Genesis of Low-Resistivity Shale Reservoirs and Its Influence on Gas-Bearing Property: A Case Study of the Longmaxi Formation in Southern Sichuan Basin

Abstract

To mitigate the exploration and development risks, it is necessary to have a deeper understanding of the formation mechanism and gas-bearing control factors of low-resistance shale reservoirs. This study focuses on typical shale gas wells (including low-resistivity wells) in Luzhou area, and identification criteria for low-resistance shale reservoirs are redefined as resistivity less than 10 Ω·m and continuous formation thickness greater than 6 m. At the macro scale, low-resistivity shale reservoirs are characterized by high clay mineral content and high water saturation with low gas content. At the micro scale, the main pore size is less than 10 nm, with a small total pore volume but a large specific surface area. Shale reservoirs close to the Class II fault have high water saturation and strong compaction, which hinders the mutual transformation between minerals, resulting in low-resistivity shale with high clay mineral content, small pore volume, and pore size, which promotes the enhancement of reservoir conductivity. The gas content of low-resistivity shale reservoirs is lower, because the distance from the Class II fault is closer, resulting in high water saturation and strong diagenesis, which is not conducive to pore development and shale gas accumulation. When the water saturation exceeds 40%, the pore volume of shale reservoirs rapidly decreases to as low as 0.0074 cm³/g. In order to reduce the risk of exploration and development of the area, the well location deployment needs to be more than 2.8 km away from the Class II fault.

1. Introduction

The exploration and development practice of deep shale gas in the Southern Sichuan basin has revealed that some wells encountered low-resistance reservoirs during drilling in the Longmaxi Formation, such as Well JYT1, H201, H206, L211, N227, etc. After hydraulic fracturing, there is a great difference in shale gas production capacity for these wells. For example, Well L211 and H201 do not produce gas, while Well JYT1 produces a small amount of gas, about 10,000 m³/d. Well N227 has a good gas production effect, with a daily gas production of 190,000 m³/d. There are individual wells with low resistance and high gas content, indicating that the exploration potential of low-resistance shale reservoirs is not the same [1,2,3,4,5,6,7]. Therefore, it is of great significance to reduce the risk of avoiding the missed selection of “desserts” to implement of the characteristics, genesis, and impact on gas bearing capacity of low-resistivity shale gas reservoirs in different areas.

Previous studies have conducted extensive research on the genesis and evaluation of conventional low-resistivity oil and gas reservoirs [8,9,10]. However, there is relatively little systematic research on the low-resistance characteristics and genesis of shale reservoirs. It is generally believed that shale reservoirs with resistivity logging values below 10 Ω·m are considered low-resistance reservoirs [7,11], but the academic community has not yet formed a unified view on its causes [3,6,12,13]. Some studies have shown that a high content of highly conductive minerals (pyrite) in rock fabrics or strong cation exchange ability of clay minerals can cause low-resistivity of shale [14,15]. Organic matter graphitization is an important reason for the decrease in resistivity of shale reservoirs [7,11,12,13,14,15,16]. Under the influence of tectonic action, high conductivity minerals or fluids can also produce low resistance when filled or injected into shale reservoirs through fractures [17,18,19]. Some scholars proposed that organic matter forms an interconnected network structure during organic–inorganic diagenesis, which enhances conductivity and leads to a decrease in shale reservoir resistance. After natural gas migration, there is a large amount of bound water stored in the small pores, which may also form low-resistance shale [7]. From this, it can be observed that rock composition, organic matter content and distribution, degree of thermal evolution, additional conductivity of clay, and water saturation may all contribute to low-resistivity shale reservoirs. This results in differences in mineral composition, degree of organic matter thermal evolution, pore structure, gas content, and other parameters between low-resistivity shale reservoirs and conventional shale reservoirs. Therefore, clarifying the genesis of low-resistance shale in different regions, the characteristics of low-resistance shale reservoirs, and their impact on gas capacity is beneficial for further guiding the exploration and development of low-resistance shale gas areas.

This article takes the rich-organic black shale layer at the bottom of the Longmaxi Formation in the Luzhou area as the research object. Based on the relationship between single well productivity, resistivity, and low-resistance shale reservoir thickness, the identification criteria for low-resistance shale reservoirs are redefined. The TOC content analysis, whole rock X-ray diffraction testing, porosity testing, resistivity experiments, field emission scanning electron microscopy maps observation, nitrogen adsorption, high-pressure mercury injection, methane isothermal adsorption, and other experimental analyses were conducted to clarify the macro and micro characteristics of low-resistance shale reservoirs. Combined with regional fault characteristics, the genesis of low-resistance shale reservoirs in the study area was identified, and the controlling factors of pore development in low-resistance shale reservoirs and their impact on gas content were elucidated.

