Introduction

Shale gas is increasingly recognized as a relatively clean natural gas resource by countries worldwide1,2. Currently, Chinese recoverable resources of shallow shale gas (below the burial depth of 4,500 m) are about 22 trillion cubic meters, making China one of the leading regions in the world to achieve large-scale shale gas development3,4, of which the proven shale gas reserves are 560,559 million cubic meters, mainly located in the southwestern region5,6. The proven reserves of shale gas in China amount to approximately 560,559 million cubic meters, primarily concentrated in the southwestern region5,7. Meanwhile, due to mountainous and other geomorphologic conditions, shale gas drilling is difficult, time-consuming and costly8,9. Therefore, the preservation conditions of shale gas not only determine the accumulation scale of shale gas, but also are a key factor in controlling the cost of shale gas exploration.

At the beginning of the 21st century, the method for evaluating the preservation conditions of North American shale gas was based on the microscopic characterization of shale reservoirs1,10. The support of a large amount of reservoir data has contributed to the success of this evaluation method for shale gas exploration in North America. The evaluation methods for shale gas selection in South China are mostly based on the Sichuan Basin and its surrounding areas6,11,12,13. The current comprehensive evaluation system of fracture-fold-stratigraphy, which is recognized by the majority of scholars, is qualitative evaluation5,14. It can only accurately evaluate the area in a small range and with full information. The continuity of this evaluation result is poor. With the expansion of shale gas exploration scope in southern China, the problem of large difference in shale gas exploration degree has been exposed1,15. Due to lack of microscopic data, the North American method is not suitable for areas and layers in the early stages of shale gas exploration (such as Silurian and Cambrian strata in Yunnan and Guizhou provinces, and Carboniferous strata in Guizhou and Guangxi provinces)16,17,18. The complex surface conditions and limited drilling data in some areas of southern China render the application of traditional fracture-fold-stratigraphic evaluation systems impractical. Consequently, there is a pressing need for a quantitative shale gas selection evaluation method that can integrate multiple factors, has reduced dependence on oil and gas data, and can be effectively implemented over large regions.

The Youjiang Basin, located in southern China, is a significant region for oil and gas resources. It also serves as the primary area of marine sedimentation during the Upper Paleozoic era19. The shale of the Middle Devonian Luofu Formation is characterized by shallow burial depth, large sediment thickness and sufficient hydrocarbon generation potential20,21. During the Middle Devonian period, the alternating paleogeographic pattern of platform and basin formations led to varied distributions of organic matter within shale. Strong tectonic activity resulted in strong heterogeneity of shale gas reservoirs22,23. Furthermore, the complex surface conditions, low exploration levels, and scarcity of information in certain areas have rendered previous shale gas selection evaluation systems obsolete24. Therefore, taking the Luofu Formation shale of the Youjiang Basin as an example, this paper proposes a method for evaluating the regional shale gas preservation conditions, by which it is easy to obtain information (using available data from regional geological maps) and process data (processed by computer software). This method is not only suitable for regions with extensive areas and limited drilling data but also enables qualitative and semi-quantitative evaluation of shale gas preservation conditions.

Materials and methods

Study area and data

Study area

The Youjiang Basin is part of the South China Plate, located at the junctions of Southeast Yunnan, Western Guangxi, and Southern Guizhou (Fig. 1a)19. It includes multiple secondary basins, such as the Nanpanjiang Basin, the Guizhong Basin and the Qiannan Basin, among others (Fig. 1b)22,23. During the Hercynian period, marine strata began to be deposited in the Youjiang Basin (Fig. 1c)25. The Late Paleozoic was the period of the greatest sea transgression26. Along with the Taghanic transgression of the Middle Devonian Givetian, the Luofu Formation shale was deposited, including siliceous shale, carbonaceous shale and so on (Fig. 1c)27. Among them, the organic-rich shale (with a thickness of tens to hundreds of meters) is a high-quality hydrocarbon source rock28,29.

Fig. 1
figure 1

(a) Location of the study area in China23; (b) Distribution of construction units23; (c) The developmental strata of the Youjiang Basin23; (d) The exposed strata on the surface (use section software to stitch 47 maps into this picture).

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Data

  1. (1)

    Geologic map data

The geological map data are all from the geological map spatial database of China Geological Survey (The data that support the findings of this study are available from [http://www.drc.cgs.gov.cn] but restrictions apply to the availability of these data, which were used under license for the current study, and so are not publicly available. Data are however available from the authors upon reasonable request and with permission of [http://www.drc.cgs.gov.cn]). This database is the first one in China based on the rules of geographic information application model (ISO19109) and geographic information spatial model (ISO19107). By comparing the information from geologic maps of different scales, it is found that 47 geologic maps with a scale of 1:250,000 cover the Youjiang Basin (Fig. 1d).

