BY 4.0 license Open Access Published by De Gruyter Open Access November 22, 2021

Origin of carbonate cements in deep sandstone reservoirs and its significance for hydrocarbon indication: A case of Shahejie Formation in Dongying Sag

Tianjiao Zhang, Yuelin Feng, Xinmin Ge, Wei Meng, Hongwei Han, Jingqiang Yu, Weizhong Zhang, Shuli Li, Xiaochen Li and Ping Gao
From the journal Open Geosciences

Abstract

Carbonate cements are primary cement types formed in deep sandstone reservoirs of Dongying Sag. We have proposed three stages of carbonate cements with different origin and material sources: carbonate cements in early stage are rim-shaped high-Mg calcite, which is the product of quasi-contemporaneous period; and calcite filled with primary pores without obvious compaction and diagenetic transformation is mudstone compaction during the drainage process. Carbonate cements in middle stage are calcite and dolomite filled with feldspar secondary dissolved pores. The rich Ca2+, Mg2+, and CO 3 2 in overpressure fluid enter the reservoir and mix with Ca2+ in the original formation water. Carbonate cements in late stage are ferrocalcite and ankerite that filled the dissolution pores of early- and middle-stage carbonate cements. They were products of CO 3 2 formed by organic acid splitting decomposition in late diagenesis and CO 3 2 formed by dissolution of carbonate cements in early and middle stages, combined with Mg2+, Ca2+, Fe2+ plasma in pore fluid. Dissolution–reprecipitation of the lacustrine carbonate rocks is responsible for obvious positive drift in the δ 13CPDB‰ values of carbonate cements. Carbonate cements in middle stage and late stage, respectively, represent the early hydrocarbon charging of Dongying Formation and the end of Guantao Formation to the present.

1 Introduction

Carbonate cements are important type of cement in sandstone diagenesis demonstrating multistage precipitation, complex and diverse genesis, and wide distribution and can be generated in different geochemical environments [1,2,3]. Carbonate cements have become vital for understanding fluid–rock reaction systems. Material sources and precipitation mechanisms of such systems have been investigated based on microscopic mineral characteristics, fluid inclusion temperature measurement, isotopes, and other methods [4,5,6,7,8]. On the one hand, due to differences in material sources and precipitation mechanisms, different oxygen isotope values during the precipitation of diverse carbonate cements indicate the presence of different fluid–rock interaction systems and strengths [9]. During the primitive deposition period, mainly precipitated entities include quasi-syngenetic micrites, dolomite formed by oolites or cuttings, and early continuous calcite that filled the primary pores of sandstone particles. The formation temperature of such carbonate cements is generally low [5,10]; carbonate cements in the middle and late stages have relatively high inclusion temperatures, indicating that they precipitate at elevated temperatures during diagenesis [5,9]. However, material sources involved in redeposition-diagenesis mainly determine carbonate cement formation, such as Ca2+, Mg2+, Fe2+, CO 3 2 , and other materials [11,12]. On the other hand, hydrocarbon inclusions in the reservoir and associated brine inclusions reflect the history of oil and gas fluid formation [13]. In particular, fluid inclusions trapped in carbonate cements have the same temperature and salinity as those of oil and gas fluid filling and can simultaneously record the properties of filled oil and gas and diagenetic fluids. Therefore, combining these inclusions and carbonate cements can help to reveal hydrocarbon charging in sedimentary basins [14,15], which has important theoretical and practical significance for oil and gas exploration.

In this work, we investigated the vertical distribution, stages, material composition, and carbonate cement sources in different layers of Dongying Sag in Bohai Bay Basin. The material sources and genesis of carbonate cements as well as their relationship with fluid inclusions are discussed based on carbonate cement petrology, mineralogy, carbon and oxygen isotopes, and homogenization temperature of inclusions using electron probe trace element analysis. The combined method is used to understand the relationship between the formation of carbonate cements in sandstone and fluid activities under deep burial conditions.

2 Geological settings

Dongying Sag is located in the southeast of Bohai Bay Basin, bordered by Chenjiazhuang uplift to the north, Binxian and Qingcheng uplifts to the west, Luxi and Guangrao uplifts to the south, and Qingtuozi uplift to the east (Figure 1a and b) with an area of 5,700 km2 and is a half graben-like fault basin, including Lijin Sub-sag, Dongying Sub-sag, Boxing Sub-sag, Minfeng Sub-sag, and the central fault belt (Figure 1c and d).

Figure 1 
               (a) Structural map of Bohai Bay Basin. (b) Tectonic setting of the Jiyang Depression. (c) Tectonic setting of Dongying Sag and the locations of sections AA′. (d) Cross section of sections AA′ showing the tectonostructural zones within Dongying Sag.

Figure 1

(a) Structural map of Bohai Bay Basin. (b) Tectonic setting of the Jiyang Depression. (c) Tectonic setting of Dongying Sag and the locations of sections AA′. (d) Cross section of sections AA′ showing the tectonostructural zones within Dongying Sag.

The Cenozoic is the main stage of the formation and evolution of Dongying Sag with intense structural activities and a set of thick continental clastic deposits [16,17,18,19], including Kongdian (Ek), Shahejie (Es), Dongying (Ed), Guantao (Ng), Minghuazhen (Nm), and Plain (Qp) Formations. Shahejie Formation can be further subdivided into four members from the top to the bottom: Es1, Es2, Es3, and Es4, respectively. In this work, we focused on Es4 and Es3.

The thickness of Es4 can reach 1,500–1,600 m, generally showing a complete coarse-fine-coarse cycle. The lower member of Es4 is composed of purple-red mudstone with brown siltstone, sandy mudstone, and thin carbonate rocks. The upper member of Es4 is composed of dark gray mudstone and shale. The thickness of Es3 is generally 700–1,000 m with the thickest part exceeding 1,200 m. It is mainly composed of gray and dark gray mudstone with sandstone, shale, and carbonaceous mudstone. Es2 is mainly composed of gray mudstone, sandstone, and gravel-bearing sandstone, while Es1 gray and dark gray mudstone, oil shale, and carbonate rock (Figure 2).

Figure 2 
               Generalized Cenozoic-Quaternary stratigraphy of Dongying Sag.

Figure 2

Generalized Cenozoic-Quaternary stratigraphy of Dongying Sag.