2. Geological Background and Experimental Methods

2.1. Geological Background

The current pattern of the Sichuan Basin is mainly controlled by two tectonic lines: northeast southwest and northwest, forming a typical diamond-shaped structure. According to the structural characteristics, the Sichuan Basin is divided into the western Sichuan low-steep structural area, the northern Sichuan low-flat structural area, the southeastern Sichuan high-steep structural area, the southwestern Sichuan low-gentle structural area, the southern Sichuan low-steep structural area, and the central Sichuan low-flat structural area [20,21]. The Luzhou area is located in the low and steep structural belt of southern Sichuan in the Sichuan Basin, on top of the Luzhou ancient uplift in the southeast of the Sichuan Basin (Figure 1a). It was formed in a large-scale compression orogenic environment during the Himalayan period. After multiple periods of tectonic movements, the area experienced strong folding deformation, forming large structural faults and multiple syncline and anticline structures, with a composite structural style. This study selected the Longmaxi Formation shale with a burial depth greater than 3500 m in the Luzhou area as the research object. There are about 10 shale gas wells, including normal-resistivity wells and low-resistivity shale gas wells (resistivity less than 10 Ω·m) (Figure 1b). The Sichuan Basin is a compressional depression basin under the background of the craton basin. The edge of the basin continues to rise under compression, and its range continues to decrease. It has transformed from an early open sea area into a limited sea area with “two uplifts and one depression”. The sedimentary environment is a low-energy static water, uncompensated strong reduction deep-water continental shelf sedimentary environment. The sea area gradually deepens from southwest to northeast, ultimately forming a basin basement sedimentary pattern of high in the south and low in the north. Overall, the bottom of the Longmaxi Formation was deposited with black shale rich in organic matter and graptolites. Based on the lithological and electrical characteristics, the target layer was divided into four small layers (Figure 1c).

2.2. Experimental Methods

The samples for this study were selected from the Long-1 sub layer, and the core samples were prepared in different forms (plug sample, cube sample, powder sample). The plug sample is used for rock electric experiments, the cube sample is used for high-pressure mercury injection and porosity testing, and the powder sample is used for TOC content testing, nitrogen adsorption, X-ray diffraction, methane isothermal adsorption, and other experiments. The basic experimental analysis of TOC content testing, porosity testing, X-ray diffraction, nitrogen adsorption, and high-pressure mercury injection can be found in reference [19], which will not be repeated in this article. The following mainly introduces the resistivity testing experiment, field emission scanning electron microscopy maps observation, and methane isothermal adsorption experiment analysis.

In order to analyze the reasons for the decrease in shale resistivity, this study tested the resistivity under dry state, initial state, and different water saturation states. The resistivity was tested by the LSR-3 Seebeck & Electric Resistance Unit produced by the German company Lindsey, which can test cylindrical samples with a diameter of 5 mm and a length of 25 mm. The static DC method was used for resistivity testing at −100~500 °C, with a conductivity range of 0.01 to 2 × 10⁵ s/cm and an accuracy of ±5%. After taking the sample on site, the plug sample (25 × 25 mm) immediately marked and the resistivity of the original state under laboratory conditions was measured. Subsequently, the rock sample was saturated under pressure of 25 MPa with a sodium chloride solution with a mineralization degree of 20,000 mg/L, and then subjected to resistivity testing in the saturated state. Finally, samples with different water saturation were obtained by gradually drying at 60 °C, and electrical resistivity testing was completed.

Shale micro-image acquisition used FEI Quanta 650 FEG (Thermo Fisher Scientific Inc., Waltham, MA, USA) field emission scanning electron microscopy. The specific steps are as follows: first, make the tested sample into rectangular blocks of 20 mm × 20 mm × 10 mm, and then use sandpaper with grit ranging from 600 to 4000 for mechanical polishing; on this basis, argon ion polishing is performed to obtain a smoother surface and improve the observation efficiency of nanopores; Finally, a brief spray coating treatment is applied to the polished sample to increase its conductivity and improve the resolution of scanning electron microscopy image observation. During the observation process, Phenom MAPS module was used to set the image storage quantity to 1024, which can achieve large-area imaging stitching without any trace, facilitating the observation of the microscopic distribution of mineral particles in the shale sample.

