In the Benxi Formation of the Carboniferous system of the Upper Paleozoic in the Ordos Basin, there are many sets of coal measures dark organic-rich shale, being marine continental transitional facies, with significant unconventional natural gas potential. Previous studies are only limited to the evaluation of tight sandstone reservoir in this set of strata, with no sufficient study on gas bearing and geological characteristics of organic-rich shale, restricting the exploration and evaluation of shale gas resources. In this study, analysis has been conducted on the organic carbon content, the major elements, the trace elements, and the mineral composition of core samples from the Benxi Formation in key drilling sections. In addition, qualitative and quantitative pore observation and characterization of core samples have been conducted. The sedimentary environments and reservoir characteristics of the shale of the Benxi Formation have been analyzed. Combined with the gas content analyzing the results of the field coring samples, the shale gas resource potentials of the Benxi Formation have been studied, and the geological characteristics of the Benxi Formation shale gas in the eastern Ordos Basin have been made clear, to provide a theoretical basis for shale gas resource evaluation of the Benxi Formation in the Ordos Basin. The results show that (1) in the Hutian Member, Pangou Member, and Jinci Member of the Benxi Formation, organic-rich shale is well developed, with the characteristics of seawater input as a whole. There is a slight difference in sedimentary redox index, which shows that the reducibility increases gradually from bottom to top. (2) There is an evident difference in the mineral characteristics of shale in these three members. The Hutian Member is rich in clay minerals, while the Jinci Member is high in quartz minerals. (3) The pores are mainly inorganic mineral intergranular pores, clay interlayer fractures, and micro fractures, and organic matter pores are developed on the surface of local organic matter. (4) The mud shale in the Jinci Member has a large cumulative thickness, has relatively high gas-bearing property, and is rich in brittle minerals. The Jinci Member is a favorable section for shale gas exploration of the Benxi Formation.

Accurately predicting permeability is important to elucidate the fluid mobility and development potential of tight reservoirs. However, for tight sandstones with the same porosity, permeability can change by nearly three orders of magnitude, which greatly increases the difficulty of permeability prediction. In this paper, we performed casting thin section, scanning electron microscopy and high-pressure mercury injection experiments to analyze the influence of pore structure parameters and fractal dimensions on the permeability of Chang 7 tight sandstones. Furthermore, the key parameters affecting the permeability were optimized, and a new permeability prediction model was established. The results show that the pore throat structure of Chang 7 tight sandstone exhibits three-stage fractal characteristics. Thus, the pore throat structure was divided into large pore throat, medium pore throat and small pore throat. The large pore throat reflects the microfracture system, whose fractal dimension was distributed above 2.99, indicating that the heterogeneity of the large pore throat was the strongest. The medium pore throat is dominated by the conventional pore throat system, and its fractal dimension ranged from 2.378 to 2.997. Small pore throats are mainly composed of the tree-shaped pore throat system, and its fractal dimension varied from 2.652 to 2.870. The medium pore throat volume and its fractal dimension were key factors affecting the permeability of Chang 7 tight sandstones. A new permeability prediction model was established based on the medium pore throat volume and its fractal dimension. Compared to other models, the prediction results of the new model are the best according to the analysis of root mean square value, average absolute percentage error and correlation coefficient. These results indicate that the permeability of tight sandstones can be accurately predicted using mesopore throat volume and fractal dimension.

Establishing a suitable classification and evaluation scheme is crucial for sweet spot prediction and efficient development of shale oil in the Chang 7_1-2 sub-member of the Longdong area. In this paper, a series of experiments, such as casting thin sections (CTS), scanning electron microscopy (SEM), low-temperature nitrogen adsorption (LTNA), high-pressure mercury intrusion porosimetry (HMIP), and nuclear magnetic resonance (NMR), were integrated to classify the pore throats and shale oil reservoirs in the study area. Moreover, the pore structure characteristics of different types of reservoirs and their contributions to productivity were revealed. The results show that the pore-throat system can be divided into four parts: large pore throats (>0.2 μm), medium pore throats (0.08~0.2 μm), small pore throats (0.03~0.08 μm), and micropore throats ( 0.1 mD), type II (Φ ≥ 8%, 0.05 mD < K < 0.1 mD), type III (Φ ≥ 5%, 0.02 mD < K < 0.05 mD) and type IV (Φ < 5% or K < 0.02 mD). From type I to IV reservoirs, the proportion of dissolved pores and intergranular pores gradually decreases, and the proportion of intercrystalline pores increases. The proportion of large pore throats gradually decreases, and the proportions of medium pore throats and small pore throats increase initially and then decrease, while the proportion of micropore throats increases successively. The NMR pore size distribution changes from the right peak to the left peak. The developed section of the type I reservoir corresponds to the oil layer, and the developed section of the type I and II reservoirs corresponds to the poor oil layer. In contrast, the developed section of the type III and IV reservoirs corresponds to the dry layer. The daily production from single wells is primarily attributable to type I and II reservoirs.