  1. (2)

    Earthquake epicenter data

Data of earthquakes with magnitude ≥ 4 in the Youjiang Basin since 2010 is from China Earthquake Networks Center (The data that support the findings of this study are available from [https://news.ceic.ac.cn] but restrictions apply to the availability of these data, which were used under license for the current study, and so are not publicly available. Data are however available from the authors upon reasonable request and with permission of [https://news.ceic.ac.cn]), including magnitude and epicenter location. Among these, there are 69 epicenters with a magnitude of 4, 87 with a magnitude of 5, 11 with a magnitude of 6, and 2 with a magnitude of 7.

  1. (3)

    Hydrothermal ore spot data

All hydrothermal ore spot data are from China Mineral Land Database (The data that support the findings of this study are available from [http://ngac-org-cn-0205gbl5e7200.wsipv6.com/kuangchandi] but restrictions apply to the availability of these data, which were used under license for the current study, and so are not publicly available. Data are however available from the authors upon reasonable request and with permission of [http://ngac-org-cn-0205gbl5e7200.wsipv6.com/kuangchandi]). A total of 51 locations for hydrothermal ore spots, encompassing four types of hydrothermal minerals (gold, antimony, mercury, and manganese), have been documented.

  1. (4)

    Hot spring data

A total of 55 hot spring data are from China Hydrogeological Survey Data (The data that support the findings of this study are available from [http://ngac-org-cn-0205gbl5e7200.wsipv6.com/Distribute/20wanSW.htm] but restrictions apply to the availability of these data, which were used under license for the current study, and so are not publicly available. Data are however available from the authors upon reasonable request and with permission of [http://ngac-org-cn-0205gbl5e7200.wsipv6.com/Distribute/20wanSW.htm]). The hot springs within the Youjiang Basin are primarily concentrated in the northeastern region of the Qiannan Basin and the northern area of the Guizhong Basin.

Methodology

The evaluation method for regional preservation conditions of shale gas is based on the fracture index, dip angle index, and stratigraphy index derived from assignment calculations. The preservation index of the formation is then calculated using a multiplication method, allowing for a quantitative assessment of the preservation conditions.

General workflow

  1. 1.

    Geological map selection and data extraction. Select appropriate geological maps and extract effective information, including exposed stratum, magmatic rock distribution, stratigraphic occurrence and surface fracture distribution.

  2. 2.

    Establishment of Standard Grid. Divide a grid with a distance of 5 km between adjacent nodes, which will be used for grid processing of data in the subsequent calculation process.

  3. 3.

    Calculation of fracture index. Extract all fracture lines in the geological map, and use the attribute information of the fracture lines to sum the length of the fracture lines within each grid, thereby calculating the fracture density within the grid. After that, the threshold value of fracture density was determined, and different fracture densities were assigned and calculated. Finally, the evaluation result of the fracture index is calculated based on the calculation results.

  4. 4.

    Calculation of dip angle index. Grid the formation dip data, and then determine the threshold value corresponding to the formation dip index and assign it a value for calculation. Finally, the evaluation result of the formation dip index is converted based on the calculation results.

  5. 5.

    Calculation of stratigraphy index. Firstly, the distribution of magmatic rock area, metamorphic rock area, target layer outcrop area, target layer denudation area and target layer burial area within the study area is determined based on the geological map. Secondly, grid processing is carried out for each region. After that, each region is assigned a value based on the grid processing results. Finally, the assigned value is the result of the stratigraphic index.

  6. 6.

    Based on the criterion and parameter data of each grid, compile the maps representing the categories of the fracture index, dip angle index and stratigraphy index.

  7. 7.

    Compute the comprehensive index based on maps by the following formula,

    $$CI=\prod_{n=1}^{3}{C}_{n}$$
    (1)

    where CI is the comprehensive index, Cn is the nth indicator.

  8. 8.

    Compare the calculated CI values with the actual exploration results to determine the accuracy of the method, and finally, to evaluate the preservation conditions of each area.

Acquisition of the evaluation metrics

  1. (1)

    The source of acquisition

In areas with a low exploration degree, the primary source of data for shale gas selection and evaluation is derived from geologic maps. Publicly available high-precision geological maps are the first choice of data source. The high-precision geologic maps should provide information, such as exposed stratum, magmatic rock distribution, stratigraphic occurrence and surface fracture distribution (Fig. 1d). The evaluation process necessitates the use of three indices: stratigraphy index, dip angle index, and fracture index. All these indices are calculated based on data extracted from a total of 47 geologic maps.

