The collection and analysis of Dawuba Shale samples were comprehensively conducted from Well QZY1, encompassing a variety of tests to assess the shale’s characteristics. A structured approach was applied in sample collection and analysis, with a total of 45 samples designated for major element analysis and 35 samples for trace element analysis. To understand the organic composition, 25 samples underwent organic petrographic characterization, and 26 samples were analyzed for vitrinite reflectance (Ro%). The total organic carbon (TOC) content was determined from 33 samples. For mineralogical composition, 60 samples were subjected to X-ray diffraction (XRD) analysis. Additionally, the study included physical property assessments on six core plug samples and gas content analysis on 16 samples. Preparation of samples varied according to the analysis requirements, with polished sections being prepared for organic petrology and vitrinite reflectance analysis, while other samples were processed into powders for various chemical analyses.

In the case of the Niutitang shales within the Guizhou area, samples and data were obtained from Well HD1, complemented by research findings from Ge et al. (2020, 2024) (Ge et al., 2020; Ge et al., 2024)and Yang et al. (2021) (Yang et al., 2021). For the Niutitang shales in the Chongqing area, data were sourced from studies by Li et al. (2015) (Li et al., 2015), Wu et al. (2016) (Wu et al., 2016), Zhang (2019) (Zhang, 2019), and Wang (2020) (Wang, 2020). This diverse dataset, combining direct sample analysis with insights from recent studies, provides a robust foundation for evaluating the geochemical, petrographic, and gas potential of these shale formations.

No unexpected or unusually high safety hazards were encountered during the testing.

For mineral content and clay composition, X-ray diffraction analysis (XRD) was conducted on crushed samples (200 mesh). Utilizing a Bruker D8 Advance XRD system with Cu-Kα radiation at 40 kV voltage and 40 mA current, this analysis semi-quantified the mineral constituents of the shales.

Porosity was determined using the CMS-300 instrument on core plugs at a pressure of 200 psi, providing insights into the storage capacity of the shales. Air permeability was assessed with a pulse-decay permeameter at a mean gas pressure of 435 psi (3 MPa), offering information on the ease with which gases can flow through the shale matrix.

Scanning electron microscopy (SEM) was employed to examine the pore morphology on freshly broken surfaces of the shales. Samples were coated with a gold/palladium (Au/Pd) alloy and observed under a Tescan Vega 3 LM system. The SEM operated at a high vacuum with 20 kV acceleration voltages and electric detectors, achieving a maximum resolution of 3 nm.

Low-pressure N2-physisorption tests were performed using a Micromeritics ASAP 2020 apparatus to characterize the pores within the shales. Crushed samples (60–80 mesh) were outgassed overnight at 110°C under vacuum to eliminate volatile substances and moisture. The N2 adsorption/desorption isotherms were then recorded at 77 K. Analysis of the adsorption data employed non-local density functional theory (NLDFT) and multipoint Brunauer–Emmett–Teller (BET) analyses for specific surface area (SSA), along with Barrett–Joyner–Halenda (BJH) analysis for pore size distribution (PSD), as detailed by Sing (1995), Lowell et al. (2004), and Barrett et al. (1951) (Barrett et al., 1951; Sing, 1995; Lowell et al., 2004). The pore-size distribution was illustrated using the dV/dlog(D) versus D plot for N2 adsorption, which effectively highlights the volume distribution of pores.

The collection of shale gas utilized a gas gathering system based on the water gas displacing method, using a saturated salt solution as the displacing medium. Pressure-core samples were immediately placed in an air collector upon extraction from the drilling borehole. The gas collected during the unheated stage was categorized as desorbed gas. After reaching a plateau in desorbed gas collection, the system was heated to gather residual gas. The quantification of lost gas was not included in this study.

The composition of major and trace elements, alongside their ratios, serves as a robust proxy for understanding the depositional environments of detrital sediments. These proxies offer valuable insights into the paleoclimate, water restriction, redox conditions, and paleoproductivity of the depositional period (Jones and Manning, 1994; Lin et al., 2008; Zeng et al., 2015; Guo et al., 2020). Specifically, the Ga/Rb and Sr/Cu ratios are instrumental in reconstructing paleoclimate conditions. Higher Ga/Rb ratios suggest warmer temperatures and reduced aridity (Lerman and Gat, 1989; Bai et al., 2015), whereas Sr/Cu ratios provide an indication of humidity levels, with values below 10.0 pointing to humid conditions and those above 10.0 suggesting aridity (Adegoke et al., 2014; Sarki Yandoka et al., 2015). Analysis of the Dawuba shales in Guizhou, as depicted in Figures 2, 3A, indicates that they were primarily deposited under conditions that were both warm and arid.

