1 Introduction

The exploration and development of shale gas are one of the primary key research in the petroleum industry, which is mainly focused on marine shale in China. As the new field of shale gas exploration and development in recent years, the global marine-continental transitional shale shows the characteristics of poor reservoir potential and low exploration and development degree compared with marine shale (Zhang et al. 2015; Hou 2020a, b; Zhai et al. 2021; He et al. 2021a, b; Wu et al. 2022; Wang et al. 2022a, b; Wang et al. 2022a, b; Yang et al. 2023). However, several sets of marine-continental transitional source rocks have been developed in the Upper Cretaceous of the San Juan Basin in the United States and the North Carnarvon Basin in Australia, and the Triassic-Jurassic of the Bonaparte Basin, both of which lack research and drilling practice. In China, the shale gas resources of marine-continental transitional shale are reached to 19.8 × 1012 m3, with the proportion of 25% of the total shale gas resources (Zhang et al. 2022a, b, c).

Marine-continental transitional shale gas shows a broad resource prospect, especially for the Permian Shanxi Formation of the Ordos Basin and the Permian Longtan Formation of the Sichuan Basin. A large number of wells drilled in the Upper Permian Longtan Formation of the Sichuan Basin and its periphery have measured varying degrees of gas, indicating the good gas-bearing properties (Zhang et al. 2021; He et al. 2021a, b). In the area of Changning-Xingwen area in the southern Sichuan Basin, 7 wells are drilled to the Longtan Formation. The ranges of gas-bearing contents are 2.61–6.02 m3/t, which is mainly adsorption gas and stored in the micropores and mesopores (Zhang et al. 2021). In the well CLD1, the gas-bearing content is controlled by lithology, and the gas-bearing content of shale are ranges from 1.78 to 9.77 m3/t (the mean of 4.88 m3/t) (Wang et al. 2022a, b). In the southeast of the Sichuan Basin, early drilling shown good gas-bearing in shale and coal seams. The TOC contents of the Longtan Formation in the well DYS1 range from 0.57 to 18.37%, and the gas-bearing contents range from 0.56 to 8.78 m3/t, with the average of 2.02 m3/t. It indicates the good gas bearing property of the Longtan Formation (He et al. 2021a, b). The gas bearing properties of the marine land transitional shale in the Shanxi Formation of the Daji Block in the Ordos Basin were studied (Jiang et al. 2023). The results indicate that pure shale is controlled by the bay environment. And the enrichment of "dual source" organic matter from marine and continental sources, as well as brittle minerals such as non-terrigenous quartz, plays a significant role in controlling the gas bearing capacity of the reservoir. Two types of rich organic matter shale development models in the marine-continental transitional facies of the Permian Shanxi Formation in the eastern margin of the Ordos Basin have been established under the constraints of the fourth order sequence (Jiao et al. 2023). Moreover, a logging identification method of multiple lithological combination shale and a quantitative evaluation model of reservoir logging parameter have been formed. Comparing with the deposition of marine shale, the sedimentary condition of marine-continental transitional shale is complex. At present, the study of marine-continental transitional shale of the Permian Longtan Formation in the Sichuan Basin is still in its infancy, with strong heterogeneity on its stratigraphic distribution, and previous studies mainly focus on the sedimentary facies and reservoir characteristics of shale formations (Xi et al. 2016; Cao et al. 2020; Xu et al. 2021; He et al. 2021a, b; Wu et al. 2022; Wang et al. 2022a, b).

The applicability of the experience of marine shale gas exploration and development in the process of marine-continental transitional shale gas exploration and development is limited, and the internal lithological heterogeneity and distribution characteristics of gas-bearing properties of the Longtan Formation shale have not been discussed in depth (Zhang et al. 2016; Liu 2018; Wang 2020). It is of great significance to study the geological characteristics and gas enrichment conditions of the Longtan shale in the Sichuan Basin and to reveal the main controlling factors of its gas bearing (Cao et al. 2022a, 2022b). Many definite and quantitative techniques are invented to analysis the pore structure of the shale (Cao et al. 2015; Zou et al. 2022; Zhang et al. 2022a, b, c). Isothermal adsorption experiment is employed to determine the adsorbed gas content and adsorption capacity of shale, and the content and storage capacity of free gas are determined by porosity and pore volume (Qiu et al. 2021). Scholars have focused on the maceral types, pore structures and nanopore development degree of shale, suggesting that these factors have certain effects on gas adsorption (Jiang et al. 2016; Qin et al. 2018; Fu et al. 2020), but the influence of lithological assemblages on gas-bearing properties has been little studied.

