Formation mechanism of dolomite reservoir controlled by fourth-order sequence in an evaporated marine environment – An example from the Lower Ordovician Tongzi Formation in the Sichuan Basin

Abstract

The high-porosity dolomite reservoirs of the Lower Ordovician Tongzi Formation (Fm.) were widely developed in the Sichuan Basin of southern China. The characteristics and developing mechanisms of the high-porosity dolomite reservoirs under the control of fourth-order sequence boundaries are discussed. In the Tongzi stage of the Early Ordovician, the Sichuan Basin was in a restricted platform facies in an evaporated shallow seawater environment. From the western to eastern regions of the basin, the Tongzi Fm. was serially developed in a tidal flat-lagoon-high-energy shoal depositional system. The evaporated seawater consequently led to dolomitization by way of the refluxing model. The Tongzi Fm. dolomites were subdivided into four coarsening-upward fourth-order sequences. Many tiny dissolution pores were formed in the dolomite beneath the four fourth-order sequence boundaries due to syn-sedimentation meteoric water erosion. Exposure above the seawater due to the short-term fall of the relative sea level consequently led to contemporaneous meteoric erosion. The Tongzi Fm. dolomites in the belt surrounding the Central Paleo-uplift were further subaerially dissolved by meteoric water due to tectonic uplift in the Guangxi Movement since the end of the Silurian period. Therefore, dolomitization, syn-sedimentation meteoric erosion under the fourth-order sequence boundaries, and meteoric karst during the Guangxi tectonic uplift jointly controlled the development of the Tongzi Formation high-porosity dolomite reservoirs. In the eastern and southeastern Sichuan Basin, the favourable reservoirs are the high-energy shoal dolomites that were eroded by meteoric water under fourth-order sequence boundaries. Around the Central Paleo-uplift, the favourable reservoirs are the dolomites dissolved by subaerial meteoric karst during the Guangxi Movement.

Keywords

Sichuan Basin lower Ordovician Tongzi formation dolomite reservoir meteoric erosion fourth-order sequence

Dolomites account for approximately 50% of all carbonate rocks (Zenger et al., 1980) and hold significant volumes of oil and gas around the world (Jiang et al., 2013; Warren, 2000). Dolomites can be formed in several geological settings, such as restricted marine carbonate platform, terrestrial alkaline lacustrine (Baker and Kastner, 1981; Land, 1980), deep burial (Wierzbicki et al., 2006) and hydrothermal environments (Smith and Davies, 2006). However, large-scale dolomitization generally occurs in marine environments (Land, 1980; Machel, 2004; Warren, 2000). The restricted marine evaporative environment is most favourable for large-scale dolomitization. In such an environment, seawater is evaporated and consequently concentrated into a hypersaline brine with high Mg/Ca, which is critical for dolomitization at the near-surface (Gregg et al., 2015; Land, 1985). As a result, massive dolomites and gypsiferous dolomites accompanying lagoon evaporation are generally precipitated (Adams and Rhodes, 1960; Saller and Nuel, 1998). Several models have been proposed to interpret the dolomitization process (Land, 1985; Warren, 2000).

Throughout the Cambrian and early Ordovician, the Sichuan Basin in southwestern China (Figure 1) was in a restricted marine platform-evaporative gypsum-salt lagoon environment (Hu et al., 2001; Li et al., 2015). Large-scale sediments of dolomite and anhydrite, as well as widespread high-energy granular shoal dolomites were formed (Li et al., 2012; Men et al., 2010; Zou et al., 2014). The dolomites, especially the granular shoal dolomites, formed around the gypsolith lagoons, creating high-porosity reservoirs (Guo, 2014; Shen et al., 2015; Zhao et al., 2014). Large gas fields have already been discovered in the Lower Cambrian Longwangmiao Formation (Fm.) and Lower Ordovician Tongzi Formation (Fm.) granular shoal dolomites (Chen, 2010; Liu et al., 2011; Ni et al., 2014; Wei et al., 2015; Zou et al., 2014) (Figures 1 and 2).

Figure 1.

Tectonic units, main drilling wells and field outcrops in the Sichuan Basin.

Figure 2.

The Paleozoic stratigraphic column of the Sichuan Basin.

During the post-sedimentation diagenetic process, karst reservoirs could be developed in carbonates due to tectonic uplift above the ground surface and subsequent meteoric erosion (Loucks, 1999; Loucks et al., 2004). A large amount of hydrocarbon has been found in the karst carbonate reservoirs (Zhao et al., 2014). The unconformity surfaces associated with post-sedimentation tectonic uplifts are generally first-, second- or third-order sequence boundaries. The post-sedimentation tectonic unconformities and the karst reservoirs have been studied in detail (Konert et al., 2001; Loucks, 1999; Mazzullo and Chilingarian, 1996).

On the basis of wells and field outcrops, the development of the Lower Ordovician Tongzi Formation high-porosity dolomite reservoirs in the Sichuan Basin was closely related to meteoric erosions. However, the meteoric erosions did not occur under first-, second- or third-order sequence boundaries but were developed under high-frequency (fourth-order) sequence boundaries during syn-sedimentation exposure above seawater. The characteristics, developing mechanisms, and the model of the high-porosity dolomite reservoirs related to the syn-sedimentation meteoric erosions under fourth-order sequence boundary in the evaporated marine lagoon environment have not been well understood.

Taking the Lower Ordovician Tongzi Fm. dolomites as an example, this article (1) discusses the sedimentary facies of the tidal flat-lagoon-high-energy shoal dolomites developed in an evaporated marine environment, (2) reveals geological and geochemical evidence for the syn-sedimentation meteoric erosions, (3) clarifies the features and formation mechanisms of the dolomite reservoirs under high-frequency (fourth-order) sequence boundaries, and (4) presents the distribution of the Tongzi Fm. dolomite reservoirs in the Sichuan Basin.

