Due to the specificity of the geological background, terrestrial strata are widely distributed in the major hydrocarbon-bearing basins in China. In addition, terrestrial shales are generally featured with high thickness, multiple layers, high TOC content, ideal organic matter types, and moderate thermal evolution, laying a solid material foundation for hydrocarbon generation. However, the quantitative characterization study on their pore structure remains inadequate. In this study, core samples were selected from the Middle Jurassic Lianggaoshan Formation in the southeastern Sichuan Basin of the Upper Yangtze Region for analyses on its TOC content and mineral composition. Besides, experiments including oil washing, the adsorption/desorption of CO2 and nitrogen, and high-pressure mercury pressure experiments were carried out. The pore structure of different petrographic types of terrestrial shales can be accurately and quantitatively characterized with these works. The following conclusions were drawn: for organic-rich mixed shales and organic-rich clay shales, the TOC content is the highest; the pore volume, which is primarily provided by macropores and specific surface area, which is provided by mesopores, was the largest, thus providing more space for shale oil and gas reservation. The pores take on a shape either close to a parallel plate slit or close to or of an ink bottle. For organic-matter-bearing shales, both the pore volume and specific surface area are the second-largest and are provided by the same sized pores with organic-rich mixed shales. Its pores take on a shape approximating either a parallel plate slit or an ink bottle. Organic-matter-bearing mixed shales have the lowest pore volume and specific surface area; its pore volume is primarily provided by macropores, and the specific surface area by mesopores and the shape of the pores are close to an ink bottle.

1. Introduction

In recent years, thanks to the development of the unconventional oil and gas theory, significant breakthroughs have been made in exploring and exploiting marine shale gas in China [15]. Similarly, terrestrial strata are widely distributed in the hydrocarbon-bearing basins. Terrestrial shales are of good kerogen type and are characterized by wide distribution, high thickness, multiple layers, high TOC content, and moderate thermal evolution [612]. All the major oil companies of China have begun to pay great attention to the geological research of terrestrial shale oil and gas, with more investment being made during the 14th Five-Year Plan period [1315].

Shale pores provide the main preservation space and seepage channels for shale oil and gas [1618]. Therefore, it is critical to find the appropriate approach for conducting studies on shale pore characteristics. Previous researchers have carried out various experiments, including nuclear magnetic resonance, CO2 adsorption, N2 adsorption, high-pressure mercury compression, and spontaneous percolation, to quantitatively characterize the shale pore’s structural characteristics [1925]. Besides, these characteristics were also observed directly using scanning electron microscopy and nano-CT. Ji et al. studied the shales of the Lower Silurian Longmaxi Formation from the Chongqing region in the southeastern Sichuan Basin and discussed the micronanopore pore structure characteristics of marine shale reservoirs and their controlling factors with the help of field emission scanning electron microscope (FE-SEM) and by CO2 and N2 low-temperature and low-pressure adsorption experiments [26]. Li et al. qualitatively and quantitatively characterized the micro- and nanopore structures of the Es3l terrestrial shale reservoir in the Zhanhua Depression by FE-SEM as well as the experiments of CO2 and N2 adsorptions and high-pressure mercury compression. The results showed that the terrestrial shale from the lower submember of the 3rd member of the Eocene Shahejie Formation in the Zhanhua Depression has four types of pores: organic-matter pore, intergranular pore, intragranular pore, and microfracture. Micropores, mesopores, and macropores are all developed in the shale. Macropores provide much more pore volume than the other two and act as the main preservation space and seepage channels for shale oil. In contrast, micropores have an absolute advantage in terms of specific surface area and are the main sites for shale oil adsorption [27]. Using the methods including FE-SEM, CO2 adsorption, N2 adsorption and high-pressure mercury analysis, and Soxhlet extraction, Li et al. carried out in-depth analyses on the difference between the terrestrial and marine shale reservoirs in the pore structure with the terrestrial shale reservoirs from the lower submember of the 3rd member of the Eocene Shahejie Formation in the Zhanhua Depression and the marine shale reservoirs from the Longmaxi Formation in the southeastern Sichuan as typical examples [28]. Wang et al. studied the shale samples from the Longmaxi and the Niutitang Formations located at the perimeter of Chongqing, respectively, to find out the organic-matter porosity and evolution characteristics of the two sets of shales herein by organic carbon content tests, whole-rock XRD analysis, equivalent vitrinite reflectance tests, FIB-SEM (focused ion beam scanning electron microscopy), and FIB-HIM (focused ion beam helium ion microscopy). Their studies also consider the strata burial history and the hydrocarbon generation evolution history [29].

