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 Lufeng Sag and Huizhou Sag, both located in the Zhu-1 Depression, have similar geographical locations, but their reservoir characteristics in the Paleogene Wenchang Formation show obvious differences. Primary intergranular pores are mainly developed in the Lufeng Sag. However, secondary pores are the main reservoir space in the Huizhou depression. Overall, the reservoir properties of the Lufeng Sag are better than those of the Huizhou Sag. To analyse the differences between the Paleogene reservoirs in these two areas, this study mainly uses assay data, such as rock thin sections, scanning electron microscope images, drilling, and logging, to analyse the differential development mechanisms of high-quality reservoirs, and two types of reservoir development models were concluded. The results show that the anti-compaction primary porosity preservation mode is mainly developed in the Lufeng Sag. High compositional maturity quartz sandstone is the congenital condition of primary porosity development. The top and bottom calcareous cementation formed of the large set of thick sand bodies increases the rock’s anti-compaction ability. The early shallow burial slows down the compaction action of overlying strata. Under the low geothermal temperature, it can delay the time for deep reservoirs to enter the middle diagenetic stage. The reservoirs in the Huizhou Sag are developed with the secondary dissolution pore development model. The Wenchang Formation reservoir in the Huizhou Sag has a large area of contact with source rocks, and organic acids can migrate to sandstone reservoirs for dissolution. Additionally, the secondary dissolution pores are more developed because the Wenchang Formation reservoirs in the Huizhou Sag contain more easily dissolved substances.

Diversity lithofacies in lacustrine shale possess different pore structures with different shale gas adsorption mechanisms. It is of great significance for lacustrine shale gas reserve prediction to clarify the adsorption mechanism of methane in lacustrine shales. A series of experiments was carried out on core samples from the Upper Triassic Xujiahe Formation in the western Sichuan Basin of China. CO 2 adsorption, N 2 adsorption, and scanning electron microscopy experiments were performed to analyze the pore structures of lacustrine shale. The pore structure of siliceous shale is more complex with higher values of pore fractal dimensions (D 1 and D 2 ) than other lithofacies, while, based on fitting curves to methane isothermal adsorption data, the method of the corrected Akaike's information criterion (AICc) measuring the goodness of the nonlinear fitting curves, pore structure, and analysis of methane adsorption layers are combined to evaluate and select methane adsorption model in this work. Additionally, the methane adsorption process could be described by a two combined first-order rate (TCFOR) model of adsorption rate. As shown by TCFOR in most samples, the normalized adsorption capacities of the fast process higher than that of the slow process (Q 1 > Q 2 ) appear during adsorption experiment. Thus, methane may not enter the micropores in large quantities under low experimental pressure (<10 MPa). The adsorption mechanism of methane in most samples is monolayer adsorption (Langmuir + Henry model) with the lower AICc value than other adsorption models. The DA + Henry model is suitable for the process of methane absorption in which the Q 2 > Q 1 appeared early in the methane adsorption experiment. Occasionally, the phenomenon of Q 2 > Q 1 could be found in the siliceous shale owing to the micropores dominating in it. Ultimately, the relationship between accumulated dV/dD (reflecting the density of different pore sizes) from N 2 adsorption experiment and the absolute adsorption amount (V abs ) calculated by models is figured out to determine the adsorption model of methane under different pore sizes. Methane adsorption in pores smaller than 3.4 nm is mainly a micropore filling mode, and filling of pores between 9.6 and 50 nm is mainly a monolayer mode. Nevertheless, the adsorption of methane in the pores between 3.4 and 9.6 nm is a transition from monolayer adsorption to micropore filling. The results enhance our understanding of the methane adsorption mechanisms in different lithofacies with different pore structures.