The widespread dolomite of the Sinian Dengying Formation in the Sichuan Basin (China) serves as one of the most important oil and gas reservoir rocks of the basin. Well WT1, as an exploration well, is recently drilled in the Kaijiang County, northeastern Sichuan Basin (SW China), and it drills through the Dengying Formation dolomite at the depth interval of 7 500–7 580 m. In this study, samples are systematically collected from the cores of that interval, followed by new analyses of carbon-oxygen isotope, major elements, trace elements, rare earth elements (REEs) and EPMA. The Dengying Formation dolomites of Well WT1 have δ13C values of 0.37‰ to 2.91‰ and δ18O values of -5.72‰ to -2.73‰, indicating that the dolomitization fluid is derived from contemporary seawater in the near-surface environment, rather than the burial environment. Based on the REE patterns of EPMA-based in-situ data, we recognized the seawater-sourced components, the mixed-sourced components and the terrigenous-sourced components, indicating the marine origin of the dolomite with detrital contamination and diagenetic alteration. Moreover, high Al, Th, and Zr contents indicate significant detrital contamination derived from clay and quartz minerals, and high Sr/Ba and Sr/Cu ratios imply a relatively dry depositional environment with extremely high seawater salinity, intensive evaporation, and strong influences of terrigenous sediment.
Dolomite is regarded as one of the most important reservoir rocks for oil and gas resources, since more than 50% of carbonate reservoirs in the world are found in dolomite, and in particular 80% of hydrocarbon resources of North America are stored in dolomite (Warren, 2000; Zenger et al., 1980). In this case, dolomite has been the research focus of mineralogists and petroleum geologists for the past 200 years. However, no consensus about the genesis of dolomite has yet been reached among previous studies based on surface outcrop samples, as dolomite is extremely vulnerable to weathering. Specifically, there are mainly two views regarding the genesis of dolomite, namely primary precipitation (Warren, 2000) and dolomitization (He et al., 2011), and the latter, which is more prevailing, can be further categorized into multiple models, such as evaporative dolomitization (McKenzie et al., 1980), mixed-water dolomitization (Badiozomani, 1973), seepage-reflux dolomitization (Adams and Rhodes, 1960), hydrothermal dolomitization (Zhu et al., 2014; (Davies and Smith, 2006), microbial dolomitization (You et al., 2011; Li et al., 2010), and deep-burial dolomitization (Guo Y et al., 2023; Guo C et al., 2022; Feng et al., 2017; Zhu et al., 2015).
A large amount of gas exploration has been achieved in the Phanerozoic of the Sichuan Basin (Wang et al., 2022; Dai et al., 2021; Hao et al., 2015). Additionally, the western margin of the Yangtze Plate of China is found with large scale widespead dolomite of the Sinian Dengying Formation, which functions as one of the most important Precambrian oil and gas reservoir rock of the Sichuan Basin (e.g., the Weiyuan gas field) (Zhao et al., 2019). Therefore, it is of utmost importance to investigate the petrogenesis of the Dengying Formation dolomite in the western margin of the Yangtze Plate, for guiding hydrocarbon exploration in deep strata. Previous studies have exploited multiple approaches (e.g., the cathode luminescence, electron probe, carbon-oxygen-strontium isotope, and rare earth element analyses) to investigate dolomite of different ages, from perspectives of tectonic evolution (Li et al., 2014), diagenesis (Shi et al., 2013), reservoir rock characteristics (Xu et al., 2020a; Si et al., 2014), and reservoir space (Yao et al., 2014) and pore evolution (Wang et al., 2000). Consequently, numerous models have been proposed, including the model based on the primary precipitation (Hu et al., 2020; Wang et al., 2020) as well as the secondary dolomitization models (Zhou et al., 2020; Peng et al., 2018; Feng et al., 2017). The controversy among these different models is, on one hand, attributed to the fact that the ancient Dengying Formation dolomite has been through complicated diagenesis, multi-stage tectonic movement, multi-stage hydrocarbon generation, multi-stage hydrocarbon charging, and multi-stage hydrothermal activity. On the other hand, previous studies mostly focus on the outcrop samples in the southern margin of the Sichuan Basin, with insufficient attention paid to the widespread Dengying Formation dolomite in the northeastern Sichuan and fresh samples from deep cores.
