Investigation of the pore structure characteristics and fluid components of Quaternary mudstone biogas reservoirs: a case study of the Qaidam Basin in China | Scientific Reports

Scientific Reports volume 14, Article number: 26512 (2024) Cite this article

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Quaternary mudstone biogas reservoirs in the Qaidam Basin have shown great potential. However, complex pore structures with high clay contents and high heterogeneity limit the understanding of the storage and migration principles of these reservoirs. In this paper, HPMI and nitrogen adsorption experiments, in combination with NMR experiments under water saturation, centrifugation, various drying temperatures and other conditions, were adopted to determine the pore structure characteristics. Specifically, the reservoir space types and pore radius distribution characteristics were clarified. The cutoff values for different types of pores were identified based on the water-saturated mudstone NMR T2 spectra for full aperture distribution scales jointly characterized by mercury injection and nitrogen adsorption experiments. Furthermore, the three pore components and the saturation of different fluids were obtained. The research results indicate that the mudstone biogas reservoir has developed various reservoir spaces, and the pore size is primarily in the micronanometer range. The average total porosity reaches 27.28%, but the proportion of movable water pores is only 9.23% with poor fluid mobility, and the fluids in the pores are mostly capillary-bound water and clay-bound water. Among the different lithologies, argillaceous sand is more likely to become a good production layer.

Biogas, also known as biogenic gas, is formed by the fermentation and synthesis of microbial communities (mainly methanogens and anaerobic bacteria) at the early stage of diagenesis or organic matter evolution (Ro less than 0.4-0.5%). The main component is methane, which includes carbon dioxide, a small amount of nitrogen, and other trace gas components1,2. Biogas fields are widely distributed in the United States, Russia, China, Canada, the Bay of Bengal and the Eastern Mediterranean3, and their total reserves account for approximately 20–30% of the natural gas resources worldwide4,5.

The Qaidam Basin, located in the northeast of the Qinghai‒Tibet Plateau, is the fourth largest basin and one of the major biogas enrichment regions in China6. The Quaternary mudstone strata in the Sanhu area of the middle east region of the basin, with a developed area of approximately 6000 m2 and a thickness of more than 3400 m, have enormous potential for natural gas exploitation. The resource amount is 9194 × 108 m3, the geological reserve is nearly 3000 × 108 m3, and it is anticipated to emerge as a promising frontier for the replacement of oil and gas resources7. The Quaternary mudstone biogas reservoirs in the Qaidam Basin are self-generated and self-preserved primary gas pools8,9, with several unique geological characteristics, such as low maturity10, weak diagenesis3, thin interbedded sand and mudstone11, indistinguishable sources and reservoirs12, and strong succession gas generation13. The lithology is mainly composed of mudstone, argillaceous siltstone, and siltstone. Due to burial depths less than 1700 m, the physical properties of the mudstone biogas reservoirs are relatively good, with average porosities between 25% and 35%. However, the large amount of clay minerals in reservoirs complicates the pore structure, leading to strong heterogeneity14, which has a significant impact on the generation, migration, and occurrence of fluid in pores15. Therefore, conducting research on the pore structure and effectiveness of mudstone biogas reservoirs and further clarifying the occurrence status of fluids in different pores and their impact on gas content are of positive significance for guiding the efficient development of mudstone biogas reservoirs.

