Introduction

Permian Maokou Formation (P1m) in Sichuan Basin is an essential natural gas exploration and development with excellent exploration potential but under-developing (Zhang et al. 2018). P1m in Sichuan Basin can be divided into four sections according to the sedimentary characteristics, lithology, and electrical characteristics. The shoal facies and weathering crust paleo-karst reservoirs of P1m2, P1m3, and P1m4 have good reservoir performance and are the main P1m natural gas exploration reservoirs. Several karst fracture-vuggy gas reservoirs have been discovered. P1m1 nodular limestone is characterized by the rapid alternate development of “Eyelid limestone” and “Eyeball limestone” (Luo 2010; Luo et al. 2019), also known as Limestone-Marl Alternations (Amberg et al. 2016), and has been recognized as a set of carbonate source rock for a long time (Hu et al. 2020). The genesis mechanism of Limestone-marl alternations is controversial, including in situ deposition (Munnecke and Westphal 2004), allochthonous deposition (Lu et al. 2014), differential diagenesis (Westphal et al. 2008), etc. Liu et al. (2011) studied nodular limestone (P1m1) in Wangcang area of northern Sichuan Basin believed that the Eyelid-Eyeball structure resulted from sedimentation and diagenesis.

Several wells in P1m1 have experienced good gas shows in conventional oil and gas exploration, but most were considered fracture gas shown in source rock and were not evaluated and tested (Hu et al. 2020). In recent years, during the exploration of shale gas in Longmaxi formation, natural gas in the first member of Maokou Formation (P1m1) has been shown for many times, and a high yield has been achieved. For example, 31 × 104 m3/day of P1m1 in well TT1 in the Chuanzhong area has been tested (Guo 2021). Vertical well acidification tests were carried out in well JS1, YH1, and DS-1 of P1m1 in the Fuling area, and the commercial gas flow obtained as 1.67 × 104 m3/day, 3.06 × 104 m3/day, and 5.4 × 104 m3/day (Zhao 2021), respectively. In addition, fracturing tests in the evaluation horizontal well FM1HF for the P1m1 deployment in the Jiaoshiba area resulted in a gas flow of 4.02 × 104 m3/day, and many wells such as JY66-1, JY30-6, and JY34-8 show a noticeable gas indicator. The well DB1, which was deployed in 2022, was cored in P1m1 and found to have high gas content in the core, and the bubble of the core immersion experiment was similar to that of the Longmaxi formation shale. And the P1m1 limestone source rock fracturing transformation of DB1 well achieved a test production capacity of 4.2 × 104 m3/day. In 2023, the DB1H horizontal well deployed later achieved a testing production capacity of 55 × 104 m3/day. The drilling results show that the P1m1 carbonate rock in the Sichuan Basin has great potential for unconventional gas exploration.

Based on the analysis of reservoir characteristics of P1m1 in well MY1 and DS-1 in the Nanchuan area of southeast Sichuan Basin, it is concluded that the P1m1 carbonate reservoir has multiple reservoir space types, such as organic pores, dissolution pores, grain margin fractures, and contraction fractures and these pores have the characteristics of source-reservoir integration and are distributed in contiguous pieces (Guo 2021; Zhang et al. 2021). Large-scale talc diagenetic pores and fissures formed by sepiolite during burial make a great contribution to the nodular limestone reservoir of P1m1 (Li et al. 2021a). In order to further exploration, many previous studies have been carried out on the sedimentary facies of P1m1, with different views, including open platform facies (Hu et al. 2012; Xiang et al. 2011), slope facies (Li et al. 2008; Chen et al. 2013), and ramp facies (Liu et al. 2019; Hao et al. 2020). The sedimentary environment of P1m1 is complex, which brings some uncertainties to gas exploration.

In view of this, the author has carried out geological geochemical analysis and testing on the core samples of P1m1 of Well DB1 in West Chongqing area. Its rock and mineral characteristics, carbon and oxygen isotope characteristics of carbonate rocks, and molecular fossil composition of soluble organic matter show that P1m1 carbonate rocks of Well DB1 may be hydrothermal sediments. The new understanding of this study is expected to provide a reference for the subsequent carbonate unconventional gas exploration.