Geological settings

The Sichuan Basin in southern China is one of the giant hydrocarbon basins in China (Figure 1). The Paleozoic strata in the Sichuan Basin includes the Cambrian, Ordovician and Silurian (Figure 2). The Niutitang Formation (Fm.) (Є₁n) of the Lower Cambrian mainly comprises black mudstones and shales. The Longwangmiao Fm. (Є₁l) of the Lower Cambrian, Shilengshui Fm. (Є₂s) of the Middle Cambrian, and Loushanguan Fm. (Є₃l) of the Upper Cambrian are predominantly dolomites, gypsiferous dolomites, and anhydrites.

The Ordovician mainly includes the Tongzi Fm. (O₁t), Honghuayuan Fm. (O₁h) and Meitan Fm. (O₁m) in the Lower Ordovician, the Shizipu Fm. (O₂s) and Baota Fm. (O₂b) in the Middle Ordovician, and the Linxiang Fm. (O₃l) and Wufeng Fm. (O₃w) in the Upper Ordovician. The Meitan Fm. is mainly composed of fine-grained mudstones with sandstone or bioclastic limestone interlayers and generally accounts for one-third of the thickness of the Ordovician. The strata above and below the Meitan Fm. are mainly carbonates. The Lower Ordovician Honghuayuan Fm. just below the Wufeng Fm. consists of mainly thick-layers of bioclastic limestones, while the Tongzi Fm. is mostly dolomites with anhydrite and shale interlayers. The Baota Fm., directly above the Meitan Fm., contains mainly bioclastic limestones and argillaceous limestones, while the Wufeng Fm. is mainly black siliceous shales.

The black mudstones of the Lower Cambrian Niutitang Fm. are widespread in the Sichuan Basin and are important hydrocarbon source rocks for the Sinian, Cambrian and Ordovician strata (Li et al., 2011; Zhao et al., 2003). The Ordovician Tongzi Fm., Meitan Fm. and Wufeng Fm. also developed mudstone and shale hydrocarbon source rocks, and they can provide a fair amount of hydrocarbons for the Ordovician layers.

Dolomites and anhydrites were successively developed throughout the Lower Cambrian Longwangmiao Fm., the Middle Cambrian Shilengshui Fm., the Upper Cambrian Loushanguan Fm. and the Lower Ordovician Tongzi Fm. The dolomites in these formations are the main hydrocarbon reservoir beds in the Sichuan Basin (Figure 2). The wells Nvji and Moshen 1 produced gas flow of 3.09×10⁴m³/day and 0.422 × 10⁴m³/day from the Tongzi Fm. dolomite reservoirs, respectively. The wells Aaping 1, Gaoke 1, Weiji, Ximen 1, Wuke 1 and He 12 revealed the Tongzi Fm. dolomites were gas-bearing layers (Figure 1).

The Ordovician sediments are divided into six third-order sequences (Yang et al., 2012). In the Early Ordovician Tongzi Stage, most of the Sichuan Basin was in the restricted platform where the sediments were mainly dolomites with a small proportion of shales (Li et al., 2015). In the Honghuayuan Stage, the Sichuan Basin evolved into an open platform, and the sediments were predominately limestones. Consequently, the Tonzi Fm. was treated as one third-order sequence.

The Ordovician experienced multiple stages of tectonic uplifts, such as the Duyun Movement in the Later Ordovician and the Guangxi Movement in the Later Silurian (Mei et al., 2005). The former was responsible for the unconformity (a third-order sequence boundary) between the Lower Silurian and Ordovician, while the latter caused the unconformity (a first-order sequence boundary) between the Permian and Silurian/Ordovician/Cambrian. Below the unconformity surfaces, the Ordovician carbonates developed karst reservoirs around the Central Paleo-uplift of the Sichuan Basin (Mei et al., 2005).

Samples and methods

The Lower Ordovician Tongzi Fm. dolomite samples were obtained from the Weihan 1, Ximen 1, Zuo 3, Wuke 1, and Chi 7 wells and the Shuanghe, Banqiao, and Honghuayuan outcrops. The dolomite samples were polished on both sides to approximately 0.03 mm thickness for microscopic observations. Microscopic observations of thin sections for petrology, mineralogy and pore structures were carried out using a Leica DM4500 microscope.

Thirteen unaltered crypto-crystalline and fine crystalline dolomite samples and 14 Lower Ordovician limestone samples were prepared for carbon, oxygen and strontium isotope analyses. Eleven medium-coarse crystalline and oolitic dolomite samples with dissolution pores beneath fourth-order sequence boundaries were also selected for carbon, oxygen and strontium isotope analyses.

The selected carbonate samples were ground into a powder of less than size 200 mesh for the isotopic analyses. Carbon and oxygen isotope analyses were carried out on a Gas Bench II apparatus connected to a MAT 253 isotope ratio mass spectrometer. Each powdered sample of approximately 200 µg was placed in a vial that was then flushed with pure He gas. Following the addition of 100% H₃PO₄, the vial was kept at 72℃ for 1 h before analysis. All C and O isotopes were reported relative to V-PDB and calibrated against NBS-18 and NBS-19 standards. Reproducibility of replicate analyses of NBS-18 and NBS-19 standards was better than ±0.1‰.

The strontium isotope analysis was performed using a Finnigan MAT Triton TI instrument. Approximately 100 mg of the powdered sample was placed in a jar, and 2 mL of 6 M HCl was added. The sample was dissolved for 24 h at a temperature of 100–110℃. The ion chromatography technique pioneered by Aldrich et al. (1953) was used to separate the strontium isotopes. Using HCl as the eluant, the AG 50 W-X12 200–400 mesh ion resin produced by Bio-Rad Corp. (USA) was used to separate and enrich the strontium isotopes. The measured ⁸⁷Sr/⁸⁶Sr values were adjusted according to the mass fractionation standard ⁸⁷Sr/⁸⁶Sr = 0.1194. The analysed ⁸⁷Sr/⁸⁶Sr values of the NBS987 standard sample averaged 0.710273 ± 0.000012.

