Characteristics and Genesis of Dolomites from the Devonian Guanwushan Formation in the Jiguanshan Section, southwest Sichuan Basin, China

doi:10.21203/rs.3.rs-2853924/v1

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Characteristics and Genesis of Dolomites from the Devonian Guanwushan Formation in the Jiguanshan Section, southwest Sichuan Basin, China

https://doi.org/10.21203/rs.3.rs-2853924/v1

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The Guanwushan Formation of Middle Devonian in Sichuan Basin is an excellent oil and gas reservoir, but its genetic mechanism is still unclear, and previous studies mainly focus on the typical profile in northwest Sichuan. In this paper, the characteristics and genesis of dolomite in Guanwushan Formation are studied by taking the Jiguanshan section in southwest Sichuan as an example. The section of Jiguanshan in Chongzhou City is located in the middle of Longmen Mountain and the southwest of Chengdu. Through field profile measurement, petrological observation, isotope analysis, REE distribution mode analysis, fluid inclusion analysis, the following understandings are obtained. 1. The Guanwushan Formation of the profile is in conformity with the overlying Shawozi Formation and the underlying Jinbaoshi Formation; 2. The dolomite types developed in Guanwushan Formation in the profile mainly include reef dolomite, and “zebra” structure dolomite can be seen. 3. On the profile, a large number of organisms can be observed, mainly including tetragonal corals, transverse corals, cladocoropsis, etc. Under the microscope, corals with intact diaphragms can be observed on thin slices; 4. The Jiguanshan section is a platform margin facies deposit, located on the continental margin side of the carbonate platform; 5. The carbon and oxygen isotope contents in the samples are all within the distribution range of carbon and oxygen isotopes of seawater in the same period of the Devonian system. It is speculated that most of the dolomite in the study section are reef quasi contemporaneous dolomite, samples’ δ¹⁸O content can be divided into three types of dolomite according to different temperatures; The distribution pattern of rare earth elements in the sample shows obvious δCe negative anomaly, speculated to be Devonian sea water, or slightly modified Devonian sea water quasi syngenetic origin, with obvious zebra dolomite filling δEu positive anomaly is a typical hydrothermal dolomite; The inclusion homogenization temperature test shows that there are two different origins of dolomite on the profile.

Sichuan Basin

Guanwushan Formation

Platform margin facies

Dolomite genesis

Dolomite reservoir is an important oil and gas reservoir all over the world. A large amount of oil and gas are enriched in dolomite reservoir all over the world, which is the focus of current oil and gas exploration. Dolomite is widely distributed in China, almost in every geological age, and is an important reservoir of oil and gas reservoirs in China (He et al. 2020). Dolomitization is of great significance in the Formation of carbonate reservoirs in Sichuan Basin (Liu et al. 2008). However, the genesis of dolomite is complex, which is a hot issue in current research. Scholars at home and abroad have also put forward a variety of different genetic models of dolomite in many years of research.

In the whole Devonian period, Sichuan Basin was under an uplifting process, Devonian deposition was denudated. In this case, the Devonian formation in Sichuan Basin could only be found in several profiles and wells along the Longmen Mountain, however, oil and gas resource were found in Middle-Devonian Guanwushan formation, the wide-developed reef facies could also be a signature of developing history of the basin.

The Devonian system in Sichuan basin is an important excellent reservoir (Hao et al. 2016). The Middle Upper Devonian dolomite in northwest Sichuan has good reservoir forming conditions, good gas storage conditions, and broad gas exploration prospects (Xiong et al. 2017). Among them, Guanwushan Formation has excellent reservoir performance (Xiong et al. 2019). The predecessors have done a lot of work on the Guanwushan Formation strata in the west of Sichuan Basin, but most of these studies focus on Ganxi, Liangshuidong and other profiles in the northwest of Sichuan, as well as ST-3 well locations. Most of them involve sequence stratigraphy, sedimentary facies palaeogeography (Xiong et al. 2019; Luo 2014; Xiong et al. 2020; Li et al. 2020; Xiong et al. 2017; Li et al. 2015), and a few involve ecological strata (Luo 2014), as well as reservoir characteristics (Zhao and Wang 2004; Xiong et al. 2017; Ya et al. 2020; Lin et al.2021) and dolomite genesis (He et al. 2020; Hao et al. 2020; Deng et al. 2018; Lai et al 2020).

The dolomite of Guanwushan Formation is also exposed in the southwest of Sichuan Basin, but there are few studies involved at present. Therefore, the Jiguanshan section in the southwest of Sichuan is selected as the research object in this study to conduct a detailed study on the origin of dolomite.

