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. 2025 Aug 23;10(35):39504–39516. doi: 10.1021/acsomega.5c00036

Elemental Geochemical Characteristics and Organic Matter Enrichment Model of Marine–Continental Transitional Shale in Longtan Formation, Southern Sichuan Basin

Wei Gao †,‡, Yong Fu †, Haiqi Li §,*, Shida Chen §
PMCID: PMC12423802  PMID: 40949184

Abstract

The Upper Permian Longtan Formation in the southern Sichuan Basin exhibits significant shale gas potential, but its paleoenvironment and organic matter enrichment mechanisms remain poorly constrained. This study integrates vitrinite reflectance, rock pyrolysis, mineralogical analysis, and major/trace element geochemistry to elucidate organic matter accumulation processes. Results indicate overmature shales (R o average 3.17%) dominated by Type III kerogen, with mineral assemblages primarily comprising clay (average 62.75% avg.) and quartz (average 22%). Geochemical proxies reveal distinct enrichment of TiO2, TFe2O3, V, Cr, Cu, Ga, and Nb, alongside the depletion of SiO2, Na2O, MnO, and Li. Warm-humid paleoclimate conditions (CIA, Rb/Sr) enhanced terrestrial organic matter input, while elevated salinity (Sr/Ba, Th/U) and persistent anoxia (redox-sensitive trace elements) promoted organic matter preservation. Organic matter enrichment in the Longtan shale was jointly controlled by high productivity under favorable climatic conditions and effective sequestration in a stratified, saline, and reducing depositional system.

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1. Introduction

China’s shale gas resources are abundant, with a geological reserve of 80 × 1012 m3, which is mainly deposited in continental, transitional depositional, and marine environments. − The Sichuan Basin has a total natural gas resource of about 40 × 1012 m3, with huge potential for exploration and development. In recent years, breakthroughs have been made in the shale gas of the Wufeng-Longmaxi Formation and the shale gas of the Wujiaping Formation in the marine phase. The Upper Permian Longtan Formation is a transitional shale in the basin, and few systematic studies have been carried out on the Permian Longtan Formation due to the limitations of exploration and development conditions. Clarifying the oil and gas enrichment mechanism in the Sichuan Basin is the basis for determining oil and gas generation and the key to evaluating oil and gas distribution. , Defining the sedimentary environment of the study area is of positive significance for understanding organic matter and preservation and enrichment, and will provide a theoretical and practical basis for shale gas exploration and development.

Transitional sedimentary-phase shales form primarily in shallow water environments, such as tidal flats, lagoons, and deltas. Sedimentary environments control the input and preservation of organic matter. The input of organic matter from terrestrial detrital sources results in complex organic matter control factors. , Paleoclimate, detrital flux, and paleoproductivity among geochemical indicators control organic matter input, and redox and paleosalinity are key parameters for organic matter preservation. , Different regions within the same basin have different factors influencing the organic matter enrichment. By way of example, paleoclimate, detrital flux input, and redox conditions in the southeastern Ordos Basin dominate organic matter aggregation, whereas the eastern transitional depositional shales are mainly influenced by the paleoproductivity and redox conditions. ,, The climate was warm and humid during the depositional period of the Longtan Formation shale. The shale in the southeastern Sichuan Basin was deposited in an oxic environment, and the main body of sedimentary water was in an oxygen-poor state with high productivity. In the northeastern Sichuan Basin, the shales were deposited in a reducing environment with low paleoproductivity and high inflow of land-source materials. , Organic matter enrichment in the Sichuan Basin was the result of a combination of detrital influx, redox conditions, and depositional rates. ,−

The aim of this study is to determine the depositional environments of shales from the Longtan Formation in the Sichuan Basin. Based on the pyrolytic and mineralogical characteristics of the rocks, the properties and fracturing characteristics of the shales were elucidated. On the basis of the parameters of major and trace elements, the paleodepositional environment was elucidated, and the enrichment pattern of Longtan Formation shale was established. This study has theoretical significance for the exploration of shale gas in the Longtan Formation in the southern Sichuan Basin.

2. Geological Setting

The Sichuan Basin is located in the Upper Yangtze Plate and consists of six tectonic zones. The well C1 in the study area is located in the low-steep tectonic zone in the southern part of the Sichuan Basin (Figure a). The Upper Permian Longtan Formation is a sedimentary layer developed on the stripping level of the Dongwu Movement, and the Kangdian old land is the main provenance area. , At the end of the Upper Permian, the sea level rose, and the study area formed a transitional facies environment dominated by land action. Well C1 develops a coastal marsh depositional zone. The Longtan Formation is limestone at both the top and bottom boundaries, with frequent interbedding of sandstone, shale, and coal , (Figure b and Table ).

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(a) Sichuan basin tectonics and location of well C1; (b) vertical profile of well C1. Reprinted with permission from ref . Copyright 2021, Elsevier. Reprinted with permission from ref . Copyright 2023, Elsevier. Reprinted with permission from ref . Copyright 2024, Springer Nature. Reprinted with permission from ref . Copyright 2024, Elsevier.