The northern margin of the Sichuan Basin is expected to be another important strategic relay area for exploration, following the Anyue giant gas field in central Sichuan, due to the recent discovery of the Xuanhan-Kaijiang paleo-uplift (Gu et al., 2016; Yang et al., 2016) and the recent recognition of the Sinian–Early Cambrian tectonic framework of the Sichuan Basin (Li et al., 2019; Zhao et al., 2017). Hence, an exploration well, Well WT1 (Figure 1), has been drilled in the southwestern Kaijiang County, northeastern Sichuan Basin, by PetroChina Southwest Oil & Gas field Company (He et al., 2021a, b). This well drills through the Dengying Formation at the depth interval of 7 500–7 592 m and provides precious samples for investigation of the Sinian fundamental geology, petroleum geology, basin structure, and sedimentary-tectonic evolution in both the basin interior and margin. In this research, a total of 20 dolomite samples are systematically collected from the interval of 7 500–7 580 m in Well WT1, followed by comprehensive petrographic, carbon and oxygen (C-O) isotopes, major and trace element and rare earth elements (REEs) to together constrain the petrogenesis of the Dengying Formation dolomite.
Figure
1.
(a) General geological and tectonic framework of the crustal blocks of China; (b) tectonic division of the Sichuan Basin and adjacent areas, and location of Well WT1 (modified from Zhao et al., 2017). Abbreviations: 1. First-order tectonic unit boundary; 2. second-order tectonic unit boundary; 3. third-order tectonic unit boundary; 4. fault; 5. anticline; 6. No. and names of faults: ① Longmenshan; ② Chengkou; ③ Pengguan; ④ Huayingshan; ⑤ Jianshi-Yujiang; ⑥Yingshan; ⑦ Wanyuan; ⑧ Longquanshan; ⑨ Qiyueshan; ⑩ South Qingling; ⑪–⑭ faults in the western margin of the Yangtze Block; 7. the boundary of Sichuan Basin; Ⅰ. West Sichuan depression; Ⅱ. Central Sichuan low-relief fold zone; Ⅲ. Southwest Sichuan low-relief fold zone; Ⅳ. North Sichuan low-relief fold zone; Ⅴ. East Sichuan steep tectonic belt; Ⅵ. South Sichuan low-relief fault-fold zone.
The Sichuan Basin is a superimposed basin developing based on the Mid–Upper Yangtze Plate. Its current geomorphology inherits from the tectonic framework first created during the Indosinian Movement, then developed during the Yanshanian Movement, and finally intensively modified during the Himalayan Movement (Liu et al., 2011).
The breakup of the Rodinia ancient continent during the Mid-Late Neoproterozoic was accompanied by the emergence of several rift basins in the Yangtze Plate (e.g., Nanhua, Kangdian, Longmenshan, and Chuanzhong rift basins) (Li et al., 2008; Luo, 1979). These rift basins were all subjected to a unified extensional stress field for a prolonged time until the Yangtze Plate merges onto the northeastern margin of the Gondwana ancient continent, although there are varied opinions regarding the mechanisms of their development (Li Z W et al., 2019; Zhao et al., 2018a, b; Li Z X et al., 2008; Zhou et al., 2002), such as superplume, back-arc extension, and post-collision extension. Meanwhile, Neoproterozoic rifts are also developed in the Sichuan Basin at the western margin of the Yangtze Plate.
The Sinian Dengying Formation in the Sichuan Basin is a set of retrogradational sedimentary successions resulting from transgression, and it presents a large-scale carbonate platform depositional system with a rimmed margin. From west to east, the depositional system consists of the paleo-land, the open platform facies, the restricted platform facies, the open platform facies, the platform margin facies, the slope facies, and the deep-water basin facies (Lan et al., 2022a). The Dengying Formation dolomite is mainly deposited in the restricted platform facies, which can be further divided into several sub-facies, including supratidal, intertidal, subtidal, lagoon, and shoal sub-facies. Specifically, the Dengying Formation dolomite mainly forms in the supratidal zone to the shallower part with a water depth of 10 m. The Dengying Formation is widely distributed across the Sichuan Basin and its periphery, with a thickness typically of 600–1 000 m.
During the geological history, the burial depth of the Dengying Formation dolomite can be up to 5 000–8 000 m, indicating deep burial. Moreover, according to the lithology, the abundance of microbialite, and the rock texture, the Dengying Formation can be divided into four members, namely the Deng Ⅰ (Z2dn1), Deng Ⅱ (Z2dn2), Deng Ⅲ (Z2dn3), and Deng Ⅳ (Z2dn4) members from top to bottom (Lan et al., 2022b; Xu et al., 2020a). The effective reservoir rock with the desired scale is mainly concentrated in Deng Ⅱ and Deng Ⅳ members (Xu et al., 2020b).
2.
SAMPLES AND EXPERIMENTS
2.1
Samples
Core samples are mainly collected from Well WT1, with its location marked in Figure 1 by a red star. The 80-meter-long section from 7 500 to 7 580 m of Well WT1 is sampled every four meters, which leads to a total of 20 core samples. The collected sample is first observed by naked eyes and under a magnifying lens, and then all dyed and put through thin-section petrographic analysis to accurately identify the lithology (Figure 2).