In this paper, geological and geochemical analysis are employed to identify the lithology and lithological assemblage types, such as sandstone, mudstone, carbonaceous mudstone, limestone and coal, for the Permian Longtan Formtaion shale in the southeastern Sichuan Basin. By comparing the petrophysics, differential gas-bearing property and its influencing factor of mudstone in the different lithological assemblages, the differential of enrichment mechanism of organic matter, petrophysics and gas-bearing property are analyzed, with the aim of providing the theoretical evidence for predicting of sweet point area of marine-continental transitional shale gas and the selection of exploration and development area.

2 Geological settings and sampling

Influenced by the Dongwu Movement, with the retreat of seawater from west to east, the western Sichuan Basin is rose to land after the Middle Permian, forming the paleogeographic pattern of “west land and east sea” with high southwest and low northeast, and from west to east, the sedimentary facies of the early Late Permian showed obvious evolution from continental to marine (Jiang et al. 2017; He et al. 2021a, b; Wang et al. 2022a, b; Wang et al. 2022a, b; Zhang et al. 2022a, b, c). Due to the difference in paleogeographic pattern and sedimentary environment, the heterogeneous features are obvious in the same period of the Permian Longtan Formation. The sedimentary facies of the whole Sichuan Basin is distributed in an arc from southwest to northeast, which is basalt eruption area, fluvial lacustrine, shore/swamp, tidal flat/lagoon, platform, slope/shelf and shelf, respectively (Fig. 1). The coal-bearing rocks with a thickness of 20–120 m were widely deposited in the Longtan Formation in southern Sichuan, which is in parallel unconformity contact with the top of the Maokou Formation. The sedimentary environments are distributed from land to sea in an east–west direction and extend from south to north, with the main sedimentary system of tidal flat-lagoon. The main lithology is black shale interbedded with thin siltstone and coalbed. 25 shale samples from the well L3 of the southern Sichuan low-steep fold belt (Fig. 1, Table 1) were sampled and measured. We included X-ray diffusion (XRD) mineralogy, total organic carbon (TOC), scanning electron microscope (SEM) observation, optical petrography and high temperature and high pressure isothermal adsorption.

Fig. 1
figure 1

A The location of Sichuan Basin in China. B Sedimentary environment of Longtan Formation and study area location. C Comprehensive colume of the Longtan Formation of Well L3 (Guo et al. 2018; Wang et al. 2022a, b)

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Table 1 The depth, organic carbon content, porosity and other information of the sample
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3 Methods

3.1 Microscopic observation

Petrology is analyzed by using Axio Scope A1 polarizing microscope to identify petrological characteristics, including the mineral types, microstructures in mudstone. The magnification is from × 5 to × 50. However, the pore of mudstone is very small, the magnification of scanning electron microscope is higher than optical microscope. A ZEISS Sigma 300 scanning electron microscope are employed to analysis the micro characteristics of pores and minerals of mudstone, which is operated in backscattered electron detector (BSED) mode with backscattered electron and Everhart–Thornley detector (ETD) mode with secondary electron. After argon-ion polishing and gold plating, structural characterization and compositional analysis are performed in a vacuum environment.

3.2 High-temperature methane adsorption

The high-temperature methane adsorption is determined by high-temperature, high-pressure gravimetric adsorption experiment (Rubotherm ISOSORP HP Static II) with reference to “High Pressure Isothermal Adsorption Test Method for Coal (GB/T 19560-2008)”. The accuracy of the test is 0.000001g. The maximum test pressure is 30 MPa, with the test temperature of 50 °C to simulate the underground pressure and temperature. Powder samples with the size of 60–80 mesh are fixed in the chamber after dewatering and introducing Helium to degas and test the buoyancy. In the range of 0–30 MPa, 10 pressure points are measured and recorded. Then, the methane with the purity of 99.99% are introduced to adsorb. The equilibration time of each pressure point is more than 2 h to ensure stable pressure during absorption. Finally, the measurement data are obtained under simulated subsurface conditions. The saturated adsorption of shale is calculated by the adsorption model based on the ternary Langmuir equation.