Scholars utilize various technical methods, such as cast thin section (CTS), scanning electron microscopy (SEM), mercury injection (MI), nuclear magnetic resonance (NMR), nitrogen adsorption (NA), and nano-CT scanning, to investigate pore structure and evaluate the effectiveness of pores16,17. Due to the limitations of low resolution in optical microscopy, CTS analysis is typically used for observing conventional micrometer-scale pores18, and the analysis results can only reflect isolated two-dimensional sections. SEM enables the observation of nanoscale pores but is confined to the part of pore morphology19,20, and human factors have a significant impact on SEM in the measurement of pore size, showing poor statistical representativeness21,22. MI is commonly used for the analysis of interconnected meso- and macropores. However, the heterogeneity of cores may cause changes in mercury surface tension and contact angle, resulting in measurement deviations23. Gas adsorption is an effective method for reflecting the distribution of nanometer-scale pores in materials and has been widely applied in the testing of pore structures in porous materials. However, the measurement accuracy of this method is influenced by the degassing temperature, time, and sample grinding degree24. The nuclear magnetic resonance (NMR) method, as a high-precision and emerging technology for studying reservoir pore structures, can provide nondestructive, fast, and information-rich superior parameters related to reservoir pore structures25,26. NMR has been widely used in the research of pore structures in reservoirs such as sandstones, carbonates, and shales27,28,29 to partition fluids, and the T2 cutoff values of capillary-bound water and clay-bound water in sandstones have been determined to be 33 ms and 3 ms, respectively. Researchers have conducted extensive research on this topic. Xiang et al.30 and Jiang et al.31 utilized the NMR technique combined with centrifugation and thermal treatment methods, respectively, to study shales and elucidate the distribution of pore fluids. Zhu et al.32 fitted centrifugal and bound water-state NMR spectra based on a normal distribution and proposed a new T2 cutoff value calculation method. Guo et al.33 investigated the pore fluid characteristics and microscopic mechanisms of tight sandstone using three-dimensional NMR. However, research on the pore structure characteristics of mudstone biogas reservoirs has not yet attracted widespread attention, which restricts the understanding of the storage and migration laws of such gas reservoirs.

Therefore, this study takes Quaternary mudstone core samples from the Qaidam Basin as the object and conducts XRD, SEM, high-pressure mercury intrusion, and nitrogen adsorption experiments, as well as low-field NMR measurements under various conditions, such as water saturation, centrifugation, and gradual drying, to determine the pore radius distribution characteristics and NMR cutoff values (T2C1 and T2C2) of the mudstone biogas reservoirs, identify the pore and fluid components, and provide a useful reference for the quantitative evaluation of different types of fluids in these reservoirs.

Core samples in the study were taken from the Quaternary mudstone strata of key wells (SS-2 and S23) in the Sebei area of the Qaidam Basin. The lithology is mainly composed of gray mudstone and sandy mudstone interbedded with gray argillaceous siltstone, siltstone, and a small amount of fine sandstone, the porosity is generally greater than 20%, and the permeability is in the range of 0.732–1.200 mD (Table 1).

Since the Quaternary mudstone biogas reservoir samples have low cementation and are loose and brittle, during the preparation process, we drilled cores after liquid nitrogen freezing to not only improve the success rate of drilling but also preserve the original occurrence state of fluids in the core to the greatest extent possible (Fig. 1a). Then, the wire cutting method was used to obtain standard plunger samples to ensure the integrity and flatness of the end face (Fig. 1b). When preparing the saturated water plunger samples, to avoid core hydrolysis and collapse, high-temperature resistant heat shrink tubes (2.57 cm diameter, double shrinkage rate, and high-temperature resistance of 175 ℃) were selected to wrap the standard plunger samples (Fig. 1c). Then, place samples in a dry balance and use needles to titrate the formation water at both ends of samples at a constant speed, allowing cores to fully self-absorbing. At the same time, observe the changes in the balance’s mass, and determine the quality for self-absorption titration at different water saturation to meet the needs of centrifugation and variable-temperature NMR experiments. At the same time, some broken blocks were collected, crushed, and subjected to XRD and nitrogen adsorption experiments.

Preparation of core samples.

Due to the high porosity and low permeability characteristics of mudstone cores, the core porosity is measured by helium gas method, while the permeability is measured by attenuation method according to the industry standard for shale porosity and permeability measurement (GB/T34533-2023).