Geology setting

The West Chongqing area is located in the east of Sichuan Basin and the upper reaches of the Yangtze River, with a geo-span between 105°–110.5° east and 28°–32.3° north, which is the transition zone between the Qinghai-Tibet plateau and the middle and lower of the Yangtze plain (Lin 2016). In tectonic position, it is located in the brush structure development area at the southwest end of the Huayingshan fault in the southeast of the Sichuan Basin, and it spans two secondary structural units, the low steep fold belt in south Sichuan basin and high steep fold belt in east Sichuan basin. In this area, the vertical folding assemblage pattern is developed, and the structural layer presents an upper, middle, and lower geological structure (middle and lower Triassic to Jurassic, the Silurian to the Permian, and the lower Cambrian). The tectonic evolution of the study area mainly includes the Caledonian, the Hercynian, the Indosinian, the Yanshanian, and the Himalayan Period. The Indosinian weak compression deformation, Yanshanian strong compression, and the Himalayan structure were fixed and formed the present stress field distribution characteristics. The regional structure was formed in the Hercynian-Indosinian Period, and the main structure was formed in the Yanshanian Period, and the structure was finalized in the Himalayan Period (Wang et al. 2013a). The secondary structural belt is a block-type fold structure with a narrow anticline and wide syncline, and from west to east, it can be divided into four groups of NNE-SW long axis anticlinal belts, Xishan-Xindianzi, Dongshan-Huangguashan-Tanziba, Xiwenquan-Huaguoshan-Liuhechang, Wennan-Linfengchang-Tanghe, and three groups of NNE-SW wide synclinal belts, Laisu, Linjiang, Bishan-Huguosi. The well DB1 is located at the southern dip of the Dongshan anticline (Fig. 1).

Fig. 1
figure 1

Structural diagram of the study area and profile of Maokou Formation of well DB1

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The tectonic movement at the end of the Devonian and Carboniferous made the sedimentary basement of the basin exposed and denuded, resulting in the absence of a lower Permian in the area (Zhang et al. 2011; Wang et al. 2013b; Hou et al. 2017; Xiao et al. 2018). In the early middle Permian, extensive transgression from east to west occurred in the area, forming the Liangshan Formation clastic shore facies deposition, and during the Qixia Period, carbonate platform deposition occurred. At the end of the Qixia Period, the sea level fell, and the sedimentary area was transformed into the exposed area, which was eluted and dissolved by atmospheric water, forming a regional exposed erosion unconformity (Wang et al. 2013b). The Maokou Formation was widely transgressive again in the early stage, and a typical limestone-marl alternations interbedding deposits developed in the lower P1m1 of the Maokou Formation (Su et al. 2020). The sedimentary strata transition to the middle and upper part to grey, dark grey bioclastic carbonate rocks, micritic bioclastic carbonate rocks, mixed with argillaceous carbonate rocks and carbonate rocks, containing more chert nodules or bands, with dolomite locally developing (Qiu and Gu 1991; Wu et al. 2015). The P1m1 is a set of continuous deposits with little sedimentary differentiation and isochronous and comparable conditions in the study area.

Sample and analysis

Samples

The track of the well DB1 traversed the fault. Logging, coring, and logging data confirmed that the formation was mutation from P1m1 into P1m3 with the main fracture surface location of 2539.2 m (Fig. 1). Coring 16.26 m of the upper wall of the P1m1 in well DB1 from 2517–2533.26 m and 29 m core from 2676.2–2688.1 m and 2717–2734.1 m of the lower wall of the P1m1 were cored. During the core extraction process of well P1m1 in DB1, immersion experiments were conducted on some cores. The results of the immersion experiment visually demonstrate that P1m1 in DB1 well is rich in gas content. For example, when the 2518.23 m core is immersed in water, there are many and continuous bubbles, and the gas output of the Marl part is significantly higher than that of the Limestone (Fig. 2a).