4 Results

4.1 Petrological characteristics

The composed of siltstone mineral clastic of Longtan Formation in the study area is mainly rock clastic from the evidences of core observation and thin section identification (Fig. 2), with generally clayized. Some clastic rock is replaced by siderite. The particle size of clastic minerals ranges from 0.25 to 0.5 mm, with a maximum of 0.65 mm. The roundness is mainly secondary roundness. The amount of quartz is small, with angular shape and scattered distribution. The composition of mudstone is mainly argillaceous, including micrystalline/cryptocrystalline clay mineral aggregation primarily and microcrystalline siliceous secondarily. A small amount of muddy clastic minerals (quartz, feldspar, etc.) are mixed into the clay mineral aggregates. The entire black carbonaceous mudstone is consisted of mud and some black organic matter. The observed partially carbonized plant debris remains preserved in the organic matter have a cellular structure of plants and are in the form of a lattice. A small amount of observed pyrite distributed in the rock as a black granular aggregation. The limestone/argillaceous limestone has a bioclastic structure. Bioclasts are mainly shell fragments, which are mostly tabular and replaced by calcite and a little quartz. Grain structure developed, with all the characteristics of the directional imbricated arrangement. From the identified results of optical microscope, the composition of coal is mainly vitrinite (Goodarzi et al. 2022). Vitrinite includes vitrinite coal and matrix mainly and organic matter vitrinite. The coal type is mostly mineralized filamentous light–dark coal and light coal.

Fig. 2
figure 2

Petrological characteristics of the core and the thin section of different lithology. A Siltstone with muddy bands. B and F Mudstone. C Carbonaceous mudstone. D and I Coal. E Siltstone. G Carbonaceous mudstone. H Clastic limestone with mud

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4.2 Lithological assemblages

From the experimental results of XRD, the primary mineral in the samples is clay minerals, with the range of 21.346–82.571% and the mean of 51.804%. The second is the clastic minerals dominated by quartz. The composition of quartz ranges from 6.729 to 45.734%, with an average of 22.782%. The carbonate minerals mainly include dolomite and calcite, with the average content of 17.795%. Meanwhile, data including continental well logging, core analysis and XRD analysis are employed to identify the lithology. Natural gamma ray logging (GR), acoustic logging (AC), compensated neutron logging (CNL), density logging (DEN), and deep investigate lateral resistivity logging (RLLD) are optimized to analyze the logging response characteristics of different lithology (Fig. 3), including coal, limestone, siltstone, carbonaceous mudstone, and shale. Different lithologies show different logging response characteristics. After core analysis, the lithology and corresponding logging value are obtained and delineated in the multiple cross plot. The significance of the effects is tested. In the cross plot of CNL-DEN, the CNL of limestone is obviously low, generally less than 10%. The CNLs of siltstone and carbonaceous mudstone/mudstone are in the range of 20–30% and from 25 to 45%, respectively. The CNL of coalbed is mainly larger than 40%, with the characteristics of high value. In DEN, the DEN of coalbed is obviously low, which is less than 2 g/cm3. The DEN of carbonaceous mudstone ranges from 2 to 2.5 g/cm3. The DEN of limestone is close to and indistinguishable from siltstone and mudstone, all of which are greater than 2.5 g/cm3. According to the cross plot of RLLD-AC, the logging characteristics of limestone show obviously high RLLD and low AC, with the RLLD greater than 100 Ω m and the AC lower than 55 μs/m. The RLLD and AC of siltstone are medium, ranging from 10 to 100 Ω m and 60–70 μs/m, respectively. The RLLDs of coal, carbonaceous mudstone and mudstone are low and indistinguishable, all of them are lower than 20 Ω m. The AC of coal is high, which is mostly greater than 100 μs/m. The ACs of carbonaceous-mudstone and mudstone are in the range of 75–95 μs/m. The lithology of the Longtan Formation is also quantitatively identified by multi-discriminant analysis technology. The multi-discriminant function of different lithology is established based on the whole model method. After comparing the calculation value of different lithology identification functions, the interpretation models are established.

$${\text{Y}}1{ }\left( {{\text{Coal}}} \right) = - 431.999 + 0.802{\text{*GR}} + 0.005{\text{*RLLD}} + 1.126{\text{*AC}} + 4.971{\text{*CNL}} + 270.424{\text{*DEN}}$$
(1)
$${\text{Y}}2{ }\left( {{\text{Limistone}}} \right) = - 461.279 + 0.639{\text{*GR}} + 0.006{\text{*RLLD}} + 1.315{\text{*AC}} + 3.221{\text{*CNL}} + 298.491{\text{*DEN}}$$
(2)
$$Y3 \left( {Siltsone} \right) = - 555.174 + 0.816*GR + 0.005*RLLD + 1.392*AC + 4.460*CNL + 322.921*DEN$$
(3)
$$\begin{aligned} &Y4 \left( {Carbonaceous mudstone} \right) \\ &\quad= - 513.395 + 0.899{\text{*GR}} + 0.005{\text{*RLLD}} \\ & \quad + 1.331{\text{*AC}} + 4.808{\text{*CNL}} + 301.595{\text{*DEN}} \\ \end{aligned}$$
(4)
$$Y5 \left( {Mudstone} \right) = - 609.208 + 0.901{\text{*GR}} + 0.005{\text{*RLLD}} + 1.433{\text{*AC}} + 5.061{\text{*CNL}} + 333.901{\text{*DEN}}$$
(5)
Fig. 3
figure 3