The methane isothermal adsorption experiment was conducted using the ISOSORP-GAS SC magnetic levitation balance high-pressure isothermal adsorption instrument manufactured by the German company Rubotherm (Herten, Germany). The maximum single-phase testing temperature and pressure of this instrument reach 200 °C and 70 Mpa, respectively, which can meet the demand for deep shale gas. The isothermal adsorption test temperature for this study is 120 °C, with 21 pressure points designed at 2.0, 4.0, 6.0, 8.0, 10.0, 12.0, 14.0, 16.0, 18.0, 20.0, 22.0, 23.9, 26.0, 28.0, 30.0, 34.9, 40.0, 44.9, 49.9, 55.0, and 60.0 MPa, respectively. Using this instrument for isothermal adsorption testing includes four steps: blank testing, sample pretreatment, buoyancy testing, and adsorption testing. Blank testing, sample pretreatment, and buoyancy testing use helium gas with a purity of 99.999%, while adsorption testing uses methane with a purity of 99.999%. The experiment can obtain excess adsorption data, and the absolute adsorption data can be obtained by fitting with a multivariate Langmuir model. The specific formula is

$$V_{ex}={\displaystyle \frac{V_L+P}{P_L+P}}\times \left(1-{\displaystyle \frac{{\rho }_g}{{\rho }_a}}\right)$$

(1)

where ρ_g is the density of free methane at a given temperature and pressure. ρ_a is the density of the adsorbed phase. V_ex is the excess adsorption capacity, P_L is Langmuir pressure, V_L represents the maximum absolute adsorption capacity, and P is the equilibrium pressure.

3. Boundary of Low-Resistivity Shale Reservoir

At present, the boundaries for low-resistivity shale reservoirs are mainly divided into two categories. One is the absolute value of resistivity logging, which refers to shale reservoirs with resistivity less than 10 Ω·m; another type is the rate of increase in resistivity, which is less than two to determine the low-resistivity shale reservoir. Different scholars define different values of low resistance intervals in different research areas, and overall, the classification of shale low-resistance thresholds is unclear, with an insufficient basis for classification [3,12,13]. Based on the relationship between resistivity and thickness, shale gas wells in Luzhou area can be divided into two categories in this study. A type represented by Well L205 (including Well L206, L208, L210, Y101H10-3, and Y101H3-8), the thickness of the formation with resistivity less than 10 Ω·m in 1~3 sub layers is 3.71 m, accounting for 33.3% of the total formation thickness (Figure 2a), and the thickness of shale with resistivity less than 10 Ω·m in 4 sub layers is 27.44 m, accounting for 44.4% (Figure 2b). Another type, represented by Well L211 (including Well Y101H88-1, Y101H91-4, and JYT1), has a formation thickness of 20.75 m with a resistivity less than 10 Ω·m in 1–3 sub layers, accounting for 100% of the total formation thickness (Figure 2a), and 51.13 m with a resistivity less than 10 Ω·m in 4 sub layers, accounting for 78.9% (Figure 2b). The resistivity of shale gas wells in the former is only partially less than 10 Ω·m, exhibiting an intermittent distribution; the majority of the latter formations have a resistivity less than 10 Ω·m and exhibit a continuous distribution. Therefore, this study utilizes previous research results and combines the thickness of the formation resistivity interval with parameters related to shale gas production to determine the low-resistance limit of shale reservoirs.

This study statistically analyzed the daily production of each well for the first three months before production in the Luzhou area and its adjacent areas, as well as the average gas content of the 1–3 sub layers. It was found that the daily average gas production of the gas producing wells was greater than 6 × 10⁴ m³, and the corresponding target production layer had an average gas content greater than 4 m³/t (Figure 3a). Through the relationship between daily average gas production, target layer average gas content, and average resistivity of 1–3 sub layers, it is shown that when the resistivity is below 10 Ω·m, the daily average gas production is less than 0.2 × 10⁴ m³, and the average gas content of 1–3 sub layers is less than 4 m³/t (Figure 3b,c). Therefore, when the resistivity of shale gas wells is below 10 Ω·m, the production capacity of shale gas wells is lower, but the gas production capacity of shale gas wells is also related to the thickness of the reservoir [3]. According to a statistical analysis of the data, when the main production layer has low resistance (resistivity below 10 Ω·m) and the formation thickness is greater than 6 m, the shale gas well production is extremely low and does not have industrial development value (Figure 3d). Based on this, it is believed that reservoirs with a resistivity less than 10 Ω·m and a formation thickness greater than 6 m are low-resistance shale reservoirs.