3 Methods

The samples were mainly collected from Shahejie Formation in Dongying Sag. Thin-section casting, quantitative statistics, cathodoluminescence, scanning electron microscopy, homogenization temperature testing, carbon and oxygen stable isotope testing, and electron probe analysis were used. All analyses and testing work were completed in the Petroleum Geology Testing Center of the Exploration and Development Research Institute of Sinopec Shengli Oilfield Co Ltd.

3.1 Type and occurrence analysis of carbonate cements

Two hundred and fifteen core samples from 60 wells in Dongying Sag were selected. Blue epoxy resin was used to polish cast sections stained with alizarin red, and the cast sections were identified by Leize polarized light microscope. At the same time, 63 typical samples were selected, and the thermal cathodoluminescence analysis and dyeing analysis of the cast thin sections were performed by cathodoluminescence (CL8200 MK5) to identify the types of carbonate cements in the core samples. The occurrence of carbonate cements and the contact accountability relationship were completed by a field emission environmental scanning electron microscope (Quanta450FEG/-J).

3.2 Electron probe analysis of carbonate cements

Sixty samples from 20 wells were selected. Electron probe microanalysis (JEOLJXA-8230) was performed to quantitatively test the chemical compositions of carbonate cements and conduct an elemental surface analysis. The main elements tested include four elements: Ca, Mg, Fe, and Mn. The analysis error was less than 3%.

3.3 Homogenization temperature testing of inclusions

Hundred core samples from 30 wells were selected to prepare double-polished slices with a thickness of 100 m to test the homogenization temperature of fluid inclusions and clarify the formation temperature and time of carbonate cements. The petrographic characteristics of fluid inclusions were observed using a polarized reflective fluorescent microscope (ZEISS DMR XP). The homogenization temperature measurement was performed on the cold and hot table (LinKam-THMS600). The refrigerant used in the test was liquid nitrogen, and standard sample calibration was performed at −196 to 600°C with an error of 0.1°C. The temperature control rate was 1–10°C/min, the room temperature was 25°C, and the humidity was 65%.

3.4 Carbon and oxygen stable isotope testing of carbonate cements

The corresponding carbonate cements from sandstone samples were dried in the air, grounded to 200 mesh, and reacted with 100% phosphoric acid, and the released CO2 was collected. A gas isotope mass spectrometer (MAT253) was used to test the stable isotopes of carbon and oxygen.

4 Results

4.1 Types and occurrence of carbonate cements

The morphological and geochemical characteristics of carbonate cements in the sandstone of Shahejie Formation in Dongying Sag were studied by thin-section casting, cathodoluminescence, backscattering, electron probe analysis, and isotope analysis. Optical microscopy and cathodoluminescence results indicated carbonate cementation in the sandstone of the deep Shahejie Formation in Dongying Sag, and mainly, early-stage calcite, middle-stage calcite and dolomite, and late-stage ferrocalcite and ankerite were found.

The carbonate cements in the early stage are mainly calcite, which can be divided into two types based on their production: the first type is mainly micritic high magnesium calcite, which usually has a good degree of automorphism, and is mostly in the form of micritic equal thickness rim, which is semi-basal-basal cementation and occurs on the surface of a small amount of clastic rock particles (Figure 3a). The second type is mainly microcrystalline calcite, mostly developed between clastic grains (Figure 3b), and the cathodoluminescence is orange (Figure 3c). Microscopic observation showed that the clastic particles were mainly distributed in the carbonate cements of this period in the form of point contact or floating, and no feldspar particles were dissolved or metasomatism clastic particles appeared. Meanwhile, the intergranular pores were relatively developed, indicating that the clastic particles had not been modified by compaction, so the formation time was earlier. The carbonate cements in the middle stage are mainly microcrystalline fine-grained calcite and dolomite. Different from carbonate cements in the early stage, carbonate cements in the middle stage are mostly dispersed and filled with pores, occupying the dissolution space of feldspar, and the secondary increased edge phenomenon of metasomatized feldspar grains and quartz can be seen (Figure 3d–f); the cathode luminescence is orange-yellow and dark red (Figure 3i). The clastic grains are mostly in point-line contact to lineal contact, which indicates that the sandstone has been strongly compacted and the cements precipitated after the dissolution of feldspar, and the precipitation time is later than that of carbonate cements in the early stage, indicating that this period is later than the formation period of carbonate cements in the early stage. The carbonate cements in the late stage are mainly fine-grained ferro-calcite and ankerite, which are porous cementation. The clastic particles are closely arranged and mostly in line contact, indicating that the sandstone has been strongly compacted and transformed; it mainly occupies the dissolution space of feldspar, carbonate cement in the early and middle stage, and some metasomatize feldspar particles and quartz enlarge edge (Figure 3e, g, and h); under cathodoluminescence, ferrocalcite is dark orange-yellow, while ankerite does not emit light (Figure 3i), indicating that it was formed later.

Figure 3 
                  Diagenesis micrograph of carbonate cements in Dongying Sag. (a) N105, 3251.66 m, micritic high-Mg calcite in early stage; (b) X154, 2934.50 m, calcite in early stage filled primary pores; (c) X154, 2934.50 m, calcite in early stage, the cathodoluminescence was bright orange-yellow; (d) G110, 2671.93 m, ferro-calcite in late stage filled the dissolution pores of calcite and dolomite in middle stage; (e) W541, 3132.98 m, dolomite in middle stage filled dissolution pores of debris, and the later dissolution was filled by ferro-calcite in late stage; (f) W58, 3023.82 m, calcite in middle stage filled feldspar dissolution pores; (g) N21, 3431.50 m, dolomite in middle stage filled the dissolution pores of feldspar particles and metasomatized part of quartz secondary enlarged edge, later was metasomatized by ankerite in late stage; (h) N24, 3174.85 m, ankerite in late stage filled the dissolution pores of calcite in middle stage and metasomatizes quartz secondary enlarged edge; and (i) W631, 3243.80 m, the cathodoluminescence of ferro-calcite is dark orange red, while ankerite does not emit light.

