Study On Geological Control Mechanism Of Gas Heterogeneity Distribution In Low Rank Bituminous Coal

The geological mechanism of gas distribution heterogeneity in the distribution areas of low rank bituminous has always been a key scientific problem to be solved for safety production of coal mines,which is of fundamental social and practical significance except its theoretical values.Therefore,this research took the coal seam No.4（low rank bituminous）of the Binchang mining area of southern Ordos Basin,where the coal and gas outburst took place frequently,as research target and conducted related experiments and analysis.This paper aims at realizing new progress in the controlling mechanism of gas occurrence and distribution heterogeneity in the low rank bituminous,and providing theoretical and technical supports for mine safety production.Specifically,the collected coal samples were tested by using measurement methods such as industrial analysis,maceral statistics,vitrinite reflectance test,argon ion polishing and scanning election microscopy,low field nuclear magnetic resonance,mercury intrusion porosimetry,low pressure N₂ adsorption,methane isotherm adsorption,and permeability.The theories and methods of coal petrography,sedimentology,structural geology,and gas geology were applied to discuss the gas occurrence characteristics and controlling geological factors of No.4 the coal seam of the study area and to reveal the geological control mechanism of gas distribution heterogeneity.The main achievements are listed as follows:1.The gas occurrence characteristics and distribution heterogeneity of the Binchang mining area were revealed.The buried depth of coal seam in the north area is generally greater than that in the south,while the gas content shows a distribution pattern of“being higher in the south-central and southeast areas and lower in the north and northwest areas”.It is in contradiction with the general rule that the gas content of coal seams increases with the increase of buried depth,suggesting the particularity of gas occurrence in the study area.There are two high-value regions in the central and southern areas of the mining area,which extend 20 km～40 km in near EW direction,and several relatively high value regions exist in some local areas of the eastern and northern part of the mining area.2.The influence of geological factors on gas occurrence was discussed.The well-developed mudstone roofs and floors in the Bingchang mining area are effective gas-resisting layers that make the gas flow gradually along the coal seams and enrich in the cores of synclines,resulting in the ribbon-shaped gas enrichment regions in the cores of synclines.The gas occurrence is significantly influenced by thickness of coal seam roof.The thick and stably distributed mudstones in the southern mining area is favorable for gas enrichment,while the thin and discontinuously distributed mudstones in the northern part bring about the low gas content,resulting in the gas content being higher in the south and lower in the north.The southeastern region of the mining area is the stagnant zone of groundwater,which also affects the heterogeneous distribution of gas content to a certain extent.However,the poor correlations between gas content and buried depth of coal seams suggest the particularity of geological factors that control gas occurrence.3.Based on the measurement results of pore-fracture structure,permeability,and methane adsorption capacity of the coal seam No.4,it is concluded that the difference of pore-fracture structure is one of the most important factors that lead to the distribution heterogeneity of gas content.The physical properties,such as coal petrology,coal deformation,and pore-fracture structure,significantly affects the gas adsorption and desorption and permeability properties of coal seams and is an important factor causing the gas occurrence heterogeneity.The areas where the coal seam No.4 shows high moisture and ash content usually have a low methane adsorption capacity and then a low gas content.The moderate brittle deformation,tectonic fractures,and mesopore～microfracture structure,are favorable for high permeability of coal seams,gas enrichment is undeveloped,and there is usually low gas content in this areas,including northern part of the Wenjiapo mine,western part of the Tingnan mine,and the Yadian mine.On the contrary,the coal seams with relatively complete coal body,undeveloped tectonic and endogenous fracture structure,and discontinuous pore size distribution usually show a low permeability and a high gas content,such as the Xiaozhuang mine,the southwestern part of the Xiagou mine.Moreover,the good aperture distribution（especially the development of10～30 nm pores）increases the methane adsorption capacity of coal seams,which was confirmed by the higher methane adsorption maximum and the higher adsorption rate at low pressures,and then,coal seams there usually show a high gas content,such as the Dafosi mine,the Xiagou mine,and the Xiaozhuang mine.4.The geological controlling mechanism of gas distribution heterogeneity in the Binchang mining area is revealed.The mine structures and their evolution is the key factor that controls gas occurrence and distribution.Controlled by the near EW-extending synsedimentary folds,the near EW-extending extra-thick and ultra-thick coal seams are well distributed in the central and southern region,while the northern region usually develops with relatively thin coal seams.The thickness of coal bearing strata in the cores of synclines is generally larger than that of anticlines.As the gas generation quantity of thick coal seams is higher than that of thin ones,the coal seams in the cores of synclines generally show high gas content.The mudstone roofs and floors are effective gas-resisting layers that are favorable for the gas sealing and preservation.The mudstone layers are generally thicker in the south than in the north,suggesting the better gas preservation condition of the southern region.Besides,coal wall observation shows that the fracture structures,mainly the fractures that cut through the whole roofs,are more developed in the coal roofs of the northern mining area.These fractures severely destroy the integrity of coal roofs,significantly decrease the gas sealing ability,and are favorable for gas dissipation.There are 48 figures,28 tables and 291 references in this dissertation.