Fig. 2
figure 2

Macroscopic and microscopic characteristics of nodular limestone in P1m1 of well DB1. a Water immersion test of 2518.23 m core in the first member of Maokou formation (P1m1) of well DB1. b Photo of limestone-marl alternations structure of 2517–2518.65 m core in P1m1 of well DB1. c The 2517.45 m silty limestone of well DB1 has a sense of granularity, with a large amount of bioclastic and a slightly higher degree of crystallization. d The foraminiferal cavity of 2517.45 m limestone in well DB1 is filled with asphalt. e Bioclastic in 2518.23 m marl of well DB1 is developed and slightly arranged in layers, indicating that secondary transport sedimentation may have occurred

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The P1m1 carbonate rock in the well DB1 can be divided into two types of fabric: “Limestone” and “Marl,” and their macroscopic and microscopic characteristics are quite different. The “Marl” often appears wrapping around the “Limestone” (Fig. 2b), and the colour of “Limestone” is relatively light (light grey-grey). The “Limestone” is a pure carbonate rock with lenticular structure and high crystal degree, and most of them are powder carbonate rock. Many shallow-water organisms were developed, including algae, gastropods, bryozoans, foraminifers, etc. Bioclasts and organisms were randomly distributed and arranged in an unoriented manner (Fig. 2c). In addition, asphalt filling was observed in the coelom of P1m1 carbonate foraminifers (Fig. 2d).

On the contrary, the colour of “Marl” is darker (dark grey-grey black), and the carbonate rock with higher argillaceous content is lamellar, and the crystallization degree is relatively low, while most of them are argillaceous micritic carbonate rock. The fossil is as developed as the “Limestone,” but the bioclasts are oriented (Fig. 2e).

Experiment

In well DB1 of P1m1, fifty-five plunger samples were collected for conventional porosity and permeability tests. Thirteen core samples were collected for organic carbon, pyrolysis, and vitrinite reflectance analysis. Based on pyrolysis and organic carbon analysis, seven samples were selected for carbon and oxygen isotope analysis and GC–MS analysis. The main experimental procedures of carbon and oxygen isotope analysis and GC–MS analysis are as follows.

GS-MS analysis

Choosing 1300-ISQLT gas chromatography-mass spectrometer, chromatographic carrier gas: 99.999% helium; inlet temperature: 300 ℃; transmission line: 280 ℃; chromatographic column: HP-5MS elastic quartz capillary column (30 m × 0.25 mm × 0.25 m); temperature programming: 80 ℃ rises to 160 ℃ at 8 ℃/min and 310 ℃ at 3 ℃/min, maintaining 10 min; flow rate of carrier gas: 1 mL/min; mass spectrum: EI-source; temperature of the ionization chamber: 280 ℃; ionization voltage: 70 eV; absolute voltage of multiplier: 1400 V; MID scanning.

Carbon and oxygen isotope analysis of carbonate rocks

Carbonate samples are cut into thin slices (thickness is within 100 μm), and the sampled surface is not polished.

Then samples were heated to about 100 ℃ in an electric heating box and then roasted for about 2 h to remove the organic matter and excess the water in the carbonate samples.

Before the laser sampling, the petrological characteristics of the sample are observed with the microscope. The structure and composition of the sample are determined.

The area to be researched and analyzed is selected, and the selected area is focused and positioned by the laser beam.

Carbon dioxide gas collected from the sample tube is fed into an isotope mass spectrometer system to determine the composition of carbon and oxygen isotopes in carbonate samples, generally represented by δ13C and δ18O.

Result

Reservoir pore characteristics

Conventional porosity–permeability analysis shows that among carbonate samples from the well DB1 P1m1, the proportion of 0–1% is 5.45%; the proportion of 1–2% is 56.36%; the proportion of 2–3% is 30.91%; and the proportion of 3–4% is 7.27%. When the conventional permeability is 0.001–0.005%, it accounts for 21.95%. When the conventional permeability is 0.005–0.01%, it accounts for 29.27%. When the conventional permeability is 0.01–0.05%, it accounts for 26.83%. When the conventional permeability is 0.05–0.1%, it accounts for 2.44%. When the conventional permeability is 0.1–0.5%, it accounts for 19.51%. In general, the porosity of well DB1 is less than 4%, mainly between 1 and 2%, and the permeability is less than 0.1%, mainly between 0.005 and 0.05% (Fig. 3).

Fig. 3
figure 3

Porosity and permeability characteristics of P1m1 limestone in well DB1

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In addition, there are no apparent pores in the thin cast section of P1m1 in the well DB1 (Fig. 4a, b). Under the SEM, intergranular pores (pore size 6 μm) and intercrystalline micropores (pore size 1 μm) were developed in the P1m1 (Fig. 4c, d). Several organic pores (aperture 70 nm) and fractures (width 150–300 nm) were observed under the Field emission scan electronic microscope (FE-SEM) (Fig. 4e, f).

