Study On The Accumulation Mechanism Of The Unconventional Natural Gas In Late Paleozoic Coal Bearing Strata Of The Northern Part Of The Ordos Basin

The unconventional natural gas(coalbed methane,shale gas and tight sandstone gas,referred to as "Three Coal Gases")in the coal bearing strata of Carboniferous and Permian has the characteristics of overlapping distribution in the plane and interdependent reservoir forming in the vertical direction in the north of Ordos Basin,which is expected to become a new field for the important strategic replacement of oil and gas resources in China.In this paper,the coal-bearing strata of Carboniferous and Permian in the north of Ordos Basin are studied by field geological survey,data collection,parameter well construction,special logging,experimental test,numerical simulation and so on from the three sources of gas,including the characteristics of reservoir space development and stacking framework,the geochemical characteristics of the "Three Coal Gases" source rocks,Gas reservoir and the gas-bearing characteristics of the coal-bearing system.The paper seeks to reveal the coupling mechanism of unconventional natural gas in coal-bearing strata.The main research results are as follows:(1)the sedimentary environment of the target layer in the study area is analyzed,and the main development tidal flat facies,delta facies and braided river facies.The thickness of the target layer has a great variation.The spatial distribution characteristics of the target layer are analyzed and controlled by the monoclinic structure,and the depth gradually increased from east to west.(2)Based on the superposition characteristics of the "Three Coal Gases" reservoirs in the coal system.The type of the "Three Coal Gases" source rocks-reservoirs-cap rocks in the study area was determined.The combination type of gas zone in the regional coal system is stressed.(3)The basic indexes of "Three Coal Gases" reservoirs in abundance,type and maturity of organic matter in the study area were systematically studied,and the hydrocarbon generation intensity of the target rock and organic matter was calculated.(4)The paper analyzes the material foundation and the fracture and permeability characteristics of the different gas reservoirs in the coal-based target layer,and discusses the stress sensitivity of different types of reservoir permeability.(5)The calculation model of the free gas and the gas content of the adsorbed gas in the coal unconventional natural gas reservoirs were established.The adsorption gas and free gas in different pore diameters were calculated and analyzed.The proportion of adsorbent gas and free gas in shale gas and coalbed methane with depth is discussed.It is pointed out that the mud-sand-mud assemblage at the bottom of Taiyuan formation is the combination type of gas zone in the regional coal system.(6)The coal-based natural gas generation and filling time is determined.The formation mechanism of gas accumulation in the Upper Paleozoic coal series of the Upper Paleozoic in the Ordos Basin is revealed by means of inversion and numerical simulation.The research results have important theoretical significance,which are of practical significance to the exploration and development of "Three Coal Gases" in coal bearing strata,and provide theoretical basis for scientific prediction and strategic deployment of exploration and development of "Three Coal Gases" favorable zones of coal measures.