  1. (2)

    Fracture index

Under the influence of strong tectonic movements during the Yanshan and Xishan phases, a large number of high-angle fractures were commonly developed in the Youjiang Basin (Fig. 2a)22,23. These fractures directly control the storage properties of shale gas. Previous studies have shown that faults exposed to the surface can become a channel for shale gas leakage30. Fractures and faults are connected to form a network, exacerbating the extent of shale gas leakage. The longer the fractures extend in a planar spread, the greater the likelihood of shale gas reservoir leakage31. Therefore, the total length of exposed faults developed per unit area, i.e., the density of exposed faults, can be used as evidence of the degree of fracture development and an indicator for evaluation.

Fig. 2
figure 2

(a) Distribution of surface fault in the Youjiang Basin; (b) Calculation results of surface fracture density; (c) The Middle Devonian fracture index. Data were extracted using Surfer 18.0 software and processed with MapGiS 6.7 software, and all pictures were created using Section software.

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Considering that the natural gas in the exploration case is dissipated in formations 8–10 km away from the fracture zone32, a 5 km × 5 km grid is used to divide the area, and the total length of the faults and the fault density is calculated over a 25 km2 area. Meanwhile, the fracture density calculation map is plotted using the natural neighbor interpolation approach33 that can better present the internal correlation of the data (Fig. 2b). The fracture density conversion maps are plotted according to the density conversion standard (Fig. 2c).

  1. (3)

    Dip angle index

The Youjiang Basin is primarily characterized by high and steep tectonic structures, a result of the significant influence of tectonic activities during the Mesozoic and Cenozoic eras23. Exploration results show that shale gas is more likely to accumulate in areas with smaller dip angles such as fold cores and horizontal strata32. Stratigraphic dip angle can reflect the deformation strength of strata, and it is also one of the important indexes for shale gas preservation condition evaluation. The formation dip data was processed through gridding (Fig. 3a) and the calculation diagram of formation dip was drawn (Fig. 3b). The dip grid density is consistent with the fracture grid density (5 km × 5 km). The stratigraphic dip conversion diagram is drawn according to the conversion standard (Fig. 3c).

Fig. 3
figure 3

(a) Distribution of stratigraphic dip measurement points in the Youjiang Basin; (b) Calculation result of dip angle; (c) The Middle Devonian dip angle index. Data were extracted using Surfer 18.0 software and processed with MapGiS 6.7 software, and all pictures were created using Section software.

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  1. (4)

    Stratigraphy index

In the case of Devonian shale, intense late tectonic activity can cause damage to the shale top and bottom plates and self-containment, affecting the production capacity of shale gas wells23. Affected by denudation, magmatic intrusion and metamorphism, the Middle Devonian shale is shallowly buried or exposed at the surface. Shale gas reservoirs cannot be formed in areas of magmatic rocks, metamorphic rocks and denudation27,28. The stratigraphy index grading evaluation is carried out according to the stratigraphic grading standard. The stratigraphy grid density is consistent with the fracture grid density (5 km × 5 km) (Fig. 4).

Fig. 4
figure 4

The Middle Devonian stratigraphy index in the Youjiang Basin. This picture was created using Mapgis Sect. 6.7 software.

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  1. (5)

    Criterion for rating the preservation condition

The threshold for the preservation condition classification is determined by combining drilling and gas logging data (Table 1).

Table 1 Rating criterion of the indicators used in the evaluation.
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Rating assignment of the evaluation metrics

Based on the existing research and experience, three indices (stratigraphy index, dip angle index and fracture index) are graded and assigned values.

  1. 1.

    Previous studies have shown that when the fracture density is greater than or equal to 30 km/km2, the shale gas reservoir is almost completely destroyed32, resulting in a fracture index of 0. When the fracture density is 0 km/km2, the fracture index is 1 (Table 1).

  2. 2.

    The exploration results show that the stratum is gentle with a dip angle of 0°, and shale gas is generally concentrated here31, so the stratum dip index is 1. When the stratum dip angle is greater than or equal to 45°, shale gas is almost completely dispersed, so the stratum dip index is 0 (Table 1).

  3. 3.

    The exploration practice has shown that the magmatic and metamorphic area and the denudation area cannot be the reservoir of shale34,35,36,37,38, so the stratigraphy index is 0. Although the Devonian strata are exposed on the surface, they may still contain potential for storage. The stratigraphy index is assigned to be 0.6. The stratigraphy index of the buried area is 1 (Table 1).