4. Characteristics of Low-Resistivity Shale Reservoirs

4.1. Macro Parameter Characteristics of Low-Resistivity Shale Reservoirs

According to the identification criteria for low-resistivity shale reservoirs determined in Section 3, it is found that low-resistivity shale reservoirs are mainly developed in Well L211, Y101H88-1, Y101H91-4, and JYT1. Therefore, these wells are called low-resistivity wells, while other wells are normal-resistivity wells, and the parameters of the low-resistivity shale reservoir and the normal-resistivity shale reservoir in the target section were statistically analyzed, as shown in

Table 1. Comparison of Low-resistivity Reservoir and Normal-resistivity Reservoir Parameters in Studied Area.

	Section	TOC (%)	Ro (%)	Quartz Content (%)	Clay Mineral Content (%)	Pyrite Content (%)	Porosity (%)	Water Saturation (%)	Gas Content (m3/t)
Low Resistivity Reservoir	1~3 sub layer	4.14	3.21	56.32	30.24	4.85	4.04	66.49	2.93
	4 sub layer	2.23	3.10	45.88	40.15	1.61	4.85	52.38	2.08
Normal-resistivity Reservoir	1~3 sub layer	4.23	3.17	59.57	19.68	2.64	4.90	22.78	6.12
	4 sub layer	2.20	3.15	44.90	34.01	2.10	4.93	43.25	3.62

. The results show that whether it is a low-resistivity shale reservoir or a normal-resistivity shale reservoir, the parameters of the 1~3 sub layers shale reservoir are better than those of the 4 sub layers. Through comparison, it was found that the difference between low-resistivity shale reservoirs and normal-resistivity shale reservoirs is mainly manifested in the 1~3 sub layers, while the difference in 4-sub-layer shale reservoirs is not significant. Overall, the low-resistivity shale reservoir of 1~3 sub layers has the characteristics of relatively high clay mineral content (30.24%), high pyrite content (4.85%), high water saturation (66.49%), and low gas content (2.93 m³/t). The TOC content, quartz content, and maturity of low-resistivity reservoirs are not much different from those of normal-resistivity shale reservoirs. In addition, the Ro of low-resistivity shale reservoirs and normal-resistivity shale reservoirs are both less than 3.5%, indicating that the shale reservoirs in the study area did not exhibit severe graphitization [22].

4.2. Microscopic Pore Structure Characteristics of Low-Resistivity Shale Reservoirs

Shale, as an unconventional reservoir that integrates source and storage, continuously generates natural gas through the thermal evolution of organic matter inside. Natural gas enriches in the internal storage space of shale after short-distance migration or no migration. The organic pore morphology of low-resistivity shale gas reservoirs in the Luzhou area is mainly circular elliptical pores, with rare irregular pores. There are many organic pores, but the pore size is small, ranging from 0 to 50 nm. Organic pores exist in isolation and have poor connectivity between pores (Figure 4a–c). In normal-resistivity shale reservoirs, organic pores are highly developed and have good connectivity, mostly consisting of sponge-shaped bubble pores and honeycomb-shaped pores, with pore sizes generally exceeding 100 nm (Figure 4d–f).

The pore structure parameters of shale reservoirs in the study area were quantitatively tested using high-pressure mercury intrusion and liquid nitrogen adsorption. The results revealed that the total pore volume in low-resistivity shale reservoirs is smaller than that in normal-resistivity shale gas reservoirs (Figure 5a). The pore volume of each well of 1–3 sub layers in the study area ranges from 0.0117 to 0.0205 cm³/g, with an average of 0.0164 cm³/g. Among them, the average pore volume of low-resistivity shale gas wells (Well Y101H88-1, L211, and Y101H91-4) of 1–3 sub layers are 0.0140 cm³/g, 0.0136 cm³/g, and 0.0158 cm³/g, respectively, with a total pore volume of relatively low. In the shale reservoir of a normal-resistivity shale gas well, the average pore volume for the 1–3 sub layers are 0.0183 cm³/g, reflecting a notably high pore volume. Within the low-resistivity shale reservoir, the shale samples from Well Y101H91-4 exhibits the most significant pore specific surface area, while the 1–3 sub layers boast an average pore specific surface area of 18.76 m²/g. Conversely, the samples from Well Y101H88-1 possess the least pore-specific surface area among the low-resistivity shales, averaging 11.31 m²/g. On the whole, the specific surface area in low-resistivity shale reservoirs is greater than 10 m²/g (Figure 5b). In normal-resistivity shale reservoirs, the average specific surface area of pores is between 8.63 and 11.76 m²/g, with an average value below 10 m²/g.