The porosity and permeability of the dolomites in two fourth-order sequences of the Banqiao outcrop profile were systematically analysed. Fifteen and 16 dolomite cylinders of 2.5 cm in diameter and 8–12 cm in length were selected from the two fourth-order sequences, respectively. The interval distance between two adjacent cylinders is approximately 1–1.5 m. The porosity of the dolomite cylinders was determined using the gas-diffusion method (Dullien, 1992), and the permeability was determined by application of Darcy's law under steady-state conditions with the methods outlined in the API RP 40 (API PR, 1998, http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/porosity-and-permeability-determination).

Based on 48 wells and 15 field outcrops, the distribution and thickness of different lithologic types (sandstone, mudstone, shale, anhydrite, dolomite and limestone) were counted; consequently, the paleo-facies of the Sichuan Basin in the Early Ordovician Tongzi Stage was determined, and the thickness of the Tongzi Fm. dolomite was drawn.

Results

Petrography and facies

Tidal flat sediments

The sediments developed in the tidal flat facies are mainly crystalline dolomites. Based on the occurrence, crystalline texture and crystal size, the Tongzi Fm. dolomites were mainly crypto-crystalline dolomites, fine-medium crystalline dolomites (Figures 3(a), (b) and 4(a) to (c)), and coarse crystalline sucrosic dolomites (Figure 3(c)). Observations of the outcrop profiles show that the crypto-crystalline dolomites are mostly grey or dark grey, thin-bedded dolomites with some interlayers of shales, for example, the Guanyinqiao outcrop (Figure 3(a)) in Tongzi County, Guizhou Province. Under a microscope, the crypto-crystalline dolomites reveal crystals with no definitive morphologies (Figure 4(a)). The fine-medium crystalline dolomites are mostly light grey or brownish grey medium-thick-bedded dolomites. The coarse crystalline sucrosic dolomites are generally thick bedded (Figure 3(c)). Crystals in the medium and coarse crystalline dolomites appear planar euhedral to subhedral in shape under the microscope, with mosaic structures (Figure 4(c)).

Figure 3.

Photos of the Lower Ordovician Tongzi Formation dolomites from field outcrops and well cores in the Sichuan Basin. (a) Thin-bedded, crypto-crystalline and fine crystalline dolomites showing horizontal bedding with thin-bedded shale layers, Guanyinqiao outcrop, Tongzi County, Guizhou Province; (b) medium bedded, fine crystalline dolomite, Guanyinqiao outcrop, Tongzi County, Guizhou Province; (c) medium–thick bedded, sucrosic, medium-coarse crystalline dolomite, Banqiao outcrop, Zunyi County, Guizhou Province; (d) Fine-medium crystalline dolomite with fine algal layers, Banqiao outcrop; (e) grey oolitic dolomite with small dissolution pores, well Weihan 1; (f) Dissolution vugs in the thick-bedded medium-coarse crystalline dolomite, Banqiao outcrop; (g) small dissolution vugs in the light-grey, medium-sized crystalline dolomite, Banqiao outcrop; (h) Grey medium crystalline dolomite with dissolution vugs, Shuanghe outcrop, Changning County, Sichuan Province.

Figure 4.

Microscopic photos of the lower Ordovician Tongzi formation dolomites from field outcrops and well cores in the Sichuan Basin. (a) Fine-medium crystalline dolomite with bitumen filling along stylolite, 50×, polarized light, Guanyinqiao outcrop, Tongzi County, Guizhou Province; (b) Peloidal wackestone bearing sand debris, 25×, polarized light, Shuanghe outcrop, Changning County, Sichuan Province; (c) fine-medium crystalline dolomite, 25×, polarized light, Shuanghe outcrop; (d) Oolitic and bioclastic packstone, 25×, well Chi 7; (e) Algal and bioclastic packstone, 25×, well Weihan 1; (f) Fine-to-medium crystalline wackestone, 25×, well Zuo 3; (g) Fine-medium crystalline dolomite with dolomite crystals filling in dissolution vugs, 5×, polarized light, Shuanghe outcrop; (h) Fine-medium sized crystalline dolomite, matrix emitting red cathodoluminescence under cathode rays, filling dolomites in vugs emitting dull blue cathodoluminescence, Shuanghe outcrop; (i) medium crystalline dolomite with intercrystalline pores, 50×, polarized light, well Wuke 1; (j) fine-to-medium crystalline dolomite with intercrystalline pores, 25×, polarized light, well Zuo 3; (k) fine crystalline dolomite with intercrystalline pores, 25×, polarized light, Shuanghe outcrop; (l) fine-medium crystalline dolomite with intercrystalline pores, 50×, polarized light, Shuanghe outcrop.

High-energy shoal sediments

In high-energy shoal facies, granular dolomites were developed. The granular dolomites are generally packstones, and the grains in the dolomites are primarily sand-sized intraclasts, oolites and algal debris (Figures 3(d), (e) and 4(d) to (f)). The oolitic dolomites have radial ooids with the size ranging from 0.5 to 1 mm. Point and line contacts between oolites are frequently found to serve as support structures.

Lagoon sediments

During the Early Ordovician Tongzi stage, several lagoons were developed in the Sichuan Basin (Figure 5), and the sediments in the lagoon facies were mostly anhydrites and anhydrite-bearing dolomites. The Chi 7, He 12, Pan 1 and Dingshan 1 wells in the Eastern Sichuan and the Yangshen 2 and Ning 2 wells in the Southeastern Sichuan all showed the deposition of lagoon gypsolith and a certain thickness of gypsiferous dolomites. For example, the well Yangshen 2 has ∼25 m thick sediments consisting of anhydrites and gypsiferous dolomites at a depth of 4257–4290 m; well Ning 2 has 14 m thick sediments at a depth of 99.5–113.5 m.

Figure 5.

Sedimentary facies of the early Ordovician Tongzi Formation in the Sichuan Basin.

Tidal flat-lagoon-shoal dolomite system

On the basis of the drilling data and field outcrops, the distribution of sediments and facies of the Tongzi Fm. in the Sichuan Basin (Figure 5) were drawn in detail. At the west side of the basin, the Tongzi Fm. has mainly littoral sediments, such as sandstones and mudstones, due to sufficient supplies of detritus coming from the Muotianling Ancient Land, Kangdian Ancient Land and uplift erosion zones. To the east of the littoral facies, the effects of terrigeneous detritus gradually weakened; near the zone from Bazhong to Weiyuan and Jianwei, the tidal flat facies were developed, and the sediments were predominantly dolomites mixed with a small amount of sandstones and shales. Anhydrites, gypsiferous shales and gypsiferous dolomites of lagoon facies were found in the zone from well Ning 2 to Yangshen 2 and Chi 7. In the eastern and southeastern Sichuan Basin, high-energy granular shoal dolomites were developed in the restricted platform and platform margin.