Figure
2.
Comprehensive stratigraphic column of core samples (typical lithology, sedimentary texture, and micro- and macro-structural features), 7 500–7 580 m, Well WT1.
The collected 20 core samples are first crushed into small cubes with a side length of ~0.5 cm. Subsequently, pure dolomite cubes are selected and ground into 200-mesh powder for elemental geochemical and C-O isotope analyses.
Major elements are measured using the Rigaku RIX 2000 X-ray fluorescence (XRF) spectrometer at the State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum (Beijing). Rare earth elements (REEs) and trace elements are tested at the National Research Center for Geoanalysis, the China Geological Survey, via a sealed acid digestion-inductively coupled plasma-mass spectrometry (ICP-MS) approach. EPMA analyses are performed at the Research Institute of Exploration and Development of PetroChina Southwest Oil & Gasfield Company, with the EPMA-1600 probe, under experimental conditions of 15 kV and 10 nA, and electron beam size of 30 μm. Analytical precision for the elements of interest is better than 1.0% relative. The raw REE data were normalized to Post-Archean Australian shale (PAAS) for comparison (Taylor and McLennan, 1985), and all of the present REE patterns are shale-normalized unless otherwise noted. The contents of Ce and Eu as well as the relative enrichment of LREEs (light REEs) to HREEs (PrSN/YbSN) were calculated as described by Lawrence et al. (2006) and Tostevin et al. (2016a).
The δ18O and δ13C values of carbonates are measured at the MLR Key Laboratory of Metallogeny and Mineral Resource Assessment at the Chinese Academy of Geological Sciences. Approximately 1 mg of powdered sample is placed into a sealed reaction vessel. Then, the samples are processed with 100% anhydrous phosphoric acid at 70 ℃ in an automated device (Thermo Finnigan Gas Bench Ⅱ) coupled to a Thermo Finnigan MAT 253 gas isotope mass spectrometer. The δ18O and δ13C values are normalized using internal laboratory standards calibrated to Vienna Pee Dee Belemnite (VPDB) per mil (‰). The test precision is better than ±0.1‰ for both δ18O and δ13C.
3.
RESULTS
As shown in Table 1, the samples of the Dengying Formation dolomite contain SiO2 (1.38 wt.%–12.59 wt.%, averaging 4.89 wt.%), Al2O3 (0.10 wt.%–2.43 wt.%, averaging 0.84 wt.%), MgO (16.44 wt.%–23.49 wt.%, averaging 20.36 wt.%, which is close to the content of pure dolomite, 21.7 wt.%), and CaO (24.07 wt.%–34.63 wt.%, averaging 29.20%, which is also close to the content of pure dolomite, 30.40 wt.%). In addition, the Dengying Formation dolomite presents a high total iron content (FeOt = 0.34 wt.%–1.04 wt.%, averaging 0.61 wt.%), a relatively high Mn content (MnO = 0.03 wt.%–0.13 wt.%, averaging 0.07 wt.%), a relatively low K content (K2O = 0.08 wt.%–0.96 wt.%, averaging 0.07 wt.%), and a relatively low Na content (Na2O = 0.06 wt.%–0.10 wt.%, averaging 0.08 wt.%).
As shown in Table 2, the average contents of Zr (2.14 ppm–33.07 ppm), Sc (0.47 ppm–1.95 ppm), Th (0.15 ppm–1.85 ppm), and Hf (0.07 ppm–0.87 ppm) of the dolomite samples from Well WT1 are all far below their counterparts in the continental crust, which are 193.00 ppm, 14.00 ppm, 10.50 ppm, and 5.30 ppm, respectively; both the contents of U (0.81 ppm–4.69 ppm) and Mo (1.78 ppm–14.35 ppm) are relatively low; the V/Sc ratio is in the range of 10.39–28.02; the content of Sr varies in the range of 94.24 ppm–252.30 ppm; and the content of Ba varies in the range of 1 417 ppm–9 841 ppm. As summarized in Table 3, the total REE content (∑REE) varies in the range of 7.37 ppm–48.22 ppm, which is consistent with the low REE characteristic of typical dolomite. As presented in Table 4, the δ13C and δ18O values of the 20 core samples of the Dengying Formation dolomite samples ranges from 0.37‰ to 2.91‰ (averaging 1.60‰) and from -5.72‰ to -2.73‰ (averaging -4.16‰).