The Jiguanshan section is located in the southwest of Sichuan Basin. The Guanwushan Formation is well exposed, rich in fossils and sedimentary tectonic phenomena, and typical stromatoporoids of reefs can be seen. It is a typical Devonian stratigraphic section. Through the observation of the field profile and microscopic thin sections, combined with the analysis of carbon and oxygen isotopes, rare earth elements and fluid inclusions, this paper has conducted a systematic study on the genesis of the dolomite in this profile, which is conducive to defining the distribution characteristics of the Guanwushan Formation strata in the Longmenshan area of Sichuan Basin, and comparing its dolomite genesis with other areas, also has certain geological significance.

The study area is located, and situated in Longmenshan fold belt between the Songpan-Ganzi orogenic belt and the low-lying fold belt of northern Sichuan in structural map.

The section of Jiguan Mountain is located in the west margin of Upper Yangtze platform with Bayan Har Ocean to the northwest and Mianlue Ocean to the north in paleogeographical map, meantime it is in the southwest of Chengdu, with its back against Longmen Mountain, facing the Chengdu Plain, and in the middle south section of the Longmen Mountain structural belt, with complex structural conditions. Longmenshan structural belt is a structural belt with strong fold and fault activity, multi-phase composite and large scale, consisting of a series of east-west, northeast, northwest and north-south uplifts, depressions, single and compound folds, and compressive and compressive torsional faults (Guo et al. 2009; Yang et al. 2020). Chongzhou is a hilly area in the central and western regions, a plain in the southeast, and a low mountain high mountain landform in the vast western regions. The low mountain areas in the west are affected by faults and folds. The rock mass is broken, the cleavage fissure is developed, the terrain is deeply cut, and the relative height difference is large.

The Guanwushan Formation on the profile is in conformity with the overlying Shawozi Formation and the underlying Jinbaoshi Formation.

The Jiguanshan section in Chongzhou is a platform margin facies deposit (Fig. 1), which is located on the continental margin side of the carbonate platform. The seawater circulation is good, the salinity is basically normal, the water energy is high, and reefs and shoals are developed. The rocks are mostly gray, dark gray, medium thick and massive, with strong biological disturbance. Nodules and horizontal bedding are clearly visible on the profile.

The lithology of the section is mainly carbonate rock, and most of the field sections are developed with various dolomite. Only a few clastic rocks are developed at the bottom of the Guanwushan Formation, and evaporites are almost not developed. The main carbonate rocks types are micritic-fine crystal dolomite, breccia dolomite, reef dolomite, bioclastic dolomite, micritic crystal limestone, bioclastic limestone, etc., of which reef dolomite is the main rock types of Guanwushan Formation in the Jiguanshan section. Various kinds of organisms could be found on the profile. The main reef building organisms are stromatoporoids and corals. The dissolution on the dolomite of most bioherms is strong, and a large number of dissolution pores are developed.

All samples come from the jiguanshan profile which is 245m long, the profile is divided into 30 layers, the samples are mainly taken in the bottom to middle of the profile, to avoid errors caused by heavy vegetation and unclear stratum boundary.

Except from microscope-observed samples that need to be cut into chips, all other samples are made into fine powder. After being taken from the profile, all samples are numbered immediately according to layers they’ve been taken from.

The carbon and oxygen isotope samples are first divided into smaller pieces to expose their fresh faces, then mark the sampling area in a 1.5-2cm² square, in order to reveal different geochemical characteristics among all types of dolomites, samples were taken from matrix, bright stripes and biological coelom. Powder was scratched off by a small drill, then collect the powder in a plastic test tube with a cover on top, each sample weights between 0.5-0.75g, and is marked according to original hand specimen.

The ICP-MS (Inductively coupled plasma mass spectrometry) was completed in the Wuhan Sample Solution Analytical Technology Co., Ltd., to obtain the REE pattern. Each powder sample weights more than 5g, and particle size is more than 200 meshes. Trace element analysis of whole rock were conducted on Agilent 7700e ICP-MS. The detailed sample-digesting procedure was as follows: (1) Sample powder (200 mesh) were placed in an oven at 105℃ for drying of 12 hours; (2) 50 mg sample powder was accurately weighed and placed in a Teflon bomb; (3) 1 ml HNO3 and 1 ml HF were slowly added into the Teflon bomb; (4) Teflon bomb was putted in a stainless steel pressure jacket and heated to 190℃ in an oven for over 24 hours; (5) After cooling, the Teflon bomb was opened and placed on a hotplate at 140℃ and evaporated to incipient dryness, and then 1 ml HNO3 was added and evaporated to dryness again; (6) 1 ml of HNO3, 1 ml of MQ water and 1 ml internal standard solution of 1ppm In were added, and the Teflon bomb was resealed and placed in the oven at 190℃ for over 12 hours; (7) The final solution was transferred to a polyethylene bottle and diluted to 100 g by the addition of 2% HNO3. All data is operated in ICPMSDATACAL 10.0.

