**2. Geological Setting**

The southern North China basin (SNCB) is a large meso-cenozoic superimposed basin, located in the southern part of the North China plate, with an area of 150,000 km<sup>2</sup> (Figure 1) [34]. The basin mainly consists of five secondary tectonic units from north to south, and the present tectonic pattern shows the NW-WNW (Figure 1).

**Figure 1.** Tectonic geology condition of southern North China basin (SNCB) and investigated well location (adapted from [34]).

The continuous uplift of the entire North China platform during the Caledonian movement led to the absence of the Upper Ordovician, Silurian and Devonian, and Lower Carboniferous strata [35]. From the Late Carboniferous to the Late Permian, affected by the regional tectonic movement, the study area underwent the depositional process from the epicontinental sea to the continental basin (Figure 2) [36]. The Taiyuan Formation (P1t), which is composed of black shale, coal, limestone and sandstone, was deposited during the early Permian, with a thickness of about 30–175 m (Figure 2) [37]. Sedimentary facies of the Taiyuan Formation are mainly lagoon, tidal flat, barrier island, and carbonate platform (Figure 2). A previous study has shown that shale and coal from Taiyuan Formation possess high organic matter content (TOC), moderate maturity, mainly type III kerogen, and high gas generation potential, which can serve as good source rocks for tight sandstone reservoirs, as well as the potential reservoirs for shale gas and coal bed methane [38].

**Figure 2.** Map showing the stratigraphic column of Permian successions in the SNCB (modified from [38]) and more details of Taiyuan Formation (well wc1), including burial depth, lithology, sedimentary facies.

#### **3. Experiments and Methods**

#### *3.1. Samples and Experiments*

A total of 5 tight sandstone samples were selected from cores obtained in wells wc1 and wpd1 (Figure 1). Cylindrical plugs, 2.5 cm in diameter and 3–4 cm in length, were cut from the cores. The core plugs were carefully cleaned and dried, to remove contamination from remnant hydrocarbon and drilling fluids. The permeability of samples was first measured using a calibrated instrument DX-07G, with a steady flow of N2 based on the Chinese Core Measurement and Analysis Method Standard (GB/T29172-2012). Secondly, the samples were placed in the brine with a salinity (~1200 mg/L), similar to formation water under vacuum for at least 24 h, until the brine saturated state. NMR signals were generated using the brine saturated samples. The samples were put in a pulsed magnetic field, and then after a brief pulse, the NMR signal gradually decayed, with a characteristic relaxation time T2. During this process, the T2 spectra under saturated conditions were recorded with a low-field NMR instrument Rec core 2500. Magnetic field strength and resonance frequency were 1200 G and 2.38 MHz, respectively. Thereafter, the saturated samples were centrifuged via an HR2500-2 apparatus, to obtain an ideal irreducible water state at a rotational speed of 5000 r/min. The experimental parameters were set as follows: the echo number 2048, the echo space 300 μs, the waiting time 6 s, and the test temperature was 25 ◦C.

Subsequently, the samples were dried again, and divided into three sections, for analysis of pore structure and mineralogical compositions, respectively. Thin sections, impregnated with blue epoxy resin under vacuum, were prepared to observe the type, geometric shapes, and distribution of the pores. In the SEM experiment, the freshly broken samples, which were polished and coated with carbon in advance, were examined with a ZEISS SIGMA field emission scanning electron microscope equipped with energy-dispersive X-ray spectra (EDX), to identify clay minerals' type, the morphologies of clay occurrence within the pore spaces and micropores associated with clay minerals [39]. The mineral compositions of the samples were determined by XRD analysis, following the Oil and Gas Industry Standards (SY/T5463-2010). Samples were initially crushed to 100 mesh size, and then they were mixed with ethanol, ground into mortar, and placed on glass slides. The measurement was carried out using the Ultima IV X-ray diffractometer. The mineral content was quantified using Jade software.

#### *3.2. Theory and Methods*