The ∑REE content of the EPMA data of in-situ compositions can be divided into three types, including the seawater-sourced components have an ∑REE range of 0.33 ppm–0.52 ppm, averaging 0.46 ppm; The mixed- sourced components have an ∑REE range of 0.66 ppm–1.76 ppm, averaging 1.16 ppm; The terrigenous-sourced components have an ∑REE range of 3.67 ppm–12.72 ppm, averaging 7.86 ppm.
4.
PETROGENESIS OF DOLOMITE
4.1
Evidence from Carbon-Oxygen Isotopes
C-O isotopes are important geochemical indicators of the origins and genetic mechanisms of dolomite, and they are mainly controlled by the C-O isotopic composition of the dolomitized rock as well as the salinity and temperature of diagenetic fluids (He et al., 2014). The C-O isotope values of dolomite, of which the dolomitization fluids are derived from seawater, are closely related to those of contemporary seawater (Qiang et al., 2017). Fairchild and Spiro (1987) and Zempolich et al. (1988) measured the Late Precambrian seawater in North America and found that the stable carbon isotope (δ13C) and stable oxygen isotope (δ18O) of the Late Precambrian seawater ranged from +5‰ to +7‰ and -0.5‰ to +0.9‰, respectively (Figure 3). The carbonates of the Sinian Dengying Formation belong to the equivalent Late Precambrian. Hence, the values represent the C-O isotope values of seawater during the deposition of Dengying Formation carbonates.
Due to their isotopic fractionation during inorganic or organic chemical precipitation, the stable C-O isotopes of carbonate sediment in the Dengying Formation (aragonite and high-magnesium calcite) have slightly lower values than those of seawater (Qiang et al., 2017). In the near-surface environment, carbonate mineral precursors (aragonite and high magnesium calcite) undergo early dolomitization to form dolomites. The carbon isotope values in dolomites are largely influenced by the carbon isotope values of carbonate precursors and indicate the source of carbon in carbonate (Zhou et al., 2020). However, carbon isotope composition in carbonate rocks is rarely affected by diagenesis, temperature, and other external conditions, and is mainly related to the fluids and mixing of different carbon sources (Wu et al., 2016). Tucker et al. (1990) believed that δ13C values between 0 and +4‰ could be regarded as typical seawater origin signals. δ13C of the dolomite samples ranges from 0.37‰ to 2.91‰, with an average of 1.60‰, and it could be seen that carbon isotope δ13C values of all the dolomite samples are within the range of 0 to +4‰.
Concerning the oxygen isotope, it is a function of both fluid properties (e.g., salinity) and temperature. Specifically, it is, to some extent, more sensitive to temperature. Generally speaking, the oxygen isotope composition of the rock mineral declines as the temperature of the rock mineral-water system increases. Studies show that when the δ18O value is less than -5‰, the original carbonate sediment is partially altered, and when the δ18O value is less than -10‰, the original carbonate sediment is strongly modified by diagenesis (Ren et al., 2016; Li et al., 2009; Kaufman and Knoll, 1995). In the burial environment, the variation of δ18O values of dolomite is mainly due to the isotopic fractionation caused by temperature increase during burial diagenesis. Therefore, δ18O values in dolomite can be used to distinguish dolomitic fluids in near-surface and buried environments. Figure 3 shows the significant difference between oxygen isotopes in near-surface and those in buried dolomitic fluids. The δ18O values of saddle dolomite formed by hydrothermal dolomitization are generally less than -10‰ (Figure 3), representing the source of hydrothermal fluids (Feng et al., 2017; Ren et al., 2016). The δ18O values of the dolomite samples range from -5.72‰ to -2.73‰, averaging -4.16‰, which indicates a near-surface environment, instead of a burial environment without hydrothermal origin.
4.2
Geochemical Evidence from REEs
Minerals in equilibrium with the aqueous solution tend to inherit the REE partition characteristics of their parent solution from which they precipitate. In other words, the REE composition of sediment is mainly dependent upon that of the liquid from which they precipitate. The terrigenous fraction is one of the most essential REE sources, and its contamination effect on the carbonate components of rocks is commonly underestimated. In modern marine environments, the ∑REE contents of detrital material, including the sand-, silt-, and clay-sized fractions, are generally one to three orders of magnitude greater than those of the pure carbonates (Bayon et al., 2015; Garzanti et al., 2011; Zhong and Mucci, 1995).
The REE compositions of the dolomite samples (Table 3) present the following characteristics.
