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Abstract

As important gas-bearing strata, Proterozoic–Paleozoic marine carbonate successions in the Sichuan Basin have experienced a long diagenetic history and multiphase hydrothermal activities. Some hydrothermal fluids were closely linked to reservoir dolomitization, which dramatically affected the physical properties of the Proterozoic–Paleozoic marine carbonates in the region. Based on comprehensive petrology, mineralogy, and geochemistry analyses, as well as analyses of the structural and depositional settings, we took as study objects three sets of typical saddle dolomites from the Upper Sinian, Middle-Upper Cambrian, and Middle Permian to understand the mechanism of hydrothermal dolomitization during the Proterozoic–Paleozoic. First, we analyzed the dolomitization stages and mineral characteristics. Then, we performed comprehensive geochemical analyses to discriminate between different saddle dolomites and utilized a fluid reconstruction method to reveal the formation mechanisms of different hydrothermal dolomitizations. We present the three sets of saddle dolomites as being characterized by obviously different lithologies, diversified mineral features, varying paragenetic mineral combinations, and distinguishable horizontal distributions. The Upper Sinian saddle dolomites originated from mantle-derived materials of high temperature and high salinity and experienced continuous and stable crystallization. Moreover, their hydrothermal dolomitization was related to the hydrothermalism of rock intrusion resulting from stress transformation during the Caledonian–Hercynian. The Middle Permian saddle dolomites originated from multisource fluids of moderate-high temperature and high salinity, and their hydrothermal dolomitization was linked to the hydrothermalism of extrusive rocks during the main body of the Hercynian. The Middle-Upper Cambrian saddle dolomites originated from fluid of moderate-low temperature and moderate salinity, and instead of hypomagma activity, their hydrothermal dolomitization might be related to the differential compaction of bottom mudstone rich in hydrothermal minerals during the diagenetic period. In summary, in this paper, we clarify the fluid characteristics and mechanisms of three-stage hydrothermal dolomitization and provide theoretical support for the exploration of hydrothermally dolomitized reservoirs of the Proterozoic–Paleozoic marine carbonate successions, which can also be used as reference for investigations of the multiple mechanisms of hydrothermal dolomitization.

Keywords

Hydrothermalism Dengying formation Cambrian Middle Permian geochemistry saddle dolomite

Introduction

Hydrothermal dolomitization was identified a long time ago by Deininger (1964) and has been proved to widely exist in sedimentary basins (Mansurbeg et al., 2016). Reservoirs related to hydrothermal dolomitization have been found in oil and gas fields in North America (Duggan et al., 2001; Katz et al., 2006; Qing and Mountjoy, 1992; Smith et al., 2006), Europe (Gomez-Rivas et al., 2014), and Asia (Wang et al., 2016), among which the Albion-Scipiol Oilfield and the Lady-fern Oilfield are super giant and indicate the obvious transformation effects of hydrothermal dolomitization on reservoirs (Jin et al., 2006; Song et al., 2009; Zhu et al., 2015). Many studies have been conducted to decipher the causes of hydrothermal fluids (Sun and Püttmann, 1997), with the dominant resulting opinion being that fractures are the main controlling factor of hydrothermal dolomitization (Davies and Smith, 2006). Scholars have proposed a genetic relationship between hydrothermalism and faults when investigating the distribution of valley-type and exhalation-sediment-type lead zinc ores in the Mississippi (Barber et al., 1985). Machel and Lonnee (2002) and Lonnee and Machel (2006) redefined hydrothermal fluids and suggested that their temperature is higher than that of surrounding rocks (5–10℃), which weakened the inherent constraint of hydrothermalism on depth and the formation modes. Although faults control hydrothermal dolomitization, discrepancies in structural features, hydrothermal sources, and flow directions make it important to consider whether all the hydrothermal mechanisms can be explained by one mode, that is, whether we can further refine the genetic model of hydrothermal dolomitization.

In this paper, we discuss the diversity of hydrothermal mechanisms. There are a series of typical saddle dolomites in the Proterozoic–Paleozoic dolostones in the Sichuan Basin, which were deposited during different periods (Upper Sinian, Middle-Upper Cambrian, and Middle Permian) and which feature distinct petrological and geochemical characteristics and can be taken as good study objects. The earliest work regarding hydrothermal dolomitization in this area was published in 1991 with the study object being the Permian Maokou formation in which the saddle dolomites distributed along the fault were comprehensively described (Zhao et al., 1991). Next came descriptions of typical hydrothermal dolomitization in the Upper Sinian Dengying Formation (Liu et al., 2008a). There had been no records of hydrothermal dolomitization in the Middle-Upper Cambrian until hydrothermal minerals such as saddle dolomites were found in those sequences, which proved the existence of hydrothermalism. At present, mainstream opinion is that all Proterozoic–Paleozoic hydrothermal dolomitizations are closely related to faults (Chen et al., 2012; Shu et al., 2012) and to magmatism resulting from large-scale tensile fractures during the Hercynian (≈259 Ma), which were the primary causes contributing to the hydrothermal dolomitization of the Middle Permian Qixia and Maokou Formations (Huang et al., 2012) and the Upper Sinian Dengying Formation (Liu et al., 2007; Song et al., 2009). Based on results from the stable isotope tracer technique, these saddle dolomites are understood to have been formed by synchronous cross-layer hydrothermal fluids (Wang and Liu, 2009). It is certain whether large-scale magmatism controlled hydrothermal dolomitization to some extent during the Hercynian, and further discussion is necessary in many respects. The cross-layer movement of hydrothermal fluids can be significantly influenced by the isolation effects of tight siliceous rocks on the top of the Upper Sinian Dengying Formation (Z₃dy⁴) and thick mudstone in the Lower Cambrian Niutitang Formation. With respect to the source of Mg²⁺, granitic pluton can be seen inside and at the bottom of the Upper Sinian sequences, and its characteristics differ from those of the extrusive rocks of the Hercynian (P₃). This suggests probably unmatched relations between the hydrothermal fluid movements in the Upper Sinian and the magmatism during the major period of the Late Permian. As for the formation mechanism, hydrothermalism during the Hercynian was characterized by upward movement along fractures and overflowing lava (Emeishan basalt) rich in heat and Mg²⁺, so that the underlayers were inevitably influenced. This has been proved by the general phenomenon of hydrothermal zebra structures in field observations close to the spill point of Emeishan basalt. However, such downward hydrothermalism has not been mentioned in previous studies. Moreover, a variety of hydrothermal associated minerals, such as pyrite, sparry, calcite, etc. is also introduced, which have been proved helpful to analysis the hydrothermal environment (Sun and Püttmann, 1997). All the above questions indicate that the hydrothermal dolomitization of Proterozoic–Paleozoic marine carbonate sequences in the Sichuan Basin may be characterized by multi-periods and multi-causes, which accords with our research on hydrothermal mechanism diversity. Accordingly, in this paper, we compare differences in the petrology, mineralogy, geochemistry, and special distribution of three sets of hydrothermal dolomites. We then reconstruct the diagenetic fluids based on their inclusion temperature, composition, and stable isotope data, analyze the source and timing of Mg²⁺ based on geological age data and structural settings, and then discuss genetic models of hydrothermalism in the Upper Sinian, the Middle-Upper Cambrian, and the Middle Permian, thereby providing a reference for the subdivision of hydrothermal dolomitization models.

Geological setting

The Sichuan Basin is located in Southwestern China (Figure 1) in which multiple sets of marine carbonate sequences developed in the middle-upper parts of the Yangtze craton during the Sinian–Paleozoic. As typical carbonate platform depositions, the Upper Sinian Dengying Formation, the Middle-Upper Cambrian, and the Middle Permian are the most important carbonate sequences, being very thick and having stable lithology. As such, we selected them as our research targets in this study. During the sedimentary and diagenetic history of the Basin, Caledonian–Himalayan multistage tectonic movements occurred, among which were two important tectonic-hydrothermal events in periods from the Late Sinian to the Early Cambrian (Z₂–∈₁) and from the Middle Devonian to the Middle Triassic (D₂–T₂) (Luo, 1981). The Xingkai taphrogenic movement from the Late Sinian to the Early Cambrian contributed to the disintegration of the ancient China platform into three plates. We noted that this ancient China platform developed continental facies and mid-acidic volcanic lithofacies in the Lower Sinian (Luo et al., 2004) and black shale rich in hydrothermal minerals in the Cambrian. The Emei taphrogenic movement from the Middle Devonian to the Middle Triassic occurred within the entire South China plate as well as in its peripheral fold belts (Luo and Fu, 2003). For example, the Emeishan basalt in the southern–western margins of the Upper Yangtze platform (covering the administrative provinces of Sichuan, Yunnan, Guizhou, and Guangxi) occupies an area of about 30 × 10⁴ km² and can be up to 2000-m thick near the fault junction. Moreover, basalts have also been found in oil and gas wells in the Sichuan Basin (e.g., Well ZG1).

Figure 1.

Simplified geological map and location of study outcrops of the Proterozoic–Paleozoic formation in the Sichuan Basin, Southwest China.

Samples and methods

For this study, we comprehensively investigated 12 Proterozoic–Paleozoic wells and outcrop profiles characterized by the development of saddle dolomites in the Sichuan Basin and its periphery (Figure 1). We collected samples from 10 profiles (Table 1) involving sequences of the Upper Sinian, the Middle-Upper Cambrian, and the Middle Permian. We prepared a total of 230 thin sections for optical characteristic analysis, 55 of which were determined to contain saddle dolomites. Next, we subjected to geochemical analysis 47 samples in which there were an abundance of saddle dolomites. We then analyzed these saddle dolomite samples both macroscopically and microscopically, using a high-expansion magnifying glass to distinguish their color, grain size, texture, and structure. We also performed cathodoluminescence (CL) analysis, electron microprobe analysis, and analyses of the rare earth elements (REEs), dolomite degrees of order, and ⁸⁷Sr/⁸⁶Sr ratios, and determined the presence of carbon and oxygen stable isotopes and fluid inclusions.

Table 1.

Sampling records in the Proterozoic–Paleozoic, Sichuan Basin.