The bar charts depicting the full pore size distribution within the shale reservoirs of normal-resistivity wells (Well Y101H10-3, Y101H3-8, and L210) reveal a broad range of pore sizes, characterized by two primary peaks at 1–6 nm and 10–50 nm, respectively (Figure 6a–c). Shale gas wells with low-resistivity, including L211, Y101H88-1, and Y101H91-4, exhibit less developed pores and smaller pore sizes compared to normal-resistivity wells. The pore volume–pore size distribution histogram reveals a predominantly single peak for the pore sizes in these low-resistivity wells, with the peak range being narrower than 10 nm (Figure 6d,e).

5. Genetic Mechanism of Low-Resistivity Shale Reservoir

Shale reservoirs are mainly composed of rock mineral frameworks and internal pore fluids, and the content and microscopic distribution of these two components control the magnitude of shale reservoir resistivity. It is generally believed that the higher the content of clay minerals and water saturation, the stronger the conductivity of the reservoir. The cause lies in the surface of clay minerals, which exhibits a strong adsorption for charged ions and binds a large number of water molecules, consequently improving conductivity. Thus, this section primarily examines the origins of low-resistivity shale reservoirs from both macroscopic and microscopic perspectives.

5.1. Resistivity Response to Reservoir Macro Parameters

This study utilized shale samples from various resistivity wells within the study area for rock-electrical experiments using brine with a saturated formation salinity of 23,000 mg/L. Experimental results are depicted in Figure 7. Shale samples from the normal-resistivity Well Y101H3-8 display high resistivity in their dry state, with values surpassing 1000 Ω·m, whereas the resistivity for low-resistivity shale gas wells (Well L211 and JYT1) is under 100 Ω·m. This suggests that the rock structure and mineral composition significantly influence resistivity (Figure 7a). The low-resistivity of dry samples from low-resistivity shale wells can be attributed to the higher clay mineral content, which enhances conductivity through internal structural water. A comparison between dry shale samples, their original states, and logging resistivities reveals that both the original state and logging resistivities are reduced compared to the dry state, highlighting the significant role of pore fluid under formation conditions in resistivity reduction.

To eliminate the impact of porosity on resistivity, shale samples from various wells with porosities between 4% and 5% were chosen for resistivity tests at varying water saturations. The findings revealed that as water saturation rose, resistivity declined, yet notable disparities in resistivity were observed across different wells. After saturation of the shale samples to SW = 100%, it was observed that the resistivity of samples from normal resistivity wells was below 100 Ω·m, and those from low resistivity shale gas wells had resistivities below 10 Ω·m (Figure 7b). Consequently, high water saturation plays a significant role in contributing to the low resistivity of shale reservoirs. Furthermore, the normal-resistivity well transitions from its original resistivity to the saturated water resistivity, with a more pronounced decrease in resistivity than that observed in low-resistivity shale samples. This is attributed to the higher water saturation in the low-resistivity shale samples under their original formation conditions, which sees less significant increase in water saturation when reaching 100%. Conversely, when the normal-resistivity shale gas well reaches 100% water saturation, the internal pore fluid increases by over 60%. Concurrently, the resistivity of 100% saturated water in low-resistivity shale samples is approximately several times lower than that of normal-resistivity samples. This suggests that besides the conductivity of pore fluids, the rock matrix is also a significant factor in reducing resistivity under water-bearing conditions. Elucidating the microscopic distribution of rock minerals can aid in unveiling the microscopic origins of low resistivity. Our understanding is consistent with the research findings of Xie et al. (2022) and Zhu et al. (2022) [23,24].

