In response to rising petroleum costs at the turn of the 21st century, the exploration and exploitation of shale gas and shale oil have become more and more important in China (Lu et al., 2018; Wang et al., 2019). Since 2010, shale gas mainly from marine in the south China has been investigated extensively including the basic geological characteristics (Zou et al., 2019), pore structure (Shang et al., 2020a; Shang et al., 2020b; Shang et al., 2020c; Liu et al., 2020), production controlling factors (Shang et al., 2019; Liu et al., 2021), etc., More recently, the shale oil from lacustrine basins in the north China has generally received significant attention (Lu et al., 2016; Zhao et al., 2018; Pu et al., 2019). However, the higher heterogeneity, lower thermal maturity, and limited geographical distributions of shale oil in China, pose greater challenges to a cost-effective development compared with marine shale oil reservoirs in North America (Ma et al., 2021).

Although the terms shale oil and tight oil are often used interchangeably in public discourse, shale formations are only a subset of all low permeability tight formations, which include sandstones and carbonates, as well as shales, as sources of tight oil production (Energy Information Administration, 2013). The restricted petrological definition of shale is fine-grained clastic sedimentary rock composed of mud and characterized by laminated structure, in field exploration, however, with the development and exploitation of shale oil and gas, it typically refers to any fine-grained sedimentary rock with total organic carbon (TOC) content greater than 1%, in a practical way.

The United States, home to the Bakken formation, Eagle Ford Formation and Barnett Formation, is the world’s hotspot of shale oil exploration (Montgomery et al., 2005; Jarvie et al., 2007; Bustin et al., 2008; Chalmers et al., 2012). As their success story unfolds, shale oil has become one of the most attractive unconventional sources all over the world. According to an EIA report released in 2013, the total reserves of technically recoverable shale oil in China is 320 × 10⁸ bbl (about 43.84 × 10⁸t), which is the third largest in the world. Nevertheless, the shale oil in China is way more difficult to extract. For comparison, the shale oil in United States is primarily light oil generated from widely-distributed and organic-rich marine shales with high maturity and low heterogeneity; while the majority of shale oil in China is relatively heavy oil generated from sparsely distributed and organic-rich lacustrine shale with low maturity and high heterogeneity (Lu et al., 2016). In terms of lithology, the mineral composition of marine shale in the United States is dominated by biogenic quartz and carbonate, and the former consists of biogenic silica which is associated with organic matter enrichment. When it comes to the lacustrine shales in China, their mineral composition is dominated by clay followed by detrital quartz and chemical carbonate. Detrital quartz is terrestrial, resulted from mechanical transportation in most cases, and usually shows no signs of organic origin (Nie et al., 2016).

Although China has tremendous shale oil reserves, extraction remains a challenge. The shale oil development approach is rather different from the American paradigm. In my opinion, the biggest difference is deposition system. With higher thermal maturity to ensure that the shale oil is abundant and light with low viscosity, and with lower heterogeneity to ensure that the pattern of storage space for shale oil is more easily to be predicted, the pivot point of marine shale oil exploration is pore structure. Compared with marine shales, the lacustrine shales in China are featured with rapidly changeable sedimentary environment, relatively low maturity and limited thickness (Liu et al., 2015; Ma et al., 2021). High heterogeneity of lacustrine shale results in different rock properties, and therefore, the heterogeneous pore structure and its controlling factors need to be studied and understood.

Since the fractal theory was put forward by Mendelbrot (1982), it has been proven to be very useful to quantitatively describe the heterogeneity of naturally occurring geometries. Katz and Thompson (1985) supported the theory, confirming fractal characteristics in several sandstones. They calculated the fractal dimension using SEM and optical data and gave correct porosity prediction based on fractal analysis. Furthermore, the experiment results from Thompson (1991) revealed the significance of characterizing a sedimentary rock’s pore geometry with fractal dimensions. Apart from the verification of fractal geometry of porous media, three fractal dimension measurement approaches were generalized: 1) the discrete methods (Mandelbrot, 1982; Orford and Whally, 1983; Kartz and Thompson, 1985; Krohn and Thompson, 1986) that use rulers of different lengths to measure the fractal dimension of an object, just like the fractal measurement of coastline at the very beginning. 2) the scattering methods (Freltoft et al., 1986; Rojanski et al., 1986; Sinha et al., 1988; Hurd et al., 1989) that use small-angle X-ray or neutron scattering to study a wide range (length scale of 0.5–50 nm) of disordered systems; 3) the adsorption methods (Avnir et, al., 1983; Pfeifer and Avnir, 1984) that characterize the fractal geometry at the molecular scale.

Fractal theory has been applied for pore size classification and reservoir evaluation by using data from SEM imaging, nitrogen adsorption, mercury intrusion, and nuclear magnetic resonance to quantify the fractal characteristics and heterogeneity of shale reservoirs (Lai & Wang, 2015; Yang et al., 2016; Sun et al., 2017; Ma et al., 2021). We based our study on the Frenkel-Halsey-Hill (FHH) theory and more recent and commonly used Brunauer-Emmett-Teller (BET) surface area approach to obtain fractal dimension.

Several methods have been proposed to calculate the fractal dimensions from nitrogen adsorption experimental results. Among them, one empirical correlation, developed on the basis of Frenkel-Halsey-Hill model, has been testified and widely used by many researchers. The equation is shown below: ln ( V V 0 ) = A [ l n ( l n ( P 0 P ) ) ] + C (1) Where V is the absorbed gas volume under equilibrium pressure P; V₀ is the absorbed volume of gas monomolecular layer; P₀ is the gas saturation pressure; A is the power-law exponent obtained as the trending line slope in the lnV vs ln (ln (P₀/P)) plot, having something to do with the fractal dimension D and adsorption mechanism.