Crossplot of well logging of different samples from Longtan Formation. A Crossplot of the CNL-DEN. B Crossplot of the RLLD-AC

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Studies have shown that the changes in the sedimentary environment lead to certain differences in the repeated superposition and spatial distribution of different gas reservoirs of the Longtan Formation in the southern Sichuan Basin, and thus to the different lithological assemblage types (Hou 2020a, b; Cao et al. 2021; Xiao et al. 2022). Multiple sets of overlapping lithological assemblages are developed in the Longtan Formation in the study area, forming multiple sets of potential source-reservoir-cap assemblages (Li et al. 2020; Hou 2020a, b; Li et al. 2021a, b, c; Li et al. 2021a, b, c; Cai et al. 2022). The lithology includes coalbed, mudstone, sandstone, and partial limestone. Tight sandstone is the primary storage space of free gas, coalbed and black mudstone are source rock and primary storage space of absorbed gas, and regional mudstone is the primary cap. For the proportion and thickness of different lithology, other lithology is identified as interbedded layer for the proportion greater than 40% and the thickness higher than 5 m in shale layer, otherwise interlayer. Thus, the lithological assemblages in the Longtan Formation are divided into four types (Fig. 4): thick mudstone with the interlayer of coalbed (Type I), rich mudstone with the interlayer of sandstone and thin coalbed (Type II), sandstone interbedded with mudstone with the interlayer of coalbed (Type III), and limestone interbedded with sandstone with the interlayer of mudstone (Type IV).

Fig. 4
figure 4

Schematic and statistical diagram of different lithological assemblage types of Longtan Formation shale in Well L3. A Thick mudstone with the interlayer of coalbed (Type I). B Rich mudstone with the interlayer of sandstone and thin coalbed (Type II). C Sandstone interbedded with mudstone with the interlayer of coalbed (Type III). D Limestone interbedded with sandstone with the interlayer of mudstone (Type IV). E Statistical analysis of different lithological assemblages. F The vertical identification results of lithological assemblage types in Well L3

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According to this criterion, the lithological assemblage type of Longtan Formation in well L is classified (Fig. 4), and it can be seen that Type IV is less developed, and it can be seen to be developed only at the top of the Longtan Formation, followed by Type I, which is mainly developed in the upper and lower parts of the Longtan Formation of the well, and the middle part of the well is mainly dominated by Type II and Type III. From the statistical results of the quantities of different lithological assemblages, the Type III is the most abundant lithological assemblage, followed by Types II and I, and the Type IV is poor.

5 Discussions

5.1 Reservoir space and gas-bearing characteristics of different lithological assemblages

5.1.1 Developing characteristics of mudstone reservoir space with different lithological assemblages

The lithology of the Permian Longtan Formation is marine-continental transitional carbonaceous mudstone with coal, and the water energy in the depositional period changed from low, medium–low to relatively low, from bottom to top (Huang et al. 2020; Yang et al. 2021, 2022). Biological productivity is high during the depositional period of the Longtan Formation due to the warm-humid climate and oxide- and oxygen-depleted sedimentary water (Cao et al. 2022b). The main macerals of organic matter are exinite and vitrinite, with the main kerogen of Type II2. Organic matter is derived from the debris of reproductive organs and epidermal tissues of terrigenous higher plants after strong degradation, with the form of detrital or partially flocculent. The ranges of TOC contents of different lithological assemblages are wide. The TOC contents vary widely in different lithological assemblages, with the range of 60%. Bimodal TOCs range from 2 to 4% and 8 to 10%, respectively (Fig. 5). The differences of thermal maturity are small, with the vitrinite reflectance (Ro) ranging from 3.11 to 3.61% (the mean of 3.39%) and high maturity (Cao et al. 2018; Yang et al. 2023).

