Vitrinite reflectance is the most widely used parameter for reconstructing the thermal history of sedimentary basins and evaluating the maturation of source rocks. However, suppression of vitrinite reflectance has also been reported, which could affect the accuracy of evaluating the degree of thermal evolution. In this article, the influence of hydrocarbon generation on vitrinite reflectance during thermal evolution is studied based on thermal simulation experiments in a closed system. The results show that hydrocarbon absorption and overpressure can lead to the suppression of vitrinite reflectance. For R ₀ values between 0.6% and 2.1%, the suppression of vitrinite reflectance is primarily attributed to the impregnation of the telocollinite texture with hydrocarbons generated from type I kerogen. At R ₀ values exceeding 2.1%, overpressure becomes the dominant cause of the anomalous reflectance. Under closed-system conditions, the retention of volatile products within the pore network of vitrinite hinders the structural reorganization, leading to reflectance suppression.

Industrial quantities of tight gas from the Carboniferous Benxi Formation have been found in the southern Ordos Basin. The source and contributions from mudstone and coal in the Benxi Formation to the tight gas are still unclear, and the hydrocarbon generation potential and kinetics of the Benxi Formation mudstone have rarely been reported, which has halted resource evaluation of tight gas. Confined pyrolysis experiments were performed to determine the yields and kinetic parameters for gaseous hydrocarbon formation for a representative sample of a Benxi Formation mudstone from the Ordos Basin, with a hydrogen index (HI) of 137 mg/g TOC and T-max of 434 degrees C. The maximum yield of C-1-C-5 hydrocarbons is 143 mg/g TOC. For samples with similar HI, Upper Paleozoic mudstone samples might have a higher total gas generation potential than Upper Paleozoic coal samples because of the more reducing environment during mudstone deposition, which is beneficial for the preservation of sedimentary lipids. Even though the HI of Upper Paleozoic coals is apparently higher than that of Upper Paleozoic mudstones, they have a similar late gas generation potential. Basin modeling shows that the amount of natural gas generated from Benxi Formation mudstone increases southward. Coaly source rocks from the Ordos Basin with a lower oil generation potential have more negative delta C-13(1) values when the vitrinite reflectance is lower than 1.4-1.7%. This phenomenon might be related to the more negative delta C-13 of wet gas or the small hydrocarbon molecules incorporated into the kerogen compared with the components generating primary cracking gases. This study deepens the understanding of gaseous hydrocarbons generated from mudstone and coal in transitional depositional environments and provides the key parameters for tight gas resources in this area.

Prediction of the H₂S distribution is critical to lower the risks during petroleum exploration and development in the exploration area. Natural gas of two wells produced from the Ordovician Majiagou Formation Ma5-6 section from Wushenqi area show great difference in H₂S content. To investigate whether the different thermal maturity of the two wells contributes to the different H₂S content, the thermal mature history and transformation ratio of gas hydrocarbons and sulfate into H₂S and CO₂ through TSR (thermochemical sulfate reduction) were simulated by using Petromod 2016. Simulation result shows that difference of thermal maturity of the two wells is about 0.6%. Propane and butane have all been consumed to generate H₂S. The transformation ratio of ethane is higher for the well with higher thermal maturity. The high H₂S content in the well with high thermal maturity and no H₂S in the well with lower thermal maturity was not caused by the different thermal maturity. Some other geological factors should be considered.

Source rocks of Shanxi Formation widely distributed in the Ordos Basin were deposited in different sedimentary facies. Different sedimentary facies differ in sedimentary environment and could have great impact on the accumulation of organic matters. TOC content and S1+S2 values of shale and coal samples from different sedimentary sub-facies of Shan 1 and Shan 2 sections of Shanxi Formation were measured to investigate the influence of sedimentary facies on organic geochemistry characteristics. The results showed that the average TOC content of shale samples from the pro-delta sub-facies of Shan 1 section was higher than other sub-facies and the shale samples from the shore shallow lake sub-facies of Shan 2 section had higher TOC content when compared with other three sub-facies. Compared with Shan 1 section, the Shan 2 section was richer in organic matter. And the hydrocarbon generation potential of source rocks from Shan 2 section was better than that of Shan 1 section. More than 50% of shale samples from the Shan 2 section was of fair to good hydrocarbon generation potential. This study could provide a framework for the identification of potential areas for conventional and unconventional petroleum exploration.

Low-rank coalbed methane (CBM) resources are abundant in China, but their commercial development is limited by insufficient understanding of the coupled controls of coal material composition, pore-fracture structure, and desorption behavior in deeper low- to medium-rank bituminous coals. This study focused on the No. II coal seam in the Longjiapu Mining Area, southeastern uplift zone of the Songliao Basin. Fifteen samples were collected from three working faces. All 15 samples were subjected to material composition analysis (proximate analysis, vitrinite reflectance, and maceral composition). Eleven representative samples were selected for reservoir physical property characterization (thin-section microfracture observation, FE-SEM, mercury intrusion porosimetry, and low-temperature N₂ adsorption), and nine of them were further used for high-pressure CH₄ adsorption experiments. The coals are low- to medium-rank bituminous coals (R_o,max = 0.63–0.80%). The 620 working face has the most favorable pore-fracture structure, with the highest total pore volume and specific surface area, the lowest fractal dimensions, and the strongest microfracture development; the 615 working face is the poorest, and b4 is intermediate. The average Langmuir volume (V_L) of the three working faces ranges from 12.40 to 16.94 m³/t, with an overall average of 15.47 m³/t; b4 has the highest value (16.94 m³/t), followed by 620 (15.91 m³/t), and 615 has the lowest value (12.40 m³/t). Partial correlation and ash-free normalization indicate that V_L is positively correlated with volatile matter, and the negative correlation between ash and V_L mainly reflects mineral dilution, suggesting that V_L is controlled by organic matter abundance and micropore development. Among the three characteristic pressures, the 620 working face has the highest sensitive pressure (1.49 MPa); b4 has the highest starting pressure and transition pressure (8.69 and 4.12 MPa); and the 615 working face has the lowest values (6.37, 3.02, and 1.05 MPa). V_L positively controls all three characteristic pressures, while coal rank negatively controls the sensitive pressure. Integrating pore-fracture development characteristics with desorption characteristic pressure parameters, the 620 working face has the best desorption performance, followed by b4, while 615 is the least favorable. This study reveals the controlling mechanism of pore-fracture structure on low-rank coal desorption behavior and provides a quantitative basis for favorable target selection of low-rank CBM in northeastern China.