Profile	Location	Matrix	Filling	Profile	Location	Matrix	Filling
Fandian (FD)	Leshan, Sichuan	8	36	Well Lin1 (Lin1)	Xishui, Guizhou	2	6
Yankong (YK)	Jinsha, Guizhou	11	48	Well Jinshi1 (JS1)	Qianwei, Sichuan	3	11
Songlin (SL)	Zunyi, Guizhou	1	5	Sanhuichang (SHC)	Nanchuan, Chongqing	4	40
Shiliu (SZ)	Shizhu, Chongqing	5	25	Zhangcun (ZC)	Ya’an, Sichuan	4	24
Well Dingshan1 (DS1)	Qijiang, Chongqing	3	8	Xibeixiang (XBX)	Guangyuan, Sichuan	6	27

We conducted most of the CL analysis at the Chengdu University of Technology. This analysis can identify differences between saddle dolomites of different series by analyzing the ratio of Fe²⁺ to Mn²⁺, with a rated voltage of 220 kV and exposure duration of 10 s for image acquisition.

To perform electron microprobe analysis, we used a Shimadzu electron probe microanalysis (EPMA)–1600 instrument at the Southwest Mineral Resources Supervision and Monitoring Center of China’s Ministry of Land and Resources, with an accelerating voltage of 15 kV and specimen current of 20 nA. We estimated the detection limits of the microprobe to be approximately 0.002 element%. This analysis can be combined with CL analysis by comparing the Fe²⁺/Mn²⁺ ratios determined by these two analyses to distinguish between the dolomites and their compositions.

We analyzed the REEs and degree of order to investigate the different geochemical characteristics of the different types of dolomites. Using a miniature bur, we took samples from fresh rock surfaces based on our observation of corresponding thin sections, taking 1 g for REE analysis and 5 g to analyze the degree of order. To analyze the trace REEs, we used an inductively coupled plasma mass spectrometer at the Australian Laboratory Services at a temperature of 22℃, humidity of 58%, and an analytical error of less than 10%. We analyzed the degree of order in the dolomites at the Chengdu University of Technology at a temperature of 20℃ and humidity of 58%.

We analyzed 14 dolomite samples (including six matrix dolomite samples) to determine their⁸⁷Sr/⁸⁶Sr ratios. To do so, we crushed samples of about 150 mg in a stainless agate mortarprior to chemical dissolution. We then leached the dolomite samples using 1 N HCl to remove exchangeable Sr from the clays and inter-crystalline calcite. We separated the Sr from other cations using 50 AL columns filled with Eichrome Sr–Spec resin, loaded the Sr on Re filaments in a solution of TaCl₅and H₃PO₄, and analyzed it on a MAT 262 multicollector mass spectrometer with a correction ratio of 88 Sr/86 Sr = 8.37521. We also routinely analyzed the Standard NBS 987, which yielded a mean value of 0.710254 + 12 (2σ).

We analyzed the carbon and oxygen stable isotopes and fluid inclusions to distinguish different types of dolomites and to reconstruct the diagenetic fluid properties by calculating the δ¹⁸O value based on the principle of isotope fractionation. To conduct the carbon and oxygen stable isotopic analyses, we used a MAT–252 mass spectrometer at the geological laboratory of the Exploration and Development Research Institute of the PetroChina Southwest Oil & Gasfield Company and examined 35 micro-samples (10–50 mg) cut with a dental drill from the surfaces of the dolomites (Table 2). We reacted the powdered micro-samples with 100% orthophosphoric acid at 50℃ for 3 h under vacuum to produce CO₂ gas. We determined the carbon and oxygen ratios of the equilibrated CO₂ gas on a VG 903 Micromass gas source stable isotope ratio mass spectrometer, with an analytical precision better than 0.011. We recorded the isotope data in standard notation relative to the standard PeeDee Belemnite (PDB) for carbon ratios and standard mean ocean water (SMOW) for oxygen ratios and we converted the δ¹⁸O (SMOW) values to δ¹⁸O (PDB) values. We analyzed the fluid inclusions using the British Linkam MDS 600 cooling–heating stage (equipped with Carl–Zeiss Axioplan 2 microscope) at the Chengdu University of Technology. Based on the thin-section observation results, we prepared a total of 59 fluid inclusion thin sections covering the main types of dolomites. We found the temperature test errors to vary with experimental temperature, that is, ±0.1℃, ±0.5℃, and ±1℃, respectively, corresponding to temperature ranges of less than 0℃, from 0 to 30℃, and greater than 30℃. We controlled the rate of temperature increase to within 10℃/min during the test stage and then to within 1℃/min when the critical phase transition temperature was reached, with a temperature precision and stability of 0.01℃/<0.01℃. In addition, due to the multistage diagenesis and mineral composition, we found no original fluid inclusions in any of the early filled dolomites.

Table 2.

δ¹³C and δ¹⁸O value (PDB ‰) of the Upper Proterozoic–Paleozoic saddle dolomites in the Sichuan Basin.

Sample.	Type	Location	Occurrence	Strata	Matrix	δ¹³C	δ¹⁸O
39-2	SD	ZC	DVSD	P2m	ML	4.2	−11.13
40-1	SD	ZC	BSD	P2m	ML	3.76	−10.83
41-2	SD	ZC	BSD	P2m	ML	2.91	−10.8
42-3	SD	ZC	BSD	P2m	ML	4.1	−10.95
129	SD	FD	DVSD	Є2-3x	MD	−1.74	−12.16
135-1	SD	FD	DVSD	Є2-3x	MD	−1.19	−11.5
137	SD	FD	DVSD	Є2-3x	MD	−2.04	−12.36
138-1	SD	FD	DVSD	Є2-3x	MD	−1.92	−12.03
140	SD	SHC	DVSD	Є2-3x	PD	−1.82	−12.26
173	SD	SHC	DFSD	Є2-3x	MD	−2.13	−12.64
48-3	SD	FD	DVSD	Z3dy	MD	−0.6	−13.61
75-2	SD	FD	DVSD	Z3dy	MD	−0.15	−13.44
77	SD	FD	FSD	Z3dy	MD	0.32	−13.24
52	SD	FD	FSD	Z3dy	MD	−0.09	−13.39
53	SD	FD	FSD	Z3dy	FMD	−0.26	−14.98
316-2	SD	YK	DFSD	Z3dy	MD	1.6	−12.27
258-2	SD	YK	DVSD	Z3dy	MD	4.39	−13.72
208	SD	YK	DVSD	Z3dy	MD	3.55	−12.84
262-2	SD	YK	FSD	Z3dy	MD	4.09	−13.68
286	SD	YK	DFSD	Z3dy	MD	2.56	−12.48
TZd37-1	SD	SZ	DVSD	Z3dy	MD	1.94	−12.18
SLZd1-25	SD	SL	DVSD	Z3dy	MD	1.01	−13.63
SLZd1-35	SD	SL	DFSD	Z3dy	MD	2.27	−14.05
TZd20-1	SD	SZ	DVSD	Z3dy	MD	1.67	−14.2
TZd31-1	SD	SZ	DVSD	Z3dy	MD	1.66	−14.39
DS1 10 9/44	SD	DS1	DFSD	Z3dy	MD	0.88	−14.43
TZd36-1	SD	SZ	DVSD	Z3dy	MD	1.99	−14.28

Note: SD: saddle dolomite; ML: matrix limestone; MD: matrix dolomite; FMD: finely crystalline matrix dolomite; DVSD: saddle dolomites in dissolved vugs; DFSD: saddle dolomites in dissolved fractures; FSD: saddle dolomites in fractures; BSD: brecciated saddle dolostone; ZC: Zhangcun; FD: Fandian.

Results

Petrography

The Sinian Dengying Formation is typically characterized by the hydrothermal mineral combination dolomite–quartz–pyrite (Liu et al., 2008b) occurring in the shape of a vein or stylolite. In the Fandian profile, we can see that an early dissolved fracture develops crust layers followed by saddle dolomites, whereas the hydrothermal dissolved fracture are directly filled by saddle dolomites with local zebra structures. In the Yankong and Jinsha profiles, the early dissolved fractures develop crust layers followed by extensive saddle dolomites and quartzes, whereas the directly filled saddle dolomites and quartzes are well developed in the hydrothermal dissolved fracture, with local developments of columnar calcite formed by low-temperature hydrothermal fluid, zebra structures, and the phenomenon of dolomites cutting through prehnites. In the cores of the Upper Sinian Dengying Formation from Well Jinshi 1, saddle dolomites and quartzes developed after the crust layers in the early dissolved fracture. In contrast, the hydrothermal dissolved fracture is filled by saddle dolomites of just one stage (Figure 2(a), to (d); Figure 3(a) to 3(c)). These geological hydrothermalism products can be transformed by stylolite and charged by asphalt in intercrystalline pores. In the outcrop of the Dengying Formation in the Dingshan–Lintanchang Structures, saddle dolomites coexist with minerals, such as lead zinc ore, barite, and fluorite. The above filling and coexistence characteristics are consistent with those of a hydrothermal environment (Davies and Smith, 2006), which suggests that the Upper Sinian Dengying Formation might possibly have experienced hydrothermal transformation at an early stage.

Figure 2.

Field photographs: (a) saddle dolomite fill in hydrothermal dissolved fracture, Upper Sinian Dengying Formation of Yankong profile in Jinsha; (b) zebra structure, Upper Sinian Dengying Formation of Yankong profile in Jinsha; (c) quartz and saddle dolomite developed in dissolved pores, Upper Sinian Dengying Formation of Yankong profile in Jinsha; (d) columnar calcites formed by low-temperature hydrothermal fluid coexist with saddle dolomites, Upper Sinian Dengying Formation of Yankong profile in Jinsha; (e) saddle dolomites developed in the mold pores or dissolved pores of gypsum salt, Upper Cambrian of Yankong profile in Jinsha; (f) saddle dolomites developed in dissolved fractures, Upper Cambrian of Sanhuichang profile in Nanchuan; (g) gypsum mold pores not entirely filled by saddle dolomites, Upper Cambrian of Well Jinshi 1; (h) hydrothermal dissolved pores not entirely filled by saddle dolomites, Middle Permian of Zhangcun profile in Hongya; (i) zebra structure, Middle Permian of Zhangcun profile in Hongya; (j) zebra structure, Middle Permian of Zhangcun profile in Hongya; (k) maculosus dolomites and hydrothermal fractures developed, Middle Permian Xixia Formation of Xiagou coal profile; and (l) hydrothermal asphalt fills in pores, Middle Permian Xixia Formation of Xiagou coal profile. SD: saddle dolomite; MD: matrix dolomite.

Figure 3.

