Gas Generation From Coal Seams And Its Contribution To Accumulation Of Coal Measure Gas In Offshore Waters Of China

The natural gas resources in the offshore area of China are rich,but the long-term research and exploration focus on conventional oil and gas,and the coal measure strata are mainly considered as hydrocarbon source rocks.However,coal seam and tight sandstone of coal measure strata in Cenozoic are also good hydrocarbon reservoirs with rich natural gas resources.In this paper,the reservoir forming characteristics and mechanism of coal measure gas in the offshore area of China are discussed for the first time by using the idea of symbiotic,co-exploration and co-exploitation of coal measure gas,and ideas and suggestions are proposed to change the exploration of natural gasThe coal-accumulating period of Cenozoic is from Eocene to Oligocene in China's sea area,and they all appeared at the end of the stage of the fault depression.The research shows that the coal accumulation of Pinghu formation,Yacheng formation and Enping formation is better than Huagang formation Lingshui formation and Zhuhai formation respectively.The Xihu depression of the East China Sea Basin,pearl river mouth basin and southeast hainan basin is dominated by littoral delta sedimentary system,river-fan delta sedimentary system and fan delta-lagoon-tidal flat sedimentary system respectively.The characteristics of coal seam development in sea area controlled by these sedimentary system and tectonics are:(1)The coal seam has many layers but the thickness of each layer is small,and it also has frequent interbedded with mudstone and sandstone,and the sulfur content and ash yield are higher;(2)The coal seam is vitrinite-rich,exinite-less and inertinite-poor,and gas-generating dominantly;(3)The hydrocarbon source rock is in the mature stage,and the maturity of local area reaches mature or over mature.The gas-generation conversation and hydrocarbon generating quantity in the Xihu depression of the East China Sea Basin is higher than the pearl river mouth basin and southeast hainan basin of the South China Sea Basin.The buried depth of coal seam from Eocene to Oligocene is generally more than 3000 m.The pressure of coal reservoir is high,and most of them are abnormal overpressure,and the features belong to typical deep and high pressure coal reservoir.Therefore,isothermal adsorption experiment is designed to simulate the environment of coal reservoir in China Sea Area,and the factors such as the rank of coal and ash yield etc.are all considered.Based on the experiment,a prediction model adapted to the gas content in deep coal reservoirs in China Sea Area is established.Simulation results show that the distribution proportion of different phase gas varies with depth in deep and super deep coal seams(buried depth from 4000 to 5000m)in China's sea basins.Response of adsorption gas to temperature and pressure is weaker,the proportion of adsorption gas content is less,and free gas becomes the main part of gas content of the coal seam with the depth increases.It is considered that the coal measures gas reservoir in Yacheng formation is similar to Surat Basin in Australia by combining the comparison with coal measures gas reservoir and the analysis of the elements of reservoir formation.The coal measures gas reservoirs develop many layers with large accumulative thickness but small each layer's thickness.A compound coal measures gas reservoir with the symbiosis of adsorbed gas and free gas is formed by coal seam,mudstone and tight sandstone.The frequent interbedded between coal and tight sandstone provides a good geological foundation for the reconstruction of reservoir seams,what's more,the mudstone layers among the frequent interbedded reservoirs are good capping reservoirs.Gas generation of coal seam provides a sufficient source of gas.We should get rid of the ideas and methods used in conventional natural gas exploration and development,and adopt new ideas in the exploration of coal measure gas in the offshore area of China.It is recommended that reservoir with high interbedded-rate and high sandstone ratio should be a priority for CBM development on the plain and the coal measures gas system should be the basic geological unit for development in the vertical direction.