volume 44, Issue 1, February 2017 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2017, 44(1): 48–57.
RESEARCH PAPER
Densification and hydrocarbon accumulation of Triassic Yanchang Formation Chang 8 Member, Ordos Basin, NW China: Evidence from geochemistry and fluid inclusions FU Jinhua1, 2, *, DENG Xiuqin1, 2, WANG Qi3, LI Jihong1, 2, QIU Junli3, HAO Lewei3, ZHAO Yande1, 2 1. National Engineering Laboratory for Exploration and Development of Low-Permeability Oil & Gas Fields, Xi’an 710018, China; 2. PetroChina Changqing Oil Field Company, Xi’an 710018, China; 3. Institute of Geology and Geophysics, Chinese Academy of Sciences, Lanzhou 730000, China
Abstract: Crushing, acid treatment and step wise separation and oil extraction were employed to obtain the different occurrence state hydrocarbons. All these fractions have been analyzed by Gas Chromatography-Mass Spectrometer (GC-MS). The fractions relationship and related oil charging process can be mirrored through the analysis of fractions weight content and geochemical characteristics, in combination with the research of inclusions homogenization temperature and fluorescence spectrum parameters. Experimental results reveal that there are four state hydrocarbons, i.e. free hydrocarbon, sealed hydrocarbon, hydrocarbon in carbonate cement, and hydrocarbon within inclusions caught by quartz grains and feldspar grains in the oil-rich sandstones of Chang8 Member, Triassic Yanchang Formation in the Ordos Basin. Among them, the overwhelming fraction is the free hydrocarbon, averaged 93.4%. Fluorescence spectrum parameters of λmax, QF535 and Q650/500 show that the crude oil maturity of the inclusions imprisoned in feldspar, quartz, and carbonate cement increase in turn, and the parameter values of the inclusions in feldspar and quartz are similar and much different from those of carbonate cement. Through analysis of C29ββ/(αα+ββ) and C2920S/(20S+20R), together with methylphenanthrene ratio, it is revealed that thermal-evolutionary degree of the hydrocarbon within inclusions, hydrocarbon in carbonate cement, sealed hydrocarbon, free hydrocarbon reflects an upward trend, and the data of the last two type are similar. Integrated study of diagenetic sequence and thermal evolutionary degree suggest that the Chang 8 sandstones had been compacted before reservoir formation and the reservoirs have experienced three phase of charge events in which the third one played the most important role for reservoir formation. Key words: Ordos Basin; Triassic; Yanchang Formation; hydrocarbon occurrence; charge period; major reservoir forming phase; tight reservoir
Introduction In recent years, great breakthroughs have been made in tight oil & gas exploration in Triassic Yanchang Formation, Ordos Basin, marked by the discovery of a series of tight oil fields, including Xifeng, Jiyuan, Huaqing oilfields etc. Previous studies have covered different aspects related to the geological conditions, main control factors and forming mechanism of the tight oil fields[17], and, these works mainly focused on the migration and accumulation of the oil & gas in the low-permeable reservoirs as well as the time sequence of the reservoir tightening and pool formation. Different researchers gave their own viewpoints on these questions, e.g. Luo et al., based on his research on diagenetic process and reservoir tar, suggested that the oil-charging before the reser-
voir tightening had changed the reservoir’s wettability, and the residual oil-pro pathway network provided favorable conduit for oil and gas migration and accumulation[3]; Ren et al. pointed out that the tight oil was formed under the overall tightness background according to several lines of evidence from inclusion temperature, K-Ar dating and saturation pressure analytical data, and so on[4]; On the contrary, Liu et al. believed that the Yanchang reservoir could not be infilled by oil under the tight state based on the fluorescence examination on inclusion, simulation on porosity evolution and oil-charging critical experiment, and concluded that Yanchang oil- pool could only be formed before the reservoir densified through inference[5]. To sum up, there is no consensus on the order of Yanchang sandstone tightening and oil-pool formation. In view of this, the forming process of Yanchang tight
Received date: 24 Dec. 2015; Revised date: 18 Dec. 2016. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the China National Science and Technology Major Project (2016ZX05050; 2011ZX05001-004). Copyright © 2017, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.
FU Jinhua et al. / Petroleum Exploration and Development, 2017, 44(1): 48–57
reservoir has been analyzed by using stepwise extraction and fluorescence examination on oil inclusions, to find the time sequence of densification of Yanchang Formation sandstone and pool formation, and get a clear understanding on the forming mechanism of Yanchang Formation tight oil reservoir.
1.
Geological setting
A giant inland depression lacustrine basin developed in Ordos area during the middle and late Triassic, where a successive clastic sequence of Yanchang Formation deposited, with a thickness of approximately 1 000 meter. There are large scale low-permeable lithological and tight oil pools in Yanchang Formation after over 200 million years of structural and diagenetic transformation. Yanchang Formation can be classified in 10 members, Chang 1 to Chang 10 from bottom to top, of which the Chang 7 Member, the deposits of maximum flooding, with abundant organic matter, is the major source rock in the Mesozoic of Ordos Basin. The Chang 8 Member, below Chang7 Member, is shallow lacustrine deltaic deposits with well-developed distributary and sub-distributary channel sandbodies widespread, stable in spatial distribution, and thick in main sandbody zone[8]. The reservoir rock of Chang 8 Member is mainly fine grained sandstone, intense compaction and carbonate cementation have caused its poor physical properties (porosity from 5.2% to 12.2%, and permeability from 0.02×103 to 5.02×103 μm2)[9]. Although Chang8 Member is poorer in physical property, the overlying Chang7 Member had a pressure coefficient of 1.21.7[1011] at the maximum palaeoburial depth, there was quite large pressure difference between Chang 7 and Chang 8 Member, which can drive oil migration downward, thus, the Chang 8 sandstone reservoir is universally oil-bearing and large in reserve scale.
2.
Sampling and experiments
Stepwise (or sequential) extraction of hydrocarbon in reservoir rock is a novel analysis technique of micro-geochemistry of oil and gas pool developed in recent years[1215]. Its theoretical basis is that the maturity of hydrocarbons in reservoir, no matter where they are originated from the same source bed or not, will change somewhat in the process of migration and accumulation, and, therefore, it can be utilized to determine the hydrocarbon source in oil-bearing reservoir rock and oil-pool forming periods. 2.1. Separation of hydrocarbons in different occurrence states First of all, 86 oil-bearing sandstone samples were chosen to make thin sections to check their fluorescence features. Four sandstone samples from Well Q22, X231, H115 and M51 (Fig. 1) were selected for stepwise extraction according to their oil content. These four samples have a porosity of 6.7%, 8.1%, 9.1% and 8.5%, and mean grain size of 3.53, 2.57, 2.44 and 2.7, respectively. Secondly, hydrocarbons in the samples
were extracted by crushing, acid processing and Soxhlet extraction methods sequentially. Four kinds of hydrocarbon have been successfully separated, including free oil, sealed oil, inclusion oil in carbonate cement ( cement oil for short hereafter), and inclusion oil in quartz and feldspar grains (inclusion oil for short hereafter). The experimental flowchart is described as follows (Fig. 2): Step 1. Free oil separation. Free oil was obtained by Soxhlet extraction method using dichloromethane–methyl alcohol solution (volume ratio 93:7) to rinse oil sandstone samples after crushing them into 0.51.5 cm in grain size. Free oil stands for the hydrocarbon in open porous system of the reservoir rock. Step 2. Sealed oil separation. The residual sandstone samples after Soxhlet extraction were re-crushed into single clastic grains by gentle manner. To control the separation of rock framework grains, microscope was used to examine the crushed sample grain size and those fragments with grain size less than the single clastic grain were sieved out in order to eliminate the released inclusion oil during crushing. And then the processed samples were extracted by dichloromethane– methyl alcohol solution (volume ratio 93:7) to obtain the sealed oil, which is the hydrocarbon preserved in unconnected pores. Step 3. Cement oil separation. Cement oil is defined as the hydrocarbon enclosed in carbonate cement. The remained samples after the extraction of the sealed oil were dissolved by 6% chloride acid to remove carbonate cement, and extracted by dichloromethane–methyl alcohol solution (volume ratio 93:7) to get cement oil. Step 4. Inclusion oil separation. The remained samples after Step 3 were rinsed by distilled water to neutral, and, furthermore, processed by potassium dichromate-concentrated sulfuric acid solution and hydrogen peroxide, and then, these samples were re-extracted by dichloromethane–methyl alcohol solution (volume ratio 93:7) to clean out surficial hydrocarbon. Finally, these samples were grinded in dichloromethane–methyl alcohol solution (volume ratio 10:1) to extract the hydrocarbon enclosed in single quartz or feldspar grains. The obtained hydrocarbon is called inclusion hydrocarbon. 2.2.
