Examination of the formation phases of coalbed methane reservoirs in the Lu'an mining area (China) based on a fluid inclusion analysis and Ro method

• The hydrocarbon accumulation phases in the coal reservoirs are investigated.
• The inclusions resulted from two intermittent injection events.
• The coal strata of the Lu'an mining area experienced two thermal maturation stages.

In this article, we employed two paleothermometric scales, fluid inclusions and vitrinite reflectance (Ro), to analyze the geohistorical restoration data of coal-bearing strata in several mine fields of the Lu'an mining area and comprehensively investigate the major hydrocarbon accumulation phases in the coal reservoirs. Our analyses revealed that the area accommodated a rich development of inclusions, with a dominance of liquid-gaseous hydrocarbons inclusions and hydrocarbon-bearing brine inclusions. A Raman spectroscopy examination revealed a carbonaceous pitch association that originated from secondary transformations of the inclusions during a late stage of high-temperature events; based on the homogenization temperatures and salinity data, the inclusions resulted from two intermittent injection events. We concluded that the coal strata of the Lu'an mining area experienced two thermal maturation stages that corresponded to two major hydrocarbon phases. During the first major phase of the Late Permian to Late Triassic, the coal-bearing strata underwent slow subsidence, temperature elevation and increased coal maturity. During the second major phase of the Late Jurassic to Early Cretaceous, the coal-bearing strata exhibited soaring temperatures and hydrocarbon generation as a result of abnormal heat influences caused by Yanshanian volcanic activities and deep intrusive rocks.