1. Information on Shale Sample Testing in Well C1 in the Southern Sichuan Basin.

sample number depth (m) R o (%) TOC (%) REP XRD major elements trace elements rare elements
t1 1780.47 √ √ √ √ √ √ √
t2 1784.07 √ √ √ √ √ √ √
t3 1791.22 √ √ √ √ √ √ √
t4 1794.46 √ √ √ √ √ √ √
t5 1796.62 √ √ √ √ √ √ √
t6 1806.43 √ √ √ √ √ √ √
t7 1808.16 √ √ √ √ √ √ √
t8 1810.32 √ √ √ √ √ √ √
t9 1812.58 √ √ √ √ √ √ √
t10 1813.87 √ √ √ √ √ √ √
t11 1816.56 √ √ √ √ √ √ √
t12 1822.82 √ √ √   √ √ √
r1 1746.89         √ √ √
r2 1748.30         √ √ √
r3 1752.09         √ √ √
r4 1757.62         √ √ √
r5 1765.95         √ √ √
r6 1769.55         √ √ √
r7 1775.70         √ √ √
r8 1831.49         √ √ √
r9 1833.00         √ √ √
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3. Sampling and Methods

A total of 12 shale cores from Well C1 were analyzed alongside cited data sets (r1–r9) to investigate organic matter enrichment mechanisms, where REP is a pyrolysis analysis experiment and XRD is an X-ray diffraction experiment. R o was observed by the optical system of a Leica DM 500 microscope and calibrated by the double-scale method with no less than 30 points; the sample preparation standards are as follows: the consolidant and sample were mixed proportionally to cure the mold, and then the whole rock was sliced directly. Polishing was carried out with a polishing solution, ensuring that the polished surface was free of stains and needle wear under the objective lens. The shale samples were ground to 80 mesh, and the carbonates were dissolved with 10% HCI at 60–80 °C, and then washed with distilled water until neutral. Based on the Chinese national standard GB/T18602–2012, the hydrocarbon content at 300 °C is free hydrocarbon (S1). The highest pyrolysis hydrocarbon peak temperature corresponds to the highest point of the measured hydrocarbon (S2) peak. Combined with TOC content, the hydrogen index (HI) was also calculated. Mineralogical characterization of the shale was carried out by using X-ray diffractometry. The pulverized sample was placed in the carrier notch, and Cu–Kα radiation was applied. The angle of the emission and scattering slits was 1°, and the scanning range was 5–45°. The contribution of the minerals was calculated from the peak areas of the diffractograms. The major elements were melted with anhydrous Li2B4O7, with NH4NO3 as the oxidizing agent, and LiF and a small amount of LiBr as the flux and release agent. The samples were melted at 1150–1250 °C in a sample melting machine to make glass samples, and the analysis error of element concentration is less than 5%. Trace element test samples were dissolved with HF and HNO3 in a closed dissolver, then sealed with HNO3 to dissolve, diluted, and then measured directly by the ICP-MS external standard method, with an analytical error typically better than ±0.5%.

4. Results

4.1. Organic Geochemical Characteristics

Organic carbon content, pyrolysis hydrocarbon yield, and potential generation are widely used as important indicators to evaluate hydrocarbon generation potential. Organic carbon determination results of 12 shale samples show that the TOC varies from 0.60 to 6.24% (average 2.49%). S2 values range from 0.05 to 0.73 mg/g. S1+S2 values vary between 0.06 and 0.76 mg/g (Table ). The source rock evaluation criteria show that the source rocks are mainly good and excellent sources (Figure ). The results of the organic-matter-type analysis of Longtan Formation samples are shown in Table . The results showed that the kerogen type was type III. The main macerals are exinite and vitrinite, with an average content of 51.58 and 40.25%, respectively. At the same time, the S2 parameters and TOC content were used to further analyze the type of organic matter as type III (Figure a). Therefore, the study sample has a good gas generation potential. Organic matter maturity is an essential evaluation index of hydrocarbon production rates. The R o ranges from 2.94 to 3.27% (average 3.17%) (Table , Figure b). The high-overmature stage facilitates massive gas generation and affords gas protection for the reservoir.

2. Shale Hydrocarbon Source Rock Analysis.

sample number R o (%) TOC (%) S1 (mg/g) S2 (mg/g) HI (mg/g) exinite (%) vitrinite (%) inertinite (%) type index type
t1 3.04 4.55 0.01 0.05 1.10 56 35 9 –7.25 III
t2 3.16 2.81 0.02 0.19 6.75 52 38 10 –12.50 III
t3 3.23 2.12 0.03 0.73 34.37 57 35 8 –5.75 III
t4 3.20 0.95 0.01 0.07 7.37 54 40 6 –9.00 III
t5 3.21 1.17 0.04 0.10 8.53 52 43 5 –11.25 III
t6 3.22 1.29 0.06 0.58 44.86 57 36 7 –5.50 III
t7 2.94 2.68 0.04 0.20 7.47 50 39 11 –15.30 III
t8 3.20 1.62 0.03 0.14 8.62 46 46 8 –19.50 III
t9 3.26 2.59 0.03 0.20 7.72 54 39 7 –9.25 III
t10 3.15 6.24 0.03 0.18 2.89 51 40 9 –13.80 III
t11 3.27 3.22 0.04 0.22 6.83 47 45 8 –18.25 III
t12 3.12 0.60 0.02 0.05 8.39 43 47 10 –23.75 III
average value 3.17 2.49 0.03 0.23 12.08 51.58 40.25 8.17 –12.59 III
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TOC and potential generation of shale samples from the Longtan Formation.

