Transitional shale gas reservoirs have become a new field of shale gas exploration in China. However, the formation and evolution of shale pores, particularly the organic matter (OM) nanopores, in this type of shale are still unclear. Simulation experiments with a typical transitional shale were conducted, and field emission scanning electron microscopy (FE-SEM) and low pressure CO₂ and N₂ adsorption (CO₂GA/N₂GA) were used to characterize the shale pore structure. Results show that the development of OM pores is controlled by the hydrocarbon generation process, and meanwhile, the evolution model in the transitional shale is established. At the oil generation stage (0.5% < Ro < 1.1%), volumes of micropores and mesopores decrease rapidly, with a minimum value of micropore volume at Ro = 1.02%, due to the infilling of extractable OMs. At the post-oil generation stage (1.1% < Ro < 1.5%), micropore volume increases due to the cracking of extractable OM, and mesopore volume decreases owing to partial extractable OM migrating into mesopores and the effect of compaction. At the wet gas generation stage (1.5% < Ro < 2%), volumes of shale pores slightly increase due to the release of the occupied pores. At the dry gas generation stage (Ro > 2%), volumes of micropores, mesopores, and macropores increase rapidly when Ro is below 2.99%, due to the porous solid bitumen formation and new pores generated in kerogen. When Ro is above 3.31%, micropore and macropore volumes decrease due to the graphitization of OM and strong compaction. The continuous increase in mesopores is probably due to the combination of micropores. This study can provide a scientific guide for transitional shale gas exploration and resource evaluation.

Hydrate-bearing sediments provide excellent materials for studying the primary sources and diagenetic alterations of organic matter. In this study, the elemental and isotopic signatures of total organic carbon (TOC), total inorganic carbon (TIC), total nitrogen (TN), and total sulfur (TS) are systematically investigated in three hydrate-bearing sediment cores (∼240 m) retrieved from the Shenhu area, South China Sea. All sediment layers from three sites are with low TOC content (average 0.35%) with marine and terrestrial mixed sources (-23.6‰ < δ¹³C_org < -21.4‰). However, the generally low δ¹⁵N (2.49–5.31‰) and C/N ratios (4.35–8.2) and their variation with depth cannot be explained by the terrestrial sources (Pearl River) and marine sources, binary end-member mixing processes. Contribution from lateral allochthonous organic matter from the mountainous river is considered after excluding other possible factors and ingeniously elucidating the organic matter origins. Furthermore, specific layers in W01B and W02B exhibit elevated S/C ratios (up to 2.39), positive bias of δ³⁴S-TS (up to +29.7‰), and negative excursion of δ¹³C-TIC (up to -8.29‰), which are the characteristics of sustained occurrence of sulfate-driven anaerobic oxidation of methane. The occurrence of coupled carbon–sulfur anomaly may be accompanied by deep hydrocarbon leakage and the formation of hydrate with high saturation.

Shale pore structure and gas resource evaluation are greatly influenced by the development and evolution of organic matter (OM) pores, which are controlled by the process of hydrocarbon generation. In this paper, hydrocarbon gas generation and OM pore evolution were investigated on a low mature Dalong Formation shale (Ro = 0.78%, TOC = 10.40%) by the high-temperature high-pressure simulation experiments. Results indicated that hydrocarbon gas production rate show a slight increase when Ro < 2.0% and then a rapid increase when Ro > 2.0%. BET surface area and pore volume of the simulated samples increase significantly with the increases of conversion of TOC, thermal maturation level and hydrocarbon production rate, suggesting that the hydrocarbon gas generation process controls the development of OM pores. With increased Ro value, OM pores are primarily developed in amorphous kerogen and solid bitumen at wet gas generation stage and dry gas generation stage, respectively. The BET surface area and pore volume normalized to TOC can be comparable to that of naturally evolved pure kerogen of Niutitang Formations at high maturity stage. After Soxhlet extraction, BET surface area and pore volume generally have significant increase, particularly at main oil generation (Ro = 1.32%) and wet gas generation stages (Ro = 1.83%), due to that the nanometer-sized OM pores are exposed after extraction. Accordingly, an integrated model of OM pore evolution in organic-rich shale was established, and OM pore evolution can be divided into three stages: initial development stage, rapid development stage and slightly destruction stage. The three stages correspond respectively to the three stages of hydrocarbon generation: liquid hydrocarbon to wet gas generation stage, dry gas generation stage, and gas depletion stage. This study further indicates that shale with extremely high maturation degree still has better OM pore development and shale gas potential.

