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The marine carbonates in the Ordovician Majiagou Formation in the Ordos Basin have significant exploration potential. Research has focused on their thermal history and hydrocarbon accumulation stages, as these are essential for guiding the exploration and development of hydrocarbons. In this paper, we study the thermal evolution history of the carbonate reservoirs of the Ordovician Majiagou Formation in the east-central Ordos Basin. Furthermore, petrographic and homogenization temperature studies of fluid inclusions were carried out to further reveal the hydrocarbon accumulation stages. The results demonstrate that the degree of thermal evolution of the Ordovician carbonate reservoirs is predominantly influenced by the deep thermal structure, exhibiting a trend of high to low values from south to north in the central region of the basin. The Fuxian area is located in the center of the Early Cretaceous thermal anomalies, with the maturity degree of the organic matter ranging from 1 to 3.2%, with a maximum value of 3.2%. The present geothermal gradient of the Ordovician Formation exhibits the characteristics of east–high and west–low, with an average of 28.6 °C/km. The average paleo-geotemperature gradient is 54.2 °C/km, the paleoheat flux is 55 mW/m2, and the maximum paleo-geotemperature reaches up to 270 °C. The thermal history recovery indicates that the Ordovician in the central part of the basin underwent three thermal evolution stages: (i) a slow warming stage before the Late Permian; (ii) a rapid warming stage from the end of the Late Permian to the end of the Early Cretaceous; (iii) a cooling stage after the Early Cretaceous, with the hydrocarbon production of hydrocarbon source rocks weakening. In the central part of the basin, the carbonate rock strata of the Majiagou Formation mainly developed asphalt inclusions, natural gas inclusions, and aqueous inclusions. The fluid inclusions can be classified into two stages. The early-stage fluid inclusions are mainly present in dissolution holes. The homogenization temperature is 110–130 °C; this coincides with the hydrocarbon charging period of 210–165 Ma, which corresponds to the end of the Triassic to the end of the Middle Jurassic. The late-stage fluid inclusions are in the dolomite vein or late calcite that filled the gypsum-model pores. The homogenization temperature is 160–170 °C; this coincides with the hydrocarbon charging period of 123–97 Ma, which corresponds to the late Early Cretaceous. Both hydrocarbon charging periods are in the rapid stratigraphic warming stage. [ABSTRACT FROM AUTHOR]

The Lower Cretaceous Bayingebi Formation at the Hari sag, Northern China is a significant bearing oil-gas tight reservoir in the Yin'E Basin. In order to understand petrophysical properties of the reservoirs, the diagenesis and hydrocarbon accumulation phases were investigated using casting thin sections, scanning electron microscopy (SEM), X-ray diffraction and fluid inclusion analysis. Additionally, the histories of burial evolution, organic matter thermal evolution and hydrocarbon charge are investigated. The results showed that the pore types include primary pore, intergranular pore and dissolution intragranular pore. The diagenesis of sandstone reservoirs in the process of burial and diagenesis mainly includes compaction, fracture, cementation, metasomatism and dissolution. The comprehensive analysis of various diagenetic stage indexes shows that Bayingebi Formation is in the middle diagenetic stage A-B. The homogenization temperature and salinity of fluid inclusions are low temperature and low salinity (120 °C ∼ 130 °C, 0% ∼ 9%), medium temperature and high salinity (140 °C ∼ 150 °C, 10% ∼ 17%) and high temperature and medium salinity (160 °C ∼ 170 °C, 9% ∼ 11%) respectively. According to the homogenization temperature of the inclusions and the history of burial evolution, the Bayingebi Formation has mainly three phases of hydrocarbon accumulation (100 ∼ 97Ma, 93 ∼ 87Ma, 84 ∼ 81Ma). [ABSTRACT FROM AUTHOR]

Gas reservoirs with high CO2 have been found in several wells in the Hailar Basin. In this paper, a composition analysis, stable carbon isotope analysis, and a rare gas helium isotope 3He/4He and argon isotope 40Ar/36Ar analysis were carried out. These comprehensive analyses show that the CO2 in the Hailar Basin is inorganic-origin gas, which generally has the characteristics of crust–mantle-mixed CO2, and the fraction of helium of mantle source can reach 15.12~18.76%. There are various types of CO2 gas reservoirs. CO2 gas mainly comes from deep crust. The distribution of gas reservoirs is mainly controlled by deep faults and volcanic rocks, as well as by reservoir properties and preservation conditions. Magmatic rocks provide gas source conditions for the formation of inorganic CO2 reservoirs. Deep–large faults provide the main migration channels for CO2 gas. The sandy conglomerate and bedrock weathering crust of the Nantun Formation and the Tongbomiao Formation provide favorable reservoir spaces for the formation of CO2 gas reservoirs. The combination of volcanic rock mass and deep–large faults creates a favorable area for CO2 gas accumulation. The age of magmatic intrusion and the homogenization temperature of oil–gas inclusions in Dawsonite-bearing sandstone indicate that 120 Ma in the Early Cretaceous was the initial gas generation period of the CO2 reservoir and that oil and gas were injected into the reservoir in large quantities in 122~88 Ma. This period is the peak period of magmatic activity in Northeast China, as well as when the crust of Northeast China greatly changed. A large-scale CO2 injection period occurred in 100~80 Ma, slightly later than the large-scale injection period of the oil and gas. Since the Cenozoic, the structure has been reversed, and the gas reservoir has been adjusted. [ABSTRACT FROM AUTHOR]

The Ordos Basin located in the Western North China Craton (WNCC) has been confirmed to have undergone large-scale lithospheric thinning at the end of the Early Cretaceous (130 ~ 90 Ma), but investigations involving its lithospheric thermal structure in the Mesozoic and Cenozoic are limited. In this study, we employed a 1D, steady-state, heat conduction equation to calculate mantle heat flow and its partitioning, the Moho temperature and the thermal lithospheric thickness of different tectonic units in the Ordos Basin based on an analysis of thermal history reconstruction and crustal layering models. The results suggest that the peak of mantle heat flow at the end of the Early Cretaceous varied between 46.65 mW/m2 and 62.97 mW/m2, far exceeding 50% of surface heat flow, which shows a typical 'hot mantle and cold crust' type of lithospheric thermal structure, and the lithospheric thinning reached a minimum thickness of 50 ~ 75 km. This was followed by a thermal decay phase in the Late Cretaceous, and a significant vertical thickening of lithosphere occurred during this period, during which the lithospheric thickness increased to 78 ~ 133 km by the end of the Late Cretaceous. Since the Cenozoic, the ratio of mantle to surface heat flow gradually declined to less than or close to 50%. At present, the average mantle heat flow in the Ordos Basin ranges from 22.10 mW/m2 to 38.76 mW/m2, the lithospheric thickness is between 81 ~ 158 km, and the Moho temperature entirely surpasses 600°C. In addition, we discuss potential factors and uncertainties in the thermal structure calculation. Combined with previous studies, we suggest that the lithosphere in the WNCC has been thinned locally due to asthenosphere upwelling. [ABSTRACT FROM AUTHOR]