The fluid inclusion homogenous temperature analysis is completed in School of Geoscience and Technology, Southwest Petroleum University, the device is THMSG 600 made by LINKAM SCIENTIFIC INSTRUMENTS, UK. The system contains geological cold and hot platform and polarizing microscope. Temperature measurement ranges from − 196℃ to 600℃; Temperature accuracy and stability is 0.2℃; Diameter of light hole is 2.0mm; Axial movement of sample X and Y: 16mm; Sample heating area is 20×20mm; Maximum heating rate is 100 ℃/min; Minimum objective/condenser working distance is 4.5/12.5mm.

4.1. Petrological characteristics of dolomite

The general category of dolomite in the studyis reef dolomite. In order to reveal the different genesis among these types, these dolomites are divided into 2 types according to their size of dolomite crystal (Fig. 2, Fig. 3).

1.Fine crystal dolomite

This type is considered as the main type of dolomite in this profile, which exists as the base of the profile, various kind of organisms could be found due to the positive sedimentary environment. Microscopic observation shows that the foggy center and bright edge is clear, biophantoms could be clearly observed through microscope, crystals are all of closely contacted unequal grain structure, with severe dolomitization, which is a semi-idiomorphic xenomorphic structure. The pores are sometimes filled with quartz.

2.Middle-coarse crystal dolomite

The bigger crystalline dolomites are often filled in dissolved biological coeloms, pores and cracks, with brighter surfaces and sharper edges than fine crystals, the idiomorphic degree is much better. On the other hand, this type of dolomite doesn’t come from former metasomatized structure, the crystals are mostly intact with clear margins, which differs from the characteristics of fine crystal dolomite. This may be caused by different dolomization machanisms between these two types.

(a) Zebra pattern dolomite, visible light and dark bands, dark bands are matrix fine crystal dolomite, bright bands are medium coarse crystal dolomite, Jiguanshan section, Guanwushan Formation; (b) Zebra striated dolomite, fissure cave development, karst cave interior is filled with medium coarse crystal dolomite, Jiguanshan section, Guanwushan Formation; (c) Reef dolomite, the matrix is fine crystal dolomite, and karst caves are developed. The interior of karst caves is filled with medium coarse crystal dolomite, Jiguanshan section, Guanwushan Formation; (d) Reef dolomite, the matrix is fine crystal dolomite, and the organism cavity is visible, and the cavity is filled with medium coarse crystal dolomite, Jiguanshan section, Guanwushan Formation

(a) JGS20-2, fine-grained bioclastic dolomite with grain phantom structure, x1, (-); (b) JGS20-2, fine-grained bioclastic dolomite, with grain phantom structure 2, x1, (-); (c) JGS20-4-1, fine crystalline bioclastic dolomite, the bioclastic interior is filled with Phase III dolomite, the first phase is fine crystalline dolomite, the second phase is medium coarse crystalline dolomite, and the third phase is mega-crystal dolomite, x5, (-); (d) JGS20-5, fine crystalline bioclastic dolomite, filled with medium coarse crystalline dolomite, calcite, asphalt, x1, (-); (e) JGS20-5-1, fine crystal dolomite, filled with medium coarse crystal dolomite, calcite, asphalt, x1, (+); (f) JGS23-1-3, fine-grained bioclastic dolomite with quartz filling, x5, (+); (g) JGS24-2, fine-grained bioclastic dolomite, sponge dolomitization, x1, (-); (h) JGS11, reef dolomite, matrix is fine crystal dolomite, fine medium crystal dolomite, x1, (-); (i) JGS17, reef dolomite, matrix is fine crystal dolomite, fine medium crystal dolomite 3, x1, (-)

Figure 3 Microscopic Characteristics of Dolomites in Guanwushan Formation of Jiguanshan Section

Table 1

Degree of order of Guanwushan Formation dolomite on Juguanshan profile
Sample No.	Degree of Order (Peak Height Ratio)	Degree of Order (Area Ratio)	CaCO₃ Mole fraction	MgCO₃ Mole fraction
JGS 20-3-A	0.36	0.38	49.38	50.62
JGS 20-3-B	0.42	0.46	49.31	50.69
JGS 23-2-A	0.76	1	50.45	49.55
JGS 23-2-B	0.40	0.40	49.27	50.73
JGS 18-1-B	0.48	0.49	50.47	49.53