(1) The bulk rock data of Dengying Formation dolomite examples have an ∑REE range of 7.37 ppm–48.22 ppm, averaging 20.26 ppm. In contrast, the EPMA data of in-situ compositions have a relatively lower ∑REE content, which can be divided into different sources according to their magnitude orders. The seawater-sourced components have an ∑REE range of 0.33 ppm–0.52 ppm, averaging 0.46 ppm. The mixed-sourced components have an ∑REE range of 0.66 ppm–1.76 ppm, averaging 1.16 ppm. The terrigenous-sourced components have an ∑REE range of 3.67 ppm–12.72 ppm, averaging 7.86 ppm. Since terrigenous impurities (e.g., clay) have a much higher ∑REE than dolomite (by several magnitude orders), a small quantity of them mixed in the dolomite can lead to considerable growth of ∑REE. Therefore, by comparing the bulk rock data with the EPMA-based in-situ data, it can be inferred that diagenetic alteration and detrital contamination could have been involved during the formation of the Dengying Formation dolomite.
(2) The PrSN/YbSN ratio has been widely used to determine the fractionation degree of light and heavy REEs. According to the EPMA-based in-situ data, the seawater-sourced components show LREE-depleted patterns (PrSN/YbSN ranges from 0.65 to 0.92, averaging 0.75), whereas the mixed-sourced components (PrSN/YbSN ranges from 0.65 to 1.57, averaging 0.95) and the terrigenous-sourced components (PrSN/YbSN ranges from 0.64 to 1.29, averaging 1.02) have the trend from LREE-depleted patterns to even slightly LREE-enriched flat patterns, indicating the LREE-enrichment is relative to exogenetic REE (porewater fluid) input of diagenetic alteration and detrital contamination.
(3) The Ce anomalies can provide a potentially useful redox proxy in carbonate-dominated marine settings (Tostevin et al., 2016a). Compared with other REE elements, Ce is unique because it can exist in both +3 and +4 oxidation states. In the presence of oxygen, Ce (Ⅲ) is partially oxidized to Ce (Ⅳ), leaving residual seawater depleted in Ce relative to other trivalent REEs (German and Elderfield, 1990). An empirical criterion is used to evaluate the Ce anomaly (< 0.9: negative anomaly; 0.9–1.3, equivocal; > 1.3: positive anomaly) (Tostevin et al., 2016b).
A weak Ce anomaly is observed in the REE partition diagram of the bulk rock data of the Dengying Formation (Figure 4), with a δCe range of 0.64–1.00, averaging 0.77. In contrast, the EPMA-based data of in-situ compositions also show Ce anomaly patterns. However, in-situ compositions from different sources vary in the range of Ce anomaly values. Among them, the seawater-sourced components have the most significant Ce anomalies, with a δCe range of 0.54–0.72, averaging 0.64. The mixed-sourced components have relatively lower Ce anomalies (δCe* ranges from 0.73 to 0.88, averaging 0.79). The terrigenous-sourced components have the lowest Ce anomalies (δCe* ranges from 0.75 to 1.05, averaging 0.90). The seawater-source components have similar Ce anomalies to modern seawater, whereas mixed-sourced and terrigenous-sourced components have relatively weaker Ce anomalies. Varied Ce anomalies in different in-situ compositions indicate that diagenetic alteration and detrital contamination may weaken the original Ce anomaly as the detrital contamination content increases.
Figure
4.
PASS-normalized (Taylor and McLennan, 1985) REE patterns of the Dengying Formation dolomite. (a) Bulk rock data of Dengying Formation samples and Duoshantuo Formation samples; (b) EPMA-based in-situ data of Dengying Formation samples of different sources. Fm. Formation.
(4) Apparently high positive Eu anomaly is observed in the REE partition diagram of the bulk rock data of Dengying Formation dolomite samples (Figure 4), with a δEu range of 1.54–19.44, averaging 6.46. In contrast, the EPMA data of in-situ compositions uniformly exhibit slightly positive Eu anomalies, ranging from 1.07 to 1.75, averaging 1.36 (seawater-sourced samples: 1.42–1.75, averaging 1.54; mixed sourced samples: 1.07–1.67, averaging 1.37; terrigenous-sourced samples: 1.13–1.51, averaging 1.26). The reason for the larger positive Eu anomalies of the bulk rock data may be related to the hydrothermal fluids mixed with seawater (Meyer et al., 2012). Eu anomalies of dolomite are mainly dependent on the diagenetic environment and subsequent fluid-driven modification. Under normal temperatures and pressures, Eu in solution mostly occurs in the form of Eu3+, except in the case of an extremely reducing environment. However, with temperatures high enough, Eu in fluids mainly occurs in the form of Eu2+ and thus differentiates from other REEs (Bau, 1991; Sverjensky, 1984).