Hydrocarbon component analysis
Firstly, weighed the separated hydrocarbons of different occurrence states. Secondly, asphaltene was precipitated using n-hexane for all hydrocarbons (excepting hydrocarbon in cement and hydrocarbon in inclusions). And then the saturated hydrocarbons and aromatic hydrocarbons fractions were obtained using column chromatography (Silica gel - alumina, v:v 4:1). At last, all these fractions were analyzed by GC-MS. GC–MS was performed using an Agilent Technologies HP6890N gas chromatograph coupled to a HP5973 Network mass selective detector. GC conditions were: Separation was achieved using fused silica capillary column J&WHP-5 (30
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Fig. 1.
Structural units and stratigraphic column of Yanchang Formation in Ordos Basin.
m×0.25 mm×0.25 µm), the primary oven temperature program was: 80 C (2 min) to 290 C (30 min) at 4 C/min. The temperature of gasifying room was 280 C. The carrier gas was helium at a constant rate of 1.2 mL/min. MS conditions were: ion source temperature 230 C, electron ionization mode at 70 eV, transfer line 280 C, spectrum library using NIST02L. 2.3.
tested by the excitation light with 365 nm wavelength to obtain the typical fluorescence spectrum of the oil inclusion using Ocean Optics spectrograph SD2000.
3. Geochemical features of hydrocarbons in different occurrence states 3.1. Content of hydrocarbons in different occurrence states
Fluorescence characteristic of the inclusions
The oil can emit visible light with a 400700 nm wavelength under the ultraviolet radiation. As oils of different properties have different fluorescence characteristics, analyzing oil and gas charge history by using maximum launch excitation wavelength, maximum strength and the red/green ratio is an important and effective method for studying the history of hydrocarbon accumulation[1620]. The experiment was done at the Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences. The spectra of the single oil inclusion was
The hydrocarbons of four occurrence states differ widely in content, of which the content of free oil is much higher than the other three kinds. The free oil has an absolute content of 7.9912.80 mg/g, 10.85 mg/g on average, and relative content of 93.4%. The sealed oil has an absolute content of 0.130.60 mg/g, on average 0.41 mg/g, and relative content from 0.96%4.96%, and 3.50% on average. Close to sealed oil in content, the cement oil has an absolute content of 0.210.48 mg/g, 0.31 mg/g on average, and relative content from 1.47%
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Fig. 2. Flow chart showing separation of hydrocarbons of different occurrence states in reservoir sandstones of Chang 8 Member and their geochemical analysis.
hydrocarbons are complete with carbon number ranging from C13-C36, main carbon number being C15-C21, presenting forward peak distribution (Table 1 and Fig. 4). C21－/C22＋ varies from 0.853.56, averaging at 1.56, of which sealed and cement oil have higher ratio, 2.03 and 1.94, respectively, reflecting the dominance of low carbon number peak group in n-alkane. The OEP and CPI value of the four kinds of hydrocarbon range from 0.79 to 1.33 and 0.88 to 1.12 respectively, and are 1.03 and 1.01 on average, reflecting no obvious odd-even predominance in n-alkanes (Table 1). It can be deduced that the organic matter is mainly sourced from aquatic organism, with minor contribution from land plants. Pr/nC17 and Ph/nC18 plot shows that the depositional environments of the source rock generating the four kinds of hydrocarbon are quite similar, being weak oxidized-weak reducing transient environment. Meanwhile, sterane mass chromatographic diagrams of four kinds of hydrocarbon (Fig. 4) show similar distribution feature of regular steranes and near values of C27 sterane and C29 sterane isomerization ratio, which also implies that they are from the same source rock (Table 1 and Fig. 5).
to 5.47%, on average 0.63%. The inclusion oil is lowest in content, with a mean absolute content of 0.055 mg/g, and relative content from 0.27% to 0.70%, on average 0.47% (Fig. 3). 3.2. Geochemical features of hydrocarbons in different occurrence states Alkane is the dominant chemical composition in source rock and saturated hydrocarbon in crude oil, and its composition and carbon number distribution can reflect the types of organic matter, thermal evolution degree and depositional environment. Peak shapes of n-alkanes from the four kinds of Table 1.
Results of n-alkanes and isoprenoid alkane from hydrocarbons of different occurrence states in Chang 8 Member sandstones
Sample Well
Q22
X231
H115
M51
Fig. 3. Content of hydrocarbons of different occurrence states in reservoir sandstones of Chang 8 Member, Yanchang Fm.
Hydrocarbon type
Main carbon
Carbon number range
Pr/Ph
Pr/nC17
Ph/nC18
OEP
CPI
C21－/C22＋
Free hydrocarbon
C20
C13—C33
1.88
0.19
0.09
0.87
0.96
1.05
Sealed hydrocarbon
C20
C13—C36
1.53
0.26
0.18
1.02
0.99
1.33
Cement hydrocarbon
C15
C13—C33
1.94
0.26
0.14
1.00
0.91
1.93
Inclusion hydrocarbon
C18
C13—C36
1.31
0.16
0.10
1.12
0.90
1.15
Free hydrocarbon
C19
C13—C36
1.09
0.28
0.21
0.93
0.88
1.24
Sealed hydrocarbon
C15
C13—C36
1.84
0.49
0.38
1.06
1.07
2.72
Cement hydrocarbon
C15
C13—C33
2.40
0.50
0.30
0.79
0.90
3.56
Inclusion hydrocarbon
C19
C13—C33
1.25
0.19
0.15
1.12
1.05
1.52
Free hydrocarbon
C21
C13—C34
2.07
0.63
0.34
0.95
0.98
0.85
Sealed hydrocarbon
C15
C13—C33
1.85
0.27
0.16
1.12
1.10
2.72
Cement hydrocarbon
C22
C13—C32
1.25
0.51
0.21
0.96
1.00
1.23
Inclusion hydrocarbon
C21
C13—C35
1.13
0.37
0.31
1.33
1.11
1.29
Free hydrocarbon
C19
C13—C33
1.49
0.53
0.33
1.04
1.05
1.07
Sealed hydrocarbon
C15
C13—C36
1.56
0.49
0.37
1.08
1.12
1.33
Cement hydrocarbon
C15
C13—C35
1.61
0.57
0.34
0.99
1.07
1.02
Inclusion hydrocarbon
C21
C13—C36
1.16
0.32
0.23
1.15
1.12
0.89
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Fig. 4. Distribution of n-alkane, isoprenoid alkane and sterane chromatography of hydrocarbons in different occurrence states in Chang 8 Member sandstone, Well M51.