Microphotographs and Cathodoluminescence microscopes in the Upper Sinian Dengying Formation, Cambrian, and Middle Permian, Sichuan Basin. (a) Bitumen immigrate in intercrystalline pores of saddle dolomites, in the Upper Sinian Dengying Formation from Well Dingshan 1; (b) saddle dolomites and authigenic quartzes precipitated in the dissolved fracture, in the Upper Sinian Dengying Formation of the Songlin profile; (c) fractures filled by saddle dolomites, in the Upper Sinian Dengying Formation from Well Dingshan 1; (d) coarse-grained saddle dolomites precipitated in the residual space of pores, in the Upper Sinian Dengying Formation from the Yankong profile; (e) coarse-grained saddle dolomites showing bright red luminescence, in the Upper Sinian Dengying Formation from the Yankong profile; (f) saddle dolomites precipitated along dissolved fractures with wavy extinction, in the Upper Cambrian of the Fandian profile in Leshan; (g) saddle dolomites with dull luminescence, which is similar to the matrix, in the Upper Cambrian of the Fandian profile in Leshan; (h) saddle dolomites precipitated along fractures, in the Middle Permian of the Zhangcun profile; and (i) saddle dolomites showing red luminescence, which obviously differs from the matrix, in the Middle Permian of the Zhangcun profile. SD: saddle dolomite; MD: matrix dolomite.

The hydrothermalism of the Middle-Upper Cambrian varies significantly between regions. In the Fandian profile of Leshan, Shawan, we can see that saddle dolomites developed in the irregular dissolved fracture in the Middle-Upper Cambrian Xixiangchi Formation. In the Yankong profile in Jiansha and Sanhuichang, Nanchuan, the Upper Cambrian (especially the upper part) developed saddle dolomites in the mold pores and dissolved pores of gypsum salt (Figure 2(e) and (f)). In the Zunyi profile, Guizhou, the Cambrian carbonate developed a lot of pyrite, millerite, and sulfur molybdenum ore, as well as relatively high quantities of exogenous hydrothermal minerals, such as ullmannite, chalcopyrite, sphalerite, barite, quartz, and gypsum, which are horizontally distributed with dolomites in the dissolved pores.

Typical hydrothermalism profiles in the Middle Permian include the Zhangcun profile in Hongya and the Xiagou coal mine profile in Xibeixiang of Guangyuan. The Permian profile of Zhangcun in Hongya is characterized by the wide development of three sets of fine-crystalline dolomite and the local development of fine-crystalline maculosus dolomites. Saddle dolomites are widely distributed in fractures and dissolved fractures (Figure 2(g) and (h)), combined with the local development of quartzes and zebra structures (Figure 2(i) and (j)). In the Xiagou coal mine profile in Xibeixiang of Guangyuan, the Permian shows the development of irregular hydrothermal fractures and schistic geodes, as well as fillings of dolomite, calcite, and asphalt (Figure 3(k) and (l)). We found reef dolomites associated with fluorite in the Permian of Duotiao in the Northern Sichuan Basin, which are considered to be hydrothermal alteration minerals (Huang et al., 2012).

Based on the above petrological and mineralogical characteristics, we see that there are widespread signs of hydrothermal petrology in the Upper Sinian Dengying Formation, the Upper Cambrian, and the Middle Permian. We note that many discrepancies still exist in these hydrothermal characteristics.

Geochemical characteristics

CL analyses

The CL images of the saddle dolomite samples show obviously different degrees of luminescence. The luminous intensity of the Dengying Formation is the strongest, followed by the Middle Permian, and then the Cambrian. The CL images of the saddle dolomites in the Upper Sinian Dengying Formation are bright red or orange and are brighter than those of the matrix. The Permian saddle dolomite CL images are red and slightly brighter than those of the matrix. The CL images of the Cambrian saddle dolomites are dull in their sector zones and are similar to those of the matrix, with a bright red rim (Figure 3).

CL intensity and color could reflect alteration in the fluid composition and mineral crystallization rate during the diagenetic stages, which are controlled by the ion contents, such as Mn²⁺, Fe²⁺, Pb²⁺, and S²⁺. Of these, the Mn²⁺ and Fe²⁺ contents and the Mn²⁺/Fe²⁺ ratio play the most important roles (Carpenter et al., 1976; Gregg and Hagni, 1987; Shu, 1993). Based on the results of our EPMA, we determined the relationship between the luminous intensity and Mn²⁺/Fe²⁺ ratio (Figure 4), which indicates that the saddle dolomites of the Upper Sinian Dengying Formation have an Fe²⁺ content of less than 1200 ppm and an Mn²⁺ content of between 70 ppm and 1000 ppm, and that these samples mainly fall in region V in the Mn²⁺/Fe²⁺ ratio and CL chart, which is consistent with the CL images. The high contents of Fe²⁺ and Mn²⁺ may indicate the source to be the deep mantle. In the same way, the red luminescence of the Permian saddle dolomites may also be related to diagenetic fluids rich in Mn²⁺. The Cambrian saddle dolomites show a luminescence similar to the matrix, possibly with an Mn²⁺ content of less than 1% or Fe²⁺ content larger than 6%. In other words, discrepancies in their luminescence characteristics may suggest that these three sets of saddle dolomites derive from different diagenetic fluids, which also show a diversity of Paleozoic hydrothermal fluid sources (Figure 4).

Figure 4.

Mn²⁺/Fe²⁺ ratio and cathode luminescence chart of saddle dolomites in the Upper Sinian Dengying Formation in the Sichuan Basin (data plotted in the chart modified from Huang, 1992).

REE compositions and degree of order

Vulcanian eruption during the Hercynian was one of the strongest hydrothermalism events in the Sichuan Basin, with numerous asthenoliths occurring in the Upper Permian within and around the Basin. It is commonly thought that both the Sinian and Middle Permian hydrothermal fluids were associated products of hydrothermal exhalation during the Hercynian. To clarify this genetic relationship, we compared the REEs and their distribution patterns with saddle dolomite samples of the Upper Sinian Dengying Formation and Middle Permian, as well as with the magmatic rocks of the Upper Permian (Figure 5(a)). The results show that despite the Middle Permian sequences being closer to the overlying Permian Emeishan basalt, the degree of rare earth enrichment of the saddle dolomites is far less than that of the Permian igneous rock samples, and also less than that of the Dengying Formation saddle dolomites.

Figure 5.

Geochemical characteristics of the Upper Proterozoic–Paleozoic saddle dolomites in the Sichuan Basin: (a) distribution of rare earth elements (the comparison between Paleozoic dolomites and the Hercynian basalt); (b) distribution of rare earth elements (the comparison of saddle dolomites in different phases); (c) the order degree of the Proterozoic–Paleozoic saddle dolomites; (d) the plot of strontium isotope (Permian); (e) the plot of strontium isotope (Upper Sinian); (f) the comparison of strontium isotopes between saddle dolomites and seawater.

I and II are non-luminous zones, III is a weakly luminous zone, IV is a middle luminous zone, V is a strongly luminous zone, 1 is a primary micrite matrix, 2 is a powder-crystal dolomite matrix, 3 is a saddle dolomite (probe data), and 4 is a saddle dolomite (microscale data from Jiang et al., 2016)

By comparing these three sets of saddle dolomites, we determined the degrees of rare earth enrichment in the Middle-Upper Cambrian. We found that saddle dolomites with a high degree of rare earth enrichment are more similar to those found in the Upper Sinian Dengying Formation and those with a low degree of rare earth enrichment are more similar to those of the Middle Permian (Figure 5(b)). The saddle dolomites of the Middle-Upper Cambrian and Middle Permian are both characterized by weak negative anomalies of Ce and Eu, whereas those of the Upper Sinian Dengying Formation show weak negative anomalies of Ce and no positive anomalies of Eu. Taking hydrothermal diagenesis as an example, there are multiple potential explanations for the Ce and Eu anomalies in sedimentary rocks.

Discrepancies in the degrees of order are obvious: the degree of order of the Upper Sinian Dengying Formation is relatively concentrated and high, with a range of 0.72–1 and an average of 0.87. That of the Middle-Upper Cambrian has a larger range (0.39–0.89) and that of most of the Middle Permian samples is less than 0.71 (Figure 5(c)). These discrepancies indicate that the crystallinity of saddle dolomites in the Upper Sinian Dengying Formation is higher than that of the other two. Moreover, its more complete crystallization may also reveal a discrepancy regarding the hydrothermal fluid sources.

Carbon, oxygen, and strontium isotopes

The δ¹⁸O values of the saddle dolomites in the Dengying Formation of the Upper Sinian, Upper Cambrian, and Middle Permian are lower than those of the matrix (You et al., 2013, 2014, 2015). Three kinds of saddle dolomite have relatively narrow δ¹⁸O value ranges, but these ranges differ significantly. The δ¹⁸O values of the Upper Sinian Dengying Formation range from −12.48 to −14.43‰ PDB, with an average of −13.53‰ PDB and that of the Middle Permian ranges between −10.80 and −11.13‰ PDB, with an average of −10.92‰ PDB (Table 2). The δ¹⁸O value range of the Cambrian saddle dolomite is between those of the above two periods, −12.64 and −11.5‰ PDB.

Differences in the δ¹³C values are also obvious (Table 2). The Upper Cambrian saddle dolomites are generally characterized by a negative carbon isotope excursion (−1.19 to −2.13‰ PDB), the δ¹³C values of saddle dolomites in the Upper Sinian Dengying Formation show a diversity of profiles (−0.6 to 4.39‰ PDB), and the δ¹³C values of the Middle Permian saddle dolomites are relatively high (2.91–4.20‰ PDB).

Saddle dolomites of the Upper Sinian Dengying Formation and the Middle Permian present a higher ⁸⁷Sr/⁸⁶Sr ratio than synchronous values in seawater. The ⁸⁷Sr/⁸⁶Sr ratios of the bioclast limestone matrix and micrite dolomite in the Middle Permian, which vary from 0.707512 to 0.707536, are in the variation range of synchronous marine carbonates (0.70662–0.70774). Saddle dolomites filling in the fracture cavity of bioclast limestone have ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.709286 to 0.709453, which is larger than the ⁸⁷Sr/⁸⁶Sr variation range of synchronous Maokou Formation marine limestone. Similarly, the ⁸⁷Sr/⁸⁶Sr variation ranges of microcrystalline–micrite algal dolomites in the Upper Sinian Dengying Formation (0.7091–0.7098) fall in the ⁸⁷Sr/⁸⁶Sr variation range of synchronous marine carbonates. However, the ⁸⁷Sr/⁸⁶Sr ratio of saddle dolomites filling in the fracture cavity ranges between 0.7120–0.7132, which is much larger than that of synchronous seawater.