This experiment was conducted according to the SY/T51632010 standard. First, the crushed core samples were soaked in distilled water, ground into 200 to 250 mesh powder, and the clay minerals and other rock minerals were separated through ultrasonic oscillation and sedimentation. After drying, a MAXima XRD-7000 X-ray diffractometer was used to irradiate the ground powder, and the corresponding types and component contents of the whole-rock minerals and clay minerals were distinguished based on the position and intensity of the diffraction peaks34.

Scanning electron microscopy uses electron beams and electromagnetic lenses to greatly magnify the fine structure of materials based on the principles of electron optics35,36. Based on the Chinese petroleum industry standard (SY/T5162-2014), this experiment used a FEI Quanta 650 field emission scanning electron microscope with a maximum resolution of 3 nm. Before observation, the core samples were naturally dried and smoothed with sandpaper, and the surfaces were polished with argon ions. Then, under vacuum conditions of less than 10 –4 mm, a Leica EM SCD500 coating machine was used for gold plating (with a thickness of approximately 15 nm) to improve the resolution37. Finally, the pore structure of the cores was observed.

Nitrogen adsorption experiments were carried out according to the Chinese national standard “Analysis of mesopores and macropores by gas adsorption” (GB/T 21650.2–2008) using an American Quadrasorb SI-type specific surface area and porosity analyzer. The pore size measurement range was 0.35–400 nm, the adsorption-desorption relative pressure range was 0.004–0.995, the specific surface area was as low as 0.0005 m2/g, and the pore volume was as low as 0.0001 cm3/g. Core samples were ground to a powder size of 60 mesh after being treated with wash oil. To eliminate the bound water and capillary water remaining in the samples, all the samples were pretreated by high-temperature evacuation at 105 ℃ for 3 h prior to the nitrogen adsorption experiment38. At the liquid-nitrogen temperature (77.35 K), the amount of nitrogen adsorbed under different relative pressures was determined using nitrogen with a purity greater than 99.999% as the adsorbent39.

Mercury injection is the most widely used method for determining the pore throat distribution. By injecting mercury into the samples and recording the pressure and volume under equilibrium conditions, the mercury injection curve and a series of pore throat characteristic parameters can be obtained40,41. The experiment was carried out using an AutoPore 9505 mercury porosimeter from the United States, with a maximum mercury injection pressure of up to 413.7 MPa, corresponding to the smallest pore throat radius of approximately 1.8 nm. Typical samples like mudstone, silty mudstone and argillaceous sand were selected based on the physical parameters, and then the samples were dried at 105 °C until a constant weight was reached. The experiments followed the standards of “Practices for core analysis” (GB/T 29172 − 2012) and “Rock capillary pressure measurement” (GB/T 29171 − 2012).

NMR was performed with a LIME-FSMAR nuclear magnetic resonance tester produced by Beijing Limecho Technology Co., Ltd., which has a main frequency of 2 MHz. To maintain the stability and uniformity of the magnetic field, the temperature of the magnetic field was controlled at 30 ± 0.5 °C. The experiments followed the standards of “Practices for core analysis” (GB/T 29172 − 2012) and “Specification for normalization measurement of core NMR parameter in laboratory” (SY/T 6490 − 2007). Considering the average clay content exceeding 50% in the Quaternary mudstone biogas reservoir samples and the short relaxation time of water in the pores42,43, the waiting time in the NMR experimental measurements was set to 3 s, the echo interval was set to 60 µs, the number of echo strings was set to 500, and the number of scans was set to 256.

The NMR experiment was divided into three stages. First, the NMR T2 spectra of the samples in the water-saturated state were measured. Then, the NMR T2 spectra of the samples after centrifugation were measured. Finally, the samples were resaturated with water, and the NMR T2 spectra were measured at various drying temperatures. The specific experimental process is as follows:

NMR measurement of water-saturated samples.