According to the data collected from Li et al. (2015) (Li et al., 2015) and Yang et al. (2021) (Yang et al., 2021), the Niutitang shales from the Chongqing area exhibit Sr/Cu ratios ranging from 1.99 to 17.41, with an average of 6.70, and Ga/Rb ratios between 0.15 and 0.22, averaging at 0.18 (Figure 3B). Similarly, the Niutitang shales from the Guizhou area display Sr/Cu ratios from 1.37 to 25.64, with an average of 4.29 (Figure 3C). These results suggest the prevalence of cooler paleo-conditions during the Cambrian Niutitang deposition.

Understanding paleosalinity, which is a key factor in reconstructing past marine environments, can be approached through the analysis of specific elemental ratios and content. The content of molybdenum (Mo) and its ratio to total organic carbon (TOC) are particularly informative. Mo tends to accumulate under anoxic reducing conditions, making its concentration and the Mo/TOC ratio valuable for assessing the degree of water restriction (Algeo and Lyons, 2022). Analysis of the Mo-TOC relationships in the Dawuba shale in Guizhou, particularly within the first member of the Dawuba Formation, suggests sedimentation occurred under conditions of restricted water flow, indicative of an enclosed environment.

Another significant indicator of paleosalinity is the (MgO/Al2O3) × 100 ratio. Higher values within this ratio are indicative of increased salinity levels (Wang X. F. et al., 2020). The classification of these ratios into <1, 1–10, and >10 correspond to fresh, transitional, and saltwater conditions, respectively. Analysis of the Dawuba shale in Guizhou, as illustrated in Figure 3A, reveals that the depositional environment predominantly ranged from transitional to saltwater. This suggests that during the deposition of the Dawuba shale, the environment transitioned to or was primarily marine.

According to the data collected from Li et al. (2015) (Li et al., 2015) and Yang et al. (2021) (Yang et al., 2021), the (MgO/Al2O3) × 100 ratios in the Niutitang shales from the Chongqing area demonstrate a range from 8.74 to 79.49, with an average of 18.72 (Figure 3B). This indicates a predominance of saltwater conditions. Furthermore, the Sr/Ba ratio, another proxy for inferring paleosalinity where higher ratios suggest greater salinity (Meng et al., 2012), supports this conclusion. In the Niutitang shales of the Guizhou area, Sr/Ba ratios range from 1.80 to 9.22, with an average of 4.03 (Figure 3C), reinforcing the interpretation that the water conditions were saltwater during the Cambrian Niutitang deposition.

The assessment of paleoredox conditions in sedimentary environments is crucial for understanding the preservation potential of OM and the historical oxygenation state of the water column. Trace elements such as vanadium (V), chromium (Cr), nickel (Ni), cobalt (Co), and uranium (U) serve as vital indicators of these conditions (Jones and Manning, 1994; Lin et al., 2008). Specifically, the concentrations of V and Ni in organic-rich sediments are highly sensitive to the prevailing redox conditions, providing insights into the oxygenation levels in various sedimentary settings (López et al., 1995; Mongenot et al., 1996). The ratio of V/(V + Ni) is particularly informative: values above 0.84 are indicative of euxinic conditions, ratios between 0.54 and 0.82 suggest anoxic environments, and those ranging from 0.46 to 0.60 point to dysoxic conditions (Hatch and Leventhal, 1992; López et al., 1995; Wang X. F. et al., 2020).

It is important to note that the enrichment of certain trace elements, such as Cr and Co, which are associated with detrital components, is less influenced by the redox state of the environment (Ross and Bustin, 2006). Thus, elevated V/Cr and Ni/Co ratios are indicative of relatively anoxic conditions. Conventionally, V/Cr ratios below two signal oxic environments, those between two and 4.25 imply dysoxic conditions, and values above 4.25 suggest anoxic conditions (Jones and Manning, 1994). Similarly, Ni/Co ratios below five denote oxic conditions, those between five and seven indicate dysoxic environments, and ratios above seven point to anoxic conditions (Jones and Manning, 1994). According to these criteria, the Dawuba shale in Guizhou, as depicted in Figure 3A, is characterized by reducing conditions, which are conducive to the preservation of OM during deposition.