tion of hydrocarbon[2122]. The plot of the two parameters (Fig. 6) shows that C2920S/(20S+20R) ratio varies from 0.39 to 0.63 except only one inclusion oil with slightly lower ratio of 0.4, and C29ββ/(αα+ββ) ratio ranges from 0.430.53. According to the finding proposed by Huang et al.[23] that the crude oil is mature when the two parameters are both in excess of 0.4, all
Fig. 5. Plot of Pr/nC17 and Ph/nC18 in hydrocarbons in different occurrence states.
Regular sterane isomerization parameters C29ββ/(αα+ββ) and C2920S/(20S+20R) are commonly used to assess maturity, and they can effectively indicate immature to peak generation stage, so the two parameters are often used to analyze the thermal maturity of source rock and crude oil. They have a clear tendency of increase with the increase of thermal evolu 52
Fig. 6.
Plot of C29ββ/(αα+ββ) and C2920S/(20S+20R).
FU Jinhua et al. / Petroleum Exploration and Development, 2017, 44(1): 48–57
the four kinds of hydrocarbon extracted from oil sandstone are mature oil, in which the maturity of inclusion oil is relatively lower, and the thermal evolution degree of inclusion oil, cement oil, sealed oil and free oil gradually increase. Based on thermal simulation experiment, Chen and Bao et al.[24] found there is a linear relationship between methylphenanthrene ratio and vitrinite reflectance. The formula between methylphenanthrene ratio and vitrinite reflectance is expressed as follows: Rc=0.594ln(MPR)+0.972 8 (Rc<1.8%) (1) According to the formula (1), the calculated vitrinite reflectance (Rc) of four kinds of hydrocarbons varies from 0.821.10% (Fig. 7), which is in line with the Ro values (0.75%1.10%) of black mudstone and shale in Yanchang Formation in Ordos Basin[12].
4. Homogenization temperature and fluorescence spectrum features of inclusions 4.1.
Homogenization temperature features of inclusions
Salt-water inclusions homochronous with hydrocarbon inclusion are selected for homogenization temperature (Th) measurement. The statistic results show Th is continuous in distribution, with a dominant range from 80 C to 120 C, and within this temperature interval, the inclusion quantity increases with the increase of homogenization temperature (Fig. 8). The inclusions with Th 90120 C account for the most proportion of over 50%.
Fig. 7. Caculated Rc by methylphenanthrene ratio from hydrocarbons in different occurrence states in Chang 8 Member, Yanchang Fm.
4.2.
Fluorescence and spectrum features of inclusions
Free oil is colorless under transmission light and yellowish white or bluish white under fluorescent light. Sealed oil is also colorless under transmission light, and bluish white or bright yellowish white under fluorescent light. Cement oil gives out bluish white light under fluorescent light, and inclusion hydrocarbons are mainly yellowish white and bluish white under fluorescent light. Two parameters are taken to quantitatively describe fluorescence spectrum in this paper: (1) main peak wavelength (λmax), represents the emission wavelength of maximum fluorescence intensity (Imax). With the increase of small molecular components and maturity, the fluorescence light will show eminent blue shifting and decrease of main peak spectrum wavelength; conversely, the main peak spectrum wavelength will increase. (2) Main red/green ratio: Main red/green ratio is defined as the ratio of red proportion with green proportion of the fluorescence color. It has two important parameters QF535 and Q650/500, and they are usually used to quantitatively describe the shape and structure of fluorescence spectrum. QF535 is the ratio of integral area of spectrum scope of emission wavelength 535750 nm with that of emission wavelength 430535 nm; and Q650/500 is the ratio of the intensity of 650 nm wavelength with that of 500 nm wavelength. The higher the QF535 and Q650/500, the lower the oil maturity will be; the lower the QF535 and Q650/500, the higher the maturity of oil will be. Analysis on features of fluorescence spectrum was merely performed on carbonate cement oil and inclusion oil within quartz and feldspar, whereas free oil and sealed oil are incapable of such examination because of their weak fluorescence light. The results show the fluorescence parameters of different inclusions in the same sample show a regular variation, in which the feldspar inclusions have the largest main peak wavelength value from 483.4534.6 nm, on average 502.1 nm, and quartz inclusions are second from 474.3530.9 nm, 496.4 nm on average, and carbonate cement oil is the lowest from 469.2491.1 nm, 476.3 nm on average. The feldspar inclusion, quartz inclusion and carbonate cement have a mean QF535 and Q650/500 of 1.08 and 0.40, 1.11and 0.37, and 0.95 and 0.35 respectively. Clearly, these inclusions have consistent variations of fluorescence spectrum wavelength and main red/green ratio, i.e. carbonate cement hydrocarbons are the lowest in these parameters, and feldspar and quartz inclusion oils are much higher and similar (Table 2, Fig. 9).
5. Relationships between reservoir densification and pool-forming history 5.1. Evolutionary sequence of hydrocarbons in different occurrence states 5.1.1. Analysis based on diagenesis and hydrocarbon occurrence states Fig. 8. Histogram of homogenization temperature of saline water inclusions in Chang 8 Member reservoir, Yanchang Fm.
Relationship between cement oil and inclusion oil: The host minerals of inclusion oil in carbonate cement of Chang 8 are
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Table 2.
Features of fluorescence spectrum parameters in inclu-
sions of Chang 8 reservoir, Yanchang Fm. Well
Depth/m
Host mineral Feldspar
H115
2 572.6
Quartz
Carbonate cement
Feldspar X231
2 092.5
Quartz Carbonate cement Feldspar
M51
2 270.8
Quartz Carbonate cement Feldspar
Q22
1 428.0
Quartz
Carbonate cement
cement oil → sealed oil → free oil. 5.1.2.