NMR T2 spectra of water-saturated samples can reveal the NMR response of all pore fluids and hydrogen-containing skeletons in the mudstone samples44. To avoid disintegration of the mudstone sample during formation water saturation and maintain the integrity of the sample, the mudstone samples were wire-cut and wrapped with high-temperature-resistant heat-shrink tubing before saturation. The samples were then dried and finally saturated with the self-absorption weight increment method45. After reaching the final saturation, the NMR T2 spectra of the water-saturated samples were measured.

NMR measurement of samples after centrifugation.

After the NMR T2 spectra of the water-saturated samples were measured, the samples were centrifuged at 10,000 r/min for 1 h. By measuring the NMR T2 spectra of the samples after centrifugation, information on the bound water and movable water in the pores of the samples was obtained.

NMR measurement of samples under varying drying temperatures.

The centrifuged samples were saturated again by the self-absorption weight increment method, and the NMR T2 spectra of the resaturated samples were measured. Then, NMR experiments were carried out under varying drying temperatures. The experiments were divided into three temperature stages, and the water in the samples gradually dissipated with increasing drying temperature.

① Low-temperature drying stage (30–60 ℃): The drying temperature interval was set to 10 ℃. According to the buried stratum temperature of the samples, two NMR experiments were carried out for each drying temperature process in the low-temperature drying stage (the time interval was 60 min), and the change in water in the core samples was obtained;

② Temperature-encrypted drying stage (65–80 ℃): Compared with the low-temperature drying stage, the drying temperature interval in this stage was set to 5 ℃, the drying time interval was set to 60 min, and the NMR T2 spectrum of each stage was measured;

③ High-temperature drying stage (90–120 ℃): The drying temperature interval was 10 ℃, the drying time interval was 60 min, and the NMR T2 spectra were measured at different temperatures to obtain information on the capillary-bound water and clay-bound water in the pores of the samples.

The XRD results show that (Table 2) the average contents of clay minerals, quartz, carbonate minerals, and feldspar in all the samples are in the following order: 46.9%, 21.3%, 15.41% and 7.67%, respectively, and small amounts of other minerals, such as aragonite and anhydrite. The clay minerals are mainly composed of mixed layers of illite and illite-smectite, with relative average contents of 38.84% and 31.22%, respectively, followed by chlorite (12.98%), smectite (10.03%), and kaolinite (6.93%). The N2 and N3 mudstone samples were not measured by XRD, and the clay content in other mudstone samples (N1, N4, and N5) exceeded 50%, while accounting for 44.72% and 38.05% in silty mudstone (N6) and argillaceous sand (N7 & N8), respectively. Excluding sample N1, which developed microcracks in mudstone (φ = 29.127, K = 0.732 mD; Fig. 1b), as the mud content increased, the physical properties of the samples gradually deteriorated (Table 1).

The CTS and SEM analyses reveal that the reservoir space types of the mudstone biogas reservoir mainly include intergranular pores, microfractures, dissolved pores (dissolved fractures) and intercrystalline pores (Fig. 2). Due to shallow burial and weak mechanical compaction, the primary intergranular pores are well preserved and mainly exist in siltstone, argillaceous siltstone and sandy strips. Most of them are supported by chemical sediments such as siderite and are in an open state, with good connectivity and a pore diameter range of 3–60 μm, which is the main effective pore type. Microfractures mainly exist in some silty mudstones and mudstones with sandy strips. The maximum visible fracture width is 20–40 μm, and the extension length can reach 1 mm. They communicate with sandy strips and provide good seepage channels. Dissolved pores (dissolved fractures) are formed by the dissolution of unstable minerals during diagenetic evolution. Their pore diameters are generally larger, but their amount is less than that of the primary intergranular pores. The maximum pore diameter can reach 1 mm, and most are in the range of 50–500 μm. Intercrystalline pores mainly exist in mud, in which pores in the illite/smectite mixed layer and between illite crystals are common. Such pores can also be found between carbonate crystals such as dolomite. The diameter of intercrystalline pores is generally small, ranging between 1 and 10 μm, and its distribution frequency in gas field reservoirs is much lower than that of intergranular pores.