Fluid inclusion petrography and microthermometry

Fluid inclusions in saddle dolomites have two phases (liquid + vapor), with vapor accounting for about 10–15% of the total inclusion volume. Measured along the longest side, the inclusions vary in size from about 7 µm to 30 µm, and the primary fluid inclusion size ranges from 10 to 18 µm. Ultraviolet-light observations reveal the presence of oil-filled fluid inclusions 8–21 µm in size.

The fluid inclusions in dolomite are both primary and secondary in origin. Primary fluid inclusions mostly show a striped distribution along the direction of crystal growth and a regular shape. Secondary inclusions often appear in isolation, vary in size, and are irregular in shape, so they are appropriate to include in statistical form.

Table 3 shows the ranges of homogenization temperatures and fluid salinities we obtained for carbonate cements. Microthermometric measurements of zoned dolomite crystals reveal that the temperature of the fluid inclusions in the tested saddle dolomites of different formations is higher than the normal burial temperature and the homogenization temperatures of replacive dolomites in the host dolostones reveal a hydrothermal cause.

Table 3.

Fluid inclusion homogenization temperature of the Upper Proterozoic–Paleozoic saddle dolomites in the Sichuan Basin.

Sample	Strata	Location	Occurrence	FI type	Shapes	Size (mm)	G/L (%)	Phase	Th aq (℃)	Sal (wt%)
E-1	Z3dy	YK	DVSD	Saline	Regular	8 × 16	≤5%	Liquid	253.0	15.0
E-2	Z3dy	YK	DVSD	Saline	Regular	8 × 21	≤5%	Liquid	257.5	18.8
E-3	Z3dy	YK	DVSD	Saline	Regular	9 × 15	≤5%	Liquid	258.4	15.2
E-4	Z3dy	YK	DVSD	Saline	Regular	11 × 20	≤5%	Liquid	260.2	15.5
E-5	Z3dy	YK	DVSD	Saline	Regular	8 × 10	≤5%	Liquid	248.0	16.0
E-6	Z3dy	YK	DVSD	Saline	Regular	5 × 15	≤5%	Liquid	244.0	16.2
E-7	Z3dy	YK	DVSD	Saline	Regular	9 × 16	≤5%	Liquid	248.2	16.1
E-8	Z3dy	YK	DVSD	Saline	Regular	8 × 18	≤5%	Liquid	260.1	16.1
E-9	Z3dy	YK	DVSD	Saline	Regular	7 × 16	≤5%	Liquid	250.0	15.0
E-10	Z3dy	YK	DVSD	Saline	Regular	6 × 22	≤5%	Liquid	247.0	16.1
E-11	Z3dy	YK	DVSD	Saline	Regular	15 × 30	≤5%	Liquid	251.0	15.5
E-12	Z3dy	YK	DVSD	Saline	Regular	8 × 18	≤5%	Liquid	260.0	15.8
E-13	Z3dy	FD	DVSD	Saline	Regular	12 × 22	≤5%	Liquid	257.6	15.0
E-14	Z3dy	FD	DVSD	Saline	Regular	6 × 8	≤5%	Liquid	250.3	15.0
E-15	Z3dy	FD	DVSD	Saline	Regular	8 × 18	≤5%	Liquid	255.0	16.1
E-16	Z3dy	FD	DFSD	Saline	Regular	7 × 10	≤5%	Liquid	258.7	15.0
E-17	Z3dy	FD	DFSD	Saline	Regular	8 × 20	≤5%	Liquid	261.0	15.0
E-18	Z3dy	FD	FSD	Saline	Regular	6 × 15	≤5%	Liquid	258.0	15.0
E-19	Z3dy	FD	FSD	Saline	Regular	11 × 16	≤5%	Liquid	258.2	15.5
E-20	Z3dy	FD	FSD	Saline	Regular	8 × 16	≤5%	Liquid	257.8	15.6
E-21	Z3dy	FD	FSD	Saline	Regular	7 × 13	≤5%	Liquid	258.6	15.9
E-22	Z3dy	FD	FSD	Saline	Regular	5 × 15	≤5%	Liquid	257.9	15.4
E-23	Z3dy	FD	FSD	Saline	Regular	10 × 16	≤5%	Liquid	258.9	15.9
E-24	Z3dy	FD	FSD	Saline	Regular	8 × 10	≤5%	Liquid	257.8	15.6
E-25	Z3dy	FD	FSD	Saline	Regular	11 × 20	≤5%	Liquid	258.8	15.8
E-26	Z3dy	FD	FSD	Saline	Regular	6 × 22	≤5%	Liquid	258.3	16.0
E-27	Z3dy	FD	FSD	Saline	Regular	12 × 25	≤5%	Liquid	257.0	16.0
E-28	Z3dy	JS1	FSD	Saline	Regular	8 × 19	≤5%	Liquid	250.5	15.3
E-29	Z3dy	JS1	FSD	Saline	Regular	10 × 21	≤5%	Liquid	251.2	15.7
E-30	Z3dy	JS1	FSD	Saline	Regular	10 × 18	≤5%	Liquid	247.7	15.2
E-31	Z3dy	JS1	DFSD	Saline	Regular	6 × 8	≤5%	Liquid	249.0	15.7
C-1	Є2-3x	SHC	FSD	Saline	Regular	9 × 13	≤5%	Liquid	100.5	Null
C-2	Є2-3x	SHC	FSD	Hydrocarbon	Regular	10 × 19	≤5%	Liquid	214.6	Null
C-3	Є2-3x	SHC	FSD	Hydrocarbon	Regular	5 × 10	≤5%	Liquid	213.0	Null
C-4	Є2-3x	SHC	FSD	Saline	Regular	5 × 12	≤5%	Liquid	129.9	Null
C-5	Є2-3x	SHC	FSD	Saline	Regular	9 × 16	≤5%	Liquid	126.0	Null
C-6	Є2-3x	SHC	FSD	Hydrocarbon	Regular	8 × 15	≤5%	Liquid	201.5	Null
C-7	Є2-3x	SHC	FSD	Saline	Regular	4 × 18	≤5%	Liquid	124.7	Null
C-8	Є2-3x	SHC	DVSD	Saline	Regular	6 × 20	≤5%	liquid	120.5	Null
C-9	Є2-3x	SHC	DVSD	Saline	Regular	15 × 22	≤5%	Liquid	110.2	Null
C-10	Є2-3x	SHC	DVSD	Saline	Regular	8 × 18	≤5%	Liquid	150.4	Null
C-11	Є2-3x	JS1	DVSD	Saline	Regular	12 × 22	≤5%	Liquid	131.5	Null
C-12	Є2-3x	JS1	DVSD	Saline	Regular	6 × 8	≤5%	Liquid	130.5	Null
C-13	Є2-3x	JS1	DVSD	Saline	Regular	8 × 18	≤5%	Liquid	115.2	Null
C-14	Є2-3x	JS1	DVSD	Saline	Regular	7 × 10	≤5%	Liquid	121.5	Null
C-15	Є2-3x	JS1	DVSD	Saline	Regular	8 × 20	≤5%	Liquid	121.6	Null
C-16	Є2-3x	JS1	DVSD	Saline	Regular	7 × 18	≤5%	Liquid	111.2	Null
C-17	Є2-3x	JS1	DVSD	Saline	Regular	10 × 16	≤5%	Liquid	112.6	Null
C-18	Є2-3x	JS1	DVSD	Saline	Regular	9 × 17	≤5%	Liquid	112.5	Null
C-19	Є2-3x	JS1	DVSD	Saline	Regular	6 × 20	≤5%	Liquid	110.6	Null
C-20	Є2-3x	JS1	DVSD	Saline	Regular	14 × 20	≤5%	Liquid	126.3	Null
C-21	Є2-3x	JS1	DVSD	Saline	Regular	17 × 22	≤5%	Liquid	128.0	Null
C-22	Є2-3x	JS1	DVSD	Saline	Regular	12 × 18	≤5%	Liquid	127.5	Null
C-23	Є2-3x	JS1	DVSD	Saline	Regular	12 × 19	≤5%	Liquid	124.5	Null
C-24	Є2-3x	JS1	DVSD	Saline	Regular	8 × 10	≤5%	Liquid	122.2	Null
C-25	Є2-3x	JS1	DVSD	Saline	Regular	7 × 8	≤5%	Liquid	129.3	Null
C-26	Є2-3x	FD	DVSD	Saline	Regular	10 × 8	≤5%	Liquid	133.1	Null
C-27	Є2-3x	FD	DVSD	Saline	Regular	6 × 8	≤5%	Liquid	130.5	Null
C-28	Є2-3x	FD	DVSD	Saline	Regular	5 × 18	≤5%	Liquid	131.6	Null
C-29	Є2-4x	FD	DVSD	Saline	Regular	10 × 20	≤5%	Liquid	111.5	Null
C-30	Є2-5x	FD	DVSD	Saline	Regular	4 × 15	≤5%	Liquid	115.4	Null
C-31	Є2-6x	FD	DVSD	Saline	Regular	6 × 12	≤5%	Liquid	115.0	Null
C-32	Є2-7x	FD	DVSD	Saline	Regular	4 × 10	≤5%	Liquid	112.6	Null
C-33	Є2-8x	FD	DVSD	Saline	Regular	10 × 23	≤5%	Liquid	140.0	Null
C-34	Є2-9x	FD	DVSD	Saline	Regular	12 × 20	≤5%	Liquid	146.5	Null
C-35	Є2-3x	FD	DVSD	Saline	Regular	8 × 10	≤5%	Liquid	122.0	Null
C-36	Є2-3x	FD	DVSD	Saline	Regular	10 × 16	≤5%	Liquid	122.1	Null
C-37	Є2-3x	FD	DVSD	Saline	Regular	9 × 20	≤5%	Liquid	125.3	Null
C-38	Є2-3x	FD	DVSD	Saline	Regular	8 × 18	≤5%	Liquid	126.0	Null
C-39	Є2-3x	FD	DVSD	Saline	Regular	10 × 15	≤5%	Liquid	114.0	Null
C-40	Є2-3x	FD	DVSD	Saline	Regular	6 × 12	≤5%	Liquid	116.5	Null
C-41	Є2-3x	FD	DVSD	Saline	Regular	10 × 21	≤5%	Liquid	111.2	Null
C-42	Є2-3x	FD	DVSD	Saline	Regular	5 × 19	≤5%	Liquid	113.6	Null
