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.

Pyrite is the most common authigenic mineral preserved in many ancient sedimentary rocks. Pyrite also widely exists in the Longmaxi and Wufeng marine shales in the middle Yangtze area in South China. The Longmaxi and Wufeng shales were mainly discovered with 3 types of pyrites: pyrite framboids, euhedral pyrites and infilled framboids. Euhedral pyrites (Py4) and infilled framboids (Py5) belong to the diagenetic pyrites. Based on the formation mechanism of pyrites, the pyrites could be divided into syngenetic pyrites, early diagenetic pyrites, and late diagenetic pyrites. Under a scanning electron microscope (SEM), the syngenetic pyrites are mostly small framboids composed of small microcrystals, but the diagenetic pyrites are variable in shapes and the diagenetic framboids are variable in sizes with large microcrystals. Due to the deep burial stage, the pore space in the sediment was sharply reduced and the diameter of the late diagenetic framboids that formed in the pore space is similar to the diameter of the syngenetic framboids. However, the diameter of the syngenetic framboid microcrystals is suggested to range mainly from 0.3 µm to 0.4 µm, and that of the diagenetic framboid microcrystals is larger than 0.4 µm in the study area. According to the diameter of the pyrite framboids (D) and the diameter of the framboid microcrystals (d), the pyrite framboids could be divided into 3 sizes: syngenetic framboids (Py1, D < 5 µm, d ≤ 0.4 µm), early diagenetic framboids (Py2, D > 5 µm, d > 0.4 µm) and late diagenetic framboids (Py3, D < 5 µm, d > 0.4 µm). Additionally, the mean size and standard deviation/skewness values of the populations of pyrite framboids were used to distinguish the paleoredox conditions during the sedimentary stage. In the study area, most of the pyrite framboids are smaller than 5 µm, indicating the sedimentary water body was a euxinic environment. However, pyrite framboids larger than 5 µm in the shales indicated that the sedimentary water body transformed to an oxic-dysoxic environment with relatively low total organic carbon (TOC: 0.4–0.99%). Furthermore, the size of the framboid microcrystals could be used to estimate the gas content due to thermochemical sulfate reduction (TSR). The process of TSR occurs with oxidation of organic matter (OM) and depletes the H bond of the OM, which will influence the amount of alkane gas produced from the organic matter during the thermal evolution. Thus, syngenetic pyrites (d ranges from 0.35 µm to 0.37 µm) occupy the main proportion of pyrites in the Wufeng shales with high gas content (1.30–2.30 m³/t), but the Longmaxi shales (d ranges from 0.35 µm to 0.72 µm) with a relatively low gas content (0.07–0.93 m³/t) contain diagenetic pyrites. Because of TSR, the increasing size of the microcrystals may result in an increase in the value of δ¹³C₁ and a decrease in the value of δ¹³C₁-δ¹³C₂. Consequently, the size of pyrite framboids and microcrystals could be widely used for rapid evaluation of the paleoredox conditions and the gas content in shales.

The Datang Formation black rock series deposited in an inter-platform basin are considered to be one of the main source rocks in Southwest China. However, their depositional conditions and organic matter enrichment in such a setting are not well understood. In this study, geochemical characteristics of total organic carbon (TOC), major, trace and rare earth elements (REEs) of thirty black rock series samples from a newly-cut roadside outcrop (Zhenning section) in Southern Guizhou, Southwest China are systematically investigated to examine provenance, paleoredox conditions, paleoclimate, paleoproductivity and sedimentary rate, and further to explore their influence on organic matter enrichment. Results show that TOC contents of the investigated samples range from 0.90% to 2.83%, with an average of 1.45%, with TOC variations being in accord with the stratigraphic cyclicity. These data also indicate that the target black rock series were deposited in a combination of continental island arc and active continental margin tectonic setting that exhibited moderate weathering, oxic/dysoxic oxygen levels, hot and arid to semiarid climate, and a higher sedimentary rate. Multiple paleoredox indicators (e.g., Ni/Co, V/Cr, U/Th and V/(V + Ni) ratios), together with slightly negative Ce anomalies and EF_U-EF_Mo covariation, totally suggest an oxic/dysoxic water environment, which is usually considered unfavorable for organic matter preservation. Average (La/Yb)_N ratios (1.41) reflect a higher sedimentary rate, resulting in shortening the exposure time of organic matter in the degradation region of aerobic bacteria under oxic/dysoxic conditions. Of the three factors that could affect organic matter enrichment, including paleoproductivity, paleoredox conditions and sedimentary rate, the relatively high paleoproductivity indicated by the relatively high TOC contents appears to be the dominant factor. In contrast, the other two factors, i.e., paleoredox conditions and the rate of sedimentation play relatively less significant roles. Based on these factors, an enrichment model of organic matter for the black rock series in Southern Guizhou Depression is established. These findings also contribute to constraining the sedimentary environment in such a setting and further deepen our understanding of the potential shale gas resources of the Southern Guizhou Province.