Terrestrial shales of low maturity usually contain shale oil, which occupies certain pore space, resulting in inaccurate characterization using the above methods and further affecting the research on shale reservation. In this paper, oil was washed away from the terrestrial shales of different petrographic types to clear the pore space occupied by shale oil, on which bases, the joint structure characterization experiments were carried out to obtain accurate characterization results. Therefore, this research is of great theoretical and practical significance for improving the theory of shale oil formation and guiding the selection of favorable areas of exploration. This paper studied the terrestrial shales from the Middle Jurassic Lianggaoshan Formation in the southeastern Sichuan Basin of the Yangtze Region in southern China. The pore structure characteristics of the terrestrial shales with different lithofacies types in TY1 Well, which is the key exploration well, were explored (Figure 1). Besides, the shale lithofacies were firstly classified according to TOC content and mineral compositions, and then, the shale samples were washed to remove shale oil from the pores. For the washed shale samples, carbon dioxide adsorption experiments, N2 adsorption experiments, and high-pressure mercury experiments were combined to accurately characterize the real micro- and nanopore structure of shale. Their pore shapes were characterized by nitrogen adsorption/desorption experiments, based on which, the pore structure of terrestrial shales of different lithofacies was accurately and quantitatively characterized [2731].

2. Geological Settings

2.1. Sedimentary and Stratum Characteristics

The underlying stratum of the Middle Jurassic Lianggaoshan Formation is the Da’anzhai Member of the Lower Jurassic Ziliujing Formation, and its overlying stratum is Member I of the Middle Jurassic Shaximiao Formation. The Lianggaoshan Formation can be divided into Members I, II, and III from the bottom to the top, and Liang Members I and II can be further divided as the upper and lower submembers [3237]. A complete lake transgression–regression cycle occurred to the Lianggaoshan Formation before, and, from the bottom to the top. It takes on a sedimentary evolution sequence of lakeside (lower submember of Liang Member I) ⟶ shallow lake and semideep lake facies (upper submember of Liang Member I—lower submember of Liang Member II) ⟶ lakeside (upper submember of Liang Member II) ⟶ delta front (Liang Member III) (Figure 2). In addition, the organic-rich dark shales are developed in the shallow lake-semi-deep lake facies of the upper submember of Liang Member I-upper submember of Liang Member II [3842].

2.2. Tectonic Characteristics

The geological structure of the southeastern Sichuan region is part of the high steep fault folds in the eastern Sichuan Basin, covering an area from east of the Huaying Mountains to west of the Qiyao Mountain [4345]. The geological structure of such a region was mainly formed during the Himalayas age with a strong tectonic fold. There is a series of high and steep anticlines stretching from north to east and nearly parallel in the structure plan, with narrow anticlines and wide and gentle synclines [4649]. The core of the high and steep anticlines has been exposed to the Upper Permian strata, and the Jurassic strata have been completely denudated. Moreover, it is completely preserved in the Lianggaoshan Formation of the syncline area at a depth of 500-2000 m, and the remaining sand shale stratum of the Shaximiao Formation of several thousand meters overlying plays the role of cap rocks, providing quality conditions for oil and gas preservations [50, 51].

3. Samples, Experiments, and Data Sources

In this study, samples were taken from shales of the Lianggaoshan Formation in TY1 Well at 14 depths and were numbered as shown in Table 1. The samples from the same depth were divided into five shares and were used for TOC content analysis experiments by a Sievers 860 TOC content analyzer and for whole-rock XRD mineral analysis by a YST-I mineral analyzer. The two kinds of analyses hereinabove were used together to delineate the shale lithofacies. For the shale samples from different depths, the shale cores were first processed by oil removal with a DY-6 instrument. After removing the oil, carbon and nitrogen dioxide adsorption experiments were carried out on the shale samples using a BSD-PM1/2 and a BSD-PS1/2/4 instrument, respectively, and a 3H-2000PS2 instrument was used for high-pressure mercury experiments. In addition, the FIB-SEM (focused ion beam-scanning electron microscopy) experiments were carried out on shale samples of different lithofacies using an instrument (Type Helios NanoLab 660) to identify the genesis types of shale pores.

4. Results and Discussion

4.1. Identification of Pore Genesis Types

The pore development characteristics in the different lithofacies of the shale can be directly observed from the FIB-SEM experimental images. As can be seen from Figure 3, the pore space of the terrestrial shales in the Lianggaoshan Formation mainly develops organic matter, clay minerals, and organic matter-clay mineral complexes.

4.2. Shale Lithofacies Classification

Previous studies proposed a shale lithofacies classification scheme based on the TOC content and mineral composition: (1) three types of shale based on TOC content: organic-lean shale (TOC content: 0%-1%), organic shale (TOC content: 1%-2%), and organic-rich shale (TOC content: ≥2%); (2) four types of shale based on mineral composition: siliceous shale (siliceous ), clay shale (clay ), calcareous shale (carbonate ), and mixed shale (siliceous minerals, clay minerals, and carbonate minerals lower than 50% each) [5255]. When the two groups of classifications are combined, there can be types of lithofacies. The shale lithofacies classified based on the experiment results of TOC content and mineral compositions of the 14 shale samples are shown in Table 2.