Figure 3

Diagenesis micrograph of carbonate cements in Dongying Sag. (a) N105, 3251.66 m, micritic high-Mg calcite in early stage; (b) X154, 2934.50 m, calcite in early stage filled primary pores; (c) X154, 2934.50 m, calcite in early stage, the cathodoluminescence was bright orange-yellow; (d) G110, 2671.93 m, ferro-calcite in late stage filled the dissolution pores of calcite and dolomite in middle stage; (e) W541, 3132.98 m, dolomite in middle stage filled dissolution pores of debris, and the later dissolution was filled by ferro-calcite in late stage; (f) W58, 3023.82 m, calcite in middle stage filled feldspar dissolution pores; (g) N21, 3431.50 m, dolomite in middle stage filled the dissolution pores of feldspar particles and metasomatized part of quartz secondary enlarged edge, later was metasomatized by ankerite in late stage; (h) N24, 3174.85 m, ankerite in late stage filled the dissolution pores of calcite in middle stage and metasomatizes quartz secondary enlarged edge; and (i) W631, 3243.80 m, the cathodoluminescence of ferro-calcite is dark orange red, while ankerite does not emit light.

4.2 Chemical composition of carbonate cements

Electron probe microanalysis and energy spectrum analysis were used to observe and identify different types of carbonate cements in the deep sandstone reservoirs of Dongying Sag and analyze their major element components. The results showed that calcite in the early stage is distributed in ring-shaped aggregates with a chlorite layer on the surface and demonstrated symbiosis with a small amount of authigenic early-stage pyrite particles (Figure 4a). The composition of carbonate cements in this stage is characterized by high-Ca, medium-Mg, and low-Fe (Figures 4b and 5): CaCO3 content was 65.35–75.15% with an average of 71.95%, MgCO3 content was 21.04–33.87% with an average of 27.11%, FeCO3 content was 0.46–1.47% with an average of 0.93%, and MnCO3 content was 0.00% (Table 1), indicating that this type of carbonate cement is micritic high-Mg calcite.

Figure 4 
                  Electron microprobe backscatter images and energy spectrum characteristics of typical carbonate cements. (a–c) W7, 2595.5 m, characteristics of back scattering and energy spectrum of micritic high-Mg calcite and calcite in early stage; (d–f) W550, 3419.48 m, characteristics of back scattering and energy spectrum of calcite in middle stage and ferrocalcite in late stage; (g–i) W58, 3026.10 m, characteristics of back scattering and energy spectrum of dolomite in middle stage and ankerite in late stage.

Figure 4

Electron microprobe backscatter images and energy spectrum characteristics of typical carbonate cements. (a–c) W7, 2595.5 m, characteristics of back scattering and energy spectrum of micritic high-Mg calcite and calcite in early stage; (d–f) W550, 3419.48 m, characteristics of back scattering and energy spectrum of calcite in middle stage and ferrocalcite in late stage; (g–i) W58, 3026.10 m, characteristics of back scattering and energy spectrum of dolomite in middle stage and ankerite in late stage.

Figure 5 
                  Composition characteristics of carbonate cements in different stages.

Figure 5

Composition characteristics of carbonate cements in different stages.

Table 1

Composition characteristics of carbonate cements in different stages

Well Depth (m) Horizon CaCO3 FeCO3 MgCO3 MnCO3 Mineral name period
W78 3426.18 Es3 97.21 1.79 0.00 0.00 Calcite middle stage
3429.84 Es3 98.50 0.17 1.12 0.21 Calcite middle stage
N876 3372.19 Es3 90.55 8.40 1.06 0.00 Ferro-calcite late stage
3375.20 Es3 84.59 15.41 0.00 0.00 Ferro-calcite late stage
N106 2595.21 Es3 98.21 1.79 0.00 0.00 Calcite middle stage
X154 2960.50 Es3 90.15 0.98 8.87 0.10 Calcite early stage
N105 3251.66 Es3 65.35 0.78 33.87 0.00 Calcite early stage
S127 2180.80 Es3 86.75 0.48 12.77 0.12 Calcite early stage
H144 2691.25 Es3 77.49 1.47 21.04 0.00 Calcite early stage
2691.25 Es3 69.27 0.46 30.27 0.00 Calcite early stage
S11 3041.20 Es3 86.18 12.03 1.79 0.00 Ferro-calcite late stage
3080.50 Es3 99.44 0.56 0.00 0.00 Calcite middle stage
H169 2840.90 Es3 51.80 0.23 47.97 0.00 Dolomite middle stage
2945.00 Es3 31.24 0.46 68.30 0.00 Dolomite middle stage
W631 2804.00 Es3 59.42 15.07 24.18 1.34 Ankerite late stage
N24 3042.50 Es4 98.24 0.56 1.20 0.22 Calcite middle stage
3082.11 Es4 87.49 11.47 1.04 0.00 Ferro-calcite late stage
N21 2987.20 Es3 58.47 13.35 25.87 2.31 Ankerite late stage
3032.60 Es3 86.23 12.13 1.63 0.00 Ferro-calcite late stage
W7 2616.10 Es4 60.25 13.54 26.21 0.00 Ankerite late stage
2616.10 Es4 50.68 15.73 32.43 1.16 Ankerite late stage
S126 2616.10 Es4 51.81 11.27 36.92 0.00 Ankerite late stage
N25 3274.10 Es3 60.90 15.46 23.13 0.51 Ankerite late stage
3274.10 Es3 51.70 1.03 47.27 0.00 Dolomite middle stage
3283.80 Es3 88.52 0.81 10.67 0.00 Calcite early stage
T174 2903.80 Es4 57.26 13.84 28.17 0.73 Ankerite late stage
W587 3456.40 Es4 55.34 12.04 30.69 1.93 Ankerite late stage
B656 3187.40 Es4 95.71 2.17 2.12 0.00 Calcite middle stage
Ch371 2693.85 Es4 96.11 0.79 3.10 0.00 Calcite early stage
FS6 3697.95 Es4 75.70 1.03 23.27 0.00 Calcite early stage

The second type of carbonate cements in the early stage fills the primary pores without obvious compaction and diagenetic transformation (Figure 4d). The composition of this carbonate cement is characterized by high Ca, low Mg, and low Fe (Figures 4c and 5). The content of CaCO3 was 86.75–96.11% with an average of 90.38%; the content of MgCO3 was 3.10–12.77% with an average of 8.85%; the content of FeCO3 was 0.48–0.98% with an average of 0.77%; the content of MnCO3 was 0.1–0.12% with an average of 0.05% (Table 1), indicating that this type of carbonate cement is calcite.