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Cross-plot of organic matter types: (a) TOC vs S2 and (b) Depth vs R o.

Clay minerals in the shale vary from 38 to 92%, with an average of 62.75%. Quartz content ranges from 3 to 41% (average 22%). Calcite content varies from 2.00 to 6.00% (average 4.58%). Small amounts of dolomite, rutile, and siderite were also detected, with average values of 0.17, 2.50, and 8.00%, respectively (Figure a). The brittle mineral method is used to calculate the brittleness index of shale minerals. − Calculate the brittleness index of shale minerals based on the mass fractions of quartz, dolomite, calcite, feldspar, and pyrite. The brittleness index of shale in well C1 is between 6 and 44, and fracture propagation needs to be improved by the transformation process. Based on the differences in organic shale fractions, shales are described as “rich” in that particular fraction (Figure b). The tested shale samples were mainly silica-dominated, carbonate-rich, mixed siliceous, and mixed shale.

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(a) Shale mineral composition of well C1; (b) Shale petrographic ternary diagram. Reprinted with permission from ref . Copyright 2022, Elsevier.

4.2. Elemental Geochemistry

4.2.1. Major Elements

The major element contents are listed in Table . SiO2 and Al2O3 are the two most dominant components, with a range of 18.58–53.70% and 6.71–33.75%, and the mean values were 41.64 and 23.51%, respectively. Other major element content of Mg, Na, K, P, Ti, Ca, TFe, and Mn is relatively low.

3. Major Element Contents (%) of Shale in the Sichuan Basin.
sample number depth (m) SiO2 TiO2 Al2O3 MgO Na2O K2O P2O5 CaO TFe2O3 MnO
t1 1780.47 44.82 3.36 32.29 0.36 0.93 0.38 0.06 0.29 2.08 0.01
t2 1784.07 44.04 4.29 33.65 0.33 1.08 0.57 0.09 0.29 1.51 0.01
t3 1791.22 44.32 5.11 32.94 0.45 0.81 0.71 0.12 0.35 1.94 0.01
t4 1794.46 44.47 3.48 22.48 1.01 0.77 1.72 0.10 0.62 13.26 0.18
t5 1796.62 44.79 4.88 33.75 0.34 0.90 0.71 0.10 0.39 1.74 0.01
t6 1806.43 53.70 4.82 24.76 0.75 0.82 2.87 0.11 0.85 2.68 0.03
t7 1808.16 43.66 3.22 21.22 1.39 1.09 2.07 0.61 3.34 8.64 0.23
t8 1810.32 43.71 3.08 15.75 1.82 0.70 1.16 0.52 2.77 15.64 0.21
t9 1812.58 39.69 4.28 26.27 0.42 0.84 1.00 0.15 0.71 10.11 0.12
t10 1813.87 45.03 4.09 29.96 0.24 1.60 1.07 0.08 0.41 1.43 0.02
t11 1816.56 50.35 3.61 27.68 0.51 1.56 1.98 0.37 0.79 2.37 0.01
t12 1822.82 42.69 7.90 32.53 0.49 0.53 0.27 0.08 0.28 3.44 0.01
r1 1746.89 18.58 1.47 13.22 0.16 0.2 0.52 0.03 8.83 33.57 0.03
r2 1748.30 26.35 2.93 22.88 0.16 0.18 0.13 0.02 0.12 24.52 0.01
r3 1752.09 45.76 4.63 25.74 0.35 0.85 0.47 0.09 0.29 1.93 0.001
r4 1757.62 50.41 3.86 23.88 0.61 1.63 1.57 0.12 0.56 2.94 0.037
r5 1765.95 19.55 5.22 23.11 1.17 1.30 0.85 0.22 0.79 25.13 0.63
r6 1769.55 45.93 2.96 10.99 1.52 0.4 0.41 0.31 1.46 12.98 0.055
r7 1775.70 48.42 3.62 16.2 2.41 0.51 0.78 0.43 3.17 12.83 0.08
r8 1831.49 39.46 3.31 17.71 1.56 0.78 1.1 0.2 2.48 15.2 0.065
r9 1833.00 38.81 0.7 6.71 0.93 0.21 0.37 0.07 2.42 15.72 0.049
PAAS   62.80 1.00 18.90 2.20 1.20 3.70 0.16 1.30 4.95 0.12
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4.2.2. Trace Elements

V (455.20 ppm), Cr (225.60 ppm), Cu (232.20 ppm), Sr (694.90 ppm), Zr (671.00 ppm), and Ba (238.70 ppm) were significantly enriched (Table ). Whereas Rb is significantly depleted, other trace element contents are close to PAAS.