The main reservoir spaces of the mudstone biogas reservoirs.

Based on the nitrogen adsorption amount measured at different relative pressures at a temperature of 77.35 K, with the relative pressure as the abscissa and the adsorption amount per unit sample mass as the ordinate, a nitrogen adsorption-desorption isotherm was constructed46. The DFT model was used to calculate the pore radius distribution curve of the core samples.

The analysis results show that the pore size distribution of the Quaternary mudstone is bimodal (Fig. 3), with a distribution ranging between 10 nm and several hundred nanometers. The main pore radius peaks are between 50 and 70 nm (N2 and N5), which are significantly smaller than those of the silty mudstone core (N6, 80–100 nm) and argillaceous sand core (N8, 90–400 nm). As the mud content increases from argillaceous sand to silty mudstone to mudstone, clay minerals with relatively small particle sizes and plastic deformation fill brittle mineral particles, reducing the storage space of the reservoir47. Therefore, it is easier to suppress pore development during the burial process, resulting in a decrease in the peak pore radius and pore volume48,49.

Pore radius distribution in Quaternary mudstone biogas reservoirs in the Qaidam Basin.

The HPMI results (Fig. 4) indicate that there are differences in the mercury intrusion curve characteristics of the mudstone, sandy mudstone, and mudstone siltstone cores. Mudstone samples N2 and N3 have higher displacement pressures and median pressures, with average values of 4.25 MPa and 15.78 MPa, respectively, and corresponding to an average pore throat radius of 0.132 μm. The curves are smooth with good sorting performance, and the physical properties are poorer, mainly with fine pore throats. The displacement pressure and the median pressure of silty mudstone (N6) are 0.62 MPa and 4.85 MPa, respectively, and corresponding to an average pore throat radius of 1.213 μm. The permeability of silty mudstone is improved compared to that of mudstone, with pore cementation being the main form. The displacement pressure and median pressure of argillaceous sand (N8) are the lowest in the samples, with values of 0.16 MPa and 2.35 MPa, respectively, and corresponding to an average pore throat radius of 2.621 μm. The mercury intrusion curve is gentle, and the porosity and permeability are relatively good, mainly due to the presence of coarse pore throats.

High-pressure mercury intrusion curves.

The NMR T2 spectra of eight mudstone biogas reservoir samples in the water-saturated state show a typical double-peak pattern with short relaxation. The T2 spectrum of the left peak is in the range of 0.01–0.3 ms, the right peak is in the range of 0.8–10 ms, and compared to the silty mudstone and mudstone samples, the argillaceous sand samples (N7 and N8) have longer T2 relaxation times in the range of 0.6–10 ms. The amplitude of the right peak is far greater than that of the left peak, accounting for more than 90% of the whole NMR T2 spectrum (Fig. 5).

NMR T2 spectra of mudstone biogas reservoir samples in the water-saturated state.

Figure 6 shows the NMR results of the water-saturated samples after centrifugation. The movable water in the macropores of the samples can be discharged after centrifugation, and the NMR results represent the NMR T2 spectra of the samples in the bound water state50. Comparative analysis revealed that after centrifugation, the T2 spectral shape of the samples was highly similar to that of the water-saturated samples, and the distribution range was basically the same. There was no obvious change in the 0.01–0.3 ms interval of the left peak, but the right peak moved slightly to the left after centrifugation. Because the samples are dominated by nanoscale pores, the movable fluid in the limited meso- and macropores is preferentially centrifuged under centrifugal pressure, causing the right peak to shift slightly to the left after centrifugation. In contrast, capillary-bound water and clay-bound water remain in the pores and are difficult to centrifuge; thus, the left peak is unchanged.

NMR T2 spectra of Quaternary mudstone biogas reservoir samples from the Qaidam Basin under the water-saturated and centrifugation states.