The carbonate cements in the middle stage are mostly filled with feldspar dissolved and compacted in residual pores (Figure 4d and g); carbonate cements in this stage can be divided into two types. The first type is characterized by high Ca, medium-low Mg, and low Fe (Figures 4e and h and 5). The content of CaCO3 was 95.71–99.44% with an average of 97.89%, MgCO3 content was 0.00–2.12% with an average of 0.74%, FeCO3 content was 0.17–2.17% with an average of 1.17%, and MnCO3 content was 0–0.22% with an average of 0.07%, indicating that this type of carbonate cement is calcite. The second type is characterized by medium-low Ca, high Mg, and low Fe in which CaCO3 content was 31.24–51.80% with an average of 44.91%, MgCO3 content was 46.04–68.30% with an average of 54.51%, FeCO3 content was 0.23–1.03% with an average of 0.57%, and MnCO3 content was 0.00% (Table 1), indicating that the type of carbonate cement is dolomite.

The carbonate cements in the late stage are mainly filled with the dissolution pores of early- and middle-stage calcite and dolomite (Figure 4d and g), and the clay-mixed layer minerals of illite and montmorillonite and high automorphic silk illite are developed on their surface. Two types of carbonate cements were identified in this stage. The first type is characterized by high Ca, medium-low Mg, and high Fe (Figure 4f). The content of CaCO3 was 84.59–90.55% with an average of 87.01%; the content of MgCO3 was 0.00–1.79% with an average of 1.10%; the content of FeCO3 was 8.40–15.41% with an average of 11.89%; the content of MnCO3 was 0.00%, indicating that it is iron calcite. The second type is characterized by medium-low Ca, high Mg, and high Fe (Figures 4i and 5) in which CaCO3 content was 50.68–60.25% with an average of 56.77%; MgCO3 content was 23.13–36.92% with an average of 28.45%; FeCO3 content was 11.27–15.73% with an average of 13.78%; MnCO3 content was 0–1% with an average of 0.57% (Table 1), indicating that it is ankerite.

4.3 Distribution characteristics of carbonate cements

In the deep sandstone of Dongying Sag, the content of early-stage micritic high-Mg calcite is low, which is only developed in a small number of samples, but its depth varies, ranging from 1,600 to 4,000 m. The early calcite is also developed in the range of 1,600–3,600 m, but mainly distributed in the depth of 1,600–2,700 m in sandstone samples buried more than 2,700 m deep. This type of carbonate is dissolved; thus, its content decreased. Calcite and dolomite in the middle stage are developed in the range of 2,000–4,500 m, and their contents decrease with the increase in burial depth. Microscopic observations showed that carbonate cement dissolution occurs in the late stage mainly in the depth range of 2,100–2,900 m, and the content of these carbonates in sandstone decreases beyond 2,900 m. In the late stage, the ferrocalcite and ankerite are mainly developed in sandstone with a depth of <3,000 m, and the development degree increases with the increase in depth (Figure 6).

Figure 6 
                  Vertical distribution characteristics of carbonate cements in different stages.

Figure 6

Vertical distribution characteristics of carbonate cements in different stages.

Carbon and oxygen isotope analysis is a widely used method to study the material source and genetic mechanism of carbonate cements in clastic rocks [20,21,22,23]. The carbon and oxygen isotopes of different types of carbonate cements in deep sandstone reservoirs in Dongying Sag showed the following characteristics: because of the low content of early-stage micritic high-Mg calcite cements, it is difficult to obtain sufficient samples for isotopic testing. The δ 13CV-PDB‰ of the medium-coarse-grained calcite cement filling the primary pores in the early stage was 1.3–2.1‰ with an average of 1.7‰, and δ 18OV-PDB‰ was −10.5 to −7.5‰ with an average of −9.1‰ (Table 2), showing positive carbon isotope characteristics. The δ 13CV-PDB‰ of the calcite cement filling the dissolved pores of feldspar and the remaining primary pores of compaction in the middle stage was −2.2 to 3.4‰ with an average of 1.9‰, and the δ 18OV-PDB‰ was −13.5 to –9.3‰ with an average of −12.2‰; the δ 13CV-PDB‰ of the dolomite cement was −1.0 to 4.3‰ with an average of 2.8‰, and the δ 18OV-PDB‰ was −11.2 to −7.1‰ with an average value of −9.7‰ (Table 2). The carbon isotope of dolomite shows a stronger positive drift than that of calcite in the same stage. The δ 13CV-PDB‰ of ferrocalcite in the late stage was −3.3 to −2.7‰ with an average of −0.6‰, the δ 18OV-PDB‰ was −15.9 to −13.3‰ with an average of −14.8‰, the δ 13CV-PDB‰ of ankerite was −1.2 to 4.3‰ with an average of 1.6‰, and the δ 18OV-PDB‰ was −13.8 to –11.5‰ with an average of −12.7‰ (Table 2). The carbon isotope of ankerite is higher than that of ferrocalcite in the same stage.

Table 2

Carbon and oxygen isotope composition and oxygen isotope temperature test results of carbonate cements(a)

Well Depth (m) Strata Type Stage δ 13CPDB (‰) δ 18OPDB (‰) δ 18OSMOW (‰) Temperature T (°C)
H155 2981.2 Es3 Calcite Middle stage 3.4 −11.8 15.5 88.3
H163 2833.4 Es3 Calcite Middle stage 2.1 −12.1 15.1 91.5
N22 3208.6 Es3 Ankerite Late stage −1.2 −13.0 13.9 134.9
S127 3214.5 Es3 Calcite Middle stage 2.8 −9.3 18.7 64.2
X154 2934.5 Es3 Calcite Early stage 1.5 −7.5 21.1 38.7
N28 3120.8 Es3 Ankerite Late stage 2.3 −13.8 12.9 147.5
H130 2780.4 Es3 Fe-Calcite Late stage 2.7 −13.3 13.5 106.3
2781.6 Es3 Dolomite Middle stage 3.7 −10.8 16.8 103.9
H156 2752.8 Es3 Dolomite Middle stage 4.3 −11.2 16.2 109.7
L881 2969.8 Es3 Ankerite Late stage 1.2 −12.8 14.1 132.5
W7 2597.5 Es4 Fe-Calcite Late stage −2.1 −15.3 10.9 134.4
2597.5 Es4 Calcite Late stage 2.1 −9.5 18.5 53.6
2597.5 Es4 Calcite Early stage 1.3 −10.5 17.2 62.0
L933 2908.7 Es3 Ankerite Late stage 4.3 −13.0 13.9 134.9
H159 2954.3 Es3 Dolomite Middle stage −1.0 −10.1 17.7 95.6
N301 2780.0 Es3 Calcite Middle stage 3.2 −13.1 13.7 104.4
2786.5 Es3 Calcite Middle stage −2.2 −12.5 14.5 97.0
HX108 3478.2 Es3 Dolomite Middle stage 3.9 −10.5 17.1 101.1
S106 3398.7 Es3 Fe-Calcite Late stage −3.3 −15.9 10.1 144.4
S121 3530.0 Es3 Calcite Middle stage 1.3 −13.5 13.2 109.3
S127 3162.8 Es3 Dolomite Middle stage 2.9 −7.1 21.6 65.3
Y67 3067.2 Es3 Calcite Middle stage 2.5 −12.8 14.1 100.6
3072.0 Es3 Ankerite Late stage 1.6 −12.3 14.8 124.4
HX108 3477.0 Es3 Dolomite Middle stage 2.8 −8.2 20.1 76.1
FS1 4322.4 Es4 Ankerite Late stage −0.4 −11.5 15.8 113.8
S106 3398.7 Es3 Ankerite Late stage 3.3 −12.7 14.3 130.1
3410.1 Es3 Fe-Calcite Late stage −3.1 −14.1 12.5 116.4
N21 2987.2 Es3 Fe-Calcite Late stage 2.2 −14.7 11.6 126.2
3032.6 Es3 Fe-Calcite Late stage −0.3 −15.4 10.7 136.9