C, O and Sr isotopes of the dolomites

The results of the carbon, oxygen and strontium isotopes of the Tongzi Fm. dolomites are listed in Table 1. For the Lower Ordovician limestone samples, the δ¹³C_V-PDB values are between −2.1‰ and −0.7‰, with an average of −1.6‰; the δ¹⁸O_V-PDB values are between −10.8‰ and −8.1‰, with an average of −9.3‰; and the ratios of ⁸⁷Sr/⁸⁶Sr are between 0.709179 and 0.709739, with an average of 0.709429. These values are consistent with previously reported data for Early Ordovician normal marine limestones (Veizer et al., 1999). For the crypto-crystalline and fine crystalline unaltered dolomite samples of the Tongzi Fm., the δ¹³C_V-PDB values are between −0.9‰ and 0.5‰, with an average of −0.1‰; the δ¹⁸O_V-PDB values are between −8.9‰ and −5.9‰, with an average of −7.5‰; and the ratios of ⁸⁷Sr/⁸⁶Sr are between 0.709360 and 0.709602, with an average of 0.709472. In comparison, the compositions of both carbon and oxygen isotopes are higher in the Tongzi Fm. dolomites than the limestones, while the ratios of the ⁸⁷Sr/⁸⁶Sr are similar (Table 1, Figures 6 and 7). For the medium-coarse crystalline dolomites and granular dolomites with numerous dissolution pores, the δ¹³C_V-PDB values are between −1.4‰ and −0.5‰, with an average of −0.9‰; the δ¹⁸O_V-PDB values are between −11.8‰ and −9.2‰, with an average of −10.2‰; and the ratios of ⁸⁷Sr/⁸⁶Sr are between 0.709961 and 0.711897, with an average of 710,547. The dissolved medium-coarse crystalline dolomites and granular dolomites are lighter in carbon and oxygen isotope compositions, while their ⁸⁷Sr/⁸⁶Sr ratios are higher than the unaltered crypto-fine crystalline dolomites (Table 1, Figures 6 and 7).

Table 1.

Carbon, oxygen and strontium isotopes of the Tongzi Fm dolomites and lower Ordovician limestones in the Sichuan Basin.

No.	Sample	Outcrop	Fm.	Petrographic Feature	δ¹³C_V-PDB (‰)	δ¹⁸O_V-PDB (‰)	⁸⁷Sr/⁸⁶Sr
Unaltered crypto-fine crystalline dolomites
1	SH-06	Shuanghe	O₁t	Brown grey fine crystalline dolomite	−0.9	−7.4	0.709602
2	SH-03	Shuanghe	O₁t	Grey crypto-fine crystalline dolomite	0.1	−7.1	0.709528
3	GYQ-36	Guanyinqiao	O₁t	Grey crypto-fine crystalline dolomite	0.2	−8.6	/
4	GYQ-37	Guanyinqiao	O₁t	Grey crypto-fine crystalline dolomite	0.1	−8.5	0.709411
5	GYQ-38	Guanyinqiao	O₁t	Grey crypto-fine crystalline dolomite	0.1	−8.9	/
6	GYQ-39	Guanyinqiao	O₁t	Grey crypto-fine crystalline dolomite	0.5	−7.5	0.709459
7	GYQ-40	Guanyinqiao	O₁t	Grey crypto-fine crystalline dolomite	0.2	−6.7	0.709360
8	PJ-O1t-5	Pojiao	O₁t	Grey crypto-fine crystalline dolomite	−0.5	−8.9	/
9	MJ-O1t-3	Majiang	O₁t	Brown grey fine crystalline dolomite	−0.9	−7.1	/
10	BQ-57	Banqiao	O₁t	Grey crypto-fine crystalline dolomite	0.3	−6.2	0.709422
11	BQ-58	Banqiao	O₁t	Grey fine crystalline dolomite	−0.2	−5.9	0.709521
12	BQ-60	Banqiao	O₁t	Grey crypto-fine crystalline dolomite	0.1	−7.1	0.709533
13	BQ-72	Banqiao	O₁t	Grey fine crystalline dolomite	−0.4	−8.2	0.709411
Average					−0.1	−7.5	0.709472
Dissolved medium-coarse crystalline and granular dolomites with dissolution vugs
1	SH-04-5	Shuanghe	O₁t	Medium crystalline dolomite	−0.7	−9.3	/
2	SH-04-6	Shuanghe	O₁t	Coarse crystalline dolomite	−0.9	−9.2	0.710375
3	SH-03-1	Shuanghe	O₁t	Intraclastic dolomite	−0.8	−10.3	0.710051
4	SH-03-2	Shuanghe	O₁t	Medium crystalline dolomite	−1.1	−9.7	/
5	SH-06-2	Shuanghe	O₁t	Intraclastic dolomite	−0.9	−10.6	0.709987
6	SH-06-3	Shuanghe	O₁t	Medium crystalline dolomite	−0.8	−9.4	/
7	SH-06-4	Shuanghe	O₁t	Intraclastic dolomite	−0.5	−10.0	0.710014
8	BQ-50	Banqiao	O₁t	Oolitic dolomite	−1.4	−10.9	0.710852
9	BQ-71	Banqiao	O₁t	Coarse crystalline dolomite	−0.6	−11.2	0.709961
10	BQ-74	Banqiao	O₁t	Medium crystalline dolomite	−1.2	−9.4	0.711235
11	BQ-75	Banqiao	O₁t	Intraclastic dolomite	−0.9	−11.8	0.711897
Average					−0.9	−10.2	0.710547
Lower Ordovician limestones
1	QL-51	Qiliao	O₁h	Grey bioclastic limestone	−1.2	−9.6	/
2	QL-40	Qiliao	O₁h	Dark grey crypto-crystalline limestone	−1.3	−8.2	/
3	QL-40	Qiliao	O₁h	Grey bioclastic limestone	−2.1	−10.7	/
4	QL-05	Qiliao	O₁t	Dark grey crypto-crystalline limestone	−1.5	−9.2	0.709739
5	QL-45	Qiliao	O₁h	Grey bioclastic limestone	−1.5	−10.8	/
6	QL-35	Qiliao	O₁h	Grey crypto-crystalline limestone	−1.4	−9.6	0.709664
7	GYQ-27-02	Guanyinqiao	O₁t	Dark grey bioclastic limestone	−1.1	−9.4	0.709278
8	GYQ-25-01	Guanyinqiao	O₁t	Dark grey crypto-crystalline limestone	−1.3	−9.2	/
9	SH-15	Shuanghe	O₁t	Grey crypto-crystalline limestone	−1.9	−9.8	0.709623
10	YW-4	Yaowan	O₁h	Dark grey crypto-crystalline limestone	−1.3	−8.6	0.709179
11	YW-10	Yaowan	O₁h	Dark grey bioclastic limestone	−0.7	−9.6	/
12	YW-11	Yaowan	O₁h	Grey crypto-crystalline limestone	−1.5	−8.1	0.709203
13	BQ-47	Banqiao	O₁h	Grey crypto-crystalline limestone	−1.8	−9.2	0.70932
14	BQ-49	Banqiao	O₁h	Grey crypto-crystalline limestone	−1.6	−8.5	0.709422
Average					−1.6	−9.3	0.709429