  4. 4.

    Rating criteria of the preservation conditions are categorized into 5 levels. The storage conditions range from good to poor, ranging between 0.7 and 1.0, 0.5–0.7, 0.3–0.5, 0.1–0.3, 0.0–0.1, respectively.

Results and discussions

Overall evaluation

Previous studies have shown that there should be some positive correlation between measured shale gas content and preservation conditions. The wells of relatively higher gas content (Well HH and Well GY) are located in areas with higher CI values (0.3–0.5), while those of lower gas content (Well DY, Well GZ and Well GTD) are located in areas with lower CI values (< 0.3) (Table 2), suggesting that CI values are indicative of shale gas preservation conditions to some extent (Fig. 5).

Table 2 Evaluation results and measured shale gas contents of the wells in the Youjiang Basin.
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Fig. 5
figure 5

Map showing the computed comprehensive indexes (CI) in different construction units of the Youjiang Basin. This picture was created using Mapgis Sect. 6.7 software.

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The distribution of hydrothermal occurrences provides evidence of regional thermal events in exposed surface zones. These thermal activities can disrupt pre-existing hydrocarbon reservoirs, potentially affecting their integrity and storage capacity. Almost all hydrothermal deposits in the Youjiang Basin are located in the region with CI of 0.0–0.1. Hot springs are hot water that flows out of the surface after infiltration of surface water and geothermal warming, so the development of hot springs can reflect the degree of opening of the related fractures. Hot springs are mainly developed in the northeast part of the Qiannan Basin and the northern part of the Guizhong Basin with CI of 0.0–0.1. The results of previous studies have shown that earthquake epicenters are generally located at the intersection of multiple fractures. There are a limited number of epicenters within the Guizhong Basin, whereas there are many more within the Nanpanjiang Basin. Regions with epicenters of earthquakes having a magnitude of 4.0 or greater tend to exhibit poor shale gas preservation conditions (Fig. 5). In summary, the computed comprehensive indexes (CI) can effectively evaluate regional shale gas preservation conditions.

Evaluation results of preservation conditions

A comprehensive evaluation based on the calculated CI values shows that regions with a CI value between 0.7 and 1.0 cover 11621.23 km2, between 0.5 and 0.7 cover 37162.67 km2, between 0.3 and 0.5 cover 57784.43 km2, between 0.1 and 0.3 cover 69303.77 km2, and between 0.0 and 0.1 cover 188995.9 km2 (Fig. 5), accounting for 3.19%, 10.19%, 15.84%, 18.99% and 51.80% of the whole area, respectively (Fig. 5). This is an effective way to qualitatively and semi-quantitatively evaluate the preservation conditions of shale gas, especially for areas with limited oil and gas geological data.

The evaluation of shale gas preservation conditions in the Youjiang Basin shows that there are significant differences in preservation conditions of different regions after the Yanshan and Xishan tectonic movements. In the northern and central parts of the Guizhong Basin, the southern part of the Xidamingshan Uplift, and the northern part of the Qiannan Basin, there are large areas with good preservation conditions (CI > 0.5) (Fig. 5). There are a few areas with better preservation conditions, but they tend to be small and scattered across various regions (Fig. 5). The preservation conditions of shale gas in the Xuefeng Uplift and Maguan Uplift are poor (CI < 0.3), and there are no shale gas preservation conditions here (Fig. 5).

Conclusions

(1) By extracting exposed stratum, magmatic rock distribution, stratigraphic occurrence and surface fracture distribution from the geological maps, the evaluation criteria for three indexes (stratigraphic index, dip index and fracture index) were established. The calculation results can represent the evaluation results of shale gas preservation conditions.

(2) The evaluation method based on the combination of stratigraphy index, dip angle index and fracture index is effective for qualitative and semi-quantitative assessment of shale gas preservation conditions. It is particularly suitable for areas with limited hydrocarbon geological data.

(3) On the basis of the calculated CI values, the comprehensive evaluation each area: 11621.23 km2 for CI of 0.7–1.0, 37162.67 km2 for CI of 0.5–0.7, 57784.43 km2 for CI of 0.3–0.5, 69303.77 km2 for CI of 0.1–0.3, and 69303.77 km2 for CI of 0.0–0.1, which account for 3.19%, 10.19%, 15.84%, 18.99% and 51.80% of the whole area, respectively.

(4) In the northern and central parts of the Guizhong Basin, the southern part of the Xidamingshan Uplift, and the northern part of the Qiannan Basin, there are large areas with good shale gas preservation conditions.