Another possible trigger of the Eu anomaly is the interference of barium (Ba) oxides formed during analysis, which can be evaluated through the measurement of Ba concentrations (Jarvis et al., 1989). During the ICP measurement process, Ba and O atoms could combine in the plasma and produce 137Ba16O, 136Ba17O, and 135Ba18O, which can interfere with the analysis of the 153Eu signal, resulting in apparently positive δEu values. The Ba content of the bulk rock data ranges from 1 417 ppm to 9 841 ppm, averaging 4 425 ppm, while EPMA-based data have a much lower Ba concentration (seawater-sourced samples: 1.79–2.28; mixed sourced samples: 3.04–32.27; terrigenous-sourced samples: 6.47–67.92). Furthermore, a significant correlation is seen between the Ba content and the δEu value (Figure 5g). The above evidence indicates that the larger positive Eu anomalies are not related to the hydrothermal fluids mixed with seawater, but to the excessive Ba formed during the analysis. The slightly positive Eu anomalies in the samples are similar to those of modern seawater, indicating the marine origin of dolomite.
Figure
5.
(a)–(d) Plots of ΣREE vs. contamination finger elements (Al, Th, Zr and Si); (e)–(f) plots of Y/Ho vs. FeT based on the bulk rock data of Dengying Formation samples and Duoshantuo Formation samples (e) and EPMA-based in-situ data of Dengying Formation samples (f); (g) plots of Eu vs. Ba.
In addition, significantly positive Eu anomalies are also present in bulk rock data of Duoshantuo Formation samples, with a Ba content range of 1 542 ppm to 2 472 ppm, averaging 2 074.6 ppm, also indicating the interference of Ba oxides. As the in-situ data for Duoshantuo Formation dolomite samples are absent, the data of Chang et al. (2020) are referred to as a supplement. The δEu values of the samples from Members Ⅲ and Ⅳ of Duoshantuo Formation carbonates average 1.17, which is similar to that of the seawater-sourced samples, indicating the Dengying Formation seawater may be inherited from the Early Ediacaran seawater.
(5) Yttrium (Y) is often included alongside REE as it has similar chemical properties to Holmium (Ho). Y anomalies present different patterns in different sedimentary settings. Larger Y anomalies occur in open marine settings, while smaller Y anomalies (33–40) are often observed in nearshore or restricted settings (Bau et al., 1997; Nozaki and Zhang, 1995; De Baar et al., 1985a, b). Y/Ho is an effective proxy to trace the origin of fluid sources. Y/Ho > 36 is a commonly used threshold value for carbonate REY signals indicating seawater source. Y/Ho = 25 represents the origin of shale. And Fe-Mn (oxyhydr) oxides may have negative Y anomalies compared with shale (< 25) (Tostevin et al., 2016b). The seawater-sourced components have a Y/Ho ratio range of 32.86 to 44.50, averaging 37.41. The mixed-sourced components have a Y/Ho ratio range of 30.66 to 47.92, averaging 40.89. The terrigenous-sourced components have a Y/Ho ratio range of 28.06 to 34.06, averaging 31.93. As a comparison to in-situ data, the bulk rock data of Dengying Formation samples have a Y/Ho ratio range of 20.85 to 35.70, averaging 29.65. In addition, the bulk rock data of Duoshantuo samples have a Y/Ho ratio range of 23.42 to 26.31, averaging 24.98. It can be inferred that both seawater-sourced and mixed-sourced components show seawater signals, whereas terrigenous-sourced components show siliciclastic signals. The bulk rock data of Dengying Formation and Duoshantuo Formation show both siliciclastic and Fe-Mn oxide signals, while the bulk rock data of Duoshantuo Formation samples only show Fe-Mn oxide signals.
The above REE data indicate that any mixing with non-carbonate phases usually results in increased REEs, weakened Ce anomalies, decreased Y/Ho ratio, and development of weak Eu anomalies (Tostevin et al., 2016a). Seawater-sourced components can be regarded as the least contaminated dolomite in this paper, of which the REE pattern is similar to that of modern seawater. The mixed-sourced components have a lower REE content, more enriched LREE, and a higher Y/Ho ratio, than terrigenous-sourced components, indicating the terrigenous-sourced components suffered more diagenetic alteration and detrital contamination than the mixed-sourced components. However, even the least contaminated seawater-sourced components may experience slight detrital contamination, as indicated by the weak Eu anomaly and the partial Y/Ho < 36 (Figure 5f). The reason for the detrital contamination in the least seawater-sourced components may be that rivers and aeolian input bring detrital contamination into the seawater. Another explanation is the release of detrital REE signatures into the porewater, which was derived from the generation of authigenic clay minerals (illite and K-feldspar) during diagenesis.