Figure 6.

Diagram of δ¹³C_V-PDB vs δ¹⁸O_V-PDB for the Tongzi Fm. dolomites and the Lower Ordovician limestones.

Figure 7.

Diagram of ⁸⁷Sr/⁸⁶Sr vs δ¹⁸O_V-PDB for the Tongzi Fm. dolomites and the Lower Ordovician limestones.

Porosity and permeability

The results of porosity and permeability for the dolomites from two fourth-order sequences of the Tongzi Fm. in the Banqiao outcrop are listed in Table 2. The two fourth-order sequences, O₁t-ii and O₁t-vi, show features of upward coarsening, changing from crypto-fine crystalline dolomites at the bottom to medium-coarse crystalline or oolitic dolomites at the top of each sequence. The medium-coarse crystalline dolomites or the oolitic dolomites beneath the fourth-order sequence boundaries generally have numerous dissolution pores and vugs. From bottom to top of each fourth-order sequence, the porosity and permeability gradually increase (Table 2). The porosity of the O₁t-ii and O₁t-vi sequences increases from 2.1% to 11.9% and 1.5% to 13.1%, respectively, and the permeability increases from 0.15 mD to 21.3 mD and 0.09 mD to 48.7 mD, respectively (Table 2).

Table 2.

Porosity and permeability of the Tongzi formation dolomites in the Banqiao outcrop in Zuiyi, Guizhou Province.

No.	Porosity (%)	Permeability (mD)	Distance to top (m)	Petrographic feature
Fourth-order sequence: O₁t-vi
1	13.1	48.7	0.2	Oolitic dolomite with dissolution pores
2	12.9	24.21	1.9	Oolitic dolomite with dissolution pores
3	11.5	29.3	3.1	Oolitic dolomite with dissolution pores
4	13.2	50.1	4.9	Medium-coarse crystalline dolomite
5	8.8	12.34	6.1	Oolite-bearing medium crystalline dolomite
6	9.6	10.21	7.8	Oolite-bearing medium crystalline dolomite
7	10.2	11.37	9.1	Medium crystalline dolomite
8	8.3	1.98	10.9	Medium crystalline dolomite
9	5.9	2.04	12	Fine-medium crystalline dolomite
10	4	1.22	13.8	Fine-medium crystalline dolomite
11	3	1.07	15.1	Oolite-bearing fine crystalline dolomite
12	4.8	1.64	16.6	Mud-bearing fine crystalline dolomite
13	3.5	0.89	18	Crypto-crystalline dolomite
14	2	0.27	19.5	Crypto-crystalline dolomite
15	2.3	0.43	21	Mud-bearing crypto-crystalline dolomite
16	1.5	0.09	22.5	Mud-bearing crypto-crystalline dolomite
Fourth-order sequence: O₁t-ii
1	11.9	21.3	0.1	Oolitic dolomite with dissolution pores
2	12.6	13.7	1.7	Oolitic dolomite with dissolution pores
3	11.3	16.15	3	Oolitic dolomite
4	9.8	6.37	4.3	Medium crystalline dolomite
5	7.6	9.52	5.7	Oolitic dolomite
6	8.4	11.25	7	Fine-medium crystalline dolomite
7	4.5	1.89	8.4	Oolitic dolomite
8	5.8	2.43	9.8	Fine crystalline dolomite
9	5.2	2.5	11	Fine crystalline dolomite
10	3.6	1.05	12.4	Fine crystalline dolomite
11	4.3	1.25	13.8	Fine crystalline dolomite
12	3	0.67	15.1	Mud-bearing fine crystalline dolomite
13	3.2	0.58	16.4	Lime crypto-crystalline dolomite
14	2	0.22	17.9	Lime crypto-crystalline dolomite
15	2.1	0.15	19.1	Bioclast-bearing lime fine crystalline dolomite

Discussion

Model of dolomitization

The occurrence of evaporative lagoons implies that the Tongzi Fm. sedimentary system was developed in a restricted shallow marine environment, which not only favoured the development of high energy shoal granular carbonates but also propelled the dolomitization process. To the east of the lagoon facies, high energy shoals, which are composed dominantly of packstones, such as oolitic or peloidal dolomites (Figure 4(b), (d), (e)), were developed at the edge of the Tongzi Fm. platform (Figures 5 and 8(a)) under the effects of high energy shallow seawater related to strong waves and tides.

Figure 8.

