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Co-existence of coalbed methane, tight gas and shale gas in the Paleozoic sea-land inter-facies coal measures of Qinshui Basin

Abstract

Recent explorations indicate that the Paleozoic marine-continental interaction coal-bearing strata in the Qinshui Basin are characterized by coalbed methane, tight gas, and shale gas symbiosis. “Three gas” may be the target of future unconventional natural gas resource exploration in coal-bearing strata, however, coalbed methane is currently only used as a research and exploration focus. The purpose of this study is to analyze the longitudinal distribution of the marine-continental coal-bearing strata in the Paleozoic of the Qinshui Basin, to reveal the coexistence mechanism of the coal-bearing gas system, to explore the main controlling factors of the co-generation reservoir, and to establish a symbiotic combination model of coalbed methane, tight gas and shale gas in coal-bearing strata. Based on the outcrop, core, logging and analytical tests, this study introduces the microfacies analysis technique of coal deposits, and uses the whole rock organic geochemical analysis to calculate the abundance of organic matter in coal seams. In this study, the organic matter type and maturity were evaluated, and the porosity and permeability characteristics of the reservoir were analyzed by mercury intrusion experiments, and comprehensive comparison was made to form the reservoir accumulation evaluation. This topic is intended to study the following key issues: (1) Exploring the geological characteristics of coal-series reservoir structure, thermal evolution, physical properties and macro-coal types in the Qinshui Basin, and explaining the basic conditions for the occurrence of coal-bearing gas systems; (2) Analyzing the typical combination mode of coal seam, shale and tight sandstone, and establishing typical reservoir formation modes such as “dual source and three reservoirs”, “dual source and double storage” and “single source and single storage”; (3) Focusing on the core geological conditions affecting the coupling mechanism of coal-bearing gas system, and the change of temperature and pressure caused by the buried depth of coal seam, which makes the dynamic transformation and tectonic uplift of different systems form a micro-drainage system in tight reservoirs, and providing a superior channel for gas migration and accumulation.