The exogenous fluid signals cannot be screened out using the available extraction techniques since these diagenetic REEs have been incorporated into the Ca2+ lattices of the secondary carbonate minerals (Zhong and Mucci, 1995).
4.3
Geochemical Evidence from Incompatible Trace Elements
The characteristics of rare earth elements and other inactive elements in carbonate rocks could reflect their tectonic environment, which is difficult to change their geochemical characteristics through later activities (Zhang et al., 2017). Table 3 summarizes the test results of incompatible trace elements in the dolomite samples, which reflects the following features.
(1) Contamination finger element (Al, Th, and Zr) analysis provides a good opportunity to trace the sources of detrital contamination minerals. The empirical criterion of Gong et al. (2021) is applied in this paper, with contamination element concentrations of Al < 100 ppm, Th < 0.5 ppm, and Zr < 0.5 ppm, which can be regarded as the seawater-sourced components with the least contamination. The seawater-sourced components, with the least REE content, meet the criterion, with finger element contents of Al (59 ppm–67 ppm, averaging 63.08 ppm), Th (0.005 3 ppm–0.009 7 ppm, averaging 0.007 7 ppm), Zr (0.035 ppm–0.103 ppm, averaging 0.071 ppm), and Si (199 ppm–420 ppm, 398.4 ppm) (Figure 5). The mixed-source components have relatively higher REE contents than the seawater-sourced components, and they have finger element contents of Al (34 ppm–217 ppm, averaging 121.10 ppm), Th (0.009 8 ppm–0.041 ppm, averaging 0.020 4 ppm), Zr (0.024 ppm–0.266 ppm, averaging 0.140 ppm), and Si (231 ppm–940 ppm, 505.1 ppm) (Figure 5). The terrigenous-sourced components, which have the highest REE content, have finger element contents of Al (146.8 ppm–6 308 ppm, averaging 1 965.6 ppm), Th (0.030 4 ppm–0.631 ppm, averaging 0.285 ppm), Zr (0.35 ppm–12.65 ppm, averaging 2.89 ppm), and Si (591 ppm–11 000 ppm, averaging 4 364.8 ppm) (Figure 5). The bulk rock data of Dengying Formation samples have finger element contents of Al (1 000 ppm–24 300 ppm, averaging 8 604.8 ppm), Th (0.151 ppm–1.852 ppm, averaging 0.606 ppm), Zr (2.14 ppm–33.07 ppm, averaging 10.67 ppm), and Si (13 800 ppm–208 500 ppm, averaging 56 481 ppm) (Figure 5). The bulk rock data of Duoshantuo Formation samples have finger element contents of Al (41 400 ppm–100 500 ppm, averaging 68 640 ppm), Th (1.864 ppm–5.516 ppm, averaging 2.914 ppm), Zr (56.61 ppm–176.1 ppm, averaging 100.85 ppm), and Si (261 400 ppm–604 400 ppm, averaging 415 640 ppm) (Figure 5). From the seawater-sourced components to the terrigenous-sourced components based on in-situ data, the REE content increases to higher orders and the contamination element concentration grows, indicating the input of detrital contamination minerals to the dolomite.
(2) Mn/Sr ratios can be used to reflect the process of diagenetic alteration and detrital contamination. Diagenesis has little effect on carbonate minerals when the Mn/Sr ratio is less than 2. When the ratio changes to 2–10, the carbonate rocks can retain the original seawater information, although they have undergone diagenetic alteration to a certain extent (Zhou et al., 2020). The seawater-sourced components have an Mn/Sr ratio range of 0.81 to 1.16, averaging 0.99, whereas the mixed-sourced components (Mn/Sr = 2.57–18.57, averaging, 7.17), the terrigenous-sourced components (Mn/Sr = 4.75–18.93, averaging 9.93), and the bulk rock data (Mn/Sr = 0.33–6.20, averaging 3.65) have much higher Mn/Sr ratios. From the seawater-sourced components to the terrigenous-sourced components, the input of detrital minerals increases, and correspondingly the Mn/Sr ratio gradually rises. It can be inferred that the Mn/Sr ratios of the bulk rock may result from mixing between seawater-sourced dolomite and detrital minerals, leading to the relatively moderate Mn/Sr ratios.
(3) The total content of Sr and Ba and the Sr/Ba ratio of dolomite can to some degree reflect the sedimentary environment. For example, the Sr/Ba ratio is suggested to be indicative of paleo-salinity. Typically, Sr/Ba > 1 is believed to indicate marine deposition, while Sr/Ba < 1 represents continental deposition (Ni et al., 2010). The seawater-sourced components have a Sr/Ba ratio range from 23.86 to 28.32, averaging 27.63, reflecting extremely high seawater salinity. In contrast, the mixed-sourced components (Sr/Ba = 1.73–27.38, averaging 10.29) and the terrigenous-sourced components (Sr/Ba = 1.03–10.82, averaging 2.95) show relatively lower Sr/Ba ratios as a result of the increased input of detrital contamination. Due to the Ba anomalies caused by analysis procedures, the Sr/Ba ratios of the bulk rock data cannot be applied.