Models for the dolomitization and reservoir development of the Tongzi Fm. in the tidal flat-lagoon-shoal sedimentary system. (a) Reflux dolomitization model. To the west of the lagoon, evaporated seawater was propelled by waves and tides on to the tidal flat and then penetrated through the sediments in the tidal flat zone. To the east of the lagoon, evaporated seawater travelled downward through sediments in the high-energy shoal zone. The evaporated seawater led to dolomitization of the sediments in the tidal flat-lagoon-shoal sedimentary system. The profile is AA’ in Figure 1. (b) Model for syn-sedimentation meteoric dissolution at the fourth-order sequence boundary. Due to short-term relative sea level drop during sedimentation, the Tongzi Fm. dolomites were exposed above seawater and consequently leached by meteoric water, leading large numbers of dissolution pores and vugs in the dolomites. The exposed and dissolved surface became the fourth-order sequence boundary. The profile is AA’ in Figure 1. (c) Model for tectonic uplifting unconformity karst. Throughout the Late Silurian and Pre-Permian, the tectonic uplift related to the Guangxi Movement caused the Tongzi Fm. dolomites to suffer from meteoric karst leaching surrounding the Central Paleo-uplift area in the Sichuan Basin. The profile is BB’ in Figure 1.

Surface seawater circulation on such a platform was severely restricted because of the presence of some barriers, such as the shoals, leading to evaporation and a medium to high-salinity brine. The restricted seawater became significantly evaporated beyond gypsum saturation, leading to precipitation of gypsum in the lagoons. The precipitation of gypsum preferentially removed Ca²⁺ from the seawater and increased the Mg/Ca ratio. The Mg/Ca ratio in normal seawater is approximately 5:1. When this ratio rose to approximately 10:1, dolomitization presumably formed (Boggs, 2009).

As evaporative concentration continued, δ¹⁸O_V-PDB of seawater gradually increased while the ⁸⁷Sr/⁸⁶Sr ratios remained stable. The CO₂ solubility generally decreases with increasing salinity (Duan and Sun, 2003) such that partial CO₂ should bubble out of seawater as salinity increases due to continuous evaporation. Based on the isotope fractionation theory, ¹²C preferentially enters into gas-phase CO₂, while ¹³C remains in seawater. Consequently, the δ¹³C_V-PDB of CO₂ or CO₃²⁻ in seawater increases with evaporation. As a result, the dolomites precipitated from evaporated seawater generally have higher δ¹³C_V-PDB and δ¹⁸O_V-PDB values than the contemporary limestones, while their ⁸⁷Sr/⁸⁶Sr ratios are similar. Analyses of the unaltered crypto-fine crystalline dolomites of the Tongzi Fm. and the Lower Ordovician limestones showed exactly the same pattern (Table 1, Figures 6 and 7), suggesting that the evaporated seawater in the tidal flat-lagoon environment during the Tongzi stage favoured the dolomitization in the Sichuan Basin.

Many dolomitization models have been proposed, including the brine-refluxing model, mixed water model, Sabkha model, and burial, biological and hydrothermal models. However, large-scale dolomitization is commonly formed in a marine seawater environment (Liu et al., 2008; Machel, 2004; Warren, 2000).

A reflux model is responsible here for dolomitization of the Tongzi Fm. sediments (Figure 8(a)). The high Mg/Ca ratio of refluxing brine helps dolomite to overcome the kinetic barrier to precipitation (Machel, 2004; Wang et al., 2016). To the east of the lagoons, the evaporated hypersaline seawater travelled downward into and seaward through the platform and high-energy shoal sediments because of its elevated density, thereby dolomitizing the penetrated sediments. To the west of the lagoons, the hypersaline seawater could be propelled periodically onto the tidal flat zone and along remnant tidal channels by strong onshore winds, as well as by storms. It refluxes through the tidal flat zone via its increased density and led to the dolomitization of the sediments in the tidal flat zone. In the lagoon-platform environment, the carbonate sediments of the Tongzi Fm. were consequently expected to be dolomitized almost completely because the refluxing brine was supersaturated with respect to dolomite (Jones et al., 2003; Saller and Nuel, 1998).

Syn-sedimentation erosion

Fourth-order sequences

Influenced by short-term relative sea level rise and fall, one third-order sequence is expected to be composed of several fourth-order sequences (Handford and Loucks, 1993; Mitchum and Van Wagoner, 1991). The third-order sequence of the Tongzi Fm. in the Banqiao outcrop in Zunyi County, Guizhou Province is approximately 112 m thick. From bottom to top of the sequence, there are four coarsening-up sedimentary cycles (Figures 9 and 10). The sediments in each cycle are generally upwardly very thin-bedded shales, thin-bedded fine-medium crystalline dolomites, medium-thick-bedded medium-coarse crystalline dolomites and granular dolomites (Figures 9 and 10). The thickness of each cycle is approximately 30 m (Figure 9).

Figure 9.

Fourth-order sequences of the Tongzi Fm. and syn-sedimentation meteoric dissolution at the fourth-order sequence boundaries in the Banqiao outcrop, Zunyi, Guizhou Province.

Figure 10.

Photos of the coarsening-up fourth-order sequence of the Tongzi Fm. in the Banqiao outcrop, Zunyi, Guizhou Province. Photos A, B and C were taken at the bottom, central and top of the O₁t-ii fourth-order sequence, respectively. (a) Thin-bedded interlayers of fine crystalline dolomite and shale; A’ and A” are microscopic photos of the fine crystalline dolomites above the shales in the photo A, ×50, polarized light; (b) thin–medium-bedded fine crystalline dolomites with shale lamina; B’ and B” are microscopic photos of the fine crystalline dolomites in the photo B, ×50, polarized light; (c) thick-bedded medium-coarse crystalline dolomites; C’ and C” are microscopic photos of the dolomites in the photo C; C’ is oolitic dolomite, ×20, polarized light; C” is medium crystalline dolomite, ×50, polarized light.