Aromatic maturity parameters were evaluated via closed-system pyrolysis experiments using a Mesozoic lacustrine source rock from the Yingen-Ejinaqi Basin, thereby ensuring a uniform source. Pulverized rock aliquots (200 mg) were reacted with water at temperatures ranging from 250 to 550 °C at 5 °C/min, and the aromatic fractions of expelled oil and extracts of the solid residue were analyzed by GC-MS. The experiments showed that the relative abundance of aromatic hydrocarbons in the oil and extractable organic matter (EOM) of source rock had different evolutionary characteristics. With the increase in the thermal evolution degree, the relative abundance of aromatic hydrocarbons in the EOM showed the characteristics of ″increased early (R_o < 0.80), unchanged middle (R_o = 0.80-2.00%), decreased lately (R_o > 2.00%)″. While the relative abundance of aromatic hydrocarbons in the expelled oils continuously increased, as the R_o values increased from 0.62 to 2.39%, the relative abundance of aromatic hydrocarbons gradually increased from 8 to 46%. With increased maturity, the relative abundance of 1-3-ring aromatic hydrocarbons continuously decreased, as observed in the phenanthrene homologs. Meanwhile, the relative abundance of 4+-ring aromatic hydrocarbons continuously increased, as seen in chrysene homologs. It was suggested that the effects of maturity on the composition of aromatic hydrocarbons might not be sufficiently obvious. The effective application range of the alkylnaphthalene-related maturity parameters (2-/1-methylnaphthalenes, (2,6- + 2,7-)/1,5-dimethylnaphthalenes, 2,3,6-/(1,4,6- + 1,3,5-) trimethylnaphthalenes, and (2,3,6- + 1,3,7-)/(1,4,6- + 1,3,5- + 1,3,6-) trimethylnaphthalenes) and the alkyldibenzothiophene maturity parameters (4-/1-methyldibenzothiophenes, 4,6-/(1,4- + 1,6-) dimethyldibenzothiophenes, and (2,6- + 3,6-)/(1,4- + 1,6-) dimethyldibenzothiophenes) was 0.84-2.06% R_o. The alkylphenanthrene-related maturity parameters had a wide application range for lacustrine source rocks with an R_o < 2.06%. These parameters included 1.5 × (2- + 3-)/(phenanthrene +1- + 9-) methylphenanthrenes, 3 × 2-/(phenanthrene + 1- + 9-) methylphenanthrenes, (2- + 3-)/(1- + 9-) methylphenanthrenes, 2-/1-methylphenanthrenes, (3- + 2-)/(1- + 2- + 3- + 9-) methylphenanthrenes, 2-/(1- + 2- + 3- + 9-) methylphenanthrenes, and 2,7-/1,8-dimethylphenanthrenes. In addition, the effective applicable range of the methylnaphthalene-related maturity parameter 3-/1-methylchrysenes was an R_o value less than 1.79%. The results clarified the validity scope of some aromatics' maturity parameters and provided a theoretical basis for the scientific application of these parameters.

In the Gaoyou Sag located within the Subei Basin, the hybrid shales from the second member of the Funing Formation (E(1)f(2)) have been identified as a prolific source of shale oil production, despite their characteristically low organic matter content (TOC < 1.5%). This observation suggests that specific macerals within these hybrid shales demonstrate a pronounced hydrocarbon generation potential, thus unveiling a new frontier for shale oil exploration endeavors. In this study, 16 samples were rigorously extracted from both mudstone and hybrid shale strata within the E(1)f(2). An exhaustive suite of organic and inorganic geochemical analyses was conducted on these specimens. The analyses elucidated several key findings: (1) The maceral composition within the hybrid shales is predominantly comprised of alginite, solid bitumen, and inertinite. Remarkably, the variability of alginite content within the hybrid shales is more pronounced than that observed in high-TOC mudstones. High-TOC mudstones are characterized by a preponderance of lamalginite and a paucity of telalginite, leading to a diminished aggregate hydrocarbon potential. (2) Biomarker ratios (e.g., sterane/hopane, C-23 tricyclic terpane/alpha beta C-30 hopane, C-24 tetracyclic terpane/C(26 )tricyclic terpane, etc.) suggest a primary derivation from lower aquatic organisms, with a secondary contribution from terrigenous organic matter within the hybrid shales. (3) The accretion of macerals is governed by an intricate set of factors, including the input of terrigenous detritus, paleosalinity, and paleoproductivity. (4) Alginite is identified as the principal constituent responsible for hydrocarbon genesis in hybrid shales. The proliferation of alginite facilitates the concurrent enrichment of shale oil and organic matter within the hybrid shales of the Subei Basin, illustrating a cooperative mechanism underlying the accumulation of shale oil and organic matter. This indicates that even hybrid shales with scant organic content exhibit considerable potential for shale oil exploration.

The pore structure and its complexity play a critical role in fluid migration and recovery efficiency. Multiple pore types, broad pore size distribution (PSD), and extremely irregular pore morphology hinder the comprehensive investigation of pore systems and their complexity in low-permeability conglomerates. In this study, the multi-scale pore system and fractal characteristics of the Permian Lower Wuerhe Formation and Triassic Baikouquan Formation conglomerates from the Junggar Basin were investigated, combining physical property analysis, casting thin sections, scanning electron microscopy, and Nuclear magnetic resonance (NMR). The results show that the pore system of conglomerates consists of residual intergranular pores (RIPs), dissolution pores (DPs), clay-related pores (CRPs), and microfractures. Three types of PSD were identified according to the shape of the T₂ spectrum. Based on the fractal characteristics derived from NMR data, pore systems in conglomerates were divided into macropores (mainly RIPs and DPs), mesopores (mainly CRPs), and micropores (reflect adsorption spaces). The fractal dimension of macropores (D3) increases with the increase of clay mineral content and the decrease of contents of quartz and feldspar. Moreover, the volume of macropores decreases with the increase of clay mineral content and the decrease of contents of quartz and feldspar. In addition, the fractal dimensions and volumes of mesopores and micropores have no obvious relationship with mineral composition. D3 and macropore volume control the physical properties and fluid mobility of conglomerates. T_2,gm shows a strong negative correlation with D3 and macropore volume. Meanwhile, the high value of D3 would reduce the volume of macropores. These results demonstrate that D3 is a good indicator to reveal the quality of pore structure in low-permeability conglomerates.