Fig. 5
figure 5

TOC distribution of mudstone of different lithological assemblages (30 data cited from Cao et al. 2018, 2022a)

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According to the pore genesis, Cao et al. (2022b) identified the reservoir space of Longtan Formation shale as inorganic matter pores, organic matter pores, and microfractures (Fig. 6). The organic matter pores are mainly distributed inside and around the organic matter, with the shape of round and elliptical and distributed isolated (Fig. 6A–D). The organic matter micropores of the Longtan Formation mudstone reservoir are mainly developed in the samples with relatively high organic matter contents, especially in the samples with high organic matter and clay contents. Microfractures are observed in the Longtan Formation, but they are totally less than in the marine shale reservoir. Microfractures are usually formed between organic matter and clay minerals due to shrinkage during the thermal evolution of organic matter. In the diagenetic process, with the transformation of clay minerals, microfractures formed by shrinkage in the clay mineral matrix are also more common. Shrinkage fractures typically have a zigzag edge and are often broken into long and linear fractures (Fig. 6G–I), with widths generally less than 50 nm and maximum lengths of several micrometers. Inorganic matter pores developed mostly between the granular of clay minerals grains and in the clastic grains (quartz, feldspar, etc.) (Fig. 6I). Curled interlayer fractures develop in the lamellar illite-montmorillonite mixed-layer aggregation (Fig. 6J). Minerals, such as quartz, pyrite, are influenced by the external environment during the growth process, resulting in the formation of intracrystalline pores in the process of crystal accumulation (Fig. 6K). The diameter of the intracrystalline pores is several to several hundred nanometers, with relatively small quantities. Occasionally, the dissolved pores between the siderite crystals or granules are formed with the shape of bay-shaped or narrow-long due to dissolution (Fig. 6L).

Fig. 6
figure 6

Reservoir space developing characteristics of mudstone in different lithological assemblages of the Longtan Formation. A Boundary fractures develop between the fragmentary organic matter and granular, and microcracks develop in the granular calcite (900×), and a few organic matter pores are developed locally (44,000×). B Pores are developed in the strip organic matter, and rhomboid granular calcite and clay minerals are interspersed (800×), and a few organic matter pores are developed locally (3000×). C Fragmentary and banded organic matter, with interlayer fractures in the lamellar clay minerals (1100×), and angular pores are developed locally in the banded organic matter (30,000×). D Clumpy organic matter (40×), and the edges of the fragmentary organic matter are jagged, with little organic matter pores locally (33,000×). E A group of microcracks nearly parallel to each other develops in the aggregation of granular clay minerals, and interlayer fractures is developed in the lamellar illite-montmorillonite-mixed-layer aggregation locally (9000×). F Intergranular microcracks and dissolved pores are developed in the granular calcite, and intragranular pores are cemented by clay minerals (400×), and interlayer pores and fractures are locally developed in the aggregation of lamella illite-montmorillonite-mixed-layer and chlorite (5000×). G Microcracks and interlayer cracks are developed in the granular clay mineral aggregation (800×), and microcracks and interlayer fractures are developed locally in lamella illite-montmorillonite-mixed-layer aggregation (9000×). H Intergranular pores of siderite is filled with clay mineral aggregation (250×), and interlayer fractures are developed locally in lamella illite-montmorillonite-mixed-layer aggregation (9000×). I Slit intragranular pores of clay minerals. J Intergranular of minerals. K Intercrystalline pore of pyrites. L Intragranular dissolved pore of minerals

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From nitrogen adsorption, the shapes of adsorption–desorption curve of different samples are varied (Fig. 7). Three types of hysteresis loops are observed, including II–I, III–II, and IV–III. The pore shapes of samples from thick mudstone with the interlayer of coalbed are cylindrical and partially ink-bottle. The specific surface area and pore volume of samples from thick mudstone with the interlayer of coal is relatively high, with the mean values of 21.0759 m2/g and 0.0262 cm3/g, respectively. The pore shapes of samples from rich mudstone with the interlayer of sandstone and thin coal and sandstone interbedded with mudstone with the interlayer of coal are cylinders or parallel-sided slits), with the mean specific surface area of 17.0374 m2/g and 6.4595 m2/g, respectively. The mean pore volume of samples from two lithological assemblages is 0.0218 cm3/g. The specific surface area and pore volume of samples from limestone interbedded with sandstone with the interlayer of mudstone are small, with the mean pore volume of 0.0047 cm3/g.