After dividing these samples into different categories, it is not hard to tell some obvious regulations among these samples from microscopic pictures. The original biological structures are usually well preserved after multiple dolomization, dolomite crystals are relatively small and closely contact, the organisms are mainly stromatoporus as main frame of the reef, the foggy core and bright edge texture indicates that these matrix fine crystals come from the earlier mud-fine limestone. In penecontemporaneous period, strong evaporation and gravity influence make the fluid in the sediment infiltrate downward, high Mg²⁺ fluid was well preserved in the limestone crystal texture, without big structural movement background, the original fine crystal texture and biological fabric are completely preserved (Table 1, Fig. 3), in later burial period, the first stage dolomization was complete.

Another characteristic is that, except the fine matrix dolomite, middle-coarse dolomite still widely exists, but only in or around cracks and secondary pores, this phenomenon indicates different genetic mechanisms. Sample JGS23-2 was taken from the bright stripe of zebra structure, showed a higher degree of order than other matrix samples, middle-coarse crystal dolomites clearly formed later than the matrix by different diagenetic fluid, the fluid could be from the deep, or original fluid that has been squeezed out under compaction, and then sedimented in these tunnels. Dissolution went along with hot fluid dolomization, however, primary porosity nearly disappeared under strong pressure during burial period, what was left are generally secondary porosity, these pores are mainly intergranular pores and intergranular dissolved pores.

4.2. Carbon and oxygen isotopes

Carbon and oxygen isotopes are often used to indicate diagenetic fluids from different sources of carbon and oxygen elements, which can be used to trace the fluid source of fluid derived rocks and play a role in tracing and environmental inversion (Chen 1998; Rollion-Bard et al. 2019). The characteristics of carbon and oxygen isotopes in carbonate rocks can reflect the sedimentary environment of ancient seawater (Neir A O and Gulbransen E A 1939; Urey et al. 1951; Allan J R and Matthews R K 1977; Scholle P A and Arthur M A 1980). According to previous studies, the carbon and oxygen isotopes in brachiopod fossils are less affected by the late transformation, and can precisely reflect the carbon and oxygen isotopic composition of ancient seawater (Carpenter S J and Lohmann K C 1995; Rollion-Bard et al. 2019; Cheng et al. 2007). Through the study of brachiopod fossils, it is concluded that the Devonian Guanwushan seawater δ¹³C value ranges from − 0.8‰ to 1.2‰ (PDB), The δ¹⁸O value ranges from − 5.8‰ to -4.2‰ (PDB) (Cheng et al. 2009).

Table 2

Carbon and oxygen isotope characteristics of dolomite samples from Guanwushan Formation in Jiguanshan Section
Sample No.	δ¹³C‰(VPDB)	δ¹⁸O‰(VPDB)	lithology
JGS18-1-A	1.5	-5.87	Fine crystal dolomite filled in the cave
JGS18-1B	1.63	-6.54	Matrix fine crystalline dolomite
JGS18-2	0.63	-7.45	“Zebra” structure, dolomite filled in fractures and caves
JGS20-1A	2.18	-5.7	Reef dolomite reef
JGS20-1B	2.45	-6.56	Dolomite filling in reef dolomite cave
JGS20-4	1.77	-6.08
JGS20-4A	2.3	-6.28	Medium fine crystal dolomite filled in the tunnel
JGS20-5-3	2.65	-6.24	Matrix dolomite (fine crystal), filled with dolomite in the tunnel but not taken out
JGS22-2-3	2.49	-5.62	Fine crystal dolomite (matrix)
JGS23-1-1A	1.84	-9.1	“Zebra” structure, dolomite filled in fractures and caves
JGS23-1-2	1.64	-9.41	“Zebra” structure, dolomite filled in fractures and caves
JGS24-1	0.67	-4.16	Dolomite in coral coelom, reef dolomite

It can be concluded from Table 1 that the oxygen isotope δ¹⁸O negative deviation of fine-grained dolomite of is small, ranging from − 6.57‰ to -5.62‰, while the δ¹⁸O negative deviation of dolomite filled in “zebra” structure is large with the value between − 9.41‰ and − 7.45‰. In the study area, fine crystal dolomite often exists as a matrix or base, and part of it is used as a filler between reefs and fractures; In the stripes of zebra dolomite, the grain size of dolomite is usually medium to coarse crystal.

Carbon isotopes of fine crystal dolomite δ¹³C value is between 1.5‰ and 2.65‰, carbon isotope in dolomite filled in zebra structure δ¹³C value is between 0.63‰ and 1.84‰, which is obviously lower than that of fine crystal dolomite.