C-43	Є2-3x	FD	DVSD	Saline	Regular	4 × 16	≤5%	Liquid	115.5	Null
C-44	Є2-3x	FD	DVSD	Saline	Regular	6 × 9	≤5%	Liquid	100.4	Null
C-45	Є2-3x	FD	DVSD	Saline	Regular	11 × 18	≤5%	Liquid	99.0	Null
C-46	Є2-3x	FD	DVSD	Saline	Regular	4 × 9	≤5%	Liquid	101.0	Null
C-47	Є2-3x	FD	DVSD	Saline	Regular	4 × 9	≤5%	Liquid	114.0	Null
C-48	Є2-3x	FD	DVSD	Saline	Regular	5 × 10	≤5%	Liquid	114.6	Null
C-49	Є2-3x	YK	DVSD	Saline	Regular	4 × 18	≤5%	Liquid	135.6	Null
C-50	Є2-3x	YK	DVSD	Saline	Regular	4 × 18	≤5%	Liquid	132.0	Null
C-51	Є2-3x	YK	DVSD	Saline	Regular	15 × 19	≤5%	Liquid	139.4	Null
C-52	Є2-3x	YK	DVSD	Saline	Regular	15 × 10	≤5%	Liquid	127.0	Null
C-53	Є2-3x	YK	DVSD	Saline	Regular	20 × 24	≤5%	Liquid	92.6	Null
C-54	Є2-3x	YK	DVSD	Saline	Regular	12 × 18	≤5%	Liquid	92.8	Null
C-55	Є2-3x	YK	DVSD	Saline	Regular	13 × 25	≤5%	Liquid	122.0	Null
C-56	Є2-3x	YK	DVSD	Saline	Regular	8 × 12	≤5%	Liquid	111.5	Null
C-57	Є2-3x	YK	DVSD	Saline	Regular	9 × 14	≤5%	Liquid	110.0	Null
C-58	Є2-3x	YK	DVSD	Saline	Regular	4 × 9	≤5%	Liquid	110.5	Null
P-1	P2m	ZC	BSD	Saline	Regular	3 × 10	≤5%	Liquid	181.0	Null
P-2	P2m	ZC	BSD	Saline	Regular	5 × 8	≤5%	Liquid	184.0	6.6
P-3	P2m	ZC	BSD	Saline	Regular	6 × 22	≤5%	Liquid	150.6	Null
P-4	P2m	ZC	BSD	Saline	Regular	8 × 9	≤5%	Liquid	124.0	7.8
P-5	P2m	ZC	BSD	Saline	Regular	4 × 13	≤5%	Liquid	147.6	11.2
P-6	P2m	ZC	BSD	Saline	Regular	19 × 25	≤5%	Liquid	157.0	7.1
P-7	P2m	ZC	BSD	Saline	Regular	19 × 22	≤5%	Liquid	125.6	9.9
P-8	P2m	ZC	BSD	Saline	Regular	20 × 24	≤5%	Liquid	185.5	8.6
P-9	P2m	ZC	BSD	Saline	Regular	4 × 7	≤5%	Liquid	162.2	6.9
P-10	P2m	ZC	BSD	Saline	Regular	4 × 7	≤5%	Liquid	163.0	8.2
P-11	P2m	ZC	BSD	Saline	Regular	5 × 9	≤5%	Liquid	128.0	Null
P-12	P2m	ZC	BSD	Saline	Regular	3 × 10	≤5%	Liquid	89.0	Null
P-13	P2m	ZC	BSD	Saline	Regular	9 × 13	≤5%	Liquid	125.5	Null
P-14	P2m	ZC	BSD	Saline	Regular	12 × 12	≤5%	Liquid	121.4	8.2
P-15	P2m	ZC	BSD	Saline	Regular	4 × 9	≤5%	Liquid	136.0	12.5
P-16	P2m	ZC	BSD	Saline	Regular	5 × 9	≤5%	Liquid	144.0	7.6
P-17	P2m	ZC	BSD	Hydrocarbon	Regular	4 × 10	≤5%	Liquid	200.0	6.9
P-18	P2m	ZC	BSD	Saline	Regular	4 × 12	≤5%	Liquid	148.8	8.2
P-19	P2m	ZC	BSD	Saline	Regular	6 × 11	≤5%	Liquid	135.5	5.7
P-20	P2m	ZC	BSD	Saline	Regular	5 × 15	≤5%	Liquid	132.0	7.7
P-21	P2m	ZC	BSD	Hydrocarbon	Regular	5 × 10	≤5%	Liquid	186.0	11.0
P-22	P2m	ZC	BSD	Saline	Regular	4 × 11	≤5%	Liquid	130.2	7.5
P-23	P2m	ZC	BSD	Saline	Regular	10 × 15	≤5%	Liquid	136.5	9.5
P-24	P2m	ZC	BSD	Saline	Regular	20 × 24	≤5%	Liquid	110.8	8.4
P-25	P2m	ZC	BSD	Hydrocarbon	Regular	12 × 18	≤5%	Liquid	180.0	7.2
P-26	P2m	ZC	BSD	Saline	Regular	13 × 25	≤5%	Liquid	120.0	8.3
P-27	P2m	ZC	BSD	Hydrocarbon	Regular	11 × 17	≤5%	Liquid	200.5	Null
P-28	P2m	ZC	BSD	Saline	Regular	12 × 17	≤5%	Liquid	147.5	Null
P-29	P2m	ZC	BSD	Saline	Regular	8 × 11	≤5%	Liquid	132.0	10.1
P-30	P2m	ZC	BSD	Saline	Regular	14 × 21	≤5%	Liquid	143.0	11.0
P-31	P2m	ZC	BSD	Saline	Regular	8 × 13	≤5%	Liquid	160.0	Null
P-32	P2m	ZC	BSD	Saline	Regular	4 × 7	≤5%	Liquid	148.5	8.3
P-33	P2m	ZC	BSD	Hydrocarbon	Regular	5 × 8	≤5%	Liquid	162.2	9.5
P-34	P2m	ZC	BSD	Hydrocarbon	Regular	4 × 15	≤5%	Liquid	167.0	8.0
P-35	P2m	ZC	BSD	Hydrocarbon	Regular	8 × 14	≤5%	Liquid	161.0	7.2
P-36	P2m	ZC	BSD	Saline	Regular	17 × 21	≤5%	Liquid	133.5	8.3
P-37	P2m	ZC	BSD	Saline	Regular	17 × 12	≤5%	Liquid	132.0	Null
P-38	P2m	ZC	BSD	Saline	Regular	23 × 27	≤5%	Liquid	136.2	7.8
P-39	P2m	ZC	BSD	Saline	Regular	16 × 22	≤5%	Liquid	131.0	11.1
P-40	P2m	ZC	BSD	Saline	Regular	11 × 23	≤5%	Liquid	95.5	7.2
P-41	P2m	ZC	BSD	Saline	Regular	6 × 10	≤5%	Liquid	95.9	9.8
P-42	P2m	ZC	BSD	Saline	Regular	6 × 11	≤5%	Liquid	92.0	8.5
P-43	P2m	ZC	BSD	Saline	Regular	6 × 8	≤5%	Liquid	124.0	7.0
P-44	P2m	ZC	BSD	Saline	Regular	5 × 12	≤5%	Liquid	117.0	8.2
P-45	P2m	ZC	BSD	Saline	Regular	7 × 10	≤5%	Liquid	129.0	Null
P-46	P2m	ZC	BSD	Saline	Regular	9 × 25	≤5%	Liquid	141.0	Null
P-47	P2m	ZC	BSD	Hydrocarbon	Regular	12 × 13	≤5%	Liquid	205.0	10.6
P-48	P2m	ZC	BSD	Saline	Regular	4 × 11	≤5%	Liquid	145.8	11.0
P-49	P2m	XBX	BSD	Saline	Regular	17 × 23	≤5%	Liquid	152.5	Null
P-50	P2m	XBX	BSD	Saline	Regular	16 × 19	≤5%	Liquid	119.0	8.2
P-51	P2m	XBX	BSD	Hydrocarbon	Regular	16 × 20	≤5%	Liquid	179.0	11.2
P-52	P2m	XBX	BSD	Saline	Regular	5 × 7	≤5%	Liquid	136.2	6.5
P-53	P2m	XBX	BSD	Saline	Regular	4 × 9	≤5%	Liquid	155.0	6.5
P-54	P2m	XBX	BSD	Saline	Regular	5 × 15	≤5%	Liquid	107.8	9.7
P-55	P2m	XBX	BSD	Hydrocarbon	Regular	5 × 12	≤5%	Liquid	182.0	Null
P-56	P2m	XBX	BSD	Saline	Regular	6 × 10	≤5%	Liquid	116.0	7.7
P-57	P2m	XBX	BSD	Saline	Regular	13 × 11	≤5%	Liquid	153.5	11.0
P-58	P2m	XBX	BSD	Hydrocarbon	Regular	13 × 17	≤5%	Liquid	161.5	7.3
P-59	P2m	XBX	BSD	Saline	Regular	8 × 11	≤5%	Liquid	125.6	9.6
P-60	P2m	XBX	BSD	Saline	Regular	13 × 20	≤5%	Liquid	147.0	8.4
P-61	P2m	XBX	BSD	Hydrocarbon	Regular	7 × 12	≤5%	Liquid	162.0	7.1
P-62	P2m	XBX	BSD	Saline	Regular	8 × 13	≤5%	Liquid	155.8	8.3
P-63	P2m	XBX	BSD	Saline	Regular	4 × 8	≤5%	Liquid	155.2	4.8
P-64	P2m	XBX	BSD	Hydrocarbon	Regular	5 × 16	≤5%	Liquid	173.0	Null
P-65	P2m	XBX	BSD	Hydrocarbon	Regular	5 × 15	≤5%	Liquid	184.0	10.7
P-66	P2m	XBX	BSD	Saline	Regular	11 × 15	≤5%	Liquid	133.2	11.0
P-67	P2m	XBX	BSD	Saline	Regular	17 × 12	≤5%	Liquid	157.0	Null
P-68	P2m	XBX	BSD	Saline	Regular	22 × 26	≤5%	Liquid	103.8	8.3
P-69	P2m	XBX	BSD	Hydrocarbon	Regular	15 × 21	≤5%	Liquid	175.0	11.5
P-70	P2m	XBX	BSD	Saline	Regular	17 × 29	≤5%	Liquid	122.0	8.4
P-71	P2m	XBX	BSD	Hydrocarbon	Regular	6 × 10	≤5%	Liquid	166.0	6.5
P-72	P2m	XBX	BSD	Saline	Regular	7 × 12	≤5%	Liquid	120.0	8.7
P-73	P2m	XBX	BSD	Saline	Regular	6 × 8	≤5%	Liquid	134.0	10.2
P-74	P2m	XBX	BSD	Saline	Regular	4 × 11	≤5%	Liquid	157.1	8.5
P-75	P2m	XBX	BSD	Saline	Regular	4 × 15	≤5%	Liquid	149.5	7.0
P-76	P2m	XBX	BSD	Saline	Regular	6 × 20	≤5%	Liquid	149.7	8.2

Note: Th aq: homogenization temperature of aqueous inclusions; Tm aq: ice melting temperature of aqueous inclusions; Sal (wt%): salinity computed from a NaCle–H2O system; FI type: fluid inclusions type; DVSD: saddle dolomites in dissolved vugs; DFSD: saddle dolomites in dissolved fractures; FSD: saddle dolomites in fractures; BSD: brecciated saddle dolostone; JS1: Well Jinshi1; SHC: Sanhuichang; XBX: Xibeixiang; ZC: Zhangcun; YK: Yankong; FD: Fandian.