A large amount of bound fluid exists in the micro- and nanopores of the mudstone biogas reservoir samples, mainly in the form of clay-bound water and capillary-bound water. Testamanti et al.51 proposed using the varying-temperature drying method in conjunction with low-field NMR experiments to obtain the content of clay-bound water to classify clay-bound water and capillary-bound water. During the drying process, the movable water in the meso- and macropores and the capillary-bound water evaporate rapidly in the early stage of drying. As the temperature increases to a certain value, the clay-bound water in the micropores continues to dissipate at a lower rate. Theoretically, as the temperature increases, the fluids in the pores evaporate completely, and only the hydrogen-containing skeleton signal is retained in the final sample.

The NMR T2 spectra measured at various drying temperatures show that with increasing temperature, the water in the meso- and macropores evaporates, the amplitude of the peaks decreases, the peaks shift significantly to the left (Fig. 7), and the transverse relaxation time of all the samples gradually decreases.

NMR T2 spectra of Quaternary mudstone biogas reservoir samples from the Qaidam Basin at varying drying temperatures

From the NMR porosity measurements of the mudstone samples at various drying temperatures (Fig. 8), it can be found that with increasing temperature, the decrease in NMR porosity exhibits distinct stage characteristics. The first stage (30–80 °C) showed a significant decrease, while the reduction rate decreased in the second stage (80–120 °C). Based on the changing magnitude of the porosity during the drying process and the fluid occurrence state in the mudstone samples, it can be concluded that the magnitudes of the two slopes in the graph reflect the fluid evaporation rates of the mudstone samples at different stages. A larger slope in the first stage indicates faster movement of pore fluids in meso- and macropores, and the movable water and capillary-bound water evaporate rapidly. During the high-temperature drying process in the second stage, the poor connectivity of micro- and nanopores hinders the movement of pore fluids, leading to a decrease in the evaporation rate of clay-bound water.

Characteristics of NMR porosity of mudstone samples at different stages.

As shown in Fig. 8, the corresponding temperature at the intersection of the two evaporation stages can be used as the evaporation temperature threshold of clay-bound water. According to the two-stage linear fitting, the evaporation temperature thresholds of clay-bound water for the eight samples are between 80 and 85 ℃ (Table 3). These thresholds represent the turning points at which the movable water and capillary bound water of the rock sample are completely evaporated and discharged, and the bound water of the clay begins to be discharged. In this state, there is no movable water or capillary-bound water in the pores of the samples, and the fluids in the pores are all in the form of clay-bound water. The nuclear magnetic resonance signal is completely derived from the clay-bound water, and the measured porosity represents the ineffective porosity.

Currently, there is a lack of a unified understanding of the classification of mudstone pores. In this study, we adopted the International Union of Pure and Applied Chemistry (IUPAC) pore classification method, which has been widely used in the field of coal and chemical industry52. The IUPAC pore size classification scheme divides the pores of porous media into three categories: micropores (< 2 nm), mesopores (2–50 nm), and macropores (> 50 nm). The Quaternary mudstone shale is dominated by micro- and nanopores.

There are differences in characterizing pore size distribution between high-pressure mercury injection and nitrogen adsorption experiments. The pore size distribution curve obtained by high-pressure mercury injection is a comprehensive reflection of the throat and its connected pores, and due to the difficulty of mercury liquid entering the micropores in mudstone, the pore size mainly exhibits the distribution characteristics of mesopores and macropores between 1.8 nm and 950 µ m in theory53. By comparison, nitrogen adsorption experiments can accurately characterize the distribution characteristics of micropores between 0.35 nm and 200 nm, but there are errors for reservoirs with larger pore sizes larger than 100 nm54.