4.3. Pore Structure Characteristics of Different Lithofacies Types of Shales

The shale oil existing in the shales of the Lianggaoshan Formation occupies the reservoir space in the shales, causing inaccurate pore structure characterization. In this paper, the shale samples were firstly washed to remove the shale oil from the shale pores and then characterized by carbon dioxide adsorption, nitrogen adsorption, and high-pressure mercury experiments for the distribution of micropores (<2 nm), mesopores (2 ~ 50 nm), and macropores (>50 nm), respectively. In addition, the pore morphology was characterized by nitrogen adsorption/desorption experiments. There are inevitably overlapping areas in the first three characterization experiments; this paper, therefore, with reference to the previous studies, deals with the pore volume and pore-specific surface area data corresponding to the overlapping area using the weighted average method [5255]. Finally, the whole-aperture pore volume, specific surface area, and pore morphology characteristics of the 14 samples were obtained, as shown in Figures 46.

4.4. Pore Volume Characteristics

The occurrence space for free shale oil is characterized by the pore volume value. The data of pore volume characteristics hereinabove are summarized in Figure 7(a). The organic-rich mixed shale and organic-rich clay shale have the highest pore volume at around 0.03 ml/g, the fine sandstone has the second-highest at around 0.025 ml/g, the organic-matter-bearing clay shale has relatively lower pore volume at around 0.02 ml/g, and the organic-matter-bearing mixed shale has the lowest at around 0.013 ml/g. As shown in Figures 7(b)7(d), macropores are the main contributor to the pore volume of each lithofacies (50%-60%), followed by mesopores (about 40%).

4.5. Pore-Specific Surface Area Characteristics

The occurrence space of shale oil in the adsorbed state is characterized by the pore-specific surface area values. The data of pore-specific surface area characteristics hereinabove were summarized, as shown in Figure 8(a). It can be seen that the organic-rich clay shale has the highest pore-specific surface area of about 12 m2/g. The pore-specific surface areas of the organic-rich mixed shale, organic-matter-bearing clay shale, and fine sandstone are around 10 m2/g. Additionally, the pore-specific surface area of the organic-matter-bearing mixed shale is about 6.5 m2/g, which is the lowest. As shown in Figures 8(b)8(d), the specific surface area of each lithofacies is mainly provided by mesopores (60% to 90%) followed by micropores (5% to 40%).

4.6. Pore Morphology Characteristics of Shale Oil Reservoir

The actual shapes of shale pores present themselves after the shale oil was washed away [5661]. The hysteresis loop characteristics formed by the nitrogen adsorption-desorption isothermal curves in Figure 6 were compared with the hysteresis loop classification, and its corresponding pore morphology characteristics were classified by the International Union of Pure and Applied Chemistry (IUPAC) [6273]. Based on the difference in pore curvature, this study further classified parallel-plate-slit pores and ink-bottle pores by adding the shales of parallel-plate-slit-approximating type and ink-bottle-approximating type (Figure 9). After the extraction of shale oil, it was found that the pore morphologies of the organic-rich mixed shales were ink-bottle-approximating and ink-bottle types, while the organic-rich clay shales and the organic-matter-bearing clay shales were of ink-bottle-approximating and parallel-slab-slit-approximating types; the organic-matter-bearing mixed shales were of ink-bottle-approximating type. Additionally, the pore morphology of the fine sandstone was of ink-bottle type (Table 3).

5. Conclusions

This paper studied the shales of the Middle Jurassic Lianggaoshan Formation in the southeastern Sichuan Basin of the Upper Yangtze Region. The lithofacies of the core samples were explored based on the TOC content and mineral compositions analyses. After removing shale oil, the shale samples were then used for carbon dioxide adsorption as well as nitrogen adsorption/desorption and high-pressure mercury experiments to accurately and quantitatively characterize the pore structure of different lithofacies types of terrestrial shale. The conclusions are as follows: (1)For shales with different lithofacies types, the reservoir characteristics vary. Organic-rich mixed shales and organic-rich clay shales have the largest pore volume and specific surface areas. Macropores are the main contributors to the pore volume, while pore-specific surface area is provided mainly by mesopores. The pore morphologies are mainly of plate-plate-slit-approximating, ink-bottle-approximating, and ink-bottle types(2)The organic-matter-bearing clay shales have large pore volume and specific surface area. Their pore volume and pore-specific surface area are provided mainly by macropores and mesopores, respectively. The pore morphology are mainly of parallel-plate-slit-approximating and ink-bottle-approximating types(3)The organic-matter-bearing mixed shales have smaller pore volume and specific surface area. Similar to the organic-matter-bearing clay shales, their pore volume and pore-specific surface area are provided mainly by macropores and mesopores, respectively. The pore morphology is mainly ink-bottle-approximating type

Data Availability

Some of the data are contained in a published source cited in the references. All the data in this article are accessible to the readers.

Conflicts of Interest

There are no conflicts of interest concerning the results of this paper.


This study was supported by the National Natural Science Foundation of China (No. 42102192, No. 42130803, and No. 42072174), the open funds from the State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development (G5800-20-ZS-KFGY012), the Open Fund of Key Laboratory of Tectonics and Petroleum Resources (China University of Geosciences), the Ministry of Education, Wuhan (TPR-2020-07), the open funds from the State Key Laboratory of Petroleum Resources and Prospecting (PRP/open-2107), the Science and Technology Cooperation Project of the CNPC-SWPU Innovation Alliance, and the Science Foundation of China University of Petroleum, Beijing (No. 2462020XKBH009).