Primary fluid inclusions in the saddle dolomite of the Sinian Dengying Formation have a narrow homogenization temperature range of 244–261℃, with an average of 254.8℃. Primary fluid inclusions in Cambrian saddle dolomite have a large temperature range of 92.6–150.4℃ and an average of 119.8℃. The maximum temperature range of the primary fluid inclusions belongs to the Middle Permian, 89–185℃, with the main peak ranging from 120 to 160℃. Neglecting fluid test error, the above test results reflect differences in the three filling sources. Also, the fluid of saddle dolomites in the Dengying Formation is characterized by a single component, stable temperature, and short migration time.

In contrast, the diagenesis of the other two kinds of saddle dolomite may be due to the mixing of multisource fluids, which led to the expansion of the fluid inclusions temperature range. We found hydrocarbon inclusions in the three sets of saddle dolomites, with that in the Cambrian Dengying Formation containing gaseous hydrocarbons with yellow–green fluorescence and a temperature range of 125–220℃. The temperature ranges of organic inclusions in the Cambrian and Permian are 201.5–214.6℃ and 161–205℃, respectively, which reflect the influence of multistage organic hydrocarbon immigration.

Saddle dolomites show significant differences in their salinity ranges. The Dengying Formation of Sinian shows a high salinity range of 15–18.8. The salinity range of the Middle Permian is 4.8–12.5, with an average of 8.52. The above figures comprehensively reflect the different origins of the three sets of saddle dolomite.

Distribution

The distributions of the three sets of saddle dolomites obviously differ (Figure 6). The Upper Sinian saddle dolomites are exposed throughout the entire sequence, are widely distributed in drilled wells and outcrops in the whole region and occur in the large dissolved vugs or fractures. The Permian saddle dolomites are typically exposed in the upper part of the Maokou Formation and are distributed among breccias or even along microcracks near large regional faults. The Middle-Upper Cambrian saddle dolomites are found in the Xixiangchi Formation and are widely distributed in dissolved fractures, with moldic pore bodies and dissolved vugs related to gypsum salt. The different occurrences of these saddle dolomites may suggest various hydrothermal alterations.

Figure 6.

Distribution of saddle dolomites in the Upper Proterozoic–Paleozoic Formation.

Discussion

Conditions of dolomitization and dolomite cementation

The δ¹⁸O–δ¹³C values of all the three sets of saddle dolomites fall within the region of fluidization and low-temperature alteration, but their distribution regions differ from that of the microcrystalline dolomite matrix (Figure 7). The δ¹⁸O value can be influenced by many factors. With respect to the matrix, it may be a comprehensive reflection of environment and latitude. The cement in the fracture cavity may reveal fluid characteristics that promote cement alteration. The δ¹⁸O values of saddle dolomites in the Upper Sinian Dengying Formation and the Upper Cambrian feature higher negative oxygen excursions than those of the Middle Permian saddle dolomites. The high homogenization temperature of the fluid inclusions indicates a less likely contribution of atmospheric water to the extremely low δ¹⁸O value of saddle dolomites. Through comparison, we found temperature to be closely related to the oxygen isotope. A higher negative oxygen isotope excursion generally corresponds to a lower homogenization temperature of the saddle dolomite inclusions, which is true in the Middle Permain and the Upper-Middle Cambrian (Figure 7), with respective inclusion peak temperatures of 120–160℃ and 110–130℃ (Table 3). Similarly, a lower negative oxygen isotope excursion corresponds to a higher homogenization temperature of the saddle dolomite inclusions in the Upper Sinian Dengying Formation (Figure 7), with inclusion peak temperatures of 200–230℃ (Table 3). Meanwhile, our comparison of the homogenization temperature of inclusions and the burial history of the host rock (evidences from Wells Kuang 2 and Lin 1) indicates that the homogenization temperature of saddle dolomite inclusions exceeds the local burial history temperature, which suggests that hydrothermal alteration may be the main factor influencing the diageneisis of the three sets of saddle dolomites.

Figure 7.

Carbon and oxygen stable isotopes and fluid inclusion homogenization temperatures of the Upper Proterozoic–Paleozoicsaddle dolomites.

Differences in the δ¹³C values are also obvious (Table 2). The Upper Cambrian saddle dolomites are generally characterized by negative carbon isotope excursions (−1.19 to −2.13‰ PDB); the δ¹³C values of saddle dolomites in the Upper Sinian Dengying Formation show a diversity of profiles (−0.6–4.39‰ PDB); and the δ¹³C values of the Middle Permian saddle dolomites are relatively high (2.91–4.20‰ PDB). We note that the δ¹³C values of saddle dolomites in the Upper Sinian Dengying Formation and the Middle Permian are positive and under biological control, as proved by the micrometer content of δ¹³C values larger than 0 in the marine environment (Huang, 2010). Also, the extremely low δ¹³C values of saddle dolomites in the Upper Cambrian may reflect a diagenetic fluid alteration related to higher terrestrial organic matter, which makes no difference to the Middle Permian and Upper Sinian sequences.

The anomaly of the high ⁸⁷Sr/⁸⁶Sr ratio indicates a contamination of radioactive Sr during the formation process. We attribute the enrichment of the Sr isotope in the hydrothermal dolomites to two mechanisms, with the first being siliciclastic sequences rich in mud or feldspar and the other originating from the basement (Shi et al., 2013). As seen in the local sedimentary record, the absence of siliciclastic deposits in the underlayer of the Middle Permian and Upper Sinian indicates that the source of the Sr isotope in the hydrothermal fluids may be the basement of the Sichuan Basin (Xie et al., 2009). Therefore, saddle dolomite cement may be formed by the influence of hydrothermal fluids. Moreover, the ⁸⁷Sr/⁸⁶Sr ratio of saddle dolomites in the Upper Sinian Dengying Formation is much higher than that in the Middle Permian, with the Δ⁸⁷Sr/⁸⁶Sr difference ranging from 0.0027 to 0.0037. This suggests that although ore-forming fluids come from brine in the deep basement, greater contact between the Upper Sinian hydrothermal fluids and the basement would also have contributed to the higher ⁸⁷Sr/⁸⁶Sr ratio than that of the Middle Permian (Figure 5(d)).

At the same time, the difference in the REEs shows that the rare earth enrichment of saddle dolomites in the Middle Permian and Sinian is much higher than that in the Cambrian, which indicates that the diagenetic fluid of saddle dolomites in the Cambrian differs from that of the former two, which also have a lower REE content. Our test results show that the rare earth enrichment in the Sinian saddle dolomites is higher than that of the Middle Permian. It is generally believed that the formation that the Middle Permian saddle dolomite covered by the Permian Emeishan basalt is closely related to the hydrothermal activity in the Hercynian period, and that the degree of mineralization of the diagenetic fluid of the Sinian saddle dolomite is higher than that of the hydrothermal solution, thereby indicating that they may come from different hydrothermal forms. On the other hand, the Permian saddle dolomite has a higher degree of abnormality with respect to Ce and Eu than the Sinian saddle dolomite, which is a good indication of its hydrothermal form. One explanation for this phenomenon argues that the hydrothermalism environments during the Middle-Upper Cambrian and the Permian might have been semi-open, whereas it was closed and reduced during the Sinian period. Some scholars have proposed that the sources of these three hydrothermal fluids may differ.

Fluid reconstruction

Oxygen isotopic fractionation, which is related to carbonate diagenesis, involves an isotope exchange reaction and isotope fractionation from evaporation (Huang, 2010). The isotope exchange reaction is a theoretical concept used to study diagenetic temperature based on the oxygen isotopic composition, which is determined by the good correlation between the δ¹⁸O value of saddle dolomites filling in the fracture cavity and the temperature referenced in the previous sentence. Therefore, we can calculate the coefficient of the oxygen isotope fractionation based on its relational expression using the temperature in the water system established by Land (1967) and Northrop (1966), and then invert the δ¹⁸O value of the primary diagenetic fluids based on the relation between the mineral δ¹⁸O value and the fractionation coefficient.

We compared the fluid inversion results with the synchronous seawater δ¹⁸O value obtained by previous authors (Cheng et al., 2013; Huang et al., 1999) (Figure 8), which indicate that the δ¹⁸O values of the diagenetic fluids of the Upper Sinian and the Middle Permian saddle dolomites are larger than those of the synchronous seawater, as well as those of the current atmospheric water (<−2‰ SMOW) (Faure, 1977; Hillgartner and Strasser, 2003). Calculated from the δ¹⁸O value of the Middle-Upper Cambrian saddle dolomites (−11.50 to −12.64‰ PDB, averaging −12.16‰ PDB), the δ¹⁸O value of the diagenetic fluids of the precipitating dolomites ranges between −1.50 and 2.75‰ SMOW. Although there is no δ¹⁸O record for seawater during the Middle-Upper Cambrian, this result is still close to the empirical δ¹⁸O value of seawater (Jacobsen and Kaufmanm 1999). The δ¹⁸O values of the hydrothermal fluids of the three sets of saddle dolomites also show significant differences, with the δ¹⁸O values of the saddle dolomite diagenetic fluids in large to small order as follows: the Upper Sinian (5.47–7.82‰ SMOW), the Middle Permian (2.09–5.79‰ SMOW), and the Middle-Upper Cambrian (−0.74–2.40‰ SMOW).

Figure 8.

Plot of the inclusion homogenization temperature (Th) vs. the oxygen isotope value of the Upper Proterozoic–Paleozoic saddle dolomite in the Sichuan Basin. The contour lines represent the water isotope composition in an equilibrium state with dolomites: (a) comparison of the saddle dolomites of the Upper Sinian Dengying Formation and the synchronous seawater and (b) comparison of the Middle Permian saddle dolomites and the synchronous seawater.