Fig. 7
figure 7

Low temperature nitrogen isothermal adsorption–desorption curves of mudstone in different lithological assemblages

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5.1.2 Gas-bearing characteristics of mudstone reservoir with different lithological assemblages

The experimental result of gas isothermal adsorption is the excess adsorption mass (Gibbs adsorption or apparent adsorption) of shale, but the actual adsorption mass is the absolute adsorption mass. In the density–temperature–pressure phase diagram of methane, the density of free methane is low when the pressure is lower than 6 MPa. The masses of excess and absolute adsorbed gas are closed, and the mass of absolute adsorbed gas is approximated with the mass of excess adsorbed gas with low pressure in the standard of isothermal adsorption. However, when the pressure is higher than 20 MPa, the density of free methane is obviously increased, and the mass of excess adsorbed gas must be corrected (Zhang et al. 2020).

From the Gibbs adsorption theory, the relationship between the mass of excess adsorbed gas and the mass of absolute adsorbed gas is given by:

$$m_{abs} = \frac{{m_{ex} }}{{1 - \frac{{\rho_{g} }}{{\rho_{a} }}}}$$
(6)

where mex is the mass of excess adsorbed gas, g; mabs is the mass of absolute adsorbed gas, g; ρg is the density of methane in different pressure, g/cm3; ρa is the density of adsorbed methane, g/cm3.

Ross and Bustin (2009) proposed the ternary Langmuir model, with the consideration of the density of adsorbed phase under high pressure and combining the definition of Gibbs adsorption with the Langmuir model. As the undetermined parameters, the volume and density of the adsorbed phase are determined by fitting the isothermal adsorption curve with the consideration of mathematical optimization.

$$m_{ex} = m_{L} \frac{p}{{p + p_{L} }}\left[ {1 - \frac{{\rho_{g} \left( {p,T} \right)}}{{\rho_{a} }}} \right] = m_{abs} \left[ {1 - \frac{{\rho_{g} \left( {p,T} \right)}}{{\rho_{a} }}} \right]$$
(7)

where mL is the Langmuir mass of adsorbed gas, g; p is the balance pressure of gas, MPa; pL is the Langmuir pressure, MPa; T is the temperature, K.

In addition to Langmuir volume and Langmuir pressure, the density of the adsorbed phase is considered as the third variable in the ternary Langmuir model. It is obvious that at low pressure, the density of free gas (ρg) is very low, and the ternary model can be simplified to a binary modal, which corrects for the influence of adsorbed phase under high pressure. The isothermal adsorption volumes of 15 samples are corrected by the ternary Langmuir model, showing that the volumes of adsorbed gas range from 0.3407 to 32.7134 cm3/g, with the mean volume of adsorbed gas being 3.9050 cm3/g.

Ji et al. (2022) proposed that the free gas in shale follows the real gas state equation. Combined with gas saturation, reservoir temperature and pressure, the volume of subsurface free gas of shale reservoir is calculated.

$$V_{f} = \frac{{pT_{std} S_{g} \varphi }}{{ZTP_{std} \rho_{r} }}$$
(8)

where Vf is the volume of free gas, cm3/g; Tstd is the standard surface temperature, equaled to 273.15 K; Sg is the gas saturation, %; φ is the porosity, %; Z is the dimensionless compressibility factor, represented the differential degree of actual gas and ideal gas; Pstd is the standard surface pressure, equaled to 0.101 MPa; ρr is the rock density, g/cm3.

Considering the reservoir temperature and pressure of the well L3 in the study area, the volumes of subsurface free gas of different samples are calculated and ranged from 0.3251 to 11.7185 cm3/g, with the mean of 4.2308 cm3/g. From the analysis of gas-bearing contents, specific surfaces and pore volumes of different samples (Fig. 8), it was found that the volume of free gas varies positively with the pore volume, also for the volume of adsorbed gas and the specific surface area and pore volume. High specific surface area represents high surface adsorption energy. High pore volume not only provides pore space for free gas, but also provides adsorption site for adsorbed gas. Due to the difference in the phase state of adsorbed gas and free gas, their assignment positions are also different. The specific surface area mainly provides adsorption sites for gas adsorption. In the analysis of Fig. 8, it can be seen than the correlation between the specific surface area values of mud shale in different lithological assemblages and the adsorbed gas content is relatively good, but the correlation with the free gas content is not very good (Fig. 8A). The free gas content is mainly related to the storage space of the shale, that is, the size of the pore volume plays a major role in the free gas content. Coal and carbonaceous mudstones are mainly distributed in type I and type II. From the analysis in Fig. 8, it can be known that carbonaceous mudstones distributed in type I and type II lithological assemblages, which have relatively large specific surface area and pore volume, show a positive correlation between the amount of adsorbed gas and the amount of free gas distribution, and the contributing role is much larger than the contributing role of mudstones in type III and type IV lithological assemblages. Its contribution to the distribution of adsorbed gas and free gas is much larger than that of mudstones in type III and type IV lithological assemblages. That is the carbonaceous mudstones in the lithological assemblages of type I and type II have stronger gas-generating capacity and more developed storage space, which provide favorable conditions for gas generation and preservation. That is shown that the gas-bearing volumes of mudstone are mainly influenced by the specific surface area and pore volume of samples.