Upon the onset of gas adsorption, the adsorption force is mainly Van de Waal’s force between solid and gas, and the relationship between fractal dimension D and slope A is: D = 3 ( 1 + A ) (2) In the equation, surface tension is not accounted for. But when the relative pressure increases, the surface tension effect between liquid and gas is no longer negligible, then the equation becomes: D = 3 + A (3)

According to Ismail and Pfeifer, (1994), when   3 ( 1 + A ) − 2 < 0 , surface tension should be considered, and the fractal dimension value D is calculated using Eq. 3, otherwise Eq. 2 is used to obtain D.

In order to characterize pore size distribution (PSD) and total porosity of the porous media, a number of methodologies have been developed by previous researchers. The fluid invasion methods, including gas adsorption and high-pressure mercury intrusion (MICP), are widely used. In addition, nuclear magnetic resonance (NMR), micro-focus X-ray computed tomography (X-ray CT), SEM and TEM (transmission electron microscopy) are popular tools for pore structure characterization (Lu et al., 2016). Last but not least, low temperature nitrogen adsorption is known as an effective method to characterize pore structure on nanometer to micrometer scale.

The pores are classified into three types according to the standard put forward by IUPAC (International Union of Pure and Applied Chemistry) in 1985, micropores (<2 nm), mesopores (2–50 nm) and macropores (>2 nm). It is claimed that different shapes of hysteresis loops at 77.35 K are often identified with different pore structures.

In the nitrogen adsorption experiment, the nitrogen molecules attached to the pore surface are forced out under gas pressure. This occurs to both monolayers and multilayers (Sing, 2001). The gas molecules form a monolayer at zero initial relative gas pressure (P/P₀ = 0) and start to fill the smallest pores. As pressure increases, they tend to fill increasingly larger pores until the entire multilayer is saturated towards the end of the adsorption process. Desorption is the reverse process of adsorption when pressure decreases, that is to say, desorption begins with the largest pores and ends with smallest ones. When the desorption process is complete, capillary condensation gives rise to the hysteresis loop (Figure 1).

The plot of adsorbed volume vs relative pressure that represents both the nitrogen adsorption and desorption processes was used to distinguish different pore types in accordance with Sing’s classification that derived from De Boer’s classification of hysteresis loops (De Boer and Lippens, 1964; Sing et al., 2001). Four theoretical patterns (H1 through H4) of hysteresis loops (curve divergence) that correspond to four typical pore shapes (cylindrical, ink-bottle, slit, and wedge) are shown in Figure 2.

Previous studies on the second member of Kongdian Formation focused mainly on geochemistry and hydrocarbon generation potential (Pu et al., 2016; Yang et al., 2018; Zhou et al., 2019; Xin et al., 2021), but not much was mentioned about pore structure especially on the nanoscale. Because of this, the characteristics of pore structure and heterogeneity of shales were poorly understood. In this paper, we used FE-SEM and low temperature nitrogen adsorption methods to characterize shale pore structure of the second member of Kongdian formation, and thereafter calculated fractal dimension based on the FHH theory. In the end, we discussed the fractal characteristics and their controlling factors of shale samples. Furthermore, we explored the relationships between fractal dimension, mineral constituents, and pore structure and the practical value of fractal dimensions.

Cangdong sag is an intracontinental sub-depression unit of the Huanghua Depression bordered by Cangxian Uplift in the west, Xuhei Salient in the east and Kongdian Salient in the north (Figure 3). The second member of Kongdian Formation was deposited when Cangdong Sag was an enclosed inland lake basin. It’s fine-grained sediments are composed of dark mudstone, thin-medium bedded siltstone and argillaceous dolomite (Pu et al., 2016; Zhao et al., 2019; Zhou et al., 2019). Hydrocarbon discoveries in recent years and several good oil wells producing from the second member of Kongdian Formation in Cangdong sag indicate great shale oil potential.

A lot of research work on pore structure and their controlling factors of terrestrial and marine shales in China has been done to date (Yang et al., 2016; Chen Q. et al., 2016; Peng et al., 2017). For the high maturity marine shales in South China, the mineral composition is dominated by quartz and clay minerals (Zou et al., 2019). Their pore types and abundances are primarily controlled by the types and contents of organic matter and clay minerals. The well-developed micropores in the organic matter and clay minerals, various pore types and a broad range of pore size distribution together lead to high shale heterogeneity, giving rise to high values of fractal dimensions D₁ and D₂ (Chen et al., 2016; Yang et al., 2016; Peng et al., 2017; Li et al., 2016).

The relationship between organic matter, mineral composition, pore structure parameters and fractal dimensions are discussed hereinafter to ascertain the controlling factors in pore structure development, paving the way for prospecting local shale plays with more confidence.

1) The low thermal maturity, low content of clay minerals and high content of brittle minerals together shape the unique pore structure characteristics of shales in the Cangdong Sag. The pore type is primarily interparticle pores caused by authigenic brittle minerals and microfractures. The organic matter pores (macropores, mesopores, and few micropores) only account for a small portion; the pore sizes are mainly distributed in the range of 2–50 nm with three peaks, 2, 5 and 25 nm. So it is fair to claim that there is a limited number of pore types.

In conclusion, the interparticle pores and intraparticle pores that are associated with a variety of brittle minerals lead to complex structures of mesopores and macropores, and the low abundance of micropores is attributed to low thermal maturity and low content of clay minerals. Because of the obvious difference in fractal dimension values between bigger pores and smaller pores, the most favorable sweet spot of shale oil reservoirs should be interparticle pores with higher fractal dimensions and larger than 20 nm in diameter and microfractures.