Next, we substituted the obtained δ¹⁸O value of the saddle dolomite diagenetic fluids into the oxygen isotope empirical chart, and results are as follows (Figure 9).

Figure 9.

δ¹⁸O isotope characteristics of different fluids in the Upper Proterozoic–Paleozoic saddle dolomites in the Sichuan Basin (Collected data plotted in the chart modified from Le, 1984 and Huang, 2010). SMOW: standard mean ocean water.

Diagenetic fluids of saddle dolomites in the Upper Sinian Dengying Formation

The δ¹⁸O values of the saddle dolomite diagenetic fluids are mostly ranged in “Carbonate sequences related to the magmatic rocks.” Dolomitization during this period was inseparable from magmation, which matches the previous geochemical characteristics and high inclusion temperature. Based on the conversion from the freezing temperature of the saddle dolomite inclusion −11 to −12℃) to salinity (15–16.05%) (Lu et al., 2016), the fluid during this period was hypersaline, which indicates sufficient contact between the asthenolith and the formation water.

Diagenetic fluids of saddle dolomites in the Middle Permian

The δ¹⁸O values of saddle dolomite diagenetic fluids in the Middle Permian are mostly ranged between those of “the carbonate sequences related to magmatic rocks” and that of “seawater,” and only a few are in the range of “the carbonate sequences related to magmatic rocks.” This may be due to the common transformation by multiple fluids. Based on the conversion from the freezing temperature of the saddle dolomite inclusion (−4.1 to −7.5℃) to salinity (6.6–11.2%), the fluid during this period was of middle–high salinity, which is obviously lower than that of the Upper Sinian Dengying Formation. Hypersalinity and high inclusion temperature may indicate contact with the asthenolith. In contrast to the salinity and inclusion temperature of the Upper Sinian Dengying Formation, those of the Permian may suggest the cooling effect of low temperature and the dilution effect of low salinity fluid apart from the contact with the asthenolith.

Diagenetic fluids of saddle dolomites in the Middle-Upper Cambrian

With respect to the δ¹⁸O values of the saddle dolomite diagenetic fluids in the Middle-Upper Cambrian, some fall within the “seawater” region and others are distributed in the zone between “the carbonate sequences related to magmatic rocks” and “seawater.” In contrast to the saddle dolomite diagenetic fluids in the Upper Sinian and the Middle Permian, we speculate that the dolomitization in the Middle-Upper Cambrian was in less close contact with magmatic rocks. However, the inclusion temperature (with the major peak distributed between 110–130℃) was higher than the fluid temperature during the depositional period, which may suggest multiple causes for the hydrothermal fluids. We propose two possible causes. One may result from the burial process, which can contribute to a maximum burial temperature of up to about 200℃, with the assumption of a surface temperature of 25℃ and average geothermal gradient of 2.5℃/100 m, as exemplified by Well Lin 1 in the Cambrian. The other may be caused by the movement of mantle magma fluid in which the baking effect might lead to an increase in the geothermal gradient and a high shallow-burial temperature.

Sources of the Mg²⁺ and timing of dolomitization

Upper Sinian

As stated above, sufficient contact between diagenetic fluids and the asthenolith is confirmed by multiple geochemical characteristics in the Dengying Formation saddle dolomites. Large numbers of ions, such as Mg²⁺, Fe²⁺ (250–1000 ppm), and Mn²⁺ (250–1000 ppm) in brine, contributed to the increased salinity of saddle dolomites. Therefore, the magmation time and Mn²⁺ enrichment process were research highlights. According to the hydrocarbon expulsion history (Xu et al., 2013), burial history, rock sequence, and inclusion temperature, the saddle dolomites in the Upper Sinian Dengying Formation were cement in the final stage, and the dolomitization timing was later than that of the medium-coarse crystalline dolomites (Shi et al., 2013) and close to or earlier than asphalt with an abundant remnant in the cleavage in the second stage. The medium-coarse crystalline dolomites were products of the burial stage with the cementation timing later than that of the asphalt in the first stage (450–430 Ma). In addition, the asphalt in the second stage has been proved by the secondary hydrocarbon generation after the Hercynian (250 Ma). Therefore, we conclude that the dolomitization timing of the saddle dolomites was not earlier than the Late Silurian and not later than the Permian during which the Caledonian movement was characterized by extrusion stress (Luo, 1981), large orogenic movement, and uplift area along the Leshan–Longnvsi region. There is no record of magmation under squeezing action, but only extensional force in the Hercynian during which the geochemical characteristics of saddle dolomites differed from the Permian hydrothermal dolomites. Therefore, we consider the saddle dolomites of the Dengying Formation to be related to Hercynian activities, but not exclusive to one single period of extensional activity.

Middle Permian

The Upper Permian Emeishan basalt (P3e) overlying the Middle Permian formation shows a close relationship with the Middle Permian hydrothermalism (Jin and Feng, 1999; Luo et al., 2004; Wang and Jin, 1997). Nevertheless, until recently, research regarding the saddle dolomites in these sequences has failed to draw sufficient attention, and the hydrothermal mechanism requires further consideration. Compared with the geochemical characteristics of the Sinian hydrothermal fluids, the Permian saddle dolomites feature relatively lower diagenetic temperature, salinity, and degrees of rare earth enrichment than those of the Sinian, which possibly suggests the impact of the asthenolith during mineralization. However, unlike the Upper Sinian Dengying Formation, the Middle Permian saddle dolomites show less contact with the asthenolith and this influencing mechanism must be further analyzed.

Permian saddle dolomite was the first phase of cementation in fractures, occurring earlier than bitumen and granular sparry calcite in the formation, which indicates that the immigration of hydrothermal fluids might have occurred in the same approximate time period as the formation of fractures. The Hercynian movement (≈ 270 Ma) was the first staged tectonic movement after the Middle Permian and featured intense magma activity. Given this it is reasonable that Mg²⁺ in the fractures came from the Emeishan basalt (P3e) and that the migration pathways were large-scale faults or microfractures related to thermal cracking, which also provided the main spaces for the saddle dolomite precipitation. There are multiple possible explanations for the Mg²⁺ migration. One is that the high-temperature Emeishan basalt erupting in the Hersynian period (250 Ma) was leached, infiltrated, and dolomitized by meteoric water. During its interplay with the Emeishan basalt, this meteoric water played important washing and cooling roles, which contributed to the increase in salinity and the escape of free O₂, based on the principle of saturation temperature. The escape of O₂ is a process known as deoxygenation, which forms a shallow burial reducing environment with a high Fe²⁺ content in saddle dolomites.

The other migration pattern may be due to the dolomitization resulting from the Hercynian Emeishan basalt, which increased the temperature of the formation water. However, no saddle dolomites have been found near the contact position of the basalt, which means that the pathway of the basalt eruption differs from that of the dolomitized fluid migration. Furthermore, there are either no or few magmatic rock compositions in the diagenetic fluid, which may be due to the relatively low temperature and salinity. Meanwhile, the diagenetic fluid inversion of the Permian saddle dolomites (Figure 9) also indicates that the formation water, perhaps with a few magmatic rock compositions, may be the main fluids that contributed to the dolomitization. These fluids were redistributed and locally rich in Mg2+ due to the baking effect of the Permian asthenolith and the uplift effect during the Hercynian. Dolomitization can occur as long as conditions are suitable, and therefore, both of the above two situations may have occurred.

Middle-Upper Cambrian

Considered to be products of the penecontemporaneous period, the granular dolosparite in early dissolved fractures and the gypsum-salt pseudomorph in the gypsum mold pores were followed by saddle dolomites. These saddle dolomites usually filled these reservoir spaces during the late stage but have not been assigned a precise time due to the lack of typical dating minerals, although diagenetic fluids were active during the early diagenetic stage (≈ 510 Ma). Moreover, the early fillings occur along the pore walls and show a serrated dissolution rim at the contact surface with saddle dolomites, which may imply that diagenetic fluids featured a dissolution capacity before or during the saddle dolomite deposition as a possible result of hydrothermal dissolution.

Geochemical characteristics suggest a differentiation of the hydrothermal dolomitization in the Upper-Middle Cambrian, the cause of which may be hydrothermal activity. On the other hand, it may be due to the isolation effect of the Lower Cambrian thick mudstone, as the hydrothermal activity was not intense in local regions. As such, fluid inversion results show the δ¹⁸O value of diagenetic fluids of the saddle dolomites to partly fall in the “seawater” zones and partly between the “carbonate sequences related to magmatic rocks” and “seawater,” while also showing differences in other geochemical characteristics.

The first cause is the “certain” transformation of Hercynian hydrothermal fluids. Hercynian movement is the largest hydrothermal activity event in the Basin, whose influence covers three provinces in Southwest China, as evident by the large amount of volcanic rocks in this region. Magmatism occurred throughout the whole Proterozoic–Paleozoic, which inevitably resulted in hydrothermal alteration in the Cambrian carbonate sequences. Nevertheless, the influence of the asthenolith on the Cambrian was limited, in contrast with its much closer contact with the Upper Sinian. An unconformable contact exists between the bottom of the Upper Sinian and the granitoid intrusion in the Weiyuan area, as well as with the intrusive rock lenses of the Upper Sinian in multiple field outcrops in the Western Sichuan Basin. Meanwhile, due to the isolation of four thick-layered siliceous rocks in the Upper Sinian Dengying Formation and the shale in the top of the Lower Cambrian Qiongzhusi and Niutitang Formations, diagenetic fluids were formed at high temperature and high salinity. Similarly, because of the overlying Upper Permian Emeishan basalt, the Middle Permian was also accessible to high temperature and Mg²⁺-rich fluids and thus to dolomitization. Despite being influenced by volcanic rocks during the Hercynian, the Middle-Upper Cambrian failed to obtain sufficient heat or Mg²⁺ resources, and therefore, its temperature and salinity were relatively lower than the other two.