Fig. 8
figure 8

The relationships of gas-bearing volume and specific surface (A, B) and pore volume (C, D)

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5.2 Influencing factors analysis of gas-bearing of different lithological assemblages

The developing differential of reservoir pore in different lithological assemblages is reflected in the pores related to the skeleton minerals and clay minerals (Fu et al. 2020; Zhao et al. 2020; Xiang et al. 2021; Wang et al. 2022a, b). From the comparative analysis, micropores are developed in the thick mudstone with the interlayer of limestone/coal. The main pore types of the rich mudstone with the interlayer of sandstone and thin coal and the sandstone interbedded with mudstone with the interlayer of coal are intergranular pore between clay minerals and partially microfractures, with large pore sizes and varied pore shapes. The main pore types of the limestone interbedded with sandstone with the interlayer of mudstone are intergranular pore between clay minerals and partially dissolved pore, with small pore sizes. Dissolved pores occur mostly on the surface of carbonate minerals with mainly micropores. The differential pore structure leads to the large differences of in specific surface area and pore volume and influences the distribution of gas-bearing contents. In the well L3, the shapes of isolated pores distributed in the mudstone are shown in the SEM images, including elliptical, drop-shaped, short tubular, and bay-shaped. The edges of the pores are smooth, with clear contours. Also, the pores are not filled, with varying depths and mostly bottomless. In addition, partially isolated pores are arranged with a narrow-long shape directionally, indicating obvious compaction. Disordered pores are partially observed and concentrated in groups, with varied pore sizes ranging from 0.1 to 5 μm widely (Fig. 9). The organic matter of the mudstone in the Longtan Formation of well L3 is mainly originated from terrigenous higher plants, with the strong ability of gas generation and physical and chemical variation (Zhu 2019; Cao et al. 2022b). According to the occurrence mode, the organic matter is divided into dispersed isolated agglomerate organic matter and intergranular symbiotic organic matter (Fig. 6). The differential occurrences of organic matter of mudstone in different lithological assemblages are exist. Some isolated organic matter is fragmented and striated (Fig. 6A, B). Some organic matter is in close contact with the surrounding minerals, or mixed in the mud (Fig. 6C, D). During the thermal evolution process, pores and microcracks are easily generated in the organic matter due to the hydrocarbon generation pressure. A few of isolated organic matter pores are developed in the higher plant debris. In the dispersed organic matter granular in the clay minerals, the organic matter pores are rounded and elliptical, with dispersed distribution and the pore size range from tens to hundreds of nanometers. Also, some narrow-long shrinkage cracks are developed at the contact edge of organic matter and clay minerals. The developments of inorganic matter pores of mudstone in different lithological assemblages are influenced by the distribution of mineral granules and diagenetic environment (Qiu et al. 2021). The differential clay mineral types and contents result in different intergranular pores and intragranular pores (Fig. 6E–H). The reason why is that during the diagenetic process, abundant intercrystalline pores are generated due to the clay minerals transformation, resulting in good connectivity, reservoir space expansion, and oil and gas migration (Xu et al. 2022; Wang et al. 2022a, b). However, during the late diagenetic evolution process, the pores morphology, compaction and cementation are affected (Ji et al. 2016; Li 2017, 2020; Li et al. 2021a, b, c). The residual pores between the mineral particles are triangular. Some residual pore spaces between large lamellar clay minerals are generally linear with large pore spaces, and the distribution of these types of pores is different under different compaction.

