Pore Structure and Fractal Characteristics of Different Shale Lithofacies in the Dalong Formation in the Western Area of the Lower Yangtze Platform

Abstract

The purpose of this article was to quantitatively investigate the pore structure and fractal characteristics of different lithofacies in the upper Permian Dalong Formation marine shale. Shale samples in this study were collected from well GD1 in the Lower Yangtze region for mineral composition, X-ray diffraction (XRD), and nitrogen adsorption–desorption analysis, as well as broad-ion beam scanning electron microscopy (BIB-SEM) observation. Experimental results showed that the TOC (total organic carbon) content and vitrinite reflectance (Ro) of the investigated shale samples were in the ranges 1.18–6.45% and 1.15–1.29%, respectively, showing that the Dalong Formation shale was in the mature stage. XRD results showed that the Dalong Formation shale was dominated by quartz ranging from 38.4% to 54.3%, followed by clay minerals in the range 31.7–37.5%, along with carbonate minerals (calcite and dolomite), with an average value of 9.6%. Based on the mineral compositions of the studied samples, the Dalong Formation shale can be divided into two types of lithofacies, namely siliceous shale facies and clay–siliceous mixed shale facies. In siliceous shale facies, which were mainly composed of organic pores, the surface area (SA) and pore volume (PV) were in the range of 5.20–10.91 m2/g and 0.035–0.046 cm3/g, respectively. Meanwhile, the pore size distribution (PSD) and fractal dimensions were in the range 14.2–26.1 nm and 2.511–2.609, respectively. I/S (illite-smectite mixed clay) was positively correlated with SA, PV, and fractal dimensions, while illite had a negative relationship with SA, PV, and fractal dimensions. I/S had a strong catalytic effect on organic matter for hydrocarbon generation, which was beneficial to the development of organic micropores, so I/S was conducive to pore structure complexity and the increase in SA and PV, while illite easily filled organic pores, which was not beneficial to the improvement of pore space. In clay–siliceous mixed shale facies, which mainly develop inorganic pores such as intergranular pores, SA and PV were in the range of 6.71–11.38 m2/g and 0.030–0.041 cm3/g, respectively. Meanwhile, PSD and fractal dimensions were in the range of 14.3–18.9 nm and 2.563–2.619, respectively. Quartz and I/S showed weak positive correlations with SA, PV, and fractal dimensions. The various compact modes between quartz particles and the disorder of I/S were conducive to the complexity of pore structure and the improvement of SA and PV. The research findings can provide a reference for the optimization and evaluation of shale gas favorable area of the Lower Yangtze Platform.

Effects of Supercritical CO2 Treatment Temperatures on Mineral Composition, Pore Structure and Functional Groups of Shale: Implications for CO2 Sequestration

“…The XRD device was used with Cu Kαradiation at 40 mA and 40 kV, and carried out in a 2θ range of 3 • to 45 • with a step size of 0.02 • and a scanning speed of 2 • /min. Quantification phase analysis was conducted with the value K method, following the Chinese oil and gas industry standard SY/T 5163-2018 [38][39][40], in which the relative deviation of test results is less than 10%, when mineral content is more than 40%. When the mineral standard and corundum are formulated into a two-phase mixture in a ratio of 1:1 by mass, the reference strength of the mineral i can be calculated as Equation (1).…”

Quantitative characterization of pore network and influencing factors of methane adsorption capacity of transitional shale from the southern North China Basin

“…It is obvious that D 2 is generally less than D 1 , indicating that the surface roughness and micropore irregularity of the pore system of transitional shale are slightly stronger than the heterogeneity of the whole pore structure. Compared with the transitional facies shale in other basins, the fractal dimension of shale samples in FB area is significantly higher than that of Dalong Formation in Lower Yangtze Platform (D ranges from 2.511 to 2.619) (Xu et al 2020) and Shihezi Formation in Huainan coalfield (D 1 ranges from 2.4750 to 2.6104) (Bu et al 2015) and slightly higher than that of Niutitang and Longmaxi Formation in Lower Cambrian strata (Hu et al 2016;Xi et al 2018). Hence, the result may suggest that the surface roughness and pore structure heterogeneity of shale samples in this study is more complex compared to other marine-continental transitional shale.…”

“…Previous studies have proposed different methods and models to obtain fractal dimension, including fractal Frenkel-Halsey-Hill (FHH) model, Langmuir model, thermodynamic model, fractal BET model and MIP (Avnir and Jaroniec 1989;Jaroniec 1995;Rouquerol et al 1994). Among above methods and models, FHH model is considered as the most effective approach to obtain the fractal dimension and describe the irregular geometry of pore structure of shale (Shang et al 2020;Sun et al 2016;Xu et al 2020). The FHH fractal dimension can be directly obtained from N 2 adsorption and can be described as follows: where V and P 0 represent the equilibrium volume of adsorbed N 2 (cm 3 /g) and the saturation pressure of N 2 (MPa), respectively.…”

Catalyst and Elemental Analysis Involving Biodiesel from Various Feedstocks

“…The nitrogen isotherms recorded in Figure 4 for CaO/Al2O3 showed hysteresis over the relative pressure. The N2 isotherms exhibited a typical s-shaped behavior of type IV with a type H1 desorption hysteresis loop as categorised by IUPAC [56][57][58], indicating a potential presence of relatively wide cylindrical pores [59,60], which shows characteristics of The specific surface area and pore volume were determined by the nitrogen adsorptiondesorption technique summarised in Table 2, using Brunauer-Emmett-Teller (BET). The nitrogen isotherms recorded in Figure 4 for CaO/Al 2 O 3 showed hysteresis over the relative pressure.…”

“…The nitrogen isotherms recorded in Figure 4 for CaO/Al 2 O 3 showed hysteresis over the relative pressure. The N 2 isotherms exhibited a typical s-shaped behavior of type IV with a type H1 desorption hysteresis loop as categorised by IUPAC [56][57][58], indicating a potential presence of relatively wide cylindrical pores [59,60], which shows characteristics of a mesoporous material [61,62].…”