The second cause of dolomitization was that actuated by tectonism. Mg²⁺ also originated from Lower Cambrian shale, as has been proved by traces of hydrothermal activity in the Lower Cambrian black shale (Liang et al., 2014). It contains large amounts of pyrite, millerite, and sulfur molybdenum ore, as well as relatively high quantities of exogenous hydrothermal minerals, such as ullmannite, chalcopyrite, sphalerite, barite, and quartz in the layers, all of which feature structures in the shapes of balls, washbasins, and pillows. Ultrastrongly enriched elements, such as Mo, As, Se, Re, and Tl, are typical vapor transport elements, and Mo is the high-temperature vapor transport element, which suggests a possible volcano emanation. Moreover, strongly enriched element combinations of Ni, U, Au, Ag, and PGE are products of ultrabasic magmation, which also indicate the presence of hydrothermal activity. The carrier of Mg²⁺ was the interlayer water enriched in mudstone and these high-salinity fluids were discharged with increases in burial depth. Due to later tectonic movement, these fluids remigrated, partly infiltrated into the Middle-Upper Cambrian carbonate along fractures, and dolomitized the primary calcite cement. The heat source might be either magmation or the increasing temperature during the burial process.

In terms of the hydrothermal timing, there were no hydrothermal conditions in the Middle-Upper Cambrian during the shallow burial stage of the Lower Cambrian. In the deep burial stage, the compaction process came to an end without any hydrothermal fluids, and in contrast, the middle burial period may have been suitable for hydrothermal activity due to the presence of Mg²⁺-rich fluids from compaction and dolomitized heat with the increasing burial temperature. From the burial history, we know that the uplift of the Cambrian–Silurian failed to reach the middle burial stage. Therefore, the hydrothermal timing was most likely the period from the end of the Caledonian uplift to the start of the Hercynian or the deposition burial stage after the Hercynian. The former was influenced by the effect of Caledonian uplift during which diagenetic fluids were redistributed and Mg²⁺ was locally enriched for dolomitization. The latter period was characterized by structural extension, which provided migration passages for diagenetic fluids, and with the increasing temperature of the asthenolith providing heat for dolomitizing primary cements.

Conceptual model of fluid flux in the basin

Formation mechanism of saddle dolomite hydrothermal fluids in the Upper Sinian

During the 280–360 Ma period, Caledonian movement came to an end and Hercynian movement began. With respect to the work area, tectonic stress changed from extrusion to extension, combined with the reopening of Caledonian faults and the upwelling of hypomagma and hyperthermal brine. However, the early weak extensional force failed to bridge the basement and the surface, therefore, the asthenolith was characterized by small-scale intrusions into the lower part of the Dengying Formation and the underlying formation. Moreover, this weak extensional force contributed to the isolation of fractures by the thick siliceous rocks in the Fourth Member of the Dengying Formation and the Lower Cambrian mudstone, which resulted in the interlaminate flow of hyperthermal brine in the Upper Sinian Dengying Formation and the saddle dolomites widely distributed in large and dissolved fractures. This isolation effect contributed to the sufficient contact between the hydrothermal fluids and rocks at a continuously high temperature to form saddle dolomite inclusions of high temperature and salinity. Despite the scant supply of surface water at deep burial depths, the main driving force was the differential uplifting tectonic movement and the hydrothermal circulation (Figure 10(a)).

Figure 10.

Proterozoic–Paleozoic hydrothermal dolomitization model and hydrocarbon migration in Sichuan Basin.

Formation mechanism of saddle dolomite hydrothermal fluids in the Middle-Upper Cambrian

The Lower Cambrian deposits of thick mudstone in the Niutitang and Qiongzhusi Formations and the total thickness in some areas can reach more than 1 km with abundant interlayer water. Evidence suggests that there were multiple sets of volcanic detritus deposits in the Lower Cambrian mudstone, which provided the necessary material conditions for fluids rich in ore and Mg²⁺. From the Early Ordovician to the Early Permian, the Lower Cambrian mudstone was further compacted and generated abnormally high pressure, thus providing the dynamic conditions necessary for fluid migration. The effect of the increasing temperature of the geothermal gradient and the baking effect of the Late Caledonian and Hercynian rocks provided the necessary temperature conditions for dolomitization to occur. In the meantime, due to differential uplifts and intense fold deformations during the Caledonian and Hercynian, many microfractures formed in the rocks. Ore-rich fluids migrated upwards along these microfractures under the abnormally high pressure and were deposited in the fractures and dissolved pores of the Middle-Upper Cambrian or replaced the early gypsum-salt pseudomorph and calcareous cement to form saddle dolomites (Figure 10(b)).

Formation mechanism of saddle dolomite hydrothermal fluids in the Middle Permian

The 250 Ma period was the main active stage of the Hercynian movement during which the whole basin was intensely uplifted. Meanwhile, a large tectonic extension force bridged the deep mantle, causing a violent eruption. During this period, the Sichuan Basin and its periphery were largely covered by extrusive rocks such as basalt and tuff, among which Emeishan basalt had a vertical thickness up to about 400 m. The combination of the asthenolith along with surface water and meteoric water contributed to the formation of fluids of high temperature and high salinity. In addition, fluids infiltrated along large-scale faults or microfractures related to thermal cracking into the Middle and Lower Permian, replacing earlier calcareous cement to form saddle dolomites (Figure 10(c)).

Hydrothermal dolomite reservoir and hydrocarbon migration

The diagenesis sequence and previous organic geochemical data show complex sequence between hydrocarbon migration and hydrothermal dolomitization (Figure 10). The main hydrocarbon source rocks was gray green shale at the bottom of the 2nd Member in Sinian Dengying Formation and the black shale in Cambrian Niutitang formation with the approximate time of hydrocarbon migration: The time of hydrocarbon migration of the Sinian shale was the Middle Cambrian epoch, and the time of Cambrian shale was Late Cambrian epoch. Both of their migration activities are terminated as the Caledonian tectonic uplift, earlier than hydrothermal dolomitization. Therefore, there was no multiple asphalt filling in these spaces relative with hydrothermal dolomitization. In the Late Permian, the area descended to deposit, and both Sinian and Cambrian source rocks recovered hydrocarbon migration, this migration activity was massive and later than hydrothermal dolomitization. So there was large massive bitumen of this migration period in vugs and seams relative hydrothermal dolomitization. The main Permian hydrocarbon source rocks was the black shale in the Longtan Formation, its hydrocarbon expulsion time was no later than Late Triassic in most areas of the basin. But these shale near the Hercynian fracture could be maturity earlier with temporal hydrocarbon expulsion, as the calefaction effect of Hercynian magmatic activities. Meanwhile, this thermal event led to hydrothermal dolomitization in the reservoir, which could make rational matching with hydrocarbon migration.

Conclusions

The main results of this paper may be summarized as follows:

We recognize three sets of saddle dolomites from Proterozoic–Paleozoic marine carbonate sequences in the Sichuan Basin, respectively, distributed in the Upper Sinian, the Middle-Upper Cambrian, and the Middle Permian. Discrepancies in the lithologies, mineral features, paragenetic mineral combinations, and planar distributions reveal the possibility of different sources and formation mechanisms of the hydrothermal dolomitization fluids. The geochemical characteristics of these saddle dolomites are obviously diverse and also differ from the matrix and cement of carbonates in the syndiagenetic stage.

The identification of fluid inversion reveals a diversity of fluid characteristics in the three sets of saddle dolomites. The Upper Sinian saddle dolomites were products of mantle-derived fluids of high temperature and salinity, and experienced continuous and stable crystallization. The Permian saddle dolomites were products of multisource fluids with middle–high temperature and high salinity and experienced fast crystallization during which the Mg²⁺ and heat required for dolomitization were inseparable from the Hercynian magmation. The Middle-Upper Cambrian dolomites were generated from fluids with middle-low temperature and mid-range salinity, and may have been in less close contact with the deep magmation.

The formation mechanism of hydrothermal dolomitization is characterized by both universality and diversity. The Middle Permian and Upper Sinian dolomitization occurred in an extensional setting, which reflects a multi-episodic feature during the Hercynian. The Upper Sinian hydrothermal dolomitization took place in the Caledonian and ended in the early Hercynian during which the extrusion stress of the whole region was transformed into extensional stress, and saddle dolomites were generated from small-scale intrusion of deep hydrothermal fluids and formation water. During the peak period of the Hercynian movement, the Middle Permian experienced intense tension and fracture and its saddle dolomites were generated from the combination of the Upper Permian extrusive rocks and meteoric water. The Cambrian hydrothermal dolomitization occurred in both the Caledonian and Hercynian. Rather than a tectonic extension force, abnormally high pressure from the compaction of mudstone could have provided the required impetus for dolomitization. The saddle dolomites of this period originated from ore-rich fluid related to the Lower Cambrian volcanic detritus mudstone.

Footnotes

Acknowledgments

We thank Professor Shen Zhongmin,Professor Yuan Haifeng and Liang Jiaju in Chengdu University of Technology for their guidance and great suggestions. We also thank Yang Yingying and He Jinhai for their help in analytical test. Special thanks to editor and reviewers.

Declaration of conflicting interests

The author(s) declare no potential conflicts of interest with respect to the research,authorship,and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research,authorship,and/or publication of this article: We acknowledge financial support from National Natural Science Foundation of China (nos: 41272150,41272159,and 41572099),China Geological Survey (grant no: 1212011120964),China Postdoctoral Science Foundation (Study on fluid diversity and genetic model of hydrothermal dolomitization,grant no: 2016M601088),Baojun Liu geoscience foundation for youths and the opening foundation of Shandong Provincial Key Laboratory of Depositional Mineralization & Sedimentary Mineral (grant no: DMSM2017041).

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Discussion of multiple formation mechanisms of saddle dolomites—Comparison of geochemical data of Proterozoic–Paleozoic dolomites

Abstract

Keywords

Introduction

Geological setting

Samples and methods

Results

Petrography

Geochemical characteristics

CL analyses

REE compositions and degree of order

Carbon, oxygen, and strontium isotopes

Fluid inclusion petrography and microthermometry

Distribution

Discussion

Conditions of dolomitization and dolomite cementation

Fluid reconstruction

Diagenetic fluids of saddle dolomites in the Upper Sinian Dengying Formation

Diagenetic fluids of saddle dolomites in the Middle Permian

Diagenetic fluids of saddle dolomites in the Middle-Upper Cambrian

Sources of the Mg2+ and timing of dolomitization

Upper Sinian

Middle Permian

Middle-Upper Cambrian

Conceptual model of fluid flux in the basin

Formation mechanism of saddle dolomite hydrothermal fluids in the Upper Sinian

Formation mechanism of saddle dolomite hydrothermal fluids in the Middle-Upper Cambrian

Formation mechanism of saddle dolomite hydrothermal fluids in the Middle Permian

Hydrothermal dolomite reservoir and hydrocarbon migration

Conclusions

Footnotes

Acknowledgments

Declaration of conflicting interests

Funding

References

Sources of the Mg²⁺ and timing of dolomitization