According to the data collected from Li et al. (2015) (Li et al., 2015), in the Niutitang shales of the Chongqing area, V/Cr ratios range from 1.22 to 30.77, with an average of 7.94, and Ni/Co ratios vary from 2.71 to 36.80, with an average of 10.60 (Figure 3B). Additionally, the V/(V+Ni) ratios range from 0.64 to 0.98, with an average of 0.80 (Figure 3B). These figures collectively indicate that the Niutitang shales were deposited under reducing conditions.

Similarly, according to the data collected from Yang et al. (2021) (Yang et al., 2021), for the Niutitang shales in the Guizhou area, V/(V+Ni) ratios range from 0.58 to 0.95, with an average of 0.76, and Ni/Co ratios span from 3.38 to 20.92, with an average of 9.67 (Figure 3C). The rare earth element (REE) index Ceanom is another proxy for redox conditions, where values exceeding −0.1 suggest cerium enrichment, a positive anomaly, and thus a reducing environment (Wright et al., 1987; Zhang et al., 2017). The Ceanom values for these shales range from −0.225 to 0.006, with an average of −0.062 (Figure 3C), further supporting the inference of reducing conditions during the Niutitang deposition in Guizhou.

As demonstrated in the cases of the Niutitang and Dawuba shales, the prevailing paleoenvironments during their deposition significantly influenced their sedimentary characteristics. In both the Chongqing and Guizhou areas, the Niutitang shale was deposited under a cooler paleoclimate, saltwater conditions, and reducing environments, consistent with a deep muddy shelf in the Early Cambrian period in the Upper Yangtze region. These conditions are corroborated by previous studies (Steiner et al., 2001). In contrast, the Dawuba shale in the Guizhou area was deposited under a warm and arid paleoclimate with transitional to marine conditions, also under reducing conditions. This aligns with the sedimentary characteristics of slope-basin facies in the Early Carboniferous period in the Upper Yangtze region, as supported by other studies (Guizhou Geological Survey Institute, 2017).

Both the Carboniferous Dawuba and Cambrian Niutitang shales experienced predominantly reducing water conditions, crucial for the preservation of organic matter (OM). Reducing environments, characterized by low oxygen levels, inhibit OM degradation by aerobic bacteria, thus enhancing its preservation. The warm and arid conditions during the Dawuba shale deposition, coupled with reducing conditions, facilitated the accumulation and preservation of OM. Conversely, the cooler and arid conditions during the Niutitang shale deposition, along with the reducing conditions, supported OM preservation. Additionally, varying paleosalinity levels influenced the type and quantity of organic matter preserved. For instance, the Dawuba shale saw a transition from transitional to predominantly saline water environments, indicating a shift towards more marine conditions conducive to OM preservation due to factors like higher primary productivity and reduced clastic dilution. On the other hand, the Niutitang shale was deposited under stable saltwater conditions, promoting a consistent marine environment favorable for OM preservation.

Furthermore, the distinct paleoclimatic conditions under which the Dawuba and Niutitang shales were deposited significantly influenced their mineralogical compositions, impacting reservoir characteristics such as brittleness—a key feature for hydraulic fracturing. The paleoclimate affected the type and deposition of minerals, with warmer, arid conditions leading to more clay deposition and cooler, arid conditions favoring the deposition of brittle minerals like quartz and carbonates. Paleosalinity also influenced the chemical precipitation and types of minerals deposited. Meanwhile, paleoredox conditions impacted the preservation of organic matter, which indirectly influences the type and paleoproductivity of organic matter and its interplay with the mineral matrix within the shale. The warm, arid climate with transitional to saline water conditions during the Dawuba shale deposition resulted in a mineralogy richer in clay, which typically confers ductility rather than brittleness. Conversely, the cooler, arid climate with stable saltwater conditions during the Niutitang shale deposition led to a mineralogy richer in brittle minerals, reflecting variations in chemical precipitation and sediment supply controlled by the paleoclimate and paleosalinity.

The thermal maturity of OM in sedimentary rocks is a critical factor in hydrocarbon generation, reflecting the extent of physical and chemical transformations under the influence of heat, pressure, microbial activity, subsidence, and time post-deposition (Ma et al., 2019). Parameters such as vitrinite reflectance (Ro%) and maximum pyrolysis peak temperature (Tmax) serve as reliable indicators of the thermal maturity of source rocks (Tissot and Welte, 1984; Bordenave et al., 1993).

In the Dawuba shales of Guizhou area, Ro% values range from 2.85% to 3.46%, with an average of 3.14%, as presented in Table 3 and illustrated in Figure 4, collectively suggesting that the Dawuba shales have attained a post-mature stage of thermal maturity.