5.2. The Impact of Reservoir Microscopic Characteristics on Resistivity

Diagenesis refers to the physical, chemical, and biological processes that occur during the long geological history of sediments from post-deposition to pre-metamorphism. Changes in formation conditions and fluid properties result in varying impacts on the pore structure, material composition, and physical properties of shale reservoirs. As the burial depth increases, various minerals within the reservoir will undergo chemical reactions with formation water, causing changes in the shale reservoir. These processes may lead to significant changes in shale resistivity.

In this study, shale composition and surface structure information were acquired at the same location using secondary electron imaging (SE2) and backscattered electron imaging (BSE). By integrating these imaging techniques, a series of high-precision (10 nm resolution), small-field-of-view (1000 × 1000 pixels), arranged in a matrix with overlaps, were obtained. Finally, these images were merged using Atlas software (version number is CSTR:17081.11.atlas), resulting in a high-precision, large-field-of-view MAPS image [25]. Shale samples from both normal- and low-resistivity wells were analyzed through large-field-of-view MAPS imaging, exhibiting the characteristics detailed below.

Normal-resistivity well: Particles and boundaries are distinct, each mineral exhibits a clear outline, and there is a lower clay mineral content, primarily distributed in organic matter in needle-like or fine-grained forms. The organic matter exhibits high surface porosity, distributed over a large area, with well-developed organic pores. Local areas showcase strawberry-shaped and single-crystal pyrite, though overall content is low (Figure 8a).

Low-resistivity well: The microscopic mineral distribution characteristics in low-resistivity wells are starkly contrasting to those in normal-resistivity wells. There is a high level of consolidation between particles, with individual mineral grains being substantial, measuring up to tens of microns in size. Clay minerals are distributed in a flowing pattern around the edges of larger mineral grains, while organic matter is fragmented into small, irregular shapes. Furthermore, along the mineral grain edges, numerous pyrite crystals have formed within the interstices of the striped clay minerals and organic matter, accompanied by localized clusters resembling strawberries (Figure 8b).

A comparative analysis reveals substantial disparities in the microscopic distribution of minerals between normal- and low-resistivity wells, potentially attributed to the intensity of diagenesis under high water saturation conditions. The formation of shale reservoirs involves intricate diagenetic processes, with the transformation between feldspar, clay, quartz, and other minerals exerting a significant influence on mineral composition. The chemical equation for the specific conversion is as follows:

Al₂SiO₅(OH)₄ (kaolinite) + KAlSi₃O₈ (Potassium feldspar) = KAl₃Si₃O₁₀(OH)₂ (Illite)+ 2SiO₂ + H₂O

(2)

3KAlSi₃O₈ (Potassium feldspar) + 2H⁺ + H₂O = KAl₃Si₃O₁₀(OH)₂ (Illite) + 6SiO₂ + 2K⁺ + H₂O

(3)

The low-resistivity shale reservoir exhibits high water saturation (as shown in Table 1) and abundant diagenetic fluids. This impedes the reaction described by chemical Equation (2) while enhancing the reaction described by chemical Equation (3). Potassium feldspar transforms into illite and siliceous minerals, with illite potentially adhering to mineral surfaces during chemical diagenesis, leading to an increase in siliceous minerals that enlarge the original particles. Concurrently, the transformation of kaolinite into illite is inhibited, culminating in distinct reservoir characteristics of low-resistivity clay minerals (as detailed in Table 1).

5.3. A Preliminary Analysis of the Causes for High Water Content in Shale Reservoirs

The low-resistivity of the shale reservoir in the study area is primarily influenced by both water saturation and diagenesis, with diagenesis further affected by diagenetic fluids. Thus, elucidating the causes of high water content in the shale reservoir is critical for unveiling the nature of low-resistivity reservoir formation in the study area. Initially, this study analyzes the relationship between the location of shale gas wells and faults, noting that the hanging wall and footwall of the same fault significantly affect resistivity, with the hanging wall’s resistivity being twice that of the footwall (Figure 9a), suggesting higher water saturation in the footwall. For the Yangshen 10 fault, a Class III fault, the fault throw does not significantly influence resistivity. Building on this, a correlation analysis between the distance from Class II faults and water saturation revealed that closer proximity to the fault corresponds to higher water saturation (Figure 9b). Consequently, faults exert a significant control over the water saturation in shale reservoirs, manifesting in both the hanging wall and footwall, as well as in the distance from Class II faults.