λmax/nm
QF535
Q650/500
498.1
1.27
0.46
483.4
1.33
0.55
483.9
0.98
0.30
483.9
1.47
0.60
478.9
1.33
0.59
475.6
1.19
0.50
534.6
1.19
0.48
531.9
1.12
0.38
530.9
1.44
0.44
528.2
1.22
0.41
491.1
1.00
0.34
490.3
0.71
0.17
488.9
0.81
0.30
474.3
0.87
0.34
484.8
1.11
0.43
469.2
0.63
0.19
494.4
0.92
0.30
494.9
1.32
0.52
491.2
0.95
0.25
494.0
0.84
0.20
469.2
0.66
0.13
473.8
0.90
0.36
Analysis based on inclusion fluorescence spectrum
The variation trends of hydrocarbon fluorescence spectrum parameters, such as λmax, QF535 and Q650/500 indicate that the carbonate cement oil is relatively higher in maturity, while quartz and feldspar inclusion oils are lower in maturity with very similar in these parameters, implying that they are formed in the similar environment and nearly contemporaneous, so they are generally believed as synchronous diagenetic products. Therefore, the trapping sequence of these oils is feldspar and quartz inclusion oil → carbonate cement oil. 5.1.3. Analysis based on geochemical parameters indicating oil maturity
Fe-rich mineral assemblages, mainly ferrocalcite and a limited amount of ankerite. Generally, the Fe-bearing cement is generated in alkaline condition in later diagenetic stage, and pervasively appear in sandstones during mesodiagenetic B stage. Silica cement and feldspar cement started to occur in the later period of early diagenetic stage, and developed widely in acidic solution media during mesodiagenetic A and B stages, thus, the forming time of quartz inclusion and feldspar inclusion is earlier than that of carbonate cement inclusion. Relationship of free oil, sealed oil and cement oil: in all hydrocarbons extracted from Chang 8 reservoir, free oil takes the absolute majority, representing the hydrocarbons charged in the main reservoir forming period. Sealed oil refers to the oil preserved in unconnected pore spaces and its abundance is largely affected by diagenesis during oil emplacement. In other words, the pores containing oil were connected with each other in the early diagenetic stage, but they are isolated and separated during late diagenesis due to cementation, clay transformation and compaction, resulting in sealed oil in the blocked and separated pores. Based on the above analysis, the trapping sequence of different oils is: feldspar and quartz inclusion oil → carbonate
The calculated vitrinite reflectance (Rc) from methylphenanthrene ratio shows that free oil and sealed oil have similar Rc, and cement oil and inclusion oil have similar Rc. The relationship between C29ββ/(αα+ββ) and C2920S/(20S+20R) reflects that sealed oil is near or slightly lower than that of free oil, but higher than cement oil in thermal evolutionary degree, implying that the forming time of sealed oil is consistent with free oil or slightly earlier, and later than the forming time of Fe-bearing carbonate cement, in other words, the sealed oil is trapped later than cement oil. Thus, the thermal evolutionary degree of the four kinds of hydrocarbons shows a gradual increase trend from inclusion oil → cement oil → sealed oil → free oil (Table 3). To sum up, the several methods mentioned above reach a consistent finding on the thermal evolutionary degree of hydrocarbons extracted from Chang 8 reservoir, that is the trapping sequence of the four kinds of oils is quartz and feldspar inclusion oil → carbonate cement oil → sealed oil → free oil. 5.2. Oil-charging phases and main oil-pool forming phase 5.2.1.
Division of oil-charging phase
The homogenization temperatures of fluid inclusions in Chang 8 reservoir are continuous, indicating the oil-charging process of Chang 8 oil–pool is continuous, and this process can be divided into three phases (Table 3): PhaseⅠ: Inclusion oil hosted in quartz and feldspar is representative of PhaseⅠoil-charging period, which is characterized by higher λmax and QF535, Q650/500, lower C29ββ/(αα+ββ) and C2920S/(20S+20R), and Rc calculated from Methylphenanthrene ratio of 0.83-1.04, implying relatively lower maturity. PhaseⅡ: Carbonate cement oil is the representative product of PhaseⅡoil-charging period, with the characteristics of lower λmax and QF535, Q650/500 values, and lower C29ββ/(αα+ββ) and C2920S/(20S+20R), indicating slightly higher maturity than that of PhaseⅠ. Phase Ⅲ: Sealed oil and free oil are typical products formed in Phase Ⅲ. Higher C29ββ/(αα+ββ) and C2920S/(20S+
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Fig. 9. Characteristics of fluorescence spectrum and fluorescence of fluid inclusions in Chang 8 reservoir sandstone (2 572.55 m), Well H115.
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FU Jinhua et al. / Petroleum Exploration and Development, 2017, 44(1): 48–57
Division of oil-charging phases in Chang 8 reservoir, Yanchang Fm.
Oil-charging phase
PhaseⅠ
Phase Ⅱ Phase Ⅲ
Hydrocarbon occurrence state
Fluorescence spectrum
C29ββ/
C2920S/
(αα+ββ)
(20S+20R)
0.430.46
0.390.47
0.831.04
0.440.51
0.460.52
0.831.04
Sealed hydrocarbon
0.460.52
0.520.58
0.871.10
Free hydrocarbon
0.480.53
0.500.63
0.901.10
Feldspar inclusion hydrocarbon Quartz inclusion hydrocarbon Carbonate cement hydrocarbon
λmax/nm
QF535
Q650/500
502.1
1.08
0.40
496.4
1.11
0.37
476.3
0.95
0.35
20R) show they are higher in thermal evolution degree, and their Rc ranging from 0.871.10, also indicate higher thermal evolutionary degree. 5.2.2.
Identification of the main oil-pool forming period
The free oil takes an absolute majority in all hydrocarbons extracted from Chang 8 tight sandstone reservoir, with an average proportion of up to 93.4%, and the next is sealed oil, with a proportion of 3.5%. The two oils combined account for 96.9% of all the hydrocarbons, therefore, they are believed to be oil-charging products in the main oil-pool forming period, i.e. Phase Ⅲ is the main oil-pool forming period. 5.3. Relationship between densification history and oil-pool forming history The Chang 8 reservoir has a porosity of 5.2%12.2%, and an average content of carbonate cement formed in the later diagenetic stage of 5.7%, indicating that the porosity loss caused by late carbonate cementation is as high as 5.7%, which leads to the densification of the Chang 8 sandstone reservoir at last. Through the above analysis, three findings have been reached. (1) The main oil-pool forming period is the latest or the third oil-charging phase represented by free oil and sealed oil in Chang 8 reservoir. (2) The thermal evolutionary degree of free oil and sealed oil is near or higher than that of Fe-bearing carbonate cement oil. (3) The late carbonate cementation is the decisive factor causing reservoir densification. Based on three cognitions above, it can be deduced that the large-scaled oil-charging and pool-forming period is after late Fe-bearing carbonate cement precipitation, therefore, the Chang 8 oil-pool is formed after the reservoir tightening.
6.
Rc/%
Thermal evolution degree
Increase
Table 3.
the same source rock formed in the similar depositional environment. The measured results of fluorescence spectrum and aromatic chromatography and mass-spectrography show that the four kinds of oils are different in thermal evolutionary degree, and increase in maturity from inclusion oil → cement oil → sealed oil → free oil. Charging of Chang 8 oil-pool is a continuous process, which can be divided into three phases, PhaseⅠis early oil-charging stage characterized by the formation of feldspar and quartz inclusions, and PhaseⅡ is the middle oil-charging stage represented by carbonate cement oil, and Phase Ⅲ is late oil-charging stage and also the main oil-pool formation period, represented by the formation of sealed oil and free oil. Thus, the Chang 8 oil-pool is formed after the reservoir tightening.
Nomenclature MPR—Methylphenanthrene ratio, dimensionless; Rc—Calculated vitrinite reflectance, %.
