3.2.2. N2GA

N2GA analyses were executed using the QUADRASORB-SI Surface Area and Pore Size Analyzer (Quantachrome Instruments Corp., Boynton Beach, FL, USA). The core plug samples were first powdered to 60–80 mesh, with a particle size of 180–250 μm, and then 3–5 g powdered samples were selected to desiccate under a vacuum environment at 110 ◦C for approximately 12 h to remove the vapor and capillary water ahead of the N2GA experiments. The static adsorption capacity method was used to measure the amount of adsorbed nitrogen at 77.3 K. The experimental data were interpreted using multipoint BET (Brunauer–Emmett–Teller) and BJH (Barrett–Joyner–Halenda) to obtain specific surface area and pore size distribution, respectively, as comprehensively described by [39].

The BJH model depicts the capillary condensation phenomenon in a cylindrical pore based on the Kelvin equation [40], which shows the relationship between the relative pressure of nitrogen (*P*/*P*0) and Kelvin radius (*rK*), as exhibited in the following equation:

$$\ln\left(\frac{P}{P\_0}\right) = -\frac{2\sigma V\_L}{RTr\_K} \tag{3}$$

where *P*/*P*0 is the relative pressure of nitrogen; σ is the surface tension of liquid nitrogen at 8.85 mN/m [40]; *VL* is the liquid molar volume of nitrogen at 34.65 mL/mol [40]; *rK* is the Kelvin radius; *R* is the gas constant (8.314 J/mol/K); and *T* is the absolute temperature (77.3K).

When the specific parameter values used in the N2GA experiment are substituted into Equation (3), we can obtain the following simplified equation:

$$r\_K = -\frac{0.9543}{\ln(P/P\_0)}\tag{4}$$

The average thickness of the adsorbed layer was determined by the Harkins–Jura equation for nitrogen:

$$t = 0.1 \times \left[ \frac{13.99}{0.034 - \lg(P/P\_0)} \right]^{1/2} \tag{5}$$

where *t* represents the average thickness of adsorbed layer.

Finally, we can obtain the pore size according to the following equation:

$$D\_p = 2 \times (r\_K + t) \tag{6}$$

where *Dp* represents the pore size.

#### 3.2.3. XRD, SEM, Porosity, and Permeability

A Bruker D8 DISCOVER Advanced X-ray Diffractometer (Bruker Nano Inc., Madison, WI, USA) was used to examine the mineral components of tight rock samples, according to the Petroleum and Gas Industry Standard of China: SY/T 5163-2010. The determination of various mineral contents is based on their different X-ray diffraction peak intensity. SEM tests were measured using a TESCAN-MIRA-3XMU Scanning Electron Microscope (TESCAN, Brno, Czech Republic) to directly view the geometric shape of pores and clay minerals. Helium-derived porosity and nitrogen-derived permeability were measured on the core samples by the CoreLab CMS-300 automatic analyzer (Core Laboratories N.V., Houston, TX, USA) at a confining pressure of around 30 MPa, in order to simulate the actual formation conditions as much as possible, according to the SY/T 6385-2016 standard. The core samples were dried in a vacuum oven at 80 ◦C for approximately 8 h based on SY/T 5336-2006 in order to generate a minimum damage to tight rock samples and to ensure the accuracy of permeability measurement.