For the Niutitang shales, according to the data collected from Wang (2020) (Wang, 2020) and Ge et al. (2020) (Ge et al., 2020), thermal maturity differs between locations. In the Chongqing area, Ro values span from 1.55% to 3.56%, with an average of 2.68%, whereas in the Guizhou area, they range from 1.43% to 2.81%, averaging at 2.37%. These findings indicate that the Niutitang shales in both areas have also reached the post-mature stage, suggesting significant hydrocarbon generation potential.

The TOC content is a primary measure of OM abundance in shale source rocks. For the Dawuba shales in Guizhou, TOC values predominantly range from 0.63 to 4.25 wt%, with an average of 1.78 wt% (Table 4). According to the source rock evaluation criteria by Peters and Cassa (1994) and Tissot and Welte (1984) (Tissot and Welte, 1984; Peters et al., 1994), these values classify the Dawuba shales in Guizhou as having fair to excellent source rock potential.

According to the data collected from Wang (2020) (Wang, 2020) and Ge et al. (2020) (Ge et al., 2020), the Niutitang shales in the Chongqing area display TOC values between 0.16 wt% and 9.62 wt%, averaging at 3.43 wt%. Similarly, in the Guizhou area, TOC values range from 0.60 wt% to 8.89 wt%, with an average of 3.55 wt%. These ranges suggest that the Niutitang shales possess predominantly good to excellent source rock potential, aligning with the criteria for evaluating source rocks.

Given that the type of OM significantly influences the properties, composition, and quantity of hydrocarbon generation (Peters et al., 1994), identifying the kerogen type within potential source rocks is crucial. The maceral compositions, namely, liptinite, vitrinite, and inertinite, serve as the basis for this classification. In the context of China, liptinite is further categorized into sapropelinite and exinite, with sapropelinite encompassing lginate and sapropelic amorphous material, and exinite including various forms like cutinite, suberinite, sporinite, resinite, exsudatinite, chlorophyllinite, liptodetrinite, and bituminite. The proportions of liptinite, vitrinite, and inertinite within the Dawuba shales in Guizhou are detailed in Table 5. Specifically, in the shales, liptinites, which are primarily composed of liptodetrinite, exhibit a range between 71% and 92%, with an average of 82.6%. Vitrinite, characterized by its distinct outlines, varies from 5% to 22%, averaging at 13.3%, as depicted in Figure 5. Inertinite, mainly consisting of fusinite, ranges from 1% to 8%, with an average of 4.1%.

The kerogen type is ascertainable through the TI index, as defined by Mukhopadhyay et al. (1995) (Mukhopadhyay et al., 1995) and Hakimi et al. (2011) (Hakimi et al., 2011): (TI = (100 × sapropelinite+50 × exinite-50 × vitrinite-75 × inertinite)/100). TI indexes are interpreted as follows: greater than 80 for Type I, between 80 and 40 for Type II1, between 40 and 0 for Type II2, and less than 0 for Type III kerogens.

The Dawuba shales in Guizhou predominantly exhibit TI indexes ranging from 0 to 40, indicative of Type II2 kerogens, as summarized in Table 5. Studies by Wang (2020) (Wang, 2020) and Ge et al. (2020) (Ge et al., 2020) suggest that the kerogen types in the Niutitang shales, specifically within the Chongqing and Guizhou areas, are predominantly Type I, with some instances of Type II1. This conclusion is supported by the observed maceral compositions (Ge et al., 2020; Wang, 2020).

Consequently, the source rocks of the Niutitang shales in both the Chongqing and Guizhou areas are characterized as post-mature, with predominantly good to excellent generation potential, and primarily contain Type I kerogens. Concurrently, the source rocks of the Dawuba shales in the Guizhou area are also post-mature but exhibit a range of fair to excellent generation potential, and contain Type II2 kerogens. These characteristics indicate that both the Niutitang and Dawuba shales possess significant gas generation potential.

The mineralogical composition of the Dawuba shales in Guizhou is characterized by a significant presence of clay minerals, ranging from 4.0% to 62.0%, with an average of 44.3%, and quartz, ranging from 15.0% to 68.0%, with an average of 33.2%. Other constituents include carbonates (0%–54.0%, Avg.=15.7%), pyrite (0%–13.0%, Avg.=4.75%), and feldspar (0%–11.0%, Avg.=2.02%) (Figure 6A; Table 6). The clay minerals are predominantly illite (Avg.=74.7%), followed by mixed-layer illite-smectite (I/S, Avg.=16.3%) and chlorite (Avg.=7.4%) (Figure 6B; Table 6). This composition suggests that, compared to the Chang-7 lacustrine shale in the Ordos Basin and the Longmaxi marine shale in South China, the Dawuba shales in Guizhou have a lower content of brittle minerals (such as quartz, feldspar, and carbonates) and a higher proportion of clays.