From the above data, it can be found that the sample δ¹³C content is positive, δ¹⁸O value is relatively negative, indicates that most of the dolomite of Guanwushan Formation on the section of Jiguanshan has undergone late transformation during burial. The carbon and oxygen isotope characteristics of fine crystal dolomite and zebra structure on the profile are obviously different, the difference of δ¹³C and δ¹⁸O value is clearly larger than that of fine crystal dolomite, which indicates that dolomite in zebra structure has been greatly reformed in the later period, or even formed by different mechanisms. For the JGS24-1 sample, the sample is taken from inside the coral body cavity, of which carbon and oxygen isotope are least affected, because the diagenetic fluid in the coral body cavity comes directly from the seawater of the same period, which is less reformed by diagenesis in the later period, the content deviation is small, and it is basically distributed in the carbon and oxygen allotropic distribution range of the ancient seawater of the same period. The oxygen isotope δ¹⁸O content will change under the influence of temperature. Therefore, the diagenetic temperature of dolomite can be classified according to the oxygen isotope content. The results show that reef dolomite and matrix fine crystal dolomite have lower diagenetic temperature and are less affected by transformation, while medium-coarse crystal dolomite has higher diagenetic temperature and is affected by high temperature in the later period (Fig. 4).

4.3. Rare earth element

The composition of rare earth elements can reflect the carbonate sedimentary environment and the source of diagenetic fluid (Nothdurft et al. 2004), and the rare earth elements in carbonate rocks are less affected by diagenesis, which can be used to analyze the origin of dolomite (German C R and Elderfield H 1990). According to Table 3, the total mass fraction of rare earth is 6.1390 ~ 25.0528, with an average of 14.5819; The mass fraction of LREEs ranges from 4.4701 to 16.7537, with an average of 10.0163; The mass fraction of heavy rare earths ranges from 1.6690 to 8.2991, with an average of 4.5656. Light rare earth elements are more enriched than heavy rare earth elements, and the average content of light rare earth elements is 2.3 times that of heavy rare earth elements.

According to the comparison of diagrams, the distribution pattern of rare earth element content in most samples is not different from that in seawater, indicating that the overall diagenetic fluid of dolomite in Jiguanshan section is Devonian ancient seawater. A few samples, such as JGS 20 − 3 and JGS 18 − 2, have obvious positive Eu anomalies (Fig. 5), reaching 2.0747 and 1.5811 respectively, which is much higher than the range of 0.2790 ~ 0.9637 for other samples, indicating that they have undergone hydrothermal transformation under high-temperature burial conditions in the later period.

Table 3

REE Distribution Pattern of Dolomite Samples of Guanwushan Formation in Jiguanshan Section
Sample No. Element	JGS 22 − 4	JGS 23-1-2	JGS 20 − 2	JGS 20-2-1	JGS 20 − 3	JGS 20-4-1	JGS 20-5-2	JGS 18 − 2	JGS 18 − 3	JGS 22-1-2
La	3.3768	2.3726	1.7727	5.4413	3.7719	2.3529	1.2951	3.4744	4.6437	3.0667
Ce	1.6970	1.0342	0.9116	2.5031	2.0192	1.0681	0.7358	2.3267	2.3603	1.6499
Pr	1.8957	1.5053	1.2380	3.3410	2.5218	1.4154	0.9149	2.8452	2.9003	1.7853
Nd	1.4488	1.1697	1.0535	2.6464	2.0894	1.1539	0.7878	2.5286	2.3400	1.4342
Sm	1.0070	0.8388	0.7013	1.8581	1.4019	0.6578	0.4160	1.5380	1.3496	0.9788
Eu	0.5397	0.7142	0.5309	0.9637	2.0747	0.2994	0.3205	1.5811	0.8598	0.4593
Gd	0.7684	0.7587	0.5432	1.5622	1.1458	0.5650	0.3624	1.3532	1.3593	0.7998
Tb	0.6733	0.5410	0.5106	1.1313	0.8467	0.4219	0.2533	1.0797	0.8265	0.5342
Dy	0.6577	0.4421	0.4224	1.1091	0.7123	0.3795	0.2247	0.9053	0.8614	0.6378
Ho	0.5813	0.4120	0.4328	1.0574	0.7129	0.3870	0.2033	0.8652	0.8368	0.4985
Er	0.5951	0.3853	0.4371	0.9554	0.6177	0.3656	0.1477	0.8836	0.7764	0.5596
Tm	0.4641	0.3282	0.3266	0.9660	0.4970	0.3193	0.2087	0.6831	0.5967	0.4870
Yb	0.3772	0.2668	0.3289	0.8462	0.4272	0.3399	0.1481	0.6067	0.5061	0.3386
Lu	0.5184	0.1530	0.3902	0.6715	0.3374	0.2598	0.1207	0.6455	0.5077	0.3464
∑REE	14.6004	10.9218	9.5997	25.0528	19.1758	9.9856	6.1390	21.3163	20.7245	13.5761
LREE	9.9649	7.6346	6.2081	16.7537	13.8788	6.9476	4.4701	14.2940	14.4537	9.3742
HREE	4.6355	3.2872	3.3916	8.2991	5.2970	3.0380	1.6690	7.0223	6.2708	4.2019
LREE/HREE	2.1497	2.3225	1.8304	3.1629	2.6201	2.2869	2.6783	2.0355	2.3049	2.2310
La_N/Yb_N	8.9514	8.8910	5.3904	6.4303	8.8284	6.9216	8.7440	5.7267	9.1762	9.0569
δEu	0.6080	0.8941	0.8532	0.5635	1.6286	0.4896	0.8235	1.0937	0.6348	0.5165
δCe	0.6437	0.5334	0.6056	0.5700	0.6417	0.5669	0.6658	0.7364	0.6258	0.6801