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Conclusions
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Four kinds of hydrocarbons were obtained from Chang 8 reservoir samples by means of crushing, acid treatment and stepwise extraction, namely, free oil, sealed oil, cement oil and inclusion oil. The four kinds of hydrocarbons are complete in peak shape of n-alkane, forward type in main peak carbon, without eminent OEP, indicating that they are in mature stage, and are sourced from aquatic organism with limited contribution of land plants. Besides, they are generated from 56
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RESEARCH PAPER
Densification and hydrocarbon accumulation of Triassic Yanchang Formation Chang 8 Member, Ordos Basin, NW China: Evidence from geochemistry and fluid inclusions FU Jinhua1, 2, *, DENG Xiuqin1, 2, WANG Qi3, LI Jihong1, 2, QIU Junli3, HAO Lewei3, ZHAO Yande1, 2 1. National Engineering Laboratory for Exploration and Development of Low-Permeability Oil & Gas Fields, Xi’an 710018, China; 2. PetroChina Changqing Oil Field Company, Xi’an 710018, China; 3. Institute of Geology and Geophysics, Chinese Academy of Sciences, Lanzhou 730000, China
Abstract: Crushing, acid treatment and step wise separation and oil extraction were employed to obtain the different occurrence state hydrocarbons. All these fractions have been analyzed by Gas Chromatography-Mass Spectrometer (GC-MS). The fractions relationship and related oil charging process can be mirrored through the analysis of fractions weight content and geochemical characteristics, in combination with the research of inclusions homogenization temperature and fluorescence spectrum parameters. Experimental results reveal that there are four state hydrocarbons, i.e. free hydrocarbon, sealed hydrocarbon, hydrocarbon in carbonate cement, and hydrocarbon within inclusions caught by quartz grains and feldspar grains in the oil-rich sandstones of Chang8 Member, Triassic Yanchang Formation in the Ordos Basin. Among them, the overwhelming fraction is the free hydrocarbon, averaged 93.4%. Fluorescence spectrum parameters of λmax, QF535 and Q650/500 show that the crude oil maturity of the inclusions imprisoned in feldspar, quartz, and carbonate cement increase in turn, and the parameter values of the inclusions in feldspar and quartz are similar and much different from those of carbonate cement. Through analysis of C29ββ/(αα+ββ) and C2920S/(20S+20R), together with methylphenanthrene ratio, it is revealed that thermal-evolutionary degree of the hydrocarbon within inclusions, hydrocarbon in carbonate cement, sealed hydrocarbon, free hydrocarbon reflects an upward trend, and the data of the last two type are similar. Integrated study of diagenetic sequence and thermal evolutionary degree suggest that the Chang 8 sandstones had been compacted before reservoir formation and the reservoirs have experienced three phase of charge events in which the third one played the most important role for reservoir formation. Key words: Ordos Basin; Triassic; Yanchang Formation; hydrocarbon occurrence; charge period; major reservoir forming phase; tight reservoir
Introduction In recent years, great breakthroughs have been made in tight oil & gas exploration in Triassic Yanchang Formation, Ordos Basin, marked by the discovery of a series of tight oil fields, including Xifeng, Jiyuan, Huaqing oilfields etc. Previous studies have covered different aspects related to the geological conditions, main control factors and forming mechanism of the tight oil fields[17], and, these works mainly focused on the migration and accumulation of the oil & gas in the low-permeable reservoirs as well as the time sequence of the reservoir tightening and pool formation. Different researchers gave their own viewpoints on these questions, e.g. Luo et al., based on his research on diagenetic process and reservoir tar, suggested that the oil-charging before the reser-
voir tightening had changed the reservoir’s wettability, and the residual oil-pro pathway network provided favorable conduit for oil and gas migration and accumulation[3]; Ren et al. pointed out that the tight oil was formed under the overall tightness background according to several lines of evidence from inclusion temperature, K-Ar dating and saturation pressure analytical data, and so on[4]; On the contrary, Liu et al. believed that the Yanchang reservoir could not be infilled by oil under the tight state based on the fluorescence examination on inclusion, simulation on porosity evolution and oil-charging critical experiment, and concluded that Yanchang oil- pool could only be formed before the reservoir densified through inference[5]. To sum up, there is no consensus on the order of Yanchang sandstone tightening and oil-pool formation. In view of this, the forming process of Yanchang tight
Received date: 24 Dec. 2015; Revised date: 18 Dec. 2016. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the China National Science and Technology Major Project (2016ZX05050; 2011ZX05001-004). Copyright © 2017, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.
FU Jinhua et al. / Petroleum Exploration and Development, 2017, 44(1): 48–57
reservoir has been analyzed by using stepwise extraction and fluorescence examination on oil inclusions, to find the time sequence of densification of Yanchang Formation sandstone and pool formation, and get a clear understanding on the forming mechanism of Yanchang Formation tight oil reservoir.
1.
Geological setting
A giant inland depression lacustrine basin developed in Ordos area during the middle and late Triassic, where a successive clastic sequence of Yanchang Formation deposited, with a thickness of approximately 1 000 meter. There are large scale low-permeable lithological and tight oil pools in Yanchang Formation after over 200 million years of structural and diagenetic transformation. Yanchang Formation can be classified in 10 members, Chang 1 to Chang 10 from bottom to top, of which the Chang 7 Member, the deposits of maximum flooding, with abundant organic matter, is the major source rock in the Mesozoic of Ordos Basin. The Chang 8 Member, below Chang7 Member, is shallow lacustrine deltaic deposits with well-developed distributary and sub-distributary channel sandbodies widespread, stable in spatial distribution, and thick in main sandbody zone[8]. The reservoir rock of Chang 8 Member is mainly fine grained sandstone, intense compaction and carbonate cementation have caused its poor physical properties (porosity from 5.2% to 12.2%, and permeability from 0.02×103 to 5.02×103 μm2)[9]. Although Chang8 Member is poorer in physical property, the overlying Chang7 Member had a pressure coefficient of 1.21.7[1011] at the maximum palaeoburial depth, there was quite large pressure difference between Chang 7 and Chang 8 Member, which can drive oil migration downward, thus, the Chang 8 sandstone reservoir is universally oil-bearing and large in reserve scale.
2.
Sampling and experiments
Stepwise (or sequential) extraction of hydrocarbon in reservoir rock is a novel analysis technique of micro-geochemistry of oil and gas pool developed in recent years[1215]. Its theoretical basis is that the maturity of hydrocarbons in reservoir, no matter where they are originated from the same source bed or not, will change somewhat in the process of migration and accumulation, and, therefore, it can be utilized to determine the hydrocarbon source in oil-bearing reservoir rock and oil-pool forming periods. 2.1. Separation of hydrocarbons in different occurrence states First of all, 86 oil-bearing sandstone samples were chosen to make thin sections to check their fluorescence features. Four sandstone samples from Well Q22, X231, H115 and M51 (Fig. 1) were selected for stepwise extraction according to their oil content. These four samples have a porosity of 6.7%, 8.1%, 9.1% and 8.5%, and mean grain size of 3.53, 2.57, 2.44 and 2.7, respectively. Secondly, hydrocarbons in the samples
were extracted by crushing, acid processing and Soxhlet extraction methods sequentially. Four kinds of hydrocarbon have been successfully separated, including free oil, sealed oil, inclusion oil in carbonate cement ( cement oil for short hereafter), and inclusion oil in quartz and feldspar grains (inclusion oil for short hereafter). The experimental flowchart is described as follows (Fig. 2): Step 1. Free oil separation. Free oil was obtained by Soxhlet extraction method using dichloromethane–methyl alcohol solution (volume ratio 93:7) to rinse oil sandstone samples after crushing them into 0.51.5 cm in grain size. Free oil stands for the hydrocarbon in open porous system of the reservoir rock. Step 2. Sealed oil separation. The residual sandstone samples after Soxhlet extraction were re-crushed into single clastic grains by gentle manner. To control the separation of rock framework grains, microscope was used to examine the crushed sample grain size and those fragments with grain size less than the single clastic grain were sieved out in order to eliminate the released inclusion oil during crushing. And then the processed samples were extracted by dichloromethane– methyl alcohol solution (volume ratio 93:7) to obtain the sealed oil, which is the hydrocarbon preserved in unconnected pores. Step 3. Cement oil separation. Cement oil is defined as the hydrocarbon enclosed in carbonate cement. The remained samples after the extraction of the sealed oil were dissolved by 6% chloride acid to remove carbonate cement, and extracted by dichloromethane–methyl alcohol solution (volume ratio 93:7) to get cement oil. Step 4. Inclusion oil separation. The remained samples after Step 3 were rinsed by distilled water to neutral, and, furthermore, processed by potassium dichromate-concentrated sulfuric acid solution and hydrogen peroxide, and then, these samples were re-extracted by dichloromethane–methyl alcohol solution (volume ratio 93:7) to clean out surficial hydrocarbon. Finally, these samples were grinded in dichloromethane–methyl alcohol solution (volume ratio 10:1) to extract the hydrocarbon enclosed in single quartz or feldspar grains. The obtained hydrocarbon is called inclusion hydrocarbon. 2.2.