Every coarsening-up sedimentary cycle developed in a period of rise and fall of relative sea level reflects a high frequency (fourth or fifth-order) sequence (Elrick, 1995). The four sedimentary coarsening-up sedimentary cycles in the Banqiao outcrop are actually four fourth-order sequences: O₁t-i, O₁t-ii, O₁t-iii and O₁t-iv (Figure 9). The development of each fourth-order sequence started as rapid rise of relative sea level, and the corresponding sediments were predominantly shales (Figures 9 and 10(a)). During the gradual fall of the relative sea level, the sediments were thin-bedded fine crystalline dolomites with shale interlayers (Figures 9 and 10(b)). At the late stage of the cycle, the relative sea level fell to its lowest stand and the sediments were mainly thick-bedded medium-coarse crystalline dolomites or high-energy shoal granular dolomites (Figures 9 and 10(c)).

Syn-sedimentation erosion at the fourth-order sequence boundary

The development and vertical stack of high-frequency sequences are probably related to sea level fluctuations induced by climate variations and/or short-term tectonic movements in the Milankovitch frequency band (Strasser, 1994). At the end of the frequency, the top of the sequence might experience subaerial exposure and meteoric erosion due to fall of relative sea level. The repeated high-frequency relative sea level oscillations must lead to repeated exposure of the carbonate sediments (Saller et al., 1994; Shen et al., 2015).

The Tongzi Fm. dolomites in the Sichuan Basin were formed in an evaporated shallow marine environment (Figure 8(a)). At the end of each fourth-order cycle, the peloidal, oolitic or coarse crystalline dolomites at the top of the fourth-order sequence were subjected to be subaerially exposed and then underwent meteoric erosion (Figure 8(b)) (Fan et al., 2007; Saller et al., 1994; Zhu et al., 2015b). After the syn-sedimentation meteoric erosion, the dolomites contained numerous dissolution pores and vugs (Figures 3(e), (f), 4(j) to (l), and 8(b)), some of which were filled with medium-coarse or giant non-planar saddle dolomite crystals (Figures 3(g), (h) and 4(f), (g)). The top of the sequence with dissolution pores and vugs was the fourth-order sequence boundary (Figure 9). After the short-term exposure, the next fourth-order sequence started as the rapid rise of relative sea level and stacked above the previous sequence.

Irregular erosional surfaces (Figure 11(a) and (b)), few dissolution grooves (Figure 11(b)), and dissolution breccia (Figure 11(c)) commonly occurred in the thick-bedded coarse crystalline dolomites or the oolitic dolomites at the top of each fourth-order sequence, suggesting that the coarse crystalline or the oolitic dolomites underwent syn-depositional erosions. Very thin layers of weathered yellow brown clays (Figure 11(b) and (d)) could be occasionally observed in the erosion zone, implying syn-sedimentation exposure and a subsequent oxidizing process (Hardie et al., 1986; Zhu et al., 2015b).

Figure 11.

Characteristics of the fourth-order sequence boundaries in the Tongzi Fm. (a) Irregular erosional surface in the dolomites, Banqiao outcrop in Zunyi, Guizhou Province; (b) dissolution groove and yellow brown very-thin layer of weathered clays in the oolitic dolomites, Yankong outcrop in Jinsha county, Guizhou Province; (c) dissolution breccia in the dolomites, Banqiao outcrop; (d) Brown ferrous and clay lamina in the medium-coarse crystalline dolomites, Shuanghe outcrop in Changning County, Sichuan Province.

Geochemical constraints on Syn-sedimentation erosion

Compared with diagenetically unaltered dolomites, the leached dolomites related to subaerial exposure and meteoric dissolution generally have relatively lighter carbon and oxygen isotope compositions (Goldstein et al., 1991; Hajikazemi et al., 2010). The δ¹³C_V-PDB and δ¹⁸O_V-PDB values of the dolomites with numerous dissolution pores and vugs at the top of each fourth-order sequence of the Tongzi Fm. are lower than those of the unaltered crypto-fine crystalline dolomites (Table 1, Figure 6), suggesting the dolomites at the fourth-order sequence boundaries underwent meteoric erosion.

When the meteoric precipitation runs off on the surface, it can pick up a fair amount of ⁸⁷Sr from sandy argillaceous debris, thereby increasing the ⁸⁷Sr/⁸⁶Sr ratio. The eroded carbonates should have high ⁸⁷Sr/⁸⁶Sr ratios due to the interactions between carbonates and meteoric precipitation (Zhu et al., 2015a). The eroded dolomites at the top of fourth-order sequences have ⁸⁷Sr/⁸⁶Sr ratios between 0.709961 and 0.711897, with an average of 710,457, significantly higher than those of the unaltered crypto-fine crystalline dolomites (Table 1, Figure 7), also implying corrosion by meteoric water.

Dolomite reservoir in the fourth-order sequence

Variations of porosity and permeability of the Tongzi Fm. dolomite reservoirs are closely related to the fourth-order sequences and changes of relative sea level (Figure 9). At the beginning of each fourth-order sequence, the thin-bedded crypto-fine crystalline dolomites formed when the relative sea level was relatively high and have low porosity and permeability. The fine-medium crystalline dolomites formed during gradual fall of relative sea level and have medium porosity and permeability. At the end of each fourth-order sequence, the medium-coarse crystalline dolomites and the granular dolomites formed in shallow high-energy environment and have high porosity and permeability as they contain a large number of dissolution pores and vugs. The dolomites of the O₁t-ii fourth-order sequence in the Banqiao outcrop have upward increasing porosity and permeability, from 2.0% to 12.6% and 0.15 mD to 21.3 mD (Table 2, Figure 9), respectively. The porosity and permeability of the O₁t-iv dolomites similarly increase upward from 1.5% to 13.1% and 0.09 mD to 48.7 mD (Table 2, Figure 9).

Several wells and outcrops in the east and southeast of the Sichuan Basin reveal the four fourth-order sequences of the Tongzi Fm. (Figure 12). The high-porosity dolomite reservoirs are commonly developed in the middle-upper part of the fourth-order sequences, especially beneath the fourth-order sequence boundaries (Figures 9 and 12), because these dolomites are coarse crystalline dolomites or oolitic dolomites and easily influenced by syn-sedimentation meteoric erosion.

Figure 12.