According to the data collected from Ge et al. (2020) (Ge et al., 2020), the Niutitang shales in the Guizhou area display a relatively high content of brittle minerals, with overall ranges from 37% to 68%, and an average of 51%. Quartz content varies from 28.0% to 53.0%, averaging 37.8%, while clay mineral content ranges from 18.0% to 58.0%, with an average of 38.3% (Figure 7A). Carbonate minerals are also present, albeit with few pyrites. The dominant clay mineral is the I/S mixed layer, which ranges from 47.0% to 64.0%, averaging 56.1% (Figure 7B), indicating strong adsorption capacity in the Niutitang Formation shale of Well HD1. Illite and a small amount of chlorite are also present, with montmorillonite absent, suggesting its complete conversion to illite under high-to over-mature conditions.

Shale classification into calcareous, siliceous, argillaceous, and mixed types is based on the relative contents of calcareous, siliceous, and argillaceous minerals, with a classification threshold set at greater than 50% (Dong et al., 2016). In the Guizhou area, both the Dawuba and Niutitang shales exhibit similar classifications. Specifically, the Dawuba shales, which are richer in clay minerals, are predominantly classified as argillaceous (31.7%) and mixed (61.7%). Conversely, the Niutitang shales, which contain higher amounts of brittle minerals, are primarily mixed shale (66.7%) and argillaceous shale (26.7%). These compositional differences have significant implications for shale gas exploration. The Niutitang shales, being rich in brittle minerals, tend to preserve more solid hosted (meso and macro) porosity compared to the more clay-rich Dawuba shales of similar maturity. This characteristic makes the Niutitang shales more amenable to hydraulic fracturing, a key technique used in shale gas extraction, due to the presence of more brittle minerals which facilitate the fracturing process.

The physical properties of shale, specifically porosity and permeability, are critical in evaluating its potential as a hydrocarbon reservoir. For the Dawuba shale samples in Guizhou, data presented in Table 7 indicate that porosity values range from 0.88% to 5.92%, averaging 3.45%. Permeability values vary from 0.0009 to 0.0441 millidarcies (mD), with an average of 0.0098 mD (Table 7). These values suggest that the Dawuba shales generally exhibit low porosity and permeability. However, a notable exception is observed within the first member of the formation, where both porosity and permeability show relatively higher values. This improvement can be attributed to the presence of various pore types, including inorganic and organic pores, as well as bedding fractures. These features collectively enhance the formation’s potential as an effective hydrocarbon reservoir by facilitating fluid flow.

According to the study conducted by Ge et al. (2020), the Niutitang shales from Well HD-1 in the Guizhou area were characterized by their porosity and permeability values. The porosity of these shales ranged from 0.2% to 1.8%, with an average value of 0.51%. Similarly, permeability values were found to range between 0.0001 mD and 0.01 mD, with an average value of 0.001 mD. These measurements categorize the reservoir as having ultra-low porosity and ultra-low permeability.

The pore structure of shale significantly influences its capacity to store and transmit hydrocarbons. In the Dawuba shales of Guizhou, pore types identified under Field Emission Scanning Electron Microscopy (FE-SEM) primarily include inter-particle pores (Inter-P), intra-particle pores (Intra-P), and OM pores, as illustrated in Figure 8. Intra-P pores are mainly inter-crystalline within pyrite and clay minerals and dissolution pores within calcite and quartz or along particle rims. Inter-P pores, situated between mineral grains, are polygonal, elongated, or slit-like and exhibit good connectivity. OM pores are predominantly spongy, with bubble-like OM pores being uniformly and independently distributed within the OM.

According to a study by Ge et al. (2024), the Niutitang shales in the Guizhou area feature micro-storage spaces dominated by matrix pores and fractures, including diagenetic fractures, layered clay mineral interlayer pores, and pyrite intercrystalline pores. Additionally, interstitial organic pores are present, featuring a size range of 10–150 nm. The pyrite observed is often in the form of strawberry-shaped or aggregate shapes, frequently coexisting with OM. Furthermore, a minor presence of residual pores and intragranular pores has also been noted (Ma et al., 2015).