4. Trace Element Contents (ppm).
sample number depth (m) Li V Cr Co Ni Cu Zn Rb Sr Ga Nb Mo Zr Ba Pb U Th
t1 1780.47 87.10 578.00 259.00 15.80 72.10 210.00 30.00 11.20 715.00 54.00 73.10 0.91 694.00 165.50 11.20 4.07 21.50
t2 1784.07 117.50 695.00 248.00 16.90 117.00 661.00 37.00 16.80 783.00 53.20 102.00 1.61 857.00 178.50 16.80 5.72 24.90
t3 1791.22 128.00 664.00 282.00 23.40 105.00 220.00 49.00 19.50 788.00 50.00 99.30 1.14 821.00 218.00 19.50 4.45 21.70
t4 1794.46 23.00 484.00 182.00 53.30 84.00 148.50 127.00 56.80 887.00 43.30 85.80 1.51 697.00 312.00 56.80 4.00 19.20
t5 1796.62 71.20 444.00 239.00 13.50 69.80 287.00 36.00 20.20 934.00 64.40 246.00 2.15 2000.00 189.50 20.20 12.90 51.70
t6 1806.43 9.80 498.00 241.00 23.50 93.30 264.00 43.00 87.70 1070.00 48.90 106.50 0.98 781.00 471.00 87.70 4.78 20.10
t7 1808.16 8.20 362.00 112.00 41.40 65.90 217.00 251.00 56.70 891.00 35.80 84.50 2.25 630.00 369.00 56.70 3.73 17.05
t8 1810.32 17.30 295.00 122.00 55.30 97.80 143.00 120.00 32.00 606.00 27.70 75.50 1.69 542.00 231.00 32.00 3.33 14.05
t9 1812.58 103.50 468.00 276.00 38.30 80.80 196.50 60.00 27.50 772.00 46.80 91.50 1.77 760.00 257.00 27.50 4.21 18.30
t10 1813.87 77.40 496.00 270.00 10.80 55.40 206.00 19.00 26.50 985.00 53.50 91.60 3.85 791.00 271.00 26.50 4.66 18.10
t11 1816.56 29.20 382.00 88.00 22.50 55.00 162.00 186.00 52.60 942.00 49.00 127.50 2.91 862.00 365.00 52.60 5.53 26.20
t12 1822.82 96.30 894.00 433.00 23.80 93.80 345.00 253.00 8.10 626.00 49.20 148.30 2.48 1070.00 149.50 8.1 6.04 26.30
r1 1746.89 54.65 302.91 321.78 78.85 211.79 179.37 70.48 14.28 186.16 38.40 41.58 2.97 263.06 111.48 64.36 4.00 9.19
r2 1748.30 152.85 665.41 496.44 53.28 121.10 376.33 106.12 3.69 153.75 43.53 87.70 0.92 542.93 40.54 72.6 7.85 17.09
r3 1752.09 106.20 441.24 216.66 19.04 85.09 308.37 36.63 17.46 557.06 49.17 97.37 1.40 537.87 187.86 10.95 3.04 15.18
r4 1757.62 38.12 356.97 161.15 18.30 59.99 167.04 66.38 48.12 956.31 41.00 74.27 0.75 453.16 404.53 7.2 4.45 19.23
r5 1765.95 34.99 327.34 138.61 45.93 64.66 165.36 163.23 21.81 681.15 28.93 60.33 1.56 457.87 304.04 7.16 2.72 11.43
r6 1769.55 23.52 270.01 222.75 57.20 103.96 127.63 135.99 13.81 451.25 33.18 56.94 1.16 378.18 121.37 8.23 4.11 10.15
r7 1775.70 27.01 322.70 181.05 63.16 108.46 126.60 153.17 25.5 593.52 32.31 72.60 1.67 418.91 218.14 7.72 2.90 12.31
r8 1831.49 29.32 365.52 179.6 98.82 151.81 150.13 155.53 35.67 697.14 31.59 59.07 1.54 390.91 336.84 37.34 2.63 11.26
r9 1833.00 28.82 246.33 66.62 46.40 96.99 216.35 41.55 10.60 317.34 15.29 13.34 2.62 144.01 110.80 20.46 3.09 7.33
PAAS   75.00 140.00 100.00 20.00 60.00 50.00 85.00 160.00 200.00 20.00 18.00 1.00          
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4.2.3. Rare Earth Elements

Rare earth element content averages 543.60 ppm, significantly higher than that of PAAS (210.00 ppm) (Table ). The LREE/HREE ratios range from 2.35 to 11.22, with a mean value of 4.42, which is higher than that of PAAS (3.73). La N /Yb N values range from 0.50 to 2.80, with slight enrichment in LREE. The degree of LREE and HREE fractionation is expressed by La N /Sm N and Gd N /Yb N values, which vary between 0.40 and 1.60 and 0.70–2.40, respectively. The weak degrees of LREE and HREE fractionation are demonstrated.