Fig. 4
figure 4

Microscopic pore structure characteristics of P1m1 limestone in well DB1. a No obvious pores can be found in 2517.7 m cast thin section of P1m1 of well DB1. b Cracks are found in 2518.55 m cast thin section of P1m1 of well DB1. c Under the scanning electron microscope, 2517.45 m of P1m1 of well DB1, intergranular pores are developed, with a pore diameter of 6 μm. d Under the scanning electron microscope, 2681.65 m of P1m1 of well DB1, intracrystalline pores are developed, with a pore diameter of 1 μm. e Under the field emission scanning electron microscope, organic pores are developed at 2718.15 m in P1m1 of well DB1, with a pore diameter of 72.64 nm. f Under the field emission scanning electron microscope, talc intergranular joint is developed at 2718.15 m in the first section of P1m1 of well DB1, with a width of 314.6 nm

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The study on the pore structure of the first member of Well JY66-1 in the Fuling area shows that the aperture ranging less than 2 nm, 2–5 nm, 5–100 nm, and larger than100 nm is the main reservoir space of the P1m1. The largest contribution rate of the aperture is in the range of 5–100 nm, accounting for 50% of the total pore volume. When the aperture range is 2–5 nm and less than 2 nm, the contribution rate of pore volume to the total pore volume is equal (20%). Furthermore, when the aperture is larger than 100 nm, the contribution rate of pore volume to the total pore volume is the least (10%) (Liu et al. 2021). Multiple regression analysis indicated that when the aperture is 5–100 nm, the absolute contribution of organic pores and talc pores was 0.001 cm3/g and 0.002 cm3/g to 0.005 cm3/g, accounting for 40–80% of the total pore volume.

The comprehensive analysis shows that the P1m1 reservoir in the well DB1 is similar to the Longmaxi Formation shale reservoir, and it is an unconventional reservoir with ultra-low porosity and ultra-low permeability. At the same time, it is dominated by nanopores and micron-pores.

Geochemistry of source rocks

All the 12 samples of P1m1 in the well DB1 were in the over-mature stage (Table 1; Fig. 5a), with Tmax between 505 and 574 ℃ and mean 543 ℃, and Ro is between 2.15 and 2.41% and a mean value 2.28%. The TOC was between 0.3 and 5.2%, and the mean value was 1.47%. The TOC of Marl was before 0.76–5.2%, and the mean value was 2.23%, while that of Limestone was before 0.3–0.69%, and the mean value was 0.41%. Hydrocarbon generation potential (S1 + S2) is between 0.05 and 1.46 mg/g, with a mean of 0.34 mg/g. S1 + S2 of Marl is before 0.08–1.46%, and the mean value is 0.08–1.46%. The S1 + S2 of Limestone is before 0.05–0.22%, with a mean value of 0.11%. Marl is the main source rock, and its organic matter content and hydrocarbon generation potential are much higher than the Limestone. The hydrogen index (HI) is between 15.94 and 70.97 mg/g, with a mean value of 20.95 mg/g. Due to the loss of the hydrogen in the over-mature source rocks, the correlation diagrams of HI-Tmax have lost the ability to distinguish the types of organic matter.

Table1 TOC, pyrolysis, and Ro parameters of P1m1 in well DB1
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Fig. 5
figure 5

Abundance, type, and maturity characteristics of organic matter in P1m1 nodular limestone of well DB1

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The content of chloroform asphalt A in the P1m1 sample of the well DB1 is between 0.045 and 0.076%, with a mean value of 0.0589%. In group composition of chloroform asphalt A, “non-hydrocarbon + asphaltene” is dominant, and the relative content is between 38.158 and 50%, with a mean value of 43.061% (Table 2). The relative content of saturated hydrocarbons ranges from 17.778 to 30.189% with a mean value of 24.386%. The relative content of aromatics ranges from 13.462 to 33.333%, with a mean value of 31.507%. The P1m1 sample is in the over-mature stage, most of the liquid chloroform asphalt A has been converted into natural gas, and non-hydrocarbon and asphaltene with larger molecular weight accumulate (Fig. 6).