4.4. Fluid inclusion

Inclusion homogenization temperature test can reflect the temperature information of diagenesis and can effectively judge the fluid diagenetic environment.

The samples in this test are all original gas-fluid inclusions, which are distributed in scattered or isolated status, in ellipse or strip shape. Except from unqualified and inaccurate samples, 11 results of 2 dolomite samples were obtained (Fig. 6, Table 4), in fact, most inclusions are preserved in calcite veins, typical dolomite inclusions are hard to observe, as a result, in later research, we used some calcite inclusions as references and comparisons. The temperature distribution range is large, with the lowest temperature of 96.5℃ and the highest temperature of 185.8℃, dolomite inclusions range from 123.5℃ to 172.4℃, but in general all results reveal high temperature, indicates that high temperature. In terms of temperature. When reef dolomite and matrix fine crystal dolomite are diagenetic, the burial depth is shallower than that of medium coarse crystal dolomite, and medium coarse crystal dolomite is formed by underground hydrothermal fluid transformation during deep burial.

Through the homogenization temperature test of inclusion, it is clear to find two different temperature sections, the low-temperature section consists of inclusions in matrix dolomite with temperature no higher than 150℃, the high-temperature section is composed of middle-coarse dolomite cement with temperature mostly above 170℃. Homogenization temperature result is consistent with the microscopic observation, carbon and oxygen isotope and REE concentration results.

Table 4

Homogeneity Temperature of Fluid Inclusions in Dolomites of Guanwushan Formation in Jiguanshan
Sample No.	Lithology	Status	Shape	Size (µm)	Temperature (℃)
JGS 18 − 2	Dolomite	Isolated	Strip	3	150.7
JGS 20-4-1	Dolomite	Isolated	Ellipse	2	153.9
JGS 20-4-1	Dolomite	Scattered	Ellipse	3	147.1
JGS 20-4-1	Dolomite	Scattered	Strip	4	150.1
JGS 20-4-1	Dolomite	Scattered	Strip	3	148.5
JGS 20-4-1	Dolomite	Scattered	Strip	4	156.7
JGS 20-4-1	Dolomite	Isolated	Strip	5	172.4
JGS 20-4-1	Dolomite	Isolated	Ellipse	3	153.5
JGS 20-4-1	Dolomite	Scattered	Ellipse	3	130.8、124.8
JGS 20-4-1	Dolomite	Isolated	Ellipse	3	136.1
JGS 20-4-1	Dolomite	Isolated	Ellipse	3	123.5

Dolomite usually shows a high potential as reservoirs, contribute a massive oil and gas production. Various kinds of fluid are the key to the genesis process of carbonates, seawater is usually considered as the main diagenetic fluid in the early time, the CaCO₃ dissolved in seawater, along with the bioclastic, clay, and mud, develop original loose sediment such as aragonite and limestone. In the later evaporation period, strong evaporation changes the fluid content by rising the potency of Mg²⁺, high Mg/Ca ratio starts the dolomization, generally the outcome would be parasyngenetic dolomite. In the burial period, fluid from the deep participates in the dolomization, these kinds of fluid usually contain different geochemical characteristics, such as isotopes, rare earth elements. Moreover, biogenesis plays another important role in the diagenetic process, forming microbialite, bioherms, and reef dolomite. Other factors are also in the process of dolomization, high diagenetic temperature will cause a different carbon and oxygen pattern, structural movements will determine the transporting direction of fluid thus influence the location and dimension of dolomization.