5. Rare Earth Element Contents (ppm).
sample number depth (m) Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho
t1 1780.47 76.10 67.70 166.00 21.00 86.20 22.30 7.37 24.20 3.61 17.85 3.03
t2 1784.07 70.50 108.50 237.00 30.30 117.50 22.00 3.74 14.55 2.39 14.40 2.82
t3 1791.22 71.70 119.00 274.00 32.60 117.50 20.30 5.24 16.15 2.63 15.30 2.79
t4 1794.46 78.20 99.50 221.00 27.70 118.00 23.30 4.80 17.40 2.56 13.90 2.77
t5 1796.62 139.00 121.00 253.00 28.40 100.50 20.10 4.44 22.50 4.24 25.20 5.08
t6 1806.43 80.10 174.50 322.00 35.80 117.50 16.15 2.87 12.05 2.15 14.30 2.97
t7 1808.16 52.00 94.00 198.00 22.40 84.40 15.45 4.27 13.35 1.93 10.75 2.05
t8 1810.32 55.60 79.70 166.00 19.60 76.80 16.80 4.98 16.75 2.23 10.85 1.97
t9 1812.58 79.90 125.50 263.00 33.10 132.50 24.40 4.21 16.70 2.57 14.85 2.88
t10 1813.87 86.10 97.30 200.00 21.00 69.20 11.60 2.48 12.05 2.35 14.70 3.13
t11 1816.56 62.00 106.50 224.00 25.20 96.90 18.60 4.13 15.65 2.38 12.50 2.29
t12 1822.82 86.90 89.10 185.00 21.60 81.60 16.30 4.47 19.10 3.05 17.05 3.16
r1 1746.89 41.50 40.60 101.30 8.70 30.30 5.50 0.90 5.60 1.10 6.60 1.70
r2 1748.30 49.00 39.70 132.80 8.40 28.20 5.70 1.30 6.60 1.50 9.30 2.20
r3 1752.09 46.20 93.50 195.40 23.90 89.80 16.90 4.40 12.20 1.90 9.90 1.90
r4 1757.62 38.30 111.90 209.10 25.50 96.30 14.40 3.80 9.80 1.60 8.10 1.80
r5 1765.95 56.70 86.50 159.20 20.80 87.20 17.40 4.00 14.40 2.50 12.30 2.60
r6 1769.55 46.10 149.50 291.00 36.70 149.40 24.70 5.00 14.40 1.90 9.40 2.10
r7 1775.70 44.60 85.00 157.60 20.00 79.10 15.00 4.10 12.80 2.00 9.50 2.00
r8 1831.49 47.00 69.80 125.80 16.70 69.10 14.40 3.00 11.40 2.00 10.00 2.10
r9 1833.00 17.30 85.10 174.00 21.10 84.70 13.10 2.80 6.10 0.80 3.60 0.80
PAAS   27.00 38.00 80.00 8.90 32.00 5.60 1.10 4.70 0.80 4.40 1.00
sample number Er Tm Yb Lu Y REE LREE HREE LREE/HREE La N /Yb N La N /Sm N Gd N /Yb N
t1 7.30 0.99 6.03 0.86 76.10 510.50 370.57 139.97 2.65 0.83 0.45 2.39
t2 7.28 1.05 6.32 0.89 70.50 639.20 519.04 120.20 4.32 1.26 0.73 1.37
t3 7.27 1.07 6.52 0.99 71.70 693.10 568.64 124.42 4.57 1.34 0.86 1.48
t4 7.33 1.02 6.08 0.90 78.20 624.50 494.30 130.16 3.80 1.21 0.63 1.70
t5 13.45 1.89 11.75 1.70 139.00 752.30 527.44 224.81 2.35 0.76 0.89 1.14
t6 7.98 1.14 6.61 0.95 80.10 797.10 668.82 128.25 5.21 1.95 1.59 1.09
t7 5.42 0.76 4.43 0.64 52.00 509.90 418.52 91.33 4.58 1.56 0.90 1.80
t8 5.05 0.68 4.09 0.60 55.60 461.70 363.88 97.82 3.72 1.44 0.70 2.44
t9 8.10 1.20 7.17 1.03 79.90 717.10 582.71 134.40 4.34 1.29 0.76 1.39
t10 8.93 1.31 7.98 1.16 86.10 539.30 401.58 137.71 2.92 0.90 1.24 0.90
t11 6.09 0.86 5.16 0.78 62.00 583.00 475.33 107.71 4.41 1.52 0.84 1.81
t12 7.90 1.03 5.83 0.81 86.90 542.90 398.07 144.83 2.75 1.13 0.81 1.95
r1 4.90 0.80 4.40 0.70 41.50 254.50 187.42 67.11 2.79 0.69 1.08 0.76
r2 5.90 1.00 5.40 0.90 49.00 297.90 216.07 81.82 2.64 0.54 1.02 0.73
r3 4.50 0.70 3.80 0.60 46.20 505.50 423.84 81.70 5.19 1.79 0.82 1.89
r4 4.70 0.70 3.90 0.60 38.30 530.20 460.85 69.32 6.65 2.14 1.15 1.52
r5 6.40 0.90 5.10 0.80 56.70 476.80 375.10 101.69 3.69 1.25 0.73 1.69
r6 5.70 0.90 4.80 0.80 46.10 742.20 656.23 85.98 7.63 2.28 0.89 1.77
r7 4.80 0.70 3.80 0.60 44.60 441.60 360.76 80.80 4.46 1.64 0.84 2.00
r8 5.10 0.70 3.90 0.60 47.00 381.50 298.80 82.68 3.61 1.32 0.71 1.74
r9 2.40 0.40 2.30 0.40 17.30 414.70 380.74 33.93 11.22 2.77 0.96 1.62
PAAS 2.90 0.40 2.80 0.40 27.00 210.00 165.60 44.40 3.73 1.31 1.21 1.00
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4.3. Chemical Index of Alteration (CIA)