Isothermal adsorption curves, which often form a hysteresis loop separating from the desorption curve within a specific pressure range, are influenced by the relative pressure of capillary condensation and evaporation, varying with pore morphology. By analyzing the shapes of hysteresis loops, it is possible to speculate about the morphology of the pore shape (Sing, 1995). In this study, the shapes of the N2 adsorption and desorption isotherms displayed minimal differences, as shown in Figures 9, 10.

N2 adsorption was employed to quantify the mesopore and macropore structures in both Dawuba and Niutitang shales in Guizhou. The N2 adsorption-desorption isotherms showed similar trends across all shale samples, indicating a broad adsorption range. The volumes of adsorption at P/P0 < 0.05 and near P/P0 = 1.0 suggest the presence of microporosity and macroporosity, respectively (Clarkson et al., 2013). As P/P0 increases to 0.5, a hysteresis loop appears between the adsorption and desorption branches, indicating the presence of mesopores (Kuila and Prasad, 2013). According to the IUPAC, the N2 adsorption isotherms follow the Type IV classification, and the hysteresis loops exhibit an intermediate type between types H2 and H3, with hysteresis loops suggesting inkbottle and slit pore shapes, although SEM analyses confirm the polymorphic nature of shale pore shapes.

Accurate characterization of the nanoscale pore system in shales requires the selection of appropriate methods for determining low-pressure gas adsorption. In this study, the DFT model was used to calculate the specific surface area (SSA) and pore volume (PV) of micropores, while the NLDFT model was applied to determine the SSA and PV of mesopores and macropores, based on the applicable conditions and principles of these methods (Sing, 1995; Clarkson et al., 2013; Kuila and Prasad, 2013).

As detailed in Table 8, the Dawuba shales in Guizhou exhibited BET-specific surface areas ranging from 8.126 to 13.937 m²/g, with an average of 11.918 m²/g. The total pore volumes ranged from 12.03×10⁻³ to 32.29 ×10⁻³ cm³/g, averaging 18.03×10⁻³ cm³/g. The average pore diameters varied from 4.194 to 9.267 nm, with a mean of 5.968 nm. The pore size distribution (PSD), plotted as dV/dlog(D) versus D curves using the BJH model on the adsorption branches, showed one sample with a bimodal curve and three samples with unimodal curves, indicating a predominance of micropores and mesopores, as depicted in Figure 11.

Conversely, the Niutitang shales in Guizhou showed BET-specific surface areas ranging from 0.178 to 7.874 m²/g, with an average of 4.691 m²/g. The total pore volumes varied from 0.634×10⁻³ to 6.52×10⁻³ cm³/g, averaging 2.63×10⁻³ cm³/g. The average pore diameters ranged from 2.46 to 16.81 nm, with a mean of 5.27 nm. The PSD, plotted using the BJH model on the adsorption branches, indicated a predominance of micropores and mesopores, as shown in Figure 12.

Consequently, the pore types observed in the Niutitang and Dawuba shales in Guizhou exhibit similarities, including the prevalence of inter-particle pores (Inter-P), intra-particle pores (Intra-P), organic matter pores (OM-P), and micro-fractures. The low-pressure N2 physisorption analyses reveal a predominance of micropores and mesopores in both the Niutitang and Dawuba shales in Guizhou.

The gas content measurements for Dawuba shales in the Guizhou area, presented in Table 9 and illustrated in Figure 13A, reveal a range in desorbed gas content from 0.07 m³/t to 1.28 m³/t, averaging at 0.46 m³/t. The residual gas contents vary from 0.09 m³/t to 0.44 m³/t, with an overall mean of 0.27 m³/t. Notably, some samples exhibited no detectable levels of residual gas. Consequently, the total gas contents in Dawuba shales range from a minimum of 0.07 m³/t to a maximum of 1.68 m³/t, with an average of 0.59 m³/t.

According to the data collected from Ge et al. (2020), the Niutitang shales in the Guizhou area, as depicted in Figure 13B, demonstrate a different gas content profile. The desorbed gas content for Niutitang shales ranges from 0.02 m³/t to 0.66 m³/t, with an average of 0.19 m³/t. The overall total shale gas content for Niutitang shales varies between 0.09 m³/t and 1.31 m³/t, averaging at 0.42 m³/t. These differences in gas content between the Dawuba and Niutitang shales can be attributed to variations in physical properties and preservation conditions, with the Dawuba shales generally exhibiting higher gas content levels.