(a) δ 13CPDB‰ is a stable carbon isotope (PDB standard); δ 18OPDB‰ is a stable oxygen isotope (PDB standard); δ 18OSMOW‰ is a stable oxygen isotope (SMOW standard). PDB is the abbreviation of Pee Dee belemnite in North America. SMOW is the standard mean seawater in Vienna. The conversion formula is δ 18OPDB = 1.03086 δ 18OSMOW – 30.86 [24]. Ferrocalcite is calculated using the formula 103ln α calcite-water = 2.78 * 106 * T −2 – 2.89 [25]; ankerite is calculated according to the formula 103 ln α calcite-water = 2.78 * 106 * T −2 + 0.11 [26].

4.4 Precipitation temperature of carbonate cement

Fluid inclusions can provide information on the precipitation temperature of authigenic minerals [10,27,28,29]. Due to the limited number of samples and inclusions in carbonate cements, the homogenization temperatures of fluid inclusions were detected from only a small number of middle- and late-stage carbonate cements (Figure 7a and c). The homogenization temperature of fluid inclusions in middle-stage carbonate cements mainly ranges from 95 to 120°C, while that in late-stage carbonate cements ranges from 135 to 150°C (Figure 8, Table 3). The homogenization temperature ranges of fluid inclusions in quartz grain healing fractures are mainly from 95 to 120°C and 135 to 155°C. The homogenization temperature ranges of aqueous inclusions in the enlarged edge of quartz are mainly from 100 to 115°C and 140 to 155°C (Figure 8, Table 3). According to the homogenization temperature distribution, two stages of hydrocarbon charging were identified in the deep sandstone reservoir of Dongying Sag: the precipitation temperatures of carbonate cements in the middle and late stages correspond to the two stages of hydrocarbon charging, respectively.

Figure 7 
                  Inclusion characteristics of quartz and carbonate cements; (a and b) H169, 3026.50 m, hydrocarbon inclusions in carbonate cements in middle stage show yellow-white fluorescence; (c and d) N21, 2987.20 m, hydrocarbon inclusions in carbonate cements in late stage show blue-white fluorescence; (e) S136, 3214.50 m, aqueous inclusions in quartz grain healing fractures; and (f) L933, 2909.10 m, aqueous inclusions in the enlarged edge of quartz.

Figure 7

Inclusion characteristics of quartz and carbonate cements; (a and b) H169, 3026.50 m, hydrocarbon inclusions in carbonate cements in middle stage show yellow-white fluorescence; (c and d) N21, 2987.20 m, hydrocarbon inclusions in carbonate cements in late stage show blue-white fluorescence; (e) S136, 3214.50 m, aqueous inclusions in quartz grain healing fractures; and (f) L933, 2909.10 m, aqueous inclusions in the enlarged edge of quartz.

Figure 8 
                  Characteristics of homogenization temperature distribution of inclusions in carbonate cements.

Figure 8

Characteristics of homogenization temperature distribution of inclusions in carbonate cements.

Table 3

Homogenization temperature data of aqueous inclusions in quartz and carbonate cements

Well Depth (m) Strata Aqueous inclusions in microfractures in quartz Aqueous inclusions in quartz overgrowth Aqueous inclusions in carbonate cements
Th°C (Number) Th°C (Number) Th°C (Number)
N21 2987.20 Es3 85–110(10), 129–146(6) 89–105(3), 140–145(2) 95–105(2), 140–150(2)
W541 3014.70 Es3 95–120(11), 135–159(5) 115(1), 145(1)
L98 3141.35 Es3 105–125(11), 145–160(7)
W7 2525.81 Es4 90–120(8), 140–155(10) 100–110(5), 145–150(4) 100–105(3), 140–155(3)
L933 2909.10 Es4 100–120(5) 110–115(2) 110(2)
H169 2845.30 Es3 98–118(4), 140–150(3) 115–120(2), 155–160(3) 95–105(3), 135–155(5)
W78 3395.93 Es3 91–115(14), 145–155(8) 99–119(5), 130–150(5)
S136 3214.50 Es3 105–120(9), 131–149(8) 115(1), 141–150(3) 110–120(3), 128–155(4)

Th: homogenization temperature; —: no available data.

5 Discussion

5.1 Formation period and diagenetic sequence of carbonate cements

The δ 18OSMOW‰ of carbonate cements is controlled by the δ 18OSMOW‰ of pore fluid and formation temperature. Determining the δ 18OSMOW‰ of pore fluid is necessary to calculate the formation temperature of carbonate cements in sandstone [25,30]. During diagenesis, the pore fluid from the sedimentary water exchanges material with clastic particles, making the δ 18OSMOW‰ of the pore fluid significantly heavier [31,32]. The δ 18OSMOW‰ of the original sedimentary water body in Dongying Sag was −4.8‰, and the δ 18OSMOW‰ of the pore fluid became −3‰ after feldspar dissolution [32,33,34].