Fig. 9
figure 9

Differential pore development and gas-bearing characteristics of different lithological assemblages

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From the distribution characteristics of lithological assemblages and the test results of gas-bearing contents. In the lithological assemblages of the Types I and II, relatively thick mudstone is ideal cap because of the strong sealing capacity. Shale gas is difficult to migration and diffusion, and storage in the micropores to form high pressure, which is beneficial to the preservation of pore morphology. Rounded bubble-shaped organic matter pore developed in the organic matter (Fig. 9). High specific surface area and pore volume correspondingly indicate the good reservoir capacity of mudstone in the two types of lithological assemblages. In the lithological assemblages of the Types III and IV, narrow-long pores are developed between granular minerals with partial direction. A few number of pores developed in the organic matter of mudstone, and the pore size is also small (Fig. 9). All the pores show obvious deformation characteristics. This is because, on the one hand, the hydrocarbon generation potential of the relatively thin coalbed and carbonaceous mudstone is low, and less methane generated; on the other hand, when the top or bottom plate is sandstone and partially limestone, the preservation condition is relatively weak. Pores formed on the basis of “gas accumulation—volume expansion—pores generation” are subjected to continuous burial and compaction (Li et al. 2020, 2022; Li et al. 2021a, b, c; Sang et al. 2022), resulting in pore collapse, pore size and reservoir space reduction, or even extinction, and low gas content.

Coalbed and carbonaceous mudstones in the different lithological assemblages are not only the source rock, but also the primary storage space of adsorbed gas. Due to the self-sealing capacity, regional mudstone can be the main cap. Therefore, the combination of coalbed, carbonaceous mudstone and regional mudstone will form multi-set of advantage source-reservoir-cap combination. The Types I is the superior lithological assemblages. Most of the natural gas generated from coalbed migrated to adjacent sandstone and mudstone, increasing the gas-bearing content of the interlayer of mudstone and sandstone. Meanwhile, the mudstone certainly has hydrocarbon generation potential. It is easy to form “microtrap” to store natural gas for the development of thick mudstone. Due to the low porosity and permeability, and high breakthrough pressure, the sealing capacity of mudstone is very strong, which provides favorable storage condition for gas. Under the dynamic condition of hydrocarbon generation, the pressure storage box is formed, accompanied with the high pressure, fine reservoir spaces and gas-bearing contents. In Type II and III, the mudstone provides some protection for the coalbed gas, preventing the gas from escaping upward. The gas is trapped in the coalbed and sandstone to form normal pressure reservoir with medium gas content. In Type IV, the relatively thin coalbed means low hydrocarbon generation potential, with the cap lithology of sandstone. Methane generated from coalbed escapes upward and downward, resulting in low gas-bearing content. The research provides a new perspective for the optimization of vertical favorable segment and the breakthrough of transitional shale gas exploration.

6 Conclusions

  1. 1.

    The lithology of the Longtan Formation is quantitatively identified by multi-discriminant analysis technology, including coal, limestone, siltstone, carbonaceous mudstone and shale. The multi-discriminant function and logging interpretation model of different lithology are established based on the whole model method. Then, the classification criteria of lithological assemblage types of shale formations are established. Four lithological assemblage types are found in the Permian Longtan Formation, including thick mudstone with the interlayer of coalbed, rich mudstone with the interlayer of sandstone and thin coalbed, sandstone interbedded with mudstone with the interlayer of coalbed, and limestone interbedded with sandstone with the interlayer of mudstone, which superimposed with each other.

  2. 2.

    Differences in pore structure characteristics and gas-bearing properties of mudstone reservoirs in the different lithological assemblages are compared. The pore shapes of mudstone in the Type I are cylindrical and partially ink-bottle, with relatively high specific surface area. The pore shapes of mudstone in the Type II and Type III are cylindrical or parallel-sided slits, and the specific surface and pore volume of Type IV samples are the smallest. The volume of free gas varies positively with the pore volume, as do the volume of adsorbed gas and the specific surface area and pore volume. High specific surface area represents high surface adsorption energy. High pore volume not only provides pore space for free gas, but also provides adsorption site for adsorbed gas. It is shown that the gas-bearing volumes of mudstone are mainly influenced by the specific surface area and pore volume of samples.

  3. 3.

    The primary factor of differential gas-bearing property is the difference in pore structure characteristics of mudstone in the different lithological assemblages. In the Type I lithological assemblages, coalbed and carbonaceous mudstone is not only the source rock, but also the primary reservoir of adsorbed gas. Relatively thick mudstone is an ideal caprock because of its strong sealing capacity. Shale gas is difficult to migrate and diffuse, and stored in the micropores to form high pressure, which is beneficial to the preservation of pore morphology. Good reservoir space and high specific surface area and pore volume, indicate good gas-bearing contents.