(4) The Sr/Cu ratio is suggested to be indicative of the paleo-climate during deposition. In general, the Sr/Cu ratio between 1.3 and 5.0 represents a humid climate, while that exceeding 5.0 implies an arid climate (Ni et al., 2010). The Sr/Cu ratio of the dolomite samples in this research is in the range of 11.19–60.90, with an average of 32.02, and thus it is safe to say that the climate of the study area is extremely dry during the deposition of the Dengying Formation dolomite.
5.
CONCLUSIONS
(1) The Dengying Formation dolomite in northeastern Sichuan presents a δ13C range of 0.37‰ to 2.91‰ and a δ18O range of -5.72‰ to -2.73‰, indicating that the dolomitization fluid is derived from contemporary seawater in a near-surface environment, instead of a burial environment, without hydrothermal origin.
(2) The REE characteristics of the bulk rock analysis and the EPMA-based in-situ analysis show significant differences. The detrital contamination of the dolomite samples may mislead our understanding of the REE patterns of primary dolomite. The in-situ technique provides a possibility for locating the least contaminated position, which may retain primary sedimentary carbonate REE signatures. With the help of the in-situ technique, the seawater-sourced components, the mixed-sourced components, and the terrigenous-sourced components can be recognized, indicating that the dolomite of the Dengying Formation of Well WT1 in northeastern Sichuan inherits its REE characteristics from the Sinian contemporary seawater, with detrital input and diagenetic alteration.
(3) Contamination finger elements (Al, Th, and Zr) have good correlations with detrital minerals, which can be used to trace the source of detrital contamination, most likely indicating clay particles and quartz minerals. Mn/Sr ratios also prove that the seawater-sourced components suffer from the least contamination. The Sr/Ba ratio of in-situ seawater-sourced components and the Sr/Cu ratio jointly reflect a relatively reducing dry environment, with extremely high seawater salinity and intensive evaporation.
ACKNOWLEDGMENTS:
This study was financially supported by the Science Foundation of China University of Petroleum, Beijing (Nos. 2462018YJRC030 and 2462020YXZZ020) and the China Sponsorship Council (No. 202306440071). We appreciate all reviewers for their constructive and critical comments that considerably help improve an early version of this manuscript. The final publication is available at Springer via https://doi.org/10.1007/s12583-022-1732-z.
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Figure 1. (a) General geological and tectonic framework of the crustal blocks of China; (b) tectonic division of the Sichuan Basin and adjacent areas, and location of Well WT1 (modified from Zhao et al., 2017). Abbreviations: 1. First-order tectonic unit boundary; 2. second-order tectonic unit boundary; 3. third-order tectonic unit boundary; 4. fault; 5. anticline; 6. No. and names of faults: ① Longmenshan; ② Chengkou; ③ Pengguan; ④ Huayingshan; ⑤ Jianshi-Yujiang; ⑥Yingshan; ⑦ Wanyuan; ⑧ Longquanshan; ⑨ Qiyueshan; ⑩ South Qingling; ⑪–⑭ faults in the western margin of the Yangtze Block; 7. the boundary of Sichuan Basin; Ⅰ. West Sichuan depression; Ⅱ. Central Sichuan low-relief fold zone; Ⅲ. Southwest Sichuan low-relief fold zone; Ⅳ. North Sichuan low-relief fold zone; Ⅴ. East Sichuan steep tectonic belt; Ⅵ. South Sichuan low-relief fault-fold zone.
Figure 2. Comprehensive stratigraphic column of core samples (typical lithology, sedimentary texture, and micro- and macro-structural features), 7 500–7 580 m, Well WT1.
Figure 4. PASS-normalized (Taylor and McLennan, 1985) REE patterns of the Dengying Formation dolomite. (a) Bulk rock data of Dengying Formation samples and Duoshantuo Formation samples; (b) EPMA-based in-situ data of Dengying Formation samples of different sources. Fm. Formation.
Figure 5. (a)–(d) Plots of ΣREE vs. contamination finger elements (Al, Th, Zr and Si); (e)–(f) plots of Y/Ho vs. FeT based on the bulk rock data of Dengying Formation samples and Duoshantuo Formation samples (e) and EPMA-based in-situ data of Dengying Formation samples (f); (g) plots of Eu vs. Ba.
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