The carbonate cements in the early stage developed in the deep sandstone reservoirs in Dongying Sag are high-Mg calcite developed in mudstone and calcite filled with early intergranular primary pores. High-Mg calcite is inferred to be a paracontemporaneous diagenetic product (Figure 5) according to its composition characteristics, and it was formed before the dissolution period of feldspar in the pore fluid with a δ 18OSMOW‰ of −4.8‰ (Figure 3a and b). The calcite in the early stage was also formed before feldspar dissolution in the pore fluid with a δ 18OSMOW‰ of −4.8‰ (Figure 3c and d). The δ 18OPDB‰ was −10.5 to −7.5‰. The calculated isotopic temperature (T°C) of carbonate cements in this stage was 38.7–62.0°C (Table 2). The carbonate cements in the middle stage are calcite and dolomite filled with feldspar dissolution pores and compacted residual pores. Because they were formed after feldspar dissolution, it was formed in the pore fluid with a δ 18OSMOW‰ of −3‰ (Figure 3e, g and h), and the δ 18OPDB‰ is −13.5 to −7.1‰. The calculated isotopic temperature (T°C) of carbonate cements was 64.2–109.7°C (Table 2), and the peak homogenization temperature of inclusions was 70–110°C (Figure 8). Combined with the research results of the burial history of Shahejie Formation in Dongying Sag [47], the carbonate cements in this stage are diagenetic products of the early Dongying Formation, and the temperature calculated by isotope is consistent with the measured homogenization temperature of inclusions. The carbonate cements in the late stage are ferrocalcite and ankerite, which are filled with feldspar dissolved in pores and metasomatized the secondary enlarged edges of quartz (Figure 3e, g, and h) formed in a pore fluid with a δ 18OSMOW‰ of −3‰ and δ 18OV-PDB‰ of −15.9 to −11.5‰. The calculated isotope temperature (T°C) of carbonate cements in this stage is 106.3–147.5°C (Table 2), and the peak homogenization temperature range of inclusions is 120–150°C (Figure 8), which coincides with each other, and they are mainly the diagenetic products of the late Guantao Formation to the present.

Therefore, our comprehensive analyses showed that the diagenetic sequence of carbonate cements and other cements in the deep sandstone reservoir of Dongying Sag is as follows: micritic high-Mg calcite in the early stage → calcite in the early stage filled with primary pores without obvious compaction and diagenetic transformation → feldspar dissolution/quartz secondary enlargement/authigenic kaolinite precipitation → calcite and dolomite in the middle stage filled with feldspar dissolved and compacted in residual pores → ferrocalcite and ankerite in the late stage filled with the dissolution pores of calcite and dolomite in the early stage and metasomatized the enlarged edge of quartz (Figure 9).

Figure 9 
                  Sequence diagram of sandstone diagenesis in the deep Shahejie Formation of Dongying Sag (Paleogeothermal data were modified from Qiu et al., 2004; burial and thermal history were modified from Song et al., 2009).

Figure 9

Sequence diagram of sandstone diagenesis in the deep Shahejie Formation of Dongying Sag (Paleogeothermal data were modified from Qiu et al., 2004; burial and thermal history were modified from Song et al., 2009).

5.2 Discussion on material source and genesis of carbonate cements

During the formation of carbonate cements, carbons in all types of organic sources are exchanged. Different types of carbon-rich acidic materials (organic acids, CO2) are formed in different evolutionary stages of source rocks. Therefore, the carbon isotope characteristics of carbonate cements can reflect the evolutionary stages of source rocks. Previous studies have shown that 60–90°C is the stage of organic acid mass formation, 90–120°C is the stage of organic acid preservation, and >120°C is the stage of organic acid destruction [35,36]. Different types of acid fluids control the geochemical properties of pore fluids and affect the formation and evolution of carbonate cements in different stages.

5.2.1 Carbonate cements in the early stage

The rim-shaped argillaceous high-Mg calcite directly precipitates from the sedimentary water in the early stage and is a product of the quasi-contemporaneous period. The isotopic temperature characteristics of calcite (Figure 11a) filled with primary pores after compaction in the early stage indicate that it was formed at 38.7–62.0°C (Table 2), and the main burial depth was about 1,600–2,700 m (Figure 6). Its δ 13CPDB‰ was 1.3–2.1‰ with an average of 1.7‰, and δ 18OV-PDB‰ was −10.5 to −7.5‰ with an average of −9.1‰ (Table 2), indicating that it is a typical lacustrine carbonate (Figure 10), that is, there is no external material supply, such as microbial activity, biological organisms, or mantle-derived materials. The shale experienced obvious mechanical compaction: the main material source of carbonate cements in this stage is Ca2+- and CO3 2–-rich pore fluid discharged from the sedimentary source during mudstone compaction.

Figure 10 
                     Isotopic characteristics of carbonate cements.

Figure 10

Isotopic characteristics of carbonate cements.

5.2.2 Carbonate cements in the middle stage

The carbonate cements in the middle stage are mainly calcite and dolomite filled with feldspar dissolution pores and compacted residual pores (Figure 3e, g and h), indicating that they were formed after large-scale feldspar dissolution. Isotopic and inclusion homogenization temperature characteristics indicate that middle-stage carbonate cements were formed at 70–110°C (Figure 8). The distribution range of the δ 13CPDB‰ was −2.2 to 4.3‰, and that of δ 18OV-PDB‰ was −13.5 to −7.1‰ (Table 2). The δ 13CPDB‰ of some carbonate cements in this stage is significantly higher than that of early-stage carbonate cements. The carbon isotope of dolomite is higher than that of the calcite cement in the same stage (Figure 10, Table 2).

Previous studies have shown that bacterial activity, internal carbon isotope fractionation of organic acids, and evaporation of lake water can lead to positive carbon isotope migration of carbonate cements. Since the precipitation temperature of carbonate cement measured by fluid inclusion in the study area exceeds the maximum temperature limit of microbial activity (70°C), the influence of bacterial activity is excluded. Carbon isotope fractionation within organic acids is believed to be the reason for the high δ 13CPDB‰ value [37]. The carbon and oxygen isotope measurements of lacustrine carbonate in Shahejie Formation in Dongying Sag showed that the δ 13CPDB‰ values of carbonate rocks in Es3 are mostly negative, while the δ 13CPDB‰ values of saltwater lacustrine carbonate rocks in Es4 are 1.20–6.29‰ with an average value of 3.75‰ [33,34]. We conclude that the positive carbon isotope migration of calcite in carbonate cements is due to the isotopic fractionation effect of organic acids; the positive migration of dolomite carbon isotope is mainly affected by organic acid isotope fractionation and the upwelling of high-salinity fluid in Es4. The formation process of carbonate cements in this stage is as follows: a large amount of organic matter in mudstone is formed, and organic acid is discharged. The existence of organic acid causes the dissolution of potash feldspar and plagioclase, providing a partial material source for the formation of carbonate cement in this period. Meanwhile, the overpressure fluid rich in Ca2+, Mg+, and part of CO 3 2 influenced by the organic carbon source enter the reservoir Ca2+ in the formation water is mixed (Figure 11b and c), leading to the precipitation of these materials in the form of medium-term carbonate cement. Due to the mixing of pore fluids from different sources, the compositions of carbonate cements are complex, including both calcite and dolomite.