Table 2 Content and composition of chloroform asphalt A in P1m1 nodular limestone of well DB1
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Fig. 6
figure 6

Composition of limestone chloroform asphalt A in P1m1 of well DB1

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Characteristics of molecular fossils

The carbon number of N-alkane in the soluble organic matter of P1m1 carbonate rock in the well DB1 ranges from nC13 to nC36, and nC28 is the optimal peak. The OEP value was 0.72–0.84, and the mean value was 0.76. The ∑c21-/∑c22+ value was between 0.07 and 0.21, and the mean value was 0.10. The (C21 + C22)/(C28 + C29) value was 0.42–0.87, and the mean value was 0.57. The P1m1 carbonate N-alkane in well DB1 presents a post-peak type distribution characteristic, with obvious even carbon dominant distribution phenomenon (Fig. 7).

Fig. 7
figure 7

Characteristics of N-alkanes in P1m1 of well DB1

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A series of monomethyl alkanes compounds with carbon number distribution ranging from C18 to C27 were detected. Figure 8a shows that 2-methylalkanes are not dominant in monomethyl alkanes with a carbon number less than 22, and 4-methylalkanes ~ 8-methylalkanes series compounds are abundant. In monomethyl alkanes with a carbon number greater than 22, 2-methylalkanes were dominant and showed even carbon dominant distribution, with 2-methylalkanes C22 (iC22) as the main peak (Fig. 8a).

Fig. 8
figure 8

Characteristics of isoalkanes and isoprenoid alkanes in P1m1 of well DB1

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The Pr/Ph of Isoprenoid alkane compounds ranged from 0.16 to 0.68, with an average value of 0.48, typical marine reducing environment deposition (Peters et al. 2005). Long-chain isoprenoid alkane (ipC21+) and Squalane are developed (Fig. 8b).

Hopane series of steranes and terpenoids derived from bacteria dominated most of the compounds, with the relative content between 69.85 and 80.42% and the mean value of 74.54%. Tricyclic terpene from algae accounted for 0.02–2.04%, with a mean value of 0.54%. Sterane of eukaryotic origin accounted for 13.80–19.62%, with an average of 15.86% (Table 3; Fig. 9).

Table 3 Parameters of steranes and terpanes in P1m1 of well DB1
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Fig. 9
figure 9

Composition characteristics of steranes and terpanes in P1m of well DB1

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At the same time, in the sterane series, rearranged sterane is not developed, and short-chain sterane from halophilic bacteria is developed (Zhang et al. 2020; Cheng et al. 2021a) (Fig. 10a). C29-regular sterane was the regular dominant sterane, and the relative content of C27-regular sterane was slightly lower (Table 3; Fig. 10b). The high content of C29-regular sterane in marine source rocks may be related to some animals (Zhang and Cheng 2021).

Fig. 10
figure 10

Composition characteristics of steranes in P1m1 of well DB1

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C29-ββ/(ββ + αα) sterane values ranged from 0.29 to 0.40, with a mean value of 0.36. C29-20S /(20S + 20R) sterane values ranged from 0.29 to 0.40, with a mean value of 0.36. The samples in C29-ββ/(ββ + αα) sterane vs. C29-20S/(20S + 20R) sterane diagram were judged to be in the low maturity stage (Table 3; Fig. 11a), and the P1m1 samples in the well DB1 had reached the mature stage. The correlation between C29-ββ/(ββ + αα) sterane and Ro suggests that isomerization in the sterane series is inhibited, rather than the isomerization reversal caused by excessive maturity (Fig. 11b, c). The inhibition of sterane isomerization is related to the source of biological parent material in saline water (Cheng et al. 2021b).

Fig. 11
figure 11

Sterane isomerization characteristics of P1m1 in well DB1

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Benzonaphthalene thiophene has a high content in aromatic compounds (2.59–28.92%, with a mean value of 17.88%). In the mass spectra of phenanthrene series and benzonaphthalene thiophene series compounds of DB1-S-2 samples, it can be intuitively seen that benzonaphthalene thiophene is dominant (Fig. 11a). At the same time, the dibenzothiophenes series dominated the inner composition of the trifluorene series, with the content ranging from 60.66 to 99.42% with an average of 79.2%, and the oxyfluorene series with the content ranging from 0.01 to 13.36% with an average of 5.60% (Fig. 11b).

Carbon and oxygen isotopic characteristics of carbonate rocks

Firstly, carbonate samples were tested for their privity, including three identification methods: trace element (Mn/Sr) identification method, oxygen isotope composition characteristic identification method, and δ13C(‰PDB) and δ18O(‰PDB) correlation identification method (Wang 2021).