6. The Impact of Low-Resistivity Shale Reservoir Characteristics on Gas-Bearing Properties

Gas content, serving as the foundation for shale gas resource assessments, has consistently been accorded significant importance by geologists as a crucial aspect of research, encompassing aspects such as storage space, total gas content, and the ratio of free to adsorbed gas. This not only influences the evaluation of shale gas resources but also impacts its extraction. During the examination of shale reservoirs’ gas-bearing characteristics, besides analyzing the total gas content and the ratio of free to adsorbed gas, factors influencing the presence of shale gas within storage spaces and the distribution of adsorbed and free gas are also emphasized. This research primarily acquires the adsorbed gas of shale lithology via methane isothermal adsorption experiments [26], and computes the free gas content in shale reservoirs using parameters such as porosity, temperature, pressure, and water saturation. The detailed calculation methods are as follows [27]:

$$n_{free}={\displaystyle \frac{P{T}_{sc}{S}_g\phi }{ZT{P}_{sc}{\rho }_r}}$$

(4)

In the formula, S_g represents the gas saturation, expressed in percentage; T_sc, the standard ground temperature, set at 273.15 K; Z, the gas compression factor; φ, the porosity, also in percentage; P_sc, the standard ground pressure, at 0.101 Mpa; and ρ_r, the density of the gas-bearing rock, g/cm³. The density of the free gas phase under specific temperature and pressure conditions can be determined using the equation of state, while the compression factor can be sourced from a table.

The gas content of the shale reservoirs in the study area exhibits significant variation. Normal-resistivity wells exhibit a free gas content exceeding 5 m³/t, reaching up to 8.86 m³/t, with a lower adsorbed gas content ranging from 1.45 to 1.85 m³/t. Conversely, low-resistivity wells display lower free and adsorbed gas contents, with both being under 3 m³/t and 1.5 m³/t, respectively (Figure 10a). Clearly, the total gas content in normal-resistivity wells significantly surpasses that of low-resistivity shale gas wells, often being 2~4 times greater. Subsequently, the study calculated the proportions of free and adsorbed gas in both normal and low-resistivity wells. The findings reveal that adsorbed gas constitutes a larger proportion in low-resistivity wells, consistently surpassing 30%, compared to the 19.62% average in normal-resistivity wells.

A comparative analysis of the results revealed that low-resistivity wells have a relatively low gas content, comprising both adsorbed and free gas volumes, but with a disproportionately high proportion of adsorbed gas. This investigation focuses on elucidating the underlying reasons for this phenomenon, examining both the macroscopic properties and the microscopic pore structure of the reservoirs. The macroscopic characteristics of low-resistivity wells include high clay content and high water saturation, as indicated in Table 1. According to Equation (3), the content of free gas is primarily influenced by temperature, pressure, porosity, and gas saturation. The study area has a considerable burial depth, with minimal variations in temperature and pressure. The key determinants of free gas quantity in shale reservoirs are porosity and water saturation, primarily accounting for the low free gas levels in low-resistivity shale reservoirs. The influence of water saturation on adsorbed gas should be differentiated from its effect on free gas. For free gas, formation water primarily fills pore spaces, diminishing its storage capacity. For adsorbed gas, formation water forms a film on pore walls, occupying methane’s adsorption sites, thus lowering the adsorbent’s capacity [28]. Currently, it is widely recognized that water saturation significantly affects adsorption capacity, with dry shale showing notably greater adsorption than its hydrated counterpart. Once water saturation reaches a certain threshold, the effect lessens [29]. Extensive research suggests that at 40% water saturation, shale’s adsorbed gas volume drops to 30% of dry shale. As water saturation rises, the variation in adsorbed gas volume becomes minimal. In normal resistivity shale gas wells, the water saturation in the shale reservoir stands at approximately 20% to 30%, whereas in low-resistivity wells, it is around 66%. These conditions result in adsorbed gas volumes of 60% and 30% for dry shale, respectively, underscoring the characteristic low adsorption capacity in low-resistivity shale.

Besides the influence of macroscopic reservoir parameters on gas-bearing properties, microscopic pore structure also plays a significant role. Typically, a higher pore volume favors the storage of free gas, while a larger specific surface area enhances the adsorption of adsorbed gas. A correlation analysis of pore volume with total, free, and adsorbed gas content in typical shale reservoirs within the study area revealed a strong positive correlation between pore volume and both total and free gas content, with a weaker positive correlation observed with adsorbed gas content (Figure 10c). In low-resistivity shale reservoirs, smaller pore volumes (Figure 5a) are unfavorable for storing free gas. Due to its smaller pore radii (Figure 6) and larger specific surface area (Figure 5b), low-resistivity shale offers numerous adsorption sites, enhancing the adsorption of shale gas, causing a high proportion of adsorbed gas (Figure 10d). Thus, even with water saturation levels reaching 65% or even 80%, low-resistivity shale reservoirs still exhibit an adsorbed gas volume exceeding 1 m³/t.