Figure 11 
                     Electron microprobe element surface characteristics of different stages of carbonate cements. (a) W7, 2595.5 m, the micritic high-Mg calcite and calcite in early stage; (b) W550, 3419.48 m, calcite in the middle stage and ferrocalcite in the late stage filled the dissolved pores; and (c) W58, 3026.10 m, dolomite in middle stage and ankerite in the late stage filled the dissolved pores.

Figure 11

Electron microprobe element surface characteristics of different stages of carbonate cements. (a) W7, 2595.5 m, the micritic high-Mg calcite and calcite in early stage; (b) W550, 3419.48 m, calcite in the middle stage and ferrocalcite in the late stage filled the dissolved pores; and (c) W58, 3026.10 m, dolomite in middle stage and ankerite in the late stage filled the dissolved pores.

5.2.3 Carbonate cements in the late stage

Carbonate cements in the late stage are mainly ferrocalcite and ankerite (Figure 3e, g and h): the distribution range of the δ 13CPDB‰ was −3.3 to 4.3‰, and that of the δ 18OV-PDB‰ was −15.9 to −11.5‰ (Table 2): the precipitation temperature range was 131.6–144.1°C (Table 3), mainly developed in sandstone with a burial depth <3,000 m (Figure 6). In the geothermal range (>120°C) of this depth, organic acids and some organic matter cracked to form a large amount of CO2, which was transformed into CO 3 2 during fluid–rock interactions [35,36]. The carbonate cements in the early and middle stages had obvious dissolution (Figure 3e, g, and h) before the formation of carbonate cements in this stage, which can also provide Mg2+, Ca2+, and CO 3 2 for carbonate cements in this stage. In this diagenetic stage, the intrusion of acidic fluid rich in organic CO2 originating from source rocks resulted in the dissolution of metamorphic rock cuttings, which could provide a large amount of Fe2+. At this stage, a large amount of Ca2+ combined with Fe2+ in pore water results in the formation of ferrocalcite (Figure 11b and c). Therefore, the ferrocalcite and ankerite in the late stage are the products of CO 3 2 formed by organic acid splitting decomposition in late diagenesis, and Ca2+, Fe2+, and Mg2+ formed by the dissolution of metamorphic rock debris in pore fluid. Similar to the carbonate cement in the middle stage, the carbon isotope of ankerite is significantly higher than that of the corresponding ferrocalcite (Figure 10, Table 2), indicating that ferrocalcite is the product of the fluid interaction of Es3, and ankerite is affected by the high-salinity fluid from Es4.

5.3 Material exchange of carbonate cement precipitation in pore fluid

Changes in the chemical composition of the pore fluid affect composition during carbonate cement precipitation. The mudstone adjacent to the carbonate cement precipitation strata has different geochemical properties discharged during the compaction process. Formation fluids and deep hydrothermal fluids flowing up into the pores along faults can affect the properties of pore fluids, leading to the precipitation of different types of carbonate cements. When the lake water of the Paleogene in Dongying Sag was in the quasi-contemporaneous period, the buried depth of the stratum was less than 500 m, the ground temperature was less than 40°C, the water itself involved a certain content of Ca2+, Mg2+, Fe3+, and SO 4 2 [38,39], and sulfate-reducing bacteria that were widely active under geothermal conditions during this period reduced Fe3+ and SO 4 2 in water to Fe2+ and S2− [4,40,41], combined with reduced Fe2+ and S2−, precipitating early authigenic pyrite, which coexists with early high-Mg calcite [38,39,42]. When the continuous buried depth of the formation reaches 500–1,500 m and the ground temperature 40–60°C due to the strong compaction of the formation, Ca2+ and CO 3 2 -rich fluid in the adjacent mudstone formation is squeezed into the adjacent sandstone in the reservoir pores [43,44], leading to early-stage calcite precipitation and the filling of the primary pores of calcite particles. As the stratum is buried 1,500–2,800 m deep and the ground temperature is 60–120°C, the organic matter in the deep mudstone of Dongying Sag will generate a large amount of organic acids into the pores of sandstone, causing the dissolution of potash feldspar and plagioclase, production of K+ and Na+, and Ca2+, Si4+, and Al3+ ions entering the pore fluid. Among them, Si4+ and Al3+ ions migrate with the fluid and precipitate in the form of secondary enlargement of kaolinite and quartz under suitable geological conditions. At the same time, due to the fault activity at this stage, high-salinity hydrothermal fluids rich in Mg2+ and Ca2+ and the organic sources of CO 3 2 enter pore fluids, depositing and producing in the form of carbonate cements in the middle stage and filling the dissolved pores of potash feldspar and plagioclase.