Under normal circumstances, marine carbonate rocks undergo late diagenesis, especially susceptible to atmospheric water, which leads to the decrease of Sr and Na contents and the increase of Fe and Mn contents in carbonate minerals. In the early sedimentary environment, when the Mn/Sr ratio increases significantly, the degree of diagenetic alteration is enhanced in the late sedimentary environment. When Mn/Sr is less than 3, it indicates that the original information of the early carbonate sedimentary environment has been well preserved (Shao et al. 2021). The Mn/Sr values of P1m1 carbonate samples in the well DB1 are all less than 1 (Table 4).

Table 4 Carbon and oxygen isotope parameters of P1m1 carbonate rock in well DB1
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It is generally believed that the δ18O(‰PDB) value in carbonate samples is less than − 5.0‰, indicating that the oxygen isotope composition has not changed, indicating that the samples have not suffered severe diagenetic alteration, which can represent the original isotope composition and accurately reflect the early sedimentary environment information of carbonate rocks (Zhang et al. 2003). The δ18O(‰PDB) values of P1m1 samples from the well DB1 range from –10.57 to − 6.34‰.

When the values of δ13C(‰PDB) and δ18O(‰ PDB) in carbonate samples are discrete, it is proved that carbonate rocks have not suffered severe diagenesis (Wang 2021). As shown in Fig. 12, the correlation between C and O isotopes of carbonate samples in the well DB1 is relatively discrete (R2 = 0.2567).

Fig. 12
figure 12

a Composition characteristics of phenanthrene series compounds and benzonaphthalene thiophene of P1m1 in well DB1. b Composition characteristics of Tri-fluorene series compounds of P1m1 in well DB1

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Considering the elements, oxygen isotope values and the correlation between carbon and oxygen isotopes, the carbon and oxygen isotopes are less affected by diagenesis and can reflect the original sedimentary information.

Discussion

Hydrothermal sedimentary minerals

There is a high talc content in P1m1 carbonate rock in the well DB1, usually converted from sepiolite. The origin of Si in sepiolite remains controversial. Some studies suggest that the main source of Si is hydrothermal vents (Yang 1992) or upwelling (Lü et al. 2010). Cai et al. (2019), based on the analysis of trace and rare earth elements, believed that the Si element originated from deep-sea hydrothermal fluid and was carried to shallow sea areas through upwelling. Recent studies suggest that the large amount of siliceous material required to form Permian lamellar and tuberculous cherts and sepiolite in the South China was mainly derived from volcanic and hydrothermal activity (Qiu and Wang 2011; Gao et al. 2020). Zhao et al. (2020) sorted out the current research results and concluded that the initial active stage of Mount Emei basalt eruption was in the early stage of Maokou Formation deposition. Some scholars found that synsedimentary faults were highly developed in the inner and edge of the Yangtze Platform in the Middle Permian (Shen et al. 2019; Luo et al. 1990; Cheng et al. 2015; Wang et al. 2019). Therefore, the Si in sepiolite of Maokou Formation may be partially derived from synsedimentary fault hydrothermal solution, which is consistent with the Eu positive anomaly of marl samples that can reflect the nature of original sedimentary water (Su et al. 2020). Under the scanning electron microscope of the P1m1 in well DB1, in addition to talc, hydrothermal fluorite was also found (Fig. 13).

Fig. 13
figure 13

Talc and fluorite mineral photos under scanning electron microscope of nodular limestone of P1m1 in well DB1

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Carbon and oxygen isotopes of hydrothermal deposits

The average δ18O (PDB) of Maokou Formation normal limestone is − 4.8‰, that of normal dolomite is − 4.0‰, and that of hydrothermal modified fine crystalline dolomite is − 7.19‰. The δ18O (PDB) values of the hydrothermal modified patch dolomite and the saddle dolomite cement are − 10.47‰ and − 10.72‰, respectively (Li et al. 2021b). The carbon and oxygen isotope distribution of P1m1 carbonate rock in the well DB1 is consistent with Maokou Formation hydrothermal dolomite in the Chuanzhong area (Fig. 14).