According to petrological characteristics, reef dolomites are widely developed, microfacies such as reef core, reef wing are clear, a large number of fossils were observed on the profile, reef-building organisms represented by stratforaminifera are widely distributed, which is a signature of a reef in an open clean ocean sedimentary environment. In Guanwushan period, the old continent was in the south-east of the area, from the south-east land to north-west sea, sedimentary facies spread as an order of shore, restricted platform, open platform, platform margin, basin-slope. Wide-developed reef dolomite of this profile indicates a wide-open shallow sea environment, widely distributed stratopodium is strong evidence that the sedimentary facies of Jiguanshan profile is a platform margin, provides a beneficial environment for reefs and marine organisms, in next 300Ma time, Sichuan Basin has gone through a series of massive tectonic movement, turned the sea into land near Jurassic, orogenic movement as well, the old sedimentary environment has been greatly changed, hot fluid from the deep was driven to move upwards through all kinds of cracks, changing surrounding sedimentary rocks, dolomite included. According to samples we found and observed under scope, two types of dolomites show us sharp distinctions, such as shapes, destruction levels, developing position, these characteristics maybe caused by different dolomization mechanisms in different periods, fine crystal dolomite formed earlier than middle-coarse dolomite, further details could be revealed by geochemical features.

We mainly used carbon and oxygen isotopes and REE distribution to analysis dolomization. Isotopes are very efficient to tracing geological fluid, many factors would cause isotope fractionation. The carbon isotope we used is ¹³C, under the effect of organic activities like photosynthesis, the heavier ¹³C will be transformed to lighter ¹²C stored in organisms, therefore marine carbonates always possess higher ¹³C content, which means δ¹³C positive anomaly, all dolomite samples we studies showed a positive anomaly, high ¹³C content indicates this profile was deposited in a typical marine environment, original sediments came from marine inorganic substance. Oxygen isotope is a frequently-used method to measure the diagenetic temperature, oxygen isotope ¹⁸O would be greatly affected by high temperature, the general trend is that, high diagenetic temperature decreases ¹⁸O content and causes a very negative oxygen isotope anomaly. In our research, δ¹⁸O anomalies showed three sections, reflects three diagenetic temperatures, this might reveal different diagenetic depths, all samples might have experienced high temperature in burial.

Rare earth elements distribution is an efficient environment and fluid indicator, behaviours of certain elements could reflect the changes of environments, due to the stability of REE, distribution patterns are reliable to understanding the geochemistry through dolomite diagenetic process.

In carbonate research, one important indicator element is Ce, the anomaly of Ce is strong evidence to conditions of marine environment. In the study of marine environment, Ce is considered as a sensitive tracer (C R German, 1990). In the anoxic or hypoxic ocean, insoluble Ce⁴⁺ will be reduced to soluble Ce³⁺ and enter the seawater solution, which shows that δCe positive anomaly; In the oxygen rich open sea environment, Ce³⁺ will be oxidized to insoluble Ce⁴⁺, which is precipitated in the form of CeO₂, forming δCe negative anomaly (hadron equivalence, 1998). The dolomite in Jiguanshan section shows obvious δCe negative anomaly indicates that the diagenetic environment is a rich and open marine environment, which is consistent with the lithological and sedimentary facies evidence of platform margin facies. Another obvious phenomenon is that, 2 of 10 samples show a clear Eu positive anomaly while other samples possess negative anomalies. High Eu content is a signature of hot fluid from deeper layers, in fact, the phenomenon could be seen in dolomite from Precambrian to Triassic in Sichuan Basin, it seems that nearly all carbonate stratums of the basin have gone through a massive hot fluid movement, developed huge amount of zebra structure dolomites, along with hydrothermal minerals, former researchers have found typical pyrite in middle-Permian stratum in north-west Sichuan Basin. Widely-developed active hydrothermal event may relate to late-Permian Emei taphrogenesis, this tectonic event caused extensive impact in the Sichuan Basin, forming the Emei mountain igneous rock province, hot magma widely migrates, driving the flow of deep fluids, at the same time, strong tectonic movements generate a large number of faults and fractures, and fluids precipitate and transform surrounding rocks in these spaces, high Eu anomaly trace was left in late time dolomites. Moreover, dolomite heavy REE/light REE ratio and La_N/Yb_N ratio indicate a characteristic that all samples possess enrichment of LREE and loss of HREE, and general REE content doesn’t present a high pattern, reflects a low REE cycling system, which further proves that the Guanwushan Formation carbonate rocks on this profile are marine carbonate rocks.

