Quantitative Characterization for the Micronanopore Structures of Terrestrial Shales with Different Lithofacies Types: A Case Study of the Jurassic Lianggaoshan Formation in the Southeastern Sichuan Basin of the Yangtze Region

State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, Beijing 100083, China Jiangxi Provincial Shale Gas Investment Company, Ltd., Nanchang 330000, China Jiangxi Provincial Natural Gas Group Company, Ltd., Nanchang 330000, China School of Geoscience and Technology, Southwest Petroleum University, Chengdu 610500, China State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, China Key Laboratory of Tectonics and Petroleum Resources (China University of Geosciences), Ministry of Education, Wuhan 430074, China Sinopec Petroleum Exploration and Production Research Institute, Beijing 100083, China PetroChina Changqing Oilfield Exploration and Development Research Institute, Xi’an 710018, China School of Environment and Resource, Southwest University of Science and Technology, Mianyang 621010, China State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China College of Geoscience, China University of Petroleum, Beijing, China


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 [1][2][3][4][5]. 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 [6][7][8][9][10][11][12]. 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 [13][14][15].
Shale pores provide the main preservation space and seepage channels for shale oil and gas [16][17][18]. 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, CO 2 adsorption, N 2 adsorption, highpressure mercury compression, and spontaneous percolation, to quantitatively characterize the shale pore's structural characteristics [19][20][21][22][23][24][25]. 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 CO 2 and N 2 low-temperature and low-pressure adsorption experiments [26]. Li et al. qualitatively and quantitatively characterized the micro-and nanopore structures of the Es 3 l terrestrial shale reservoir in the Zhanhua Depression by FE-SEM as well as the experiments of CO 2 and N 2 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, CO 2 adsorption, N 2 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 characteris-tics 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, N 2 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 [27][28][29][30][31] [32][33][34][35][36][37]. 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 [38][39][40][41][42]. 2 Geofluids  Figure 1 for the well locations. Modified from references [8].

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 [43][44][45]. 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 [46][47][48][49]. 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].

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.

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 matterclay mineral complexes.

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 [52][53][54][55]. Finally, the whole-aperture pore volume, specific surface area, and pore morphology characteristics of the 14 samples were obtained, as shown in Figures 4-6. 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 secondhighest 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 , 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 porespecific 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 m 2 /g. The pore-specific surface areas of the organic-rich mixed shale, organic-matter-bearing clay shale, and fine sandstone are around 10 m 2 /g. Additionally, the pore-specific surface area of the organic-matter-bearing mixed shale is about 6.5 m 2 /g, which is the lowest. As shown in Figures 8 , the specific surface area of each lithofacies is mainly provided by mesopores (60% to 90%) followed by micropores (5% to 40%).

Pore Morphology Characteristics of Shale Oil Reservoir.
The actual shapes of shale pores present themselves after the shale oil was washed away [56][57][58][59][60][61]. The hysteresis loop characteristics formed by the nitrogen adsorptiondesorption 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) [62][63][64][65][66][67][68][69][70][71][72][73]. 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-slitapproximating 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-slabslit-approximating types; the organic-matter-bearing mixed shales were of ink-bottle-approximating type. Additionally, the pore morphology of the fine sandstone was of inkbottle type (Table 3).

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 inkbottle-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.