Hydrocarbon component analysis
Firstly, weighed the separated hydrocarbons of different occurrence states. Secondly, asphaltene was precipitated using n-hexane for all hydrocarbons (excepting hydrocarbon in cement and hydrocarbon in inclusions). And then the saturated hydrocarbons and aromatic hydrocarbons fractions were obtained using column chromatography (Silica gel - alumina, v:v 4:1). At last, all these fractions were analyzed by GC-MS. GC–MS was performed using an Agilent Technologies HP6890N gas chromatograph coupled to a HP5973 Network mass selective detector. GC conditions were: Separation was achieved using fused silica capillary column J&WHP-5 (30
49
FU Jinhua et al. / Petroleum Exploration and Development, 2017, 44(1): 48–57
Fig. 1.
Structural units and stratigraphic column of Yanchang Formation in Ordos Basin.
m×0.25 mm×0.25 µm), the primary oven temperature program was: 80 C (2 min) to 290 C (30 min) at 4 C/min. The temperature of gasifying room was 280 C. The carrier gas was helium at a constant rate of 1.2 mL/min. MS conditions were: ion source temperature 230 C, electron ionization mode at 70 eV, transfer line 280 C, spectrum library using NIST02L. 2.3.
tested by the excitation light with 365 nm wavelength to obtain the typical fluorescence spectrum of the oil inclusion using Ocean Optics spectrograph SD2000.
3. Geochemical features of hydrocarbons in different occurrence states 3.1. Content of hydrocarbons in different occurrence states
Fluorescence characteristic of the inclusions
The oil can emit visible light with a 400700 nm wavelength under the ultraviolet radiation. As oils of different properties have different fluorescence characteristics, analyzing oil and gas charge history by using maximum launch excitation wavelength, maximum strength and the red/green ratio is an important and effective method for studying the history of hydrocarbon accumulation[1620]. The experiment was done at the Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences. The spectra of the single oil inclusion was
The hydrocarbons of four occurrence states differ widely in content, of which the content of free oil is much higher than the other three kinds. The free oil has an absolute content of 7.9912.80 mg/g, 10.85 mg/g on average, and relative content of 93.4%. The sealed oil has an absolute content of 0.130.60 mg/g, on average 0.41 mg/g, and relative content from 0.96%4.96%, and 3.50% on average. Close to sealed oil in content, the cement oil has an absolute content of 0.210.48 mg/g, 0.31 mg/g on average, and relative content from 1.47%
50
FU Jinhua et al. / Petroleum Exploration and Development, 2017, 44(1): 48–57
Fig. 2. Flow chart showing separation of hydrocarbons of different occurrence states in reservoir sandstones of Chang 8 Member and their geochemical analysis.
hydrocarbons are complete with carbon number ranging from C13-C36, main carbon number being C15-C21, presenting forward peak distribution (Table 1 and Fig. 4). C21－/C22＋ varies from 0.853.56, averaging at 1.56, of which sealed and cement oil have higher ratio, 2.03 and 1.94, respectively, reflecting the dominance of low carbon number peak group in n-alkane. The OEP and CPI value of the four kinds of hydrocarbon range from 0.79 to 1.33 and 0.88 to 1.12 respectively, and are 1.03 and 1.01 on average, reflecting no obvious odd-even predominance in n-alkanes (Table 1). It can be deduced that the organic matter is mainly sourced from aquatic organism, with minor contribution from land plants. Pr/nC17 and Ph/nC18 plot shows that the depositional environments of the source rock generating the four kinds of hydrocarbon are quite similar, being weak oxidized-weak reducing transient environment. Meanwhile, sterane mass chromatographic diagrams of four kinds of hydrocarbon (Fig. 4) show similar distribution feature of regular steranes and near values of C27 sterane and C29 sterane isomerization ratio, which also implies that they are from the same source rock (Table 1 and Fig. 5).
to 5.47%, on average 0.63%. The inclusion oil is lowest in content, with a mean absolute content of 0.055 mg/g, and relative content from 0.27% to 0.70%, on average 0.47% (Fig. 3). 3.2. Geochemical features of hydrocarbons in different occurrence states Alkane is the dominant chemical composition in source rock and saturated hydrocarbon in crude oil, and its composition and carbon number distribution can reflect the types of organic matter, thermal evolution degree and depositional environment. Peak shapes of n-alkanes from the four kinds of Table 1.
Results of n-alkanes and isoprenoid alkane from hydrocarbons of different occurrence states in Chang 8 Member sandstones
Sample Well
Q22
X231
H115
M51
Fig. 3. Content of hydrocarbons of different occurrence states in reservoir sandstones of Chang 8 Member, Yanchang Fm.
Hydrocarbon type
Main carbon
Carbon number range
Pr/Ph
Pr/nC17
Ph/nC18
OEP
CPI
C21－/C22＋
Free hydrocarbon
C20
C13—C33
1.88
0.19
0.09
0.87
0.96
1.05
Sealed hydrocarbon
C20
C13—C36
1.53
0.26
0.18
1.02
0.99
1.33
Cement hydrocarbon
C15
C13—C33
1.94
0.26
0.14
1.00
0.91
1.93
Inclusion hydrocarbon
C18
C13—C36
1.31
0.16
0.10
1.12
0.90
1.15
Free hydrocarbon
C19
C13—C36
1.09
0.28
0.21
0.93
0.88
1.24
Sealed hydrocarbon
C15
C13—C36
1.84
0.49
0.38
1.06
1.07
2.72
Cement hydrocarbon
C15
C13—C33
2.40
0.50
0.30
0.79
0.90
3.56
Inclusion hydrocarbon
C19
C13—C33
1.25
0.19
0.15
1.12
1.05
1.52
Free hydrocarbon
C21
C13—C34
2.07
0.63
0.34
0.95
0.98
0.85
Sealed hydrocarbon
C15
C13—C33
1.85
0.27
0.16
1.12
1.10
2.72
Cement hydrocarbon
C22
C13—C32
1.25
0.51
0.21
0.96
1.00
1.23
Inclusion hydrocarbon
C21
C13—C35
1.13
0.37
0.31
1.33
1.11
1.29
Free hydrocarbon
C19
C13—C33
1.49
0.53
0.33
1.04
1.05
1.07
Sealed hydrocarbon
C15
C13—C36
1.56
0.49
0.37
1.08
1.12
1.33
Cement hydrocarbon
C15
C13—C35
1.61
0.57
0.34
0.99
1.07
1.02
Inclusion hydrocarbon
C21
C13—C36
1.16
0.32
0.23
1.15
1.12
0.89
51
FU Jinhua et al. / Petroleum Exploration and Development, 2017, 44(1): 48–57
Fig. 4. Distribution of n-alkane, isoprenoid alkane and sterane chromatography of hydrocarbons in different occurrence states in Chang 8 Member sandstone, Well M51.