5.4 Hydrocarbon indicating the significance of carbonate cements

The thermal evolution and diagenesis of source rocks in sedimentary basins are interactive and organic processes. Organic acids produced during the thermal evolution of the source rock directly affect the stability of carbonate cements in the overlying reservoir sandstone [36]. Therefore, the charging of hydrocarbon fluids inevitably leads to secondary changes in the reservoir, and carbonate cements and hydrocarbon inclusions present in carbonate cements can well record these changes. The content of calcite in deep sandstone reservoirs in Dongying Sag is similar in sandstone samples with and without hydrocarbon display; the content of dolomite, ferrocalcite, and ankerite in sandstone samples with hydrocarbon display is much higher than that in sandstone samples without hydrocarbon display (Figure 12). In addition, the early micritic high-Mg calcite is a product of the concentration of sedimentary water and the product of the compression fluid of the calcite mudstone filling the dissolution pores in the early stage, both of which are not symbiotic with asphaltenes and have no relation with hydrocarbon fluids. The calcite and dolomite in the middle stage are associated with asphaltenes and precipitate in the pores after feldspar dissolution (Figure 13a and b); ferrocalcite and ankerite in the late stage are also associated with asphaltenes, filling the dissolution pores of carbonate cements in the early and middle stages (Figure 13c and d). Thus, carbonate cements in the two stages are all diagenetic products formed during the filling of hydrocarbon fluids: Dongying Sag has two periods of hydrocarbon charging, namely the early charging period of the Dongying Formation and the late Guantao Formation–the present charging period [45,46]. In this work, hydrocarbon inclusions in the early hydrocarbon charging period were recorded as yellow-brown fluorescence in middle-stage carbonate cements (Figure 7b), and the measured peak homogenization temperature range was 95–120°C (Figure 8). Combined with the research results of the burial history of Shahejie Formation in the study area [47], these results show that the carbonate cements in this stage are the early diagenetic products of Dongying Formation. It coincides with the early hydrocarbon charging period of the early Dongying Formation, proving that the carbonate cements in this stage represent the early hydrocarbon charging of Dongying Formation. During the late oil and gas charging period, the hydrocarbon inclusions were recorded in the form of blue-white fluorescence in the late carbonate cement (Figure 7d), and the measured peak homogenization temperature range was 135–150°C (Figure 8), indicating that the carbonate cements in this stage are a diagenetic product from the end of Guantao Formation to the present, indicating that the carbonate cements in this stage represent the end of Guantao Formation to the present. Hydrocarbon charging and the formation of carbonate cement precipitation in the late stage indicate continuous hydrocarbon accumulation.

Figure 12 
                  Relationship between the content of carbonate cements and the display of hydrocarbon.

Figure 12

Relationship between the content of carbonate cements and the display of hydrocarbon.

Figure 13 
                  Relationship between carbonate cements in different stages and hydrocarbon charging; (a) H130, 2722.31 m, calcite in the middle stage associated with asphaltene; (b) W541, 3035.40 m, dolomite in the middle stage associated with asphaltene; (c) N116, 3091.64 m, ferrocalcite in the late stage associated with asphaltene; and (d) L65, 3219.20 m, ankerite in the late stage associated with asphaltene.

Figure 13

Relationship between carbonate cements in different stages and hydrocarbon charging; (a) H130, 2722.31 m, calcite in the middle stage associated with asphaltene; (b) W541, 3035.40 m, dolomite in the middle stage associated with asphaltene; (c) N116, 3091.64 m, ferrocalcite in the late stage associated with asphaltene; and (d) L65, 3219.20 m, ankerite in the late stage associated with asphaltene.

6 Conclusions

  1. (1)

    The deep sandstone reservoirs in Dongying Sag are mainly composed of three stages of carbonate cements. Carbonate cements in the early stage can be divided into two types: the first type is micritic high-Mg calcite, which is rim-shaped and coexists with a small amount of authigenic pyrite particles; the second type is medium-coarse-grained calcite filled with primary pores. The carbonate cements in the middle stage are calcite and dolomite filled with secondary dissolution pores of feldspar, and the carbonate cements in the late stage are ferrocalcite and ankerite filled with the dissolution pores of early and middle stage carbonate cements, respectively.

  2. (2)

    The formation stages of the three stages of carbonate cements are micritic high-Mg calcite in the early stage → calcite in the early stage filled with primary pores without obvious compaction and diagenetic transformation → feldspar dissolution/quartz secondary enlargement/authigenic kaolinite precipitation → calcite and dolomite in the middle stage filled with feldspar dissolved and compacted in residual pores → ferrocalcite and ankerite in the late stage filling the dissolved pores in calcite and dolomite, and metasomatic quartz secondary enlargement.

  3. (3)

    Ca2+, Mg2+, and CO 3 2 in the pore fluid in the early stage entered the rock skeleton in the form of high-Mg calcite; Ca2+ and CO 3 2 precipitate in sandstone in the form of calcite in the early stage during the compaction and drainage of mudstone; influenced by the intrusion of the overpressure of the fluid, Ca2+ and Mg2+ rich in the fluid and CO 3 2 that is partly affected by the organic carbon source enter the reservoir. These substances mix and precipitate with Ca2+ in the original formation water to form middle-stage carbonate cements. The mixing of pore fluids from different sources makes the composition of carbonate cements complex in this period, involving both calcite and dolomite; carbonate cements in the late stage are the product of the combination of CO 3 2 formed by organic acid splitting decomposition in late diagenesis, CO 3 2 formed by carbonate cement dissolution in early and middle stages, and Mg2+, Ca2+, and Fe2+ plasma formed by the dissolution of metamorphic rock cuttings in the pore fluid.

  4. (4)

    High-Mg calcite and calcite in the early stage did not coexist with asphaltenes and are not related to hydrocarbon fluids. The carbonate cements in the middle and late stages are both diagenetic products formed during the charging process of hydrocarbon fluids. The carbonate cements in the middle stage are the early diagenetic products of Dongying Formation; it coincides with the early hydrocarbon charging period of the early Dongying Formation, proving that the carbonate cements in this stage represent the early hydrocarbon charging of Dongying Formation. The carbonate cements in the late stage are diagenetic products from the end of Guantao Formation to the present, which coincides with the period of hydrocarbon charging since the end of the Guantao Formation, proving that carbonate cements of this stage represent the hydrocarbon charging at the end of Guantao Formation to the current.

Acknowledgments

This study was financially supported by the National Science and Technology Special Grant (No. 2016ZX05006-003). Thanks also go to the following individuals and institutions: Dr. Yuelin Feng and Dr. Wei Meng of Shengli Oilfield Exploration and Development Research Institute of Sinopec; Shengli Oilfield Company of Sinopec provided all the related core samples and some geological data of Dongying Sag.

  1. Funding information: This study was financially supported by the National Science and Technology Special Grant (No. 2016ZX05006-003).

  2. Author contributions: Yuelin Feng and Wei Meng designed the experiments; Hongwei Han, Jingqiang Yu, and Weizhong Zhang carried them out. Shuli Li and Xiaochen Li drew the figures and carried out the tables. Ping Gao reviewed and edited. Tianjiao Zhang offered the article thoughts and prepared the full manuscript with contributions from all co-authors. The authors applied the SDC approach for the sequence of authors.

  3. Conflict of interest: Authors state no conflict of interest.

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Received: 2020-11-14
Revised: 2021-06-01
Accepted: 2021-07-20
Published Online: 2021-11-22

© 2021 Tianjiao Zhang et al., published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.