Based on the stability of the felsic provenance of the Kangdian ancient land, the exclusion of potassium metasomatism by XRD clay mineral assemblages, and the vertical continuous gradient of Chemical Index of Alteration (CIA), the CIA value of Longtan Formation can effectively indicate the chemical weathering intensity, and the influence of provenance-diagenesis-recycling can be ignored. CIA is a valid parameter to indicate paleoclimate. ,, CIA is calculated by molar [(Al2O3)/(Al2O3+CaO*+Na2O+K2O)] × 100, where CaO* represents only the content of CaO in silicate minerals. In this study, the obtained P2O5 data were used for the preliminary calibration of CaO (CaO* = molar CaO–molar P2O5 × 10/3). When the residual molar content is less than Na2O, the CaO value is used as the CaO* value. Otherwise, Na2O is used as the CaO* value. Longtan Shale CIA values range from 76.55 to 98.16% (Table ), reflecting deposition of sediments under intense chemical weathering conditions and a warm-humid paleoclimate.

6. Geochemical Parameters of the Longtan Shale.

sample number depth (m) CIA Sr/Cu Rb/Sr Ni/Co V/(V + Ni) V/Cr δU P/Ti Babio Sr/Ba Th/U
t1 1780.47 95.28 3.40 0.02 4.56 0.89 2.23 0.72 0.18 165.33 4.32 5.28
t2 1784.07 94.55 1.18 0.02 6.92 0.86 2.80 0.82 0.21 178.28 4.39 4.35
t3 1791.22 94.63 3.58 0.02 4.49 0.86 2.35 0.76 0.23 217.74 3.61 4.88
t4 1794.46 87.85 5.97 0.06 1.58 0.85 2.66 0.77 0.29 311.82 2.84 4.80
t5 1796.62 94.41 3.25 0.02 5.17 0.86 1.86 0.86 0.20 189.25 4.93 4.01
t6 1806.43 84.51 4.05 0.08 3.97 0.84 2.07 0.83 0.23 470.76 2.27 4.21
t7 1808.16 76.55 4.11 0.06 1.59 0.85 3.23 0.79 1.89 368.84 2.41 4.57
t8 1810.32 77.28 4.24 0.05 1.77 0.75 2.42 0.83 1.68 230.84 2.62 4.22
t9 1812.58 91.15 3.93 0.04 2.11 0.85 1.70 0.82 0.35 256.78 3.00 4.35
t10 1813.87 90.68 4.78 0.03 5.13 0.90 1.84 0.87 0.19 270.79 3.63 3.88
t11 1816.56 86.47 5.81 0.06 2.44 0.87 4.34 0.78 1.02 364.82 2.58 4.74
t12 1822.82 96.79 1.81 0.01 3.94 0.91 2.06 0.82 0.10 149.10 4.19 4.35
r1 1746.89 93.49 1.04 0.08 2.69 0.59 0.94 1.13 0.20 111.41 1.67 2.30
r2 1748.30 98.16 0.41 0.02 2.27 0.85 1.34 1.16 0.07 40.39 3.79 2.18
r3 1752.09 94.11 1.81 0.03 4.47 0.84 2.04 0.75 0.19 187.63 2.97 4.99
r4 1757.62 86.40 5.73 0.05 3.28 0.86 2.22 0.82 0.31 404.33 2.36 4.32
r5 1765.95 88.71 4.12 0.03 1.41 0.84 2.36 0.83 0.42 303.78 2.24 4.20
r6 1769.55 90.08 3.54 0.03 1.82 0.72 1.21 1.10 1.04 121.22 3.72 2.47
r7 1775.70 90.00 4.69 0.04 1.72 0.75 1.78 0.83 1.18 217.96 2.72 4.24
r8 1831.49 86.94 4.64 0.05 1.54 0.71 2.04 0.82 0.60 336.67 2.07 4.28
r9 1833.00 89.47 1.47 0.03 2.09 0.72 3.70 1.12 1.00 110.76 2.86 2.37
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5. Discussion

5.1. Element Enrichment

Major element enrichment in shales is characterized by a concentration coefficient (CC), which is given as rates that test the trace element content against the corresponding element content in post-Australian shales. Figure a shows the standardized sample data showing distribution patterns of major elements. TiO2 is the most enriched, with an average CC of 3.85; TFe2O3 is moderately enriched (CC = 2.02); the oxides P2O5 and CaO are close to PAAS, and slightly enriched; SiO2, Na2O, and MnO are moderately depleted (0.50 < CC < 1.00).

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Distribution pattern of Sichuan shale elements: (a) major elements; (b) trace elements; and (c) rare earth elements.

The degree of enrichment of trace elements shows that V (CC = 3.25), Cr (CC = 2.26), Cu (CC = 4.64), Ga (CC = 2.12), and Nb (CC = 5.01) are enriched. Other elements, Li, are significantly depleted with CC < 0.50 (Figure b).

The distribution pattern of the normalized REE is shown in Figure c. Dy and Ho geochemical characteristics are similar, so the Dy is also plotted in Figure c. δEu is an effective parameter reflecting the degree of Eu anomalies in the samples. The mean δEu (δEu = Eu N /(Sm N × Gd N )∧0.5) is 1.19, which is a positive Eu anomaly. Ce anomalies in sedimentary systems can effectively reflect changes in water oxidation–reduction conditions. The mean δCe (δCe = Ce N /(La N × Pr N )∧0.5) is 1.00, which demonstrates that shale depositional environments are reducing environments.