tion of hydrocarbon[2122]. The plot of the two parameters (Fig. 6) shows that C2920S/(20S+20R) ratio varies from 0.39 to 0.63 except only one inclusion oil with slightly lower ratio of 0.4, and C29ββ/(αα+ββ) ratio ranges from 0.430.53. According to the finding proposed by Huang et al.[23] that the crude oil is mature when the two parameters are both in excess of 0.4, all
Fig. 5. Plot of Pr/nC17 and Ph/nC18 in hydrocarbons in different occurrence states.
Regular sterane isomerization parameters C29ββ/(αα+ββ) and C2920S/(20S+20R) are commonly used to assess maturity, and they can effectively indicate immature to peak generation stage, so the two parameters are often used to analyze the thermal maturity of source rock and crude oil. They have a clear tendency of increase with the increase of thermal evolu 52
Fig. 6.
Plot of C29ββ/(αα+ββ) and C2920S/(20S+20R).
FU Jinhua et al. / Petroleum Exploration and Development, 2017, 44(1): 48–57
the four kinds of hydrocarbon extracted from oil sandstone are mature oil, in which the maturity of inclusion oil is relatively lower, and the thermal evolution degree of inclusion oil, cement oil, sealed oil and free oil gradually increase. Based on thermal simulation experiment, Chen and Bao et al.[24] found there is a linear relationship between methylphenanthrene ratio and vitrinite reflectance. The formula between methylphenanthrene ratio and vitrinite reflectance is expressed as follows: Rc=0.594ln(MPR)+0.972 8 (Rc<1.8%) (1) According to the formula (1), the calculated vitrinite reflectance (Rc) of four kinds of hydrocarbons varies from 0.821.10% (Fig. 7), which is in line with the Ro values (0.75%1.10%) of black mudstone and shale in Yanchang Formation in Ordos Basin[12].
4. Homogenization temperature and fluorescence spectrum features of inclusions 4.1.
Homogenization temperature features of inclusions
Salt-water inclusions homochronous with hydrocarbon inclusion are selected for homogenization temperature (Th) measurement. The statistic results show Th is continuous in distribution, with a dominant range from 80 C to 120 C, and within this temperature interval, the inclusion quantity increases with the increase of homogenization temperature (Fig. 8). The inclusions with Th 90120 C account for the most proportion of over 50%.
Fig. 7. Caculated Rc by methylphenanthrene ratio from hydrocarbons in different occurrence states in Chang 8 Member, Yanchang Fm.
4.2.
Fluorescence and spectrum features of inclusions
Free oil is colorless under transmission light and yellowish white or bluish white under fluorescent light. Sealed oil is also colorless under transmission light, and bluish white or bright yellowish white under fluorescent light. Cement oil gives out bluish white light under fluorescent light, and inclusion hydrocarbons are mainly yellowish white and bluish white under fluorescent light. Two parameters are taken to quantitatively describe fluorescence spectrum in this paper: (1) main peak wavelength (λmax), represents the emission wavelength of maximum fluorescence intensity (Imax). With the increase of small molecular components and maturity, the fluorescence light will show eminent blue shifting and decrease of main peak spectrum wavelength; conversely, the main peak spectrum wavelength will increase. (2) Main red/green ratio: Main red/green ratio is defined as the ratio of red proportion with green proportion of the fluorescence color. It has two important parameters QF535 and Q650/500, and they are usually used to quantitatively describe the shape and structure of fluorescence spectrum. QF535 is the ratio of integral area of spectrum scope of emission wavelength 535750 nm with that of emission wavelength 430535 nm; and Q650/500 is the ratio of the intensity of 650 nm wavelength with that of 500 nm wavelength. The higher the QF535 and Q650/500, the lower the oil maturity will be; the lower the QF535 and Q650/500, the higher the maturity of oil will be. Analysis on features of fluorescence spectrum was merely performed on carbonate cement oil and inclusion oil within quartz and feldspar, whereas free oil and sealed oil are incapable of such examination because of their weak fluorescence light. The results show the fluorescence parameters of different inclusions in the same sample show a regular variation, in which the feldspar inclusions have the largest main peak wavelength value from 483.4534.6 nm, on average 502.1 nm, and quartz inclusions are second from 474.3530.9 nm, 496.4 nm on average, and carbonate cement oil is the lowest from 469.2491.1 nm, 476.3 nm on average. The feldspar inclusion, quartz inclusion and carbonate cement have a mean QF535 and Q650/500 of 1.08 and 0.40, 1.11and 0.37, and 0.95 and 0.35 respectively. Clearly, these inclusions have consistent variations of fluorescence spectrum wavelength and main red/green ratio, i.e. carbonate cement hydrocarbons are the lowest in these parameters, and feldspar and quartz inclusion oils are much higher and similar (Table 2, Fig. 9).
5. Relationships between reservoir densification and pool-forming history 5.1. Evolutionary sequence of hydrocarbons in different occurrence states 5.1.1. Analysis based on diagenesis and hydrocarbon occurrence states Fig. 8. Histogram of homogenization temperature of saline water inclusions in Chang 8 Member reservoir, Yanchang Fm.
Relationship between cement oil and inclusion oil: The host minerals of inclusion oil in carbonate cement of Chang 8 are
53
FU Jinhua et al. / Petroleum Exploration and Development, 2017, 44(1): 48–57
Table 2.
Features of fluorescence spectrum parameters in inclu-
sions of Chang 8 reservoir, Yanchang Fm. Well
Depth/m
Host mineral Feldspar
H115
2 572.6
Quartz
Carbonate cement
Feldspar X231
2 092.5
Quartz Carbonate cement Feldspar
M51
2 270.8
Quartz Carbonate cement Feldspar
Q22
1 428.0
Quartz
Carbonate cement
cement oil → sealed oil → free oil. 5.1.2.
