Adsorbed and free gas occurrence characteristics and controlling factors of deep shales in the southern Sichuan Basin,China

Deep shale gas(3500-4500 m)will be the important succeeding field for the growth of shale gas production in China.Under the condition of high temperature and high pressure in deep shale gas reservoirs,its gas occurrence characteristics are markedly different from those of medium and shallow layers.To elucidate the gas occurrence characteristics and controlling factors of deep shales in the Wufeng-Longmaxi Formation,methane adsorption,low-temperature N2,and cO2 adsorption experi-ments were conducted.The results show that in deep shales,the mesopores provide approximately 75%of the total specific surface area(SA)and 90%of the total pore volume(PV).Based on two hypotheses and comparing the theoretical and actual adsorption capacity,it is speculated that methane is adsorbed in deep shale in the form of micropore filling,and free gas is mainly stored in the mesopores.Correlation analysis demonstrated that ToC is the key material constraint for the adsorption capacity of deep shale,and micropore SSA is the key spatial constraint.Other minerals and mesopore parameters have limited effect on the amount of adsorbed gas.Moreover,the free gas content ranges from 2.72 m^(3)/t to 6.20 m^(3)/t,with an average value of 4.60 m^(3)/t,and the free gas content ratio is approximately 58%,suggesting that the deep shale gas reservoirs are dominated by free gas.This ratio may also increase to approximately 70%when considering the formation temperature effect on adsorbed gas.Gas density,porosity,and gas saturation are the main controlling factors of free gas content,resulting in significantly larger free gas content in deep shale than in shallower formations.

Restoration of Eroded Thickness of the Neogene Strata in the Western Qaidam Basin and Its Significance for Oil and Gas Occurrence

During the Pleistocene, the western Qaidam Basin has largely experienced strong structural reconstruction and strong erosion. First, the eroded thickness of Neogene strata was restored approximately by the stratigraphic profile comparison method and plane trend surface restoring method; then, accurate calculation of erosion was recovered using vitrinite reflectance, and the erosion that was restored by the trend surface restoring method was corrected; finally, a distribution map of cumulative erosion was produced. This study marks an important achievement in that one of the most important parameters of basin tectonic evolution, sedimentary evolution, and oil and gas accumulation history has been obtained, and that a basic geological problem has been solved in the Qaidam Basin. The areas with high erosion and low erosion are shown in the map and a close relation between the distribution of oil and gas fields and erosion was recognized. Large and medium oil and gas fields are mainly distributed in areas with medium and low erosion. It is difficult to form large-scale oil and gas accumulation in areas in which erosion is more than 2000 m. The mechanism of the relation between oil and gas distribution and erosion is explained. This study will be of use in predicting the distribution of oil and gas.

Coupling mechanism of THM fields and SLG phases during the gas extraction process and its application in numerical analysis of gas occurrence regularity and effective extraction radius

The analysis of the coupling mechanism of thermal-hydraulic-mechanical(THM)fields,and solid-liquidgas(SLG)phases during gas extraction process is of profound significance to explore its numerical application in the gas occurrence regularity and its effective extraction radius.In this study,the Hudi coal mine in Qinshui basin is taken as the research area,the influencing factors of gas occurrence were analyzed,the differences in overburden load for gas pressure distribution and the factors influencing the effective extraction radius were further discussed by using the COMSOL software.The results show that the derivation of mathematical model in gas extraction shows that the process is a process the THM fields restrict each other,and the SLG phases influence each other.The longer the extraction time,the larger the influencing range of borehole,and the better the extraction effect.The larger the diameter of borehole,the larger the effective extraction radius,and the influence on gas extraction effect is smaller in the early stage and larger in the late stage.The borehole arrangement should be flexibly arranged according to the actual extraction situation.The higher the porosity,the higher the permeability,the better the gas extraction effect.The larger the overburden load of reservoir,the stronger the effective stress,which will result in the more severe the strain,and the closure of pore and fracture,which in turn will lead to the decrease of permeability and slow down the gas extraction.The relationship among extraction time,borehole diameter,negative pressure of gas extraction,permeability with effective extraction radius is exponential.This study has important theoretical and practical significance for clarifying and summarizing the gas occurrence regularity and its engineering practice.

Gas Occurrence and Accumulation Characteristics of Cambrian–Ordovician Shales in the Tarim Basin, Northwest China

The Tarim Basin is located in northwestern China and is the biggest basin in China with huge oil and gas resources. Especially the Lower to Middle Cambrian and Middle to Upper Ordovician possess the major marine source rocks in the Tarim Basin and have large shale gas resource potential. The Cambrian–Ordovician shales were mainly deposited in basin–slope facies with thicknesses between 30–180 m. For shales buried shallower than 4500 m, there is high organic matter abundance with TOC（total organic carbon） mainly between 1.0% and 6.0%, favorable organic matter of Type I and Type II, and high thermal maturity with RoE as 1.3%–2.75%. The mineral composition of these Cambrian–Ordovician shale samples is mainly quartz and carbonate minerals while the clay minerals content is mostly lower than 30%, because these samples include siliceous and calcareous shale and marlstone. The Cambrian and Ordovician shales are compacted with mean porosity of 4% and 3%, permeability of 0.0003×10^（-3）–0.09×10^（-3） μm^2 and 0.0002×10^（-3）–0.11×10^（-3） μm^2, and density of 2.30 g/m^3 and 2.55 g/m^3, respectively. The pores in the shale samples show good connectivity and are mainly mesopore in size. Different genetic types of pores can be observed such as intercrystal, intergranular, dissolved, organic matter and shrinkage joint. The reservoir bed properties are controlled by mineral composition and diagenesis. The maximum adsorption amount to methane of these shales is 1.15–7.36 cm^3/g, with main affecting factors being organic matter abundance, porosity and thermal maturity. The accumulation characteristics of natural gas within these shales are jointly controlled by sedimentation, diagenesis, hydrocarbon generation conditions, reservoir bed properties and the occurrence process of natural gas. The natural gas underwent short-distance migration and accumulation, in-place accumulation in the early stage, and adjustment and modification in the later stage. Finally, the Yulin（well Y1） and Tazhong（well T1） areas are identified as the targets for shale gas exploration in the Tarim Basin.