Based on the above analysis, it is believed that the main reasons for the low gas content in low-resistance shale reservoirs are high water saturation and low pore volume. From the analysis of the formation mechanism of low-resistance shale reservoirs, it is believed that the low pore volume is mainly due to the proximity of the reservoir to faults, which leads to the escape of shale gas and an increase in water saturation [30]. Under high temperature and pressure conditions, compaction and cementation are promoted, resulting in larger quartz particles, reduced pores, and decreased pore volume. Figure 11 shows that when the water saturation of shale reservoirs is less than 40%, the average pore volume is 0.0196 cm³/g; When the water saturation exceeds 40%, the pore volume rapidly decreases and eventually drops to 0.0074 cm³/g. According to relevant literature, as the burial depth increases, the porosity of shale reservoirs decreases slowly, and there is a phenomenon of increased porosity in some areas [31]. Meanwhile, the difference in burial depth of the shale reservoir studied is less than 200 m. So, the burial depth is not the main reason for the decrease in pore volume of shale reservoirs. Previous studies have shown that when the water saturation is high, in situ geological conditions can promote diagenesis, causing quartz mineral particles to increase and compress existing pores, resulting in a smaller pore volume [32,33]. This study has recognized that low-resistivity shale reservoirs have a higher water saturation. This provides environmental conditions for rapid or strong diagenesis of low-resistance shale reservoirs, resulting in about 10% clay mineral content higher than normal-resistivity shale gas reservoirs. Due to the strong plasticity of clay minerals, they are easily compacted under strong diagenesis, but can provide a large number of micropores. Therefore, in the study area, when the water saturation exceeds 40%, the pore volume rapidly decreases under strong diagenesis, which is not conducive to shale gas storage. Through the relationship diagram between the distance between the reservoir and the fault and the water saturation (Figure 9b), it was found that the distance corresponding to a water saturation of 40% is 2.8 km.

7. Conclusions

(1) Based on previous studies, the boundary for low-resistivity shale reservoirs is defined as a resistivity below 10 Ω·m and a continuous formation thickness exceeding 6 m. This is established based on the relationship between reservoir resistivity, its range of values, and its correlation with gas content and gas well output. The low-resistivity shale reservoirs within the production section are characterized by a notably high clay mineral content (30.24%), elevated pyrite content (4.85%), a high water saturation level (66.49%), and a lower gas content (2.93 m³/g).

(2) In the low-resistivity shale reservoir, organic matter is primarily distributed in sporadic and fragmented chunks, with poorly developed internal organic pores. The pores, predominantly isolated and circular, have poor interconnectivity, resulting in low surface porosity. Additionally, micro-fractures are underdeveloped. The predominant pore sizes in the overall pore distribution are under 10 nm, characterized by tiny pores. The total pore volume is under 0.015 cm³, yet the specific surface area exceeds 8 m²/g.

(3) The primary causes for the low resistivity of shale reservoirs in the Luzhou area are high water saturation, elevated clay mineral content, and diagenetic processes. High water saturation causes a swift decline in shale reservoir resistivity, intensifies the dissolution of potassium feldspar, and impedes the transformation of kaolinite to illite. This results in a low-resistivity shale reservoir where clay minerals are distributed in a flow-like pattern around large quartz grains, interspersed with recrystallized pyrite crystals, creating a comprehensive conductive network that lowers resistivity. The development of high water saturation is primarily influenced by the proximity of gas wells to Class II faults, as well as the fault’s hanging and footwalls.

(4) The low-resistivity shale reservoir exhibits low gas potential, with both free and adsorbed gas content under 3 m³/t; yet, a significant proportion of adsorbed gas, over 30%, mainly due to high water saturation and low pore volume. When the water saturation exceeds 40%, the pore volume rapidly decreases, which is not conducive to shale gas storage, and the corresponding fault distance is 2.8 km. Therefore, in order to reduce the exploration and development risks of shale gas in the Luzhou area, the well deployment needs to be 2.8 km away from the Class II fault.