5.2. Paleoclimate

Ternary phase diagrams of Al2O3–(CaO*+Na2O)–K2O were plotted for medium-high degree chemical weathering products and low degree chemical weathering products for CIA values of 80–100, 50–70, respectively (Figure ). Weathering products produce a number of columns of CIA values distributed along the trend of the weathering line. The Longtan Shale varies along the weathering line, indicating a warm and humid climate. In addition, Sr/Cu and Rb/Sr elemental ratios can also reflect paleoclimate. The Longtan Shale is characterized by a high Sr/Cu ratio (mean 3.50%) and a low Rb/Sr ratio (mean 0.04%), implying warm-humid paleoclimatic conditions. ,

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Ternary phase diagram of the Longtan shale Al2O3–(CaO*+Na2O)–K2O. Reprinted with permission from ref . Copyright 2022, Elsevier.

5.3. Paleoenvironmental Reconstruction

5.3.1. Detrital Flux

The elements titanium (Ti) and aluminum (Al) in the sediments are representative of the detrital flux since they are not susceptible to weathering. Ti is mainly associated with heavy minerals and clay minerals, and Al is usually found in fine-grained aluminosilicate clay minerals. The TiO2 content ranges from 0.70 to 7.90%. The Al2O3 content varies from 6.71 to 33.75%. Both of them are much higher than that of PAAS, which implies a strong input of terrestrial-sourced debris.

The Al2O3 and TiO2 show a significant positive correlation (Figure a), proving that terrestrial-derived detrital material is the main source of Ti in Longtan shale. The SiO2 and TiO2 show a weak positive correlation (Figure b), indicating a small input of terrestrial source debris for Si elements in shales.

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Terrestrial source clastic correlation diagram for the Longtan Shale: (a) Al2O3-TiO2 and (b) SiO2-TiO2.

5.3.2. Redox

Redox-sensitive elements in sediments are used to identify paleodepositional environments. Elemental ratios are resistant to perturbation and relatively stable in determining paleo-redox conditions compared to the content of individual elements. ,

Here, Ni/Co ratios (1.41–6.92, mean 3.09) indicate an oxidizing environment for the Longtan Shale paleoenvironment, V/Cr ratios (0.94–4.34, mean 2.25) reflect an eutrophic environment, V/(V + Ni) ratios (0.59–0.91, mean 0.82) indicate a stable reducing environment, and δU (0.72–1.16, mean 0.87) <1 for an oxidizing environment (Table ).

7. Criteria for Determining Redox Conditions (Modified from ref in Accordance with the RightsLink Printable License).
environment ratios index
paleoclimate Sr/Cu wet and warm (Sr/Cu < 10) dry and hot (Sr/Cu > 10)  
paleosalinity Sr/Ba fresh water (Sr/Ba < 0.5) brackish water (0.5 < Sr/Ba < 1) saline water (Sr/Ba > 1)
redox V/(V+Ni) suboxic or anoxic (V/(V+Ni) > 0.6) dysoxic (0.46 < V/(V+Ni) < 0.6) oxic (V/(V+Ni) < 0.46)
Ni/Co suboxic or anoxic (Ni/Co > 7) dysoxic (5 < Ni/Co < 7) oxic (Ni/Co < 5)
V/Cr suboxic or anoxic (V/Cr > 4.25) dysoxic (2 < V/Cr < 4.25) oxic (V/Cr < 2)
δU suboxic or anoxic (δU > 1)   oxic (δU < 1)
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Paleo-redox conditions revealed by MoEF–UEF covariation in response to uncertainties in elemental ratio judgments of aquatic environments. The MoEF–UEF cross-plot showed that the Mo/U ratio was mainly in the range of 0.1–0.3 × SW, indicating anoxic conditions (Figure a). Mo and U exhibit different chemical behaviors in different oxygenated environments. Mo and U exist in the form of thiomolybdate and uranium oxide complexes under hypoxic conditions, and in the form of molybdate and uranium carbon complexes in oxidizing environments. The rendezvous map of MoEF–UEF can effectively avoid the effects of fragmented inputs. The Longtan Shale displays a low Mo/TOC ratio indicative of deposition in a strongly constrained basin. The lack of H2S in the anoxic environment limits Mo accumulation. Combined with other elemental ratios (Figure ), the Longtan Shale is in a stable anoxic environment.

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(a) Cross-plots of MoEF–UE. (b) Relationship between TOC and Mo content. Reprinted with permission from ref . Copyright 2016, Elsevier. Reprinted with permission from ref . Copyright 2012, Elsevier.

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Well C1 paleoenvironmental characterization.

5.3.3. Paleoproductivity

Plankton, plant debris, and bacteria are the primary sources of organics in lake basins. Phosphorus (P) and barium (Ba) can accurately characterize paleoproductivity and govern the forming of hydrocarbon rocks. ,

P is an essential element for biological development. Plankton productivity is high when there is an adequate supply of material, and the use of P/Ti to characterize the paleoproductivity of the water column eliminates the influence of autochthonous minerals. , The P/Ti ranges from 0.07 to 1.78 (average 0.55) (Table ). The P/Ti ratios are indicative of high paleoproductivity during the sedimentary period.