Shale gas was recently found in the Lower Cambrian Niutitang Formation (LCNF) of the Micangshan tectonic zone of south Shaanxi (MTZSS), but not in commercial quantities. To determine the laws governing the generation, enrichment, and desorption of shale gases in overmatured shale strata in the LCNF of MTZSS, we carried out in situ desorption experiments on nine shale core samples and got 168 desorbed gas samples at different phases of desorption. Also measured were the chemical and carbon isotopic compositions of these desorbed gas samples and the geochemical parameters of the shale core samples. CH4 was the predominant hydrocarbon shale gas identified in the 82.06-98.48% range, suggesting that the gases were mainly dry. The nonhydrocarbon gases found were CO2 and H-2. The CH4 content of the desorbed gas samples dropped continuously during desorption, lowering the dryness index to 98.48 and 92.26% of the first and last desorbed shale gas, respectively. The change in the gas ratio during shale gas desorption proved that the adsorbability of the LCNF to the various gases follows the trend H-2 > CO2 > C2H6 > CH4 > He. Further, delta(C2H6)-C-13 and delta(CH4)-C-13 become heavier during desorption, showing isotopic fractionation arising from the desorption-diffusion coeffect. As the desorption temperature increases, the value of delta(CH4)-C-13 increases because (CH4)-C-12 is more sensitive to temperature than (CH4)-C-13, so it is with the ethane. Similar to the LCNF shale gas in other areas of China, the desorbed shale gases are characteristic of carbon isotope reversal (CIR) (delta(CH4)-C-13 > delta(C2H6)-C-13). The cracking of the residual soluble organic matter at the high overmaturity stage mixed with the cracking of kerogen at the early stage of maturation, causing CIR. Furthermore, the desorbed gas content was proportionally and inversely related to the CIR degree and final dryness index of the desorbed gas, respectively. Moreover, the carbon isotope fractionation degree of CH4 and delta C-13(1) of the last desorbed gas correlated positively with the desorbed gas content and the desorbed time of the gas. In conclusion, the four parameters are effective parameters for identifying shale gas sweet spots.

Organophosphorus esters (OPEs), widely utilized as flame retardants and plasticizers, are physically incorporated into those products and exhibit semi-volatility, resulting in release throughout their lifecycle. The ocean serves as a significant sink and plays a pivotal role in the global distribution and environmental fate of OPEs. However, the OPEs’ behavior and ecological effects in marine systems are not well understood. This review systematically examines recent advances in the sources, transport pathways, transformation mechanisms, and distributions of OPEs in the marine environment, and it also addresses current research limitations and suggests directions for future work. It is found that OPEs predominantly enter the marine environment through terrestrial input and in situ release; the transportation means include river input, long-range atmospheric transport, air–sea exchange, and oceanic circulation; and the degradation processes of OPEs are recognized as hydrolysis, photodegradation, and biodegradation. The distributions of OPEs in marine environments vary in different media, with their concentrations observed to range from pg m⁻³ to ng m⁻³ in marine air, ng L⁻¹ to hundreds of ng L⁻¹ in seawater, and pg g⁻¹ dw to ng g⁻¹ dw in sediments. The distributions of different species of OPEs are affected by many factors, such as compound properties, environmental conditions, and policy regulations. Comparisons between different regions and different seasons need to be further studied, and predictive models should be developed to better assess ecological risks and exposure pathways of OPEs.