The origin of high and variable concentrations of heavy hydrocarbon gases in coal from the Enhong syncline of Yunnan, China

Abstract

The composition of coalbed methane (CBM) in the Enhong syncline of Yunnan China has abnormally high concentrations of heavy hydrocarbon gases (C2+). The distribution of the C2+ gases is variable in the syncline, some areas contain high concentration of C2+ gases from 2.9 to 36.98% which are called “wet gas area”, and in some areas, C2+ gases can't be examined which are called “dry gas area”. The reason for the high concentration of C2+ gases is connected with the coal maceral composition, the barkinites. The barkinites have the oil-prone character and have generated a lot of oil and exsudatinite in the process of coal maturation, which have been observed under the microscope. Accompanied by the oil and exsudatinite, many C2+ gases are generated from the coal, which can be proved by a large amount of C2+ gases generated from coal in Enhong syncline during the pyrolysis simulation experiments (Py-Gc). The reason of the uneven distribution of C2+ gases in the study area may be caused by different degrees of microbial degradation. The gases in the “dry gas area” become drier because of more microbial degradation and presenting biogenic gases character with little wet gas and lighter carbon isotopic methane, while gases in the “wet gas area” keep the character of thermogenic gases. The different degrees of microbial degradation may be caused by different gas preservation conditions such as coal pore structure. Mercury injection experiments show that coals in the “wet gas area” have larger pore specific surface areas and more micropores but fewer macropores. The larger pore volume of micropore and transitional pore may help the coal in the “wet gas area” protect the wet gases, maybe avoid the wet gases being degraded by the microorganisms.