Barium (Ba) generally occurs as Barite (BaSO4), and the deposition rates are related to paleoproductivity. The total Ba content of sediments is mainly derived from terrestrial clastic inputs and biological sources, and biological barium is considered a reliable indicator of paleoproductivity. The formula for calculating biological Ba is Babio = Batot–(Titot × (Ba/Ti)PAAS). Babio content varies from 40.39 to 470.76 ppm with an average of 238.50 ppm. Babio < 200 ppm is considered low productivity, 200 ≤ Babio < 1000 ppm is considered medium productivity, and Babio ≥ 1000 ppm is considered high productivity. So the Longtan Shale depositional period has medium productivity (Figure ).

5.3.4. Paleosalinity

Paleosalinity is an excellent indicator in paleodepositional environments, and higher salinity favors organic matter preservation. Sr/Ba and Th/U are valid parameters for determining paleosalinity. , Sr/Ba ratios range from 1.67 to 4.92 with a mean value of 3.11; Th/U ratios range from 2.18 to 5.28 with a mean value of 4.05 (Table ). The average Sr/Ba ratio is 3.11 > 1.00, indicating deposition in a salty water environment. The average Th/U ratio of 4.05 ranges from 1.00 to 10.00, indicating deposition in a brackish environment. The Longtan Shale is deposited in a distinctly saline environment (Figure ), which provides for the preservation of organics.

5.4. Organic Matter Enrichment Mechanism

Paleodepositional environments control the input and preservation of organics, , which accumulates through complex physicochemical processes over long periods of deposition. , Paleoclimate, detrital flux, and paleoproductivity are the primary factors influencing organic inputs, while redox and paleosalinity control organic matter preservation. ,−

Sources throughout the depositional period indicate warm and humid paleoclimatic conditions, which were favorable for plant growth and reproduction, showing a high TOC content (Figure a). Warm and humid environments increase primary paleoproductivity while indirectly enhancing the terrestrial source detrital flux input (Figure b). The comprehensive analysis of paleoproductivity parameters is related to TOC (Figure c,d). Paleoclimate is negatively correlated with paleosalinity (Figure e). High paleosalinity leads to low productivity (Figure f). During the depositional period, organic-rich shales were formed as sediments accumulated and compacted. These organic matter-rich shales are an important source of paleo-oil and paleo-gas. High salinity depositional environments favor the preservation of organic matter after deposition and therefore have no obvious relationship with organic matter (Figure g,h,i).

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Cross-plots of paleoenvironmental indicator parameters: (a) TOC vs CIA; (b) TOC vs Al2O3; (c) TOC vs P/Ti; (d) TOC vs Babio; (e) Rb/Sr vs Sr/Ba; (f) Sr/Ba vs Babio; (g) TOC vs V/(V+Ni); (h) TOC vs V/Cr; and (i) TOC vs Ni/Co.

Corresponding depositional patterns were obtained from elemental geochemical characterization of well C1. Organic matter is deposited under warm and humid climatic conditions. Paleoclimate provides the basis for organic matter accumulation for plant growth, the Kangdian old land provided debris input, and the high salinity environment provided conditions for organic matter preservation. During the depositional period, the salinity of the water body in the shallow strata of the Longtan Formation (depth less than 1760 m) was low (Figure ). The oxic environment was characterized by algal blooms in the water body and poor conditions for the preservation of organic matter (Figure ). At a depth greater than 1760 m, the high water salinity does not lead to stable salinity stratification, but the bottom of the sedimentary water body presents anoxic conditions as a whole, and the overall anoxic conditions at the bottom of the sedimentary water column are more conducive to the preservation of organic matter.

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Model of organic matter enrichment.

6. Conclusions

  • (1)

    The shale within the Longtan Formation exhibits high organic matter abundance and is predominantly composed of type III kerogen. The mineral composition of the shale is mainly composed of clay minerals and quartz, with a combined content ranging from 67–95%.

  • (2)

    A series of indicators (CIA, Al2O3, TiO2, Ni/Co, V/(V+Ni), V/Cr, δU, Babio, Sr/Ba, Th/U) indices that the climate during the deposition of the Longtan Shale was warm and humid, with a strong input of terrestrial-derived detritus, stable anoxia, medium productivity, and a salty water environment.

  • (3)

    The prevailing warm and humid climate established a fundamental prerequisite for the accumulation of organic matter. The high salinity characteristic of the deep-seated sedimentary waters, in tandem with the overall anoxic environmental conditions, created a milieu highly conducive to the preservation of organic matter.

Acknowledgments

This research financing was supported by the Guizhou Provincial Science and Technology Plan Project (Qiankehe Platform Talents-CXTD[2022]016; Qiankehe basis-ZK[2024] commonly 687; Qiankehe support-ZK[2023] commonly 287; Qiankehe support-ZK[2025] commonly 062).

W.G. and H.L. conceived and designed this project; H.L., Y.F., and S.C. conducted the field sampling; W.G., Y.F., H.L., and S.C. carried out all of the measurements; and W.G., H.L., and S.C. wrote the draft of the paper.

The authors declare no competing financial interest.

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