Therefore, a high-pressure mercury injection method combined with nitrogen adsorption experiments was used in this study to quantitatively characterize the micro- and nanopores in Quaternary mudstone biogas reservoir samples at multiple scales. Due to the different methods used by high-pressure mercury injection and nitrogen adsorption methods to calculate the pore size distribution, there is a superposition phenomenon in the characterization results. Therefore, we using 100 nm as the dividing boundary, the high-pressure mercury injection method was used to obtain the parameters of the macropores, while the nitrogen adsorption experiment was carried out to obtain the parameters of the mesopores and micropores55,56. The statistical results of the samples show that the pore size distribution range is between 2 nm and 5 μm (Fig. 9), and the pore sizes are mainly in the pattern of double peaks, with the main peak in the range of 50–150 nm. In terms of pore types, the macropores contribute the most to the pore volume, with a proportion of up to 67.2%, followed by mesopores, with a proportion of 32.6%. The contribution of micropores to the pore volume is negligible.

Pore radius distribution of mudstone biogas reservoirs.

The NMR signals of porous media are characterized by solids and fluids, with solid signals mainly from hydrogen-bearing solids in the rock matrix and dry clay57,58, while fluid signals mainly from clay-bound water, capillary-bound water, and movable water59. According to the composition of fluid components in mudstone reservoirs (Fig. 10), there are two thresholds in the NMR T2 spectra, i.e., the movable fluid threshold T2C1 and the capillary-bound fluid threshold T2C260. When T2 > T2C1, the fluid is movable; when T2C2 < T2 < T2C1, the fluid is capillary-bound; and when T2 < T2C2, the fluid is clay-bound.

Model of fluid components in mudstone biogas reservoirs.

By comparing the NMR T2 spectra under the water-saturated state with those under the centrifugation state, the cutoff value T2C1 for distinguishing the bound water from the movable water in mudstones can be determined, and the movable water content can be obtained. By comparing the NMR T2 spectra at the temperature threshold obtained from NMR experiments at various drying temperatures with the NMR T2 spectra under the water-saturated state, the cutoff value of the clay-bound water, T2C2, can be determined. That is, capillary-bound water can be discharged at this temperature threshold, and the corresponding NMR T2 spectra at this threshold represent the NMR response characteristics of the clay-bound water. The analyses show that (Table 4) the distributions of the T2c1 values of the eight mudstone biogas reservoir samples are in the range of 2.6–4.7 ms, with an average value of 3.3 ms, and the distributions of the T2c2 values are in the range of 1.5–2.5 ms, with an average value of 1.76 ms. The average values of T2C1 and T2C2 are 2.82 ms and 1.58 ms, 3.4 ms and 1.7 ms, and 4.35 ms and 2.25 ms from mudstone, silty mudstone to argillaceous sand, respectively, which presents an increasing trend.

After determining the NMR T2 cutoff values, the pore components of the mudstone biogas reservoirs were further quantitatively classified (Table 4). The total NMR porosity includes all reservoir fluids, while the effective porosity includes capillary-bound fluid pores and movable fluid pores but not clay-bound fluid pores (Fig. 10). Biogas can be transported in effective pores while the fluid confined within clay-bound fluid pores cannot move. The quantitative calculations of the three pore components show that the NMR total porosity distribution of the mudstone reservoirs ranges from 25.07 to 29.17%. The effective porosity distribution ranges from 12.88 to 16.83%, with an average value of 14.55%, accounting for 53.34% of the total porosity, but the porosity of movable water only accounts for 9.23%.

Further analysis of the proportion of effective pores in samples of different lithologies shows that, the proportion of effective pores in the mudstone samples is 50.8%, which is less than that in the silty mudstone (59.57%) and argillaceous sand (56.52%) samples, and the proportion of moveable porosity in the mudstone samples is only 6.83%, which is far less than that in the silty mudstone (11.93%) and argillaceous sand (13.87%) samples. In other words, as the clay content increases, the proportion of ineffective pores in the sample gradually increases, and the physical properties and fluidity of the reservoir gradually deteriorate. This is mainly because the hydration effect of the high content of clay minerals causes the blockage of formation water, which causes the clay surface of the mudstone samples and the internal micropores in the clay to be occupied by bound water, resulting in a decrease in the number of effective pores with a larger amount of clay-bound water. In addition, in view of the limited number of silty mudstone samples, the phenomenon in which the proportion of ineffective pores is lower than that in muddy siltstone requires further experimental verification.