λmax/nm
QF535
Q650/500
498.1
1.27
0.46
483.4
1.33
0.55
483.9
0.98
0.30
483.9
1.47
0.60
478.9
1.33
0.59
475.6
1.19
0.50
534.6
1.19
0.48
531.9
1.12
0.38
530.9
1.44
0.44
528.2
1.22
0.41
491.1
1.00
0.34
490.3
0.71
0.17
488.9
0.81
0.30
474.3
0.87
0.34
484.8
1.11
0.43
469.2
0.63
0.19
494.4
0.92
0.30
494.9
1.32
0.52
491.2
0.95
0.25
494.0
0.84
0.20
469.2
0.66
0.13
473.8
0.90
0.36
Analysis based on inclusion fluorescence spectrum
The variation trends of hydrocarbon fluorescence spectrum parameters, such as λmax, QF535 and Q650/500 indicate that the carbonate cement oil is relatively higher in maturity, while quartz and feldspar inclusion oils are lower in maturity with very similar in these parameters, implying that they are formed in the similar environment and nearly contemporaneous, so they are generally believed as synchronous diagenetic products. Therefore, the trapping sequence of these oils is feldspar and quartz inclusion oil → carbonate cement oil. 5.1.3. Analysis based on geochemical parameters indicating oil maturity
Fe-rich mineral assemblages, mainly ferrocalcite and a limited amount of ankerite. Generally, the Fe-bearing cement is generated in alkaline condition in later diagenetic stage, and pervasively appear in sandstones during mesodiagenetic B stage. Silica cement and feldspar cement started to occur in the later period of early diagenetic stage, and developed widely in acidic solution media during mesodiagenetic A and B stages, thus, the forming time of quartz inclusion and feldspar inclusion is earlier than that of carbonate cement inclusion. Relationship of free oil, sealed oil and cement oil: in all hydrocarbons extracted from Chang 8 reservoir, free oil takes the absolute majority, representing the hydrocarbons charged in the main reservoir forming period. Sealed oil refers to the oil preserved in unconnected pore spaces and its abundance is largely affected by diagenesis during oil emplacement. In other words, the pores containing oil were connected with each other in the early diagenetic stage, but they are isolated and separated during late diagenesis due to cementation, clay transformation and compaction, resulting in sealed oil in the blocked and separated pores. Based on the above analysis, the trapping sequence of different oils is: feldspar and quartz inclusion oil → carbonate
The calculated vitrinite reflectance (Rc) from methylphenanthrene ratio shows that free oil and sealed oil have similar Rc, and cement oil and inclusion oil have similar Rc. The relationship between C29ββ/(αα+ββ) and C2920S/(20S+20R) reflects that sealed oil is near or slightly lower than that of free oil, but higher than cement oil in thermal evolutionary degree, implying that the forming time of sealed oil is consistent with free oil or slightly earlier, and later than the forming time of Fe-bearing carbonate cement, in other words, the sealed oil is trapped later than cement oil. Thus, the thermal evolutionary degree of the four kinds of hydrocarbons shows a gradual increase trend from inclusion oil → cement oil → sealed oil → free oil (Table 3). To sum up, the several methods mentioned above reach a consistent finding on the thermal evolutionary degree of hydrocarbons extracted from Chang 8 reservoir, that is the trapping sequence of the four kinds of oils is quartz and feldspar inclusion oil → carbonate cement oil → sealed oil → free oil. 5.2. Oil-charging phases and main oil-pool forming phase 5.2.1.
Division of oil-charging phase
The homogenization temperatures of fluid inclusions in Chang 8 reservoir are continuous, indicating the oil-charging process of Chang 8 oil–pool is continuous, and this process can be divided into three phases (Table 3): PhaseⅠ: Inclusion oil hosted in quartz and feldspar is representative of PhaseⅠoil-charging period, which is characterized by higher λmax and QF535, Q650/500, lower C29ββ/(αα+ββ) and C2920S/(20S+20R), and Rc calculated from Methylphenanthrene ratio of 0.83-1.04, implying relatively lower maturity. PhaseⅡ: Carbonate cement oil is the representative product of PhaseⅡoil-charging period, with the characteristics of lower λmax and QF535, Q650/500 values, and lower C29ββ/(αα+ββ) and C2920S/(20S+20R), indicating slightly higher maturity than that of PhaseⅠ. Phase Ⅲ: Sealed oil and free oil are typical products formed in Phase Ⅲ. Higher C29ββ/(αα+ββ) and C2920S/(20S+
54
FU Jinhua et al. / Petroleum Exploration and Development, 2017, 44(1): 48–57
Fig. 9. Characteristics of fluorescence spectrum and fluorescence of fluid inclusions in Chang 8 reservoir sandstone (2 572.55 m), Well H115.
55
FU Jinhua et al. / Petroleum Exploration and Development, 2017, 44(1): 48–57
Division of oil-charging phases in Chang 8 reservoir, Yanchang Fm.
Oil-charging phase
PhaseⅠ
Phase Ⅱ Phase Ⅲ
Hydrocarbon occurrence state
Fluorescence spectrum
C29ββ/
C2920S/
(αα+ββ)
(20S+20R)
0.430.46
0.390.47
0.831.04
0.440.51
0.460.52
0.831.04
Sealed hydrocarbon
0.460.52
0.520.58
0.871.10
Free hydrocarbon
0.480.53
0.500.63
0.901.10
Feldspar inclusion hydrocarbon Quartz inclusion hydrocarbon Carbonate cement hydrocarbon
λmax/nm
QF535
Q650/500
502.1
1.08
0.40
496.4
1.11
0.37
476.3
0.95
0.35
20R) show they are higher in thermal evolution degree, and their Rc ranging from 0.871.10, also indicate higher thermal evolutionary degree. 5.2.2.
Identification of the main oil-pool forming period
The free oil takes an absolute majority in all hydrocarbons extracted from Chang 8 tight sandstone reservoir, with an average proportion of up to 93.4%, and the next is sealed oil, with a proportion of 3.5%. The two oils combined account for 96.9% of all the hydrocarbons, therefore, they are believed to be oil-charging products in the main oil-pool forming period, i.e. Phase Ⅲ is the main oil-pool forming period. 5.3. Relationship between densification history and oil-pool forming history The Chang 8 reservoir has a porosity of 5.2%12.2%, and an average content of carbonate cement formed in the later diagenetic stage of 5.7%, indicating that the porosity loss caused by late carbonate cementation is as high as 5.7%, which leads to the densification of the Chang 8 sandstone reservoir at last. Through the above analysis, three findings have been reached. (1) The main oil-pool forming period is the latest or the third oil-charging phase represented by free oil and sealed oil in Chang 8 reservoir. (2) The thermal evolutionary degree of free oil and sealed oil is near or higher than that of Fe-bearing carbonate cement oil. (3) The late carbonate cementation is the decisive factor causing reservoir densification. Based on three cognitions above, it can be deduced that the large-scaled oil-charging and pool-forming period is after late Fe-bearing carbonate cement precipitation, therefore, the Chang 8 oil-pool is formed after the reservoir tightening.
6.
Rc/%
Thermal evolution degree
Increase
Table 3.
the same source rock formed in the similar depositional environment. The measured results of fluorescence spectrum and aromatic chromatography and mass-spectrography show that the four kinds of oils are different in thermal evolutionary degree, and increase in maturity from inclusion oil → cement oil → sealed oil → free oil. Charging of Chang 8 oil-pool is a continuous process, which can be divided into three phases, PhaseⅠis early oil-charging stage characterized by the formation of feldspar and quartz inclusions, and PhaseⅡ is the middle oil-charging stage represented by carbonate cement oil, and Phase Ⅲ is late oil-charging stage and also the main oil-pool formation period, represented by the formation of sealed oil and free oil. Thus, the Chang 8 oil-pool is formed after the reservoir tightening.
Nomenclature MPR—Methylphenanthrene ratio, dimensionless; Rc—Calculated vitrinite reflectance, %.
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