After analyzing fluid properties using geochemical characteristics, still, there is another question: we’ve proved the diagenetic fluid is mainly seawater in an aerobic environment, but inclusions results indicate high diagenetic temperature, which cannot match the thermal signature of seawater quasi-syngenetic dolomite, these inclusions are all come from fine crystal dolomites, the matrix is the first-stage product of dolomization, yet it shows an abnormal diagenetic temperature. This may be caused by the fast changes of geological condition, the Guanwushan formation quickly entered burial stage after completion of Guanwushan period, meanwhile the environment had transformed into lagoon of a restricted platform, which means the starting point of strong evaporation, the evaporation continuously rose the Mg²⁺ content, along with the high temperature of the shallow-burial stage, started the dolomization, similarly, the main pore types observed in hand specimens and microscopic slices are all secondary pores dominated by dissolution pores, with almost no primary pores, and fractures are also secondary fractures cut through the original fabric, In general, the origin of fine crystalline matrix is clear now, the strong evaporation and sedimentary environment rose the Mg/Ca ratio of the fluid, then the origin sediment quickly went into burial period, under the affection of heat and pressure, diagenetic fluid was driving by multiple dynamics, fluid preserved in original sediment started dolomization, the product was fine crystal dolomite, which is the result of first stage dolomization, in this stage, limestone crystal texture, original fine crystal texture and biological fabric were preserved and dolomized as we observed under microscope, the second stage occurred under a tectonic movement background, hot fluid from the deep transported through cracks and faults, started the second stage dolomization, the main product is middle-coarse crystal dolomite with a positive δEu anomaly and negative δ¹⁸O anomaly.

Throughout the research, lithological and geochemical evidence shows the existence of two distinguish types of dolomites, one is matrix fine crystalline dolomite that formed in a shallow sea platform margin environment, the other one is a middle-coarse crystalline dolomite formed in a deep burial environment. In the process of dolomization, the diagenetic fluid is a critical factor, the fluid controls the types, characteristics, even the deposit locations. The early fluid that shaped the reef dolomite frame was the original seawater, the isotopes from the coral body and matrix dolomite are not showing an obvious difference, inside the organism isotopes are least effected, the small δ¹⁸O deviation of matrix dolomite could be caused during a burial period, high temperature will change the content of oxygen isotope.

After the petrological macro characteristics, micro characteristics and geochemical analysis of the Jiguanshan section in Chongzhou City, Sichuan Basin, the following conclusions are drawn:

Guanwushan Formation in Jiguanshan section is a platform margin facies deposit, with a large number of dolostones including reef dolostones. The ancient water body is clear, the hydrodynamic conditions are strong, and the biological disturbance is strong, with corals, stromatoporoids and other organisms visible.

Through separate petrological characteristics, carbon and oxygen isotope characteristics δ Eu positive anomaly analysis shows that the dolomite of Guanwushan formation in Jiguanshan section is of multi-stage diagenesis, the first stage is of shallow burial origin, diagenetic fluid is original seawater, which is transformed by high temperature underground, mainly composed of fine crystal dolomite; The second stage is hydrothermal fluid origin, which is filled with medium-coarse crystal dolomite and quartz and other hydrothermal minerals in fracture cavity and zebra pattern bright zone.

According to the analysis of Ce negative anomaly, it is believed that the overall diagenetic fluid of the dolomite of Guanwushan formation in the Jiguanshan section is the seawater from the platform edge at the same time, which can also be reflected by a large number of reef dolomite and biological fossils. After shallow burial in the later period and transformation of hydrothermal high-temperature fluid, finally formed the phenomenon of multiple types and multiple stages of formation of dolomites in the Guanwushan formation of the modern Jiguanshan section.

Statements and Declarations:

All authors involved in the work declared no conflict of interest.

Data Availability

Data will be made available on request.

Funding

This work was supported by Sinopec “Reservoir-forming conditions and zonal evaluation of Devonian-Permian system in central and western Sichuan Basin” program.

Acknowledgements

We are grateful to Kun Peng and Pan Lin for their field work and geochemical experiment operation. Finally, we are grateful to all editors and reviewers for their professional knowledge and precious suggestions to this manuscript.

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Characteristics and Genesis of Dolomites from the Devonian Guanwushan Formation in the Jiguanshan Section, southwest Sichuan Basin, China

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Abstract

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1. Introduction

2. Geological background

3. Samples and methods

4. Results

4.1. Petrological characteristics of dolomite

1.Fine crystal dolomite

2.Middle-coarse crystal dolomite

4.2. Carbon and oxygen isotopes

4.3. Rare earth element

4.4. Fluid inclusion

5. Discussion

6. Conclusion

Declarations

References

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