Based on the obtained double T2 spectra cutoff values (T2C1 and T2C2), the saturation of different types of fluids can be calculated. Assuming that S1 is the clay-bound water saturation, S2 is the capillary-bound water saturation, and S3 is the movable water saturation in the mudstone biogas reservoirs, the calculation formula can be expressed as:

where T2min and T2max are the minimum and maximum T2 spectral relaxation signals, respectively, ms; T2C1 is the T2 spectral signal corresponding to the cutoff values of bound water and movable water in mudstones, ms; and T2C2 is the T2 spectral signal corresponding to the cutoff values of clay-bound water and capillary-bound water, ms.

From the NMR T2 cutoff values and the saturation calculation results for the different fluids in the mudstone biogas reservoirs (Table 5), it is found that capillary-bound water and clay-bound water are the main fluids in the mudstone biogas reservoirs, and they together account for 90.65% of the total fluid saturation, whereas the movable water saturation accounts for only 9.35%. This indicates that in the mudstone reservoir, micro- and nanopores are the main reservoir spaces, capillary-bound water and clay-bound water are the dominant fluids in the pores, and the movable water content is low. The movable water saturation in the mudstone samples (S3 = 7.33%) is lower than that in the silty mudstone (S3 = 8.49%) and argillaceous sand (S3 = 14.84%) samples, indicating that argillaceous sand is more likely to constitute a good production layer.

The Quaternary mudstone biogas reservoirs in the Qaidam Basin have developed various reservoir spaces, such as intergranular pores, micropores, dissolved pores (dissolved fractures), and intercrystalline pores. The pore size distribution of the rock samples ranges between 2 nm and 5 μm, showing a pattern of double peaks, with the main peak pore size ranging between 50 and 150 nm. The pore size peak range of the mudstone samples is significantly smaller than that of the silty clay layer and the muddy silt layer.

The cutoff values of capillary bound fluid (T2C2) in the Quaternary mudstone biogas reservoir were determined to be 1.5–2.5 ms, with an average of 1.76 ms, and the cutoff values of movable fluid (T2C1) were 2.6–4.7 ms, with an average of 3.28 ms. The cutoff value shows an increasing trend from mudstone to mudstone siltstone.

Due to the high content of clay minerals blocking micro/nanopores and throats, the Quaternary strata with an average total porosity of 27.28% only have 9.28% movable pores, and the average core permeability is only 0.403 mD. Mudstone and sandstone are more likely to become good production layers.

The original data presented in the study are included in the article; further inquiries can be directed to the corresponding author.

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This study was funded jointly by the Natural Science Foundation of Sichuan Province, China P R (Grant Nos. 2023NSFSC0260), the Open foundation of the Sinopec Key Laboratory of Shale Oil/Gas Exploration and Production Technology (No. 33550000-22-ZC0613-0209), and the Scientific Research Project of Mianyang Normal University (Grant No. QD2019A03).

College of Resource & Environmental Engineering, Mianyang Normal University, Mianyang, 621000, Sichuan, China

Jia Jun

State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu, Sichuan, 610059, China

Wang Liang

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Jia Jun: Investigation, Writing original draft & Editing, and Review, Funding acquisition.Wang Liang: Conceptualization, Writing review & editing, Funding acquisition.

Correspondence to Jia Jun.

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Jun, J., Liang, W. Investigation of the pore structure characteristics and fluid components of Quaternary mudstone biogas reservoirs: a case study of the Qaidam Basin in China. Sci Rep 14, 26512 (2024). https://doi.org/10.1038/s41598-024-78010-4

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Received: 25 April 2024

Accepted: 28 October 2024

Published: 03 November 2024

DOI: https://doi.org/10.1038/s41598-024-78010-4

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