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Geological structure has become a hot research topic in gas-hydrate exploration. Until now, gas hydrate has not been found in the Mohe permafrost. Lack of information about geological structures that are closely related to the potential gas-hydrate accumulation in the permafrost, seems to be one of the limitations to the gas-hydrate exploration. In this paper, imaging logging and isotope dating are adopted to indicate that geological structures, associated with the potential gas-hydrate accumulations, including faults, fractures, and anticlines, are highly developed in the permafrost. The north-dipping normal faults and the unfilled fractures below 700 m, are considered as structure passages for the migration of hydrocarbon gas in the permafrost, while reverse faults, unfilled fractures, and anticlines of approximately 100–700 m provide the majority of reservoir spaces for the potential gas-hydrate accumulation. The area is surrounded by Luoguhe Village, Laoyeling Hill, Beihong Village, Shuangjianshan Hill, and Ershibazhan Forest Farm and is conducive for the accumulation of the potential gas-hydrate in the Mohe permafrost, because it contains the most developed permafrost and a large number of wide-space anticlines and faults. Furthermore, the depths of 300–700 m, where the unfilled high-angle fractures have been developed, are identified as the most favourable interval for the potential gas-hydrate occurrence. Therefore, it is concluded that the reverse faults, unfilled fractures, and the anticlines at the depth of 300–700 m in this area can be considered as the primary drilling objects for the gas-hydrate exploration. The importance of this study is to minimise the cost of gas-hydrate exploration drilling in the Mohe permafrost. These findings can also be used for future gas-hydrate exploration projects in similar permafrost zones. • The geological structures associated with potential gas-hydrate accumulation are highly developed in the Mohe permafrost. • The reactivations of the structures in the Cenozoic are conducive to potential gas-hydrate accumulation in the Permafrost. • The normal and reverse faults and the unfilled crevices could be passages for gas migration during the gas-hydrate formation. • The reverse faults, unfilled crevices and anticlines provided the reservoir spaces for potential gas-hydrate accumulation. • The anticlines and north-dipping faults determine the regional distributions of potential hydrate, while the reverse faults and unfilled high-angle crevices restrict its occurrence. [ABSTRACT FROM AUTHOR]

The central Junggar Basin in western China is unique because it provides insights into the geochemistry of oil from Jurassic coal measures. The analysis of TOC, Rock-Eval pyrolysis, gas chromatography-mass spectrometry (GC-MS), and carbon isotopes was performed to characterize the oil geochemistry and evaluate the hydrocarbon generation potential of source rocks in a coal-bearing formation. Fluid inclusions, fluorescence microscopy, and burial-thermal history were also used to study the oil generation and charging histories. The results show that the mudstone of the Badaowan Formation is the main source rock of the Jurassic and has relatively high hydrocarbon generation potential. The Jurassic oil in the Zhong-4 Block is moderately mature, with an equivalent Ro of about 0.75%. The oil is characterized by heavy carbon isotopes, a predominance of Pr over the Ph, low abundances of trisnorhopane (Ts), tricyclic terpanes and gammacerane, predominance of C 29 steranes among regular steranes, a preponderance of naphthalene and its alkylated analogues in aromatic compounds, and abundant polycyclic compounds. Such geochemical data indicate a contribution of Jurassic humic type organic matter to the oil. The mudstone in the Badaowan Formation started generating hydrocarbons in the Early Cretaceous and reached the peak of hydrocarbon generation in the middle Cretaceous. One major hydrocarbon charging episode is suggested in the Late Cretaceous and another in the early Neogene. The Fukang Depression in the south is the source kitchen, and oil migrates in a direction of southwest to northeast through faults and sandstone carrier beds. • The Jurassic mudstones and oils are mostly within the oil window. • The mudstone in Badaowan Formation is the primary Jurassic source rock. • Unique biomarker characteristics of the Jurassic oil are demonstrated. • The Jurassic oil shows heavy carbon isotopes (δ 13 C > -28‰). • Two major hydrocarbon charging periods for the Jurassic reservoirs are suggested. [ABSTRACT FROM AUTHOR]

Understanding fluid mobility during spontaneous imbibition (SI) and flooding is important for the enhancement of oil recovery. Many researches on SI, water flooding (WF), and N 2 flooding (NF) have been published, but an in-depth understanding of the oil mobility at the pore scale is still inadequate in low-permeability reservoirs. To solve this problem, SI, WF, and NF experiments were performed on the low-permeability conglomerates of the Baikouquan Formation and the Urho Formation in the Mahu Sag. Moreover, online NMR was introduced to monitor the oil volume in pores with different sizes in real-time. Considering the wettability, mineral compositions, and pore structure, we established a schematic diagram to reveal the fluid mobility mechanism. The results show that the micropores in the low-permeability conglomerates are more hydrophilic, whereas the other pores are more lipophilic. During SI, the movable oil is mainly distributed in the micropores in the form of clay-bound oil (CBO), which is discharged from the rock through the meso/macropores. High hydrophilicity contributes to a high imbibition rate and oil recovery from SI, while the pore structure only affects the imbibition rate. During WF, the movable oil is contributed by CBO in the micropores and free oil (FO) in the pores controlled by throats >0.3 μm. During NF, the movable oil is mainly distributed in the pores controlled by > 0.1 μm throats, existing as FO and part capillary-bound oil (CAO). The results demonstrate that NF is effective to enhance the oil recovery of tight reservoirs, while WF is only effective for tight reservoirs with better pore structure (high V r > 0.3 μm). • A series of simulation experiments were performed to analyze the relationship between the recovered oil and the pore sizes and fluid types combining online NMR. • A schematic diagram were established to reveal the fluid transport mechanism during spontaneous imbibition, water flooding and nitrogen flooding. • The controlling factors of the recovery factor were analyzed with respect to the wettability, pore structure and mineral compositions of low-permeability conglomerates. [ABSTRACT FROM AUTHOR]