Fig. 14
figure 14

Comparison of C and O isotopic compositions between P1m1 limestone in well DB1 and hydrothermal carbonate in other areas of Sichuan Basin

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Hydrothermal biomolecular fossils

In general, the odd–even dominance of N-alkanes in source rocks gradually decreases and disappears with the increase of thermal evolution (Bao et al. 2016). The N-alkane in the over-mature soluble organic matter of P1m1 carbonate rock in the well DB1 shows that nC28 is the main peak and has a rare phenomenon of even carbon dominant distribution. Zhu et al. (2003) analyzed the distribution characteristics of even-carbon-dominant n-alkanes in the E23 salt lacustrine carbonate strata in the Qaidam Basin and believed that it was mainly related to the biological source of organic matter. The Cambrian Niutitang Formation developed hydrothermal sedimentary shale in northern Guizhou (Liang et al. 2014). N-alkanes with dominant carbon distribution have been observed in the Niutitang Formation over-mature shale of Cambrian in northern Guizhou. It is speculated that P1m1 carbon-dominant N-alkanes in the well DB1 may be derived from extreme environment tolerant microorganisms in a hydrothermal sedimentary environment.

iC22 was the main peak in monomethyl alkanes of the well DB1, and 4-methylalkanes ~ 8-methylalkanes was the input of cyanobacteria in monomethyl alkanes of low carbon number (C14-C21) (Shiea et al. 1990). The iC22 is mainly derived from salt-tolerant bacteria, and heterotrophic bacteria contribute (Connan et al. 1986). The iC22+ with even carbon dominance comes from heterotrophic bacteria (Cheng et al. 2019a). Long-chain acyclic isoprenoid alkane (ipC21+) compounds and Squalane of P1m1 carbonate in the well DB1 are abundant. ipC21 + is generally believed to be derived from archaea, including halophiles, thermophiles, and methanogens. It is the diagenetic evolution product of C30, C 40 or higher carbon precursors (Thompson and Kennicuttii 1992).

Hopane was dominant among steranes and terpenoids (69.85–80.42%), indicating that organic matter was mainly derived from bacteria. In addition, the content of C29-regular sterane in P1m1 of the well DB1 was higher than that of C27-regular sterane, and sterane isomerization was significantly inhibited. The strategy of organisms at high temperatures is general to increase the carbon number of skeleton compounds (Arakawa et al. 2001; Sprott et al. 1991), and steroid analysis of modern hydrothermal organisms suggests the presence of a precursor of C29 sterane (Kawai et al. 2007). Therefore, abnormally high C29-regular sterane with isomerization inhibition characteristics may come from hydrothermal animals (Cheng et al. 2019b; Zhang and Cheng 2021). In addition, the abundance of sulfur-containing compounds in P1m1 aromatic compounds in the well DB1 was high, and dibenzothiophenes in Trifluorene series compounds were overwhelmingly dominant (61.47–99.42%), indicating that the sedimentary environment of P1m1 organic matter was highly reductive and had a high concentration of H2S or sulfate.

Based on the molecular fossil analysis of the P1m1, the sedimentary environment of the P1m1 was a hydrothermal sedimentary environment rich in H2S and sulfates. Chemoautotrophic bacteria and archaea were primary producers, forming a hydrothermal ecosystem with many heterotrophic bacteria and hydrothermal animals. High-quality gas source rocks were deposited and preserved in a strong reductive sedimentary environment.

Conclusion

  1. 1.

    Well DB1 P1m1 carbonate Source rock has high organic matter abundance, good organic matter type and is in the over mature stage, and has formed a large amount of natural gas. The reservoir is an ultra-low porosity ultra-low permeability reservoir, which has the material basis and reservoir formation conditions for forming unconventional natural gas reservoirs in carbonate rocks.

  2. 2.

    Hydrothermal talc and fluorite minerals exist in the P1m1 carbonate rock in the well DB1, and the carbon and oxygen isotopic compositions of the carbonate rock are similar to those of the hydrothermal dolomite in central Sichuan.

  3. 3.

    The Source rock of P1m1 carbonate rock in Well DB1 has even carbon dominant n-alkanes and 2-methylalkanes, high abundance of Squalane, and long-chain acyclic Isoprenoid alkane (iPC21+), as well as abnormally high abundance of C29 regular sterane and high abundance of sulfur compounds in Hopane and Sterane series. These molecular fossil evidence indicates that the P1m1 sedimentary environment in the West Chongqing region has strong reducibility, with a large amount of H2S and sulfate substances in the water body, chemoautotrophic bacteria, a large number of heterotrophic bacteria, and hydrothermal animals jointly forming the hydrothermal ecosystem.