Correlation of the fourth-order sequences and dolomite reservoirs of the Tongzi Fm. in the east and southeast of the Sichuan Basin. The location of this profile is marked as CC’ in Figure 3.

Tectonic unconformity

During the Late Caledonian (Late Silurian to pre-Permian), the Sichuan Basin went through a significant tectonic uplift event, i.e. the Guangxi Movement. This had a profound influence on the landscape of the Sichuan Basin. From west to the central basin, an angular unconformity came between the Permian and the pre-Permian strata, including the Sinian, Cambrian, Ordovician and Silurian (Figure 13). The unconformity is the first-order sequence boundary in the western and central Sichuan Basin.

Figure 13.

Anshui88 seismic section shows the unconformity surface related to the Guangxi Movement in the central Sichuan Basin. The location of this section is shown as DD’ in Figure 1. It is a layer-levelling section along the TP₁ unconformity surface. TЄ₁, TO, TS and TP₁ stand for the seismic reflection boundaries below the Lower Cambrian, Ordovician, Silurian and Lower Permian strata, respectively.

The Guangxi Movement caused the Sinian, Cambrian and Ordovician carbonates in the west and central basin to become subaerially exposed and corroded by meteoric water. The Tongzi Fm. strata were stripped off in most of the eroded area but remained in a belt surrounding the Central Paleo-uplift. The remaining Tongzi Fm. dolomites were further altered by meteoric karst (Figure 8(c)).

Many drilling wells have revealed karst reservoirs of the Tongzi Fm. dolomites surrounding the Central Paleo-uplift below the unconformity surface. For example, well Nvji has a 39 m-thick Tongzi Fm. dolomite remnant at the depth of 4518–4557 m, containing abundant intercrystalline pores and corrosion vugs. Black dry asphalt, quartz and dolomites are often found filling in the vugs. It was tested and produces gas at 3.09×10⁴m³ per day. Abundant karst pores and vugs were found in the Tongzi Fm. dolomites at approximately 4154 m in the well Anping 1. The vugs have an average size of 6–50 mm. At depth of 4539.47–4550.24 m, the Tongzi Fm. dolomite cores from the well Nvshen 5 contain large amounts of karst caves. This well produces gas at 719 m³/day from the Tongzi Fm. dolomite reservoirs.

Distribution of dolomite reservoirs

The sedimentation of the tidal flat-lagoon-shoal system under a shallow evaporated marine environment is the basis for the large-scale development of the Tongzi Fm. dolomite reservoirs in the Sichuan Basin. The dolomites became high-porosity reservoirs because they not only contain abundant primary intercrystalline, intergranular and intragranular pores but were also secondarily eroded by meteoric water under the fourth-order sequence boundaries and the tectonic unconformity.

Based on data from drilling wells and field outcrops, the thickness distribution of the Tongzi Fm. dolomites in the Sichuan was sketched (Figure 14). As seen in Figure 14, the Tongzi Fm. dolomites are broadly distributed within the Sichuan Basin, except for the unconformity zone in the west and the Central Paleo-uplift. The dolomites in the east and southeast of the Sichuan Basin are relatively thicker. For example, well Jinshi 1 in the southeast revealed Tongzi Fm. dolomites of approximately 73 m, well Ximen 1 in the east had a dolomite layer of 97 m, and well Dingshan 1 had a dolomite layer of 110 m.

Figure 14.

Distribution of the Ordovician Tongzi Fm. dolomite reservoirs in the Sichuan Basin.

In the east and southeast of the Sichuan Basin, the development of high-porosity dolomite reservoirs was controlled dominantly by the high-energy shoal facies as well as the syn-sedimentation meteoric erosion. According to the sedimentary facies (Figures 5 and 14), there are several high-energy granular shoals surrounding evaporative lagoons. The shoal dolomites were further eroded by meteoric water during syn-sedimentation subaerial exposure (Figure 8(b)), and consequently, the high-porosity dolomite reservoirs mainly occurred at the middle–top part of each of the fourth-order sequences, especially under the fourth-order sequence boundaries (Figures 9 and 12). The thickness of the dolomite reservoirs in each of fourth-order sequences is approximately 10–20 m (Figures 9 and 12).

In the Central Paleo-uplift, the development of the Tongzi Fm. high-porosity dolomite reservoirs was controlled by meteoric karst related to the tectonic uplift in the Guangxi Movement (Figure 8(c)). The distribution of karst dolomite reservoirs was limited in a narrow belt surrounding the Central Paleo-uplift (Figure 14). Below the unconformity surface, the Tongzi Fm. dolomite reservoirs are generally 20–50 m thick, containing abundant asphalt and gas.

Conclusion

During the early Ordovician Tongzi stage, tidal flat, lagoon, and high-energy shoal facies were developed in a shallow evaporative marine environment in the Sichuan Basin. The high-energy shoals commonly surround the evaporative lagoons. The process of the dolomitization in this environment can be depicted by the refluxing model.

In the shallow evaporative marine environment, the Tongzi Fm. dolomites were subject to syn-sedimentation subaerial exposure and meteoric erosion due to short-term fall of relative sea level, and consequently, high-porosity dolomite reservoirs were developed principally beneath the fourth-order sequence boundaries.

Petrography variations, carbon, oxygen, and strontium isotope features and the presence of an erosion groove, weathered clay lamina, breccia and dissolution vugs support the syn-sedimentation subaerial meteoric erosion under fourth-order sequence boundaries.

High-porosity dolomite reservoirs controlled by high-energy shoal facies plus syn-sedimentation meteoric erosion under fourth-order sequence boundaries were widely distributed in the east and southeast of the Sichuan Basin, while the dolomite reservoirs in the narrow belt surrounding the Central Paleo-uplift were further influenced by unconformity meteoric karst related to tectonic uplift during the Guangxi Movement.

Footnotes

Declaration of conflicting interests

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

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship and/or publication of this article: This work was financially supported by the Strategic Priority Research Program of the Chinese Academy of Science (Grant No. XDA14010201) and National Natural Science Foundation of China (Grant Nos. 41372149, 41625009 and U1663209).

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