Mechanism of stepwise tectonic control on gas occurrence: A study in North China

To gain an understanding of gas occurrence, distribution is the fundamental basis for preventing gas disasters. Presently, how tectonic structures control gas occurrence remains problematic. This study proposes the theory and elucidates the mechanism of stepwise tectonic control on gas occurrence according to the characteristics of gas occurrence and the patterns of gas distribution in coal mines in North China. On the one hand, tectonic compression and shearing lead to stress concentration and thus deform the coal and reduce the coal seam permeability, further contributing to gas preservation. On the other hand, tectonic extension and rifting lead to stress release and thus improve the coal seam permeability, further contributing to gas emission. Therefore, the distribution zones of tectonic compression, ubiquitous coal deformation, and gas accumulation have been step-wisely revealed, and the coal-gas outburst proneness zones are finally identified. The proposed theory of step-wise tectonic control on gas occurrence is of practical significance for gas prediction and control.

Analysis on geological structures influencing gas occurrence at Qidong coalmine

The occurrence state of methane is mostly controlled by coalfield geologicalstructures.The coal-bearing strata at Qidong coalmine experienced many tectonic cyclessince their formation.The gas content made by the complicated structural geologic systemat the coalfield is very different, which is obviously higher on the north side of the Weimiaofracture belt than that on the south side and near itself.This thesis discussed the gas occurrenceregularity based on the geometric characteristics of the geological structure andits regional tectonic evolution.This study can provide a foundation for coalfield exploitationand deal with coal and gas outburst.

Gas emission quantity prediction and drainage technology of steeply inclined and extremely thick coal seams

Gas emissions of workfaces in steeply inclined and extremely thick coal seams differ from those under normal geological conditions, which usually feature a high gas concentration and a large emission quantity. This study took the Wudong coal mine in Xinjiang province of China as a typical case. The gas occurrence of the coal seam and the pressure-relief range of the surrounding rock(coal) were studied by experiments and numerical simulations. Then, a new method to calculate the gas emission quantity for this special geological condition was provided. Based on the calculated quantity, a further gas drainage plan, as well as the evaluation of it with field drainage data, was finally given. The results are important for engineers to reasonably plan the gas drainage boreholes of steeply inclined and extremely thick coal seams.

Controlling factors for gas hydrate occurrence in Shenhu area on the northern slope of the South China Sea

Temperature and pressure on seafloor of the northern slope in the South China Sea are suitable for gas hydrate formation, but bottom simulation reflector (BSR), an indication of gas hydrate occurrence, only occurred in limited areas of the slope. Drillings in the BSR-distributed area (the District S) on the northern slope of the South China Sea suggested that gas hydrate only occurred at Sites SH2, SH3, and SH7 with high saturation (up to 20%-40%), and there is no hydrate at Sites SH1 and SH5 although the distance between SH1 to SH3 is only 500m. In this paper, we investigated seafloor gradient, fault development, temperature, and pressure in the District S on the northern slope of the South China Sea to understand the possible factors con- trolling BSR distribution and gas hydrate occurrence. The District S is a structurally fractured continental slope zone and its seafloor gradient varied greatly. The BSR-occurred areas have an average gradient of 19.89×10 2 whereas the BSR-free zone has the average gradient of 10.57×10 2 . The calculated relative structural intensities from fault densities and displacements show that the BSR-distributed areas tend to occur in the areas with a moderately high structural intensity, where faults frequently developed close to the seafloor that are possibly favored for lateral migration of gases. On the basis of temperatures and pressures at drilling sites, hydrate-occurred Sites SH2, SH3, and SH7 are located within the thermodynamically stable area for methane hydrate, and hydrate-absent Sites SH1 and SH5 are out of the thermodynamically stable area for methane hydrate formation, suggesting that both BSR and the thermodynamic stability are necessary for hydrate occurrence in the subsurface.

Influence of foraminifera on formation and occurrence characteristics of natural gas hydrates in fine-grained sediments from Shenhu area, South China Sea

Marine gas hydrates accumulate primarily in coarse-grained, high-permeability layers; however, highly saturated natural gas hydrates have been discovered in the fine-grained sediments of Shenhu area, South China Sea(SCS). This may be explained by key factors, such as the great abundance of foraminifera shells. In this paper, by analyzing the SCS foraminifera structure and performing hydrate formation experiments in the foraminifera shells, the contribution of foraminifera to hydrate accumulation in the SCS was investigated from a microscopic point of view. Simulations of hydrate formation were carried out in both pure SCS foraminifera shells and the host sediments. Pore structures in typical foraminifera were studied by use of micro-focus X-ray computed tomography(CT) and scanning electron microscopy(SEM). Hydrate growth and occurrence characteristics in the foraminifera shells were observed in-situ. The results showed that the presence of foraminifera significantly enhanced the effective porosity of the SCS sediments. Moreover, while the hydrates grew preferentially in the chambers of the coarse-grained foraminifera by adhering to the inner walls of the foraminifera shells, no apparent hydrate accumulation was observed in the fine-grained or argillaceous matrix. These findings provide a basis for further studies on the accumulation mechanism of hydrates and physical properties of hydrate reservoir in the South China Sea.