The origin of abnormally high benzene in light hydrocarbons associated with the gas from the Kuqa depression in the Tarim Basin, China

The chemical and carbon isotopic compositions of C-1-C-3 and C-6-C-7 light hydrocarbons were analyzed in 14 natural gas samples from the Kela 2, Dabei and Dina 2 gas fields in the Kuqa depression of the Tarim Basin. The relative content of light aromatic compounds (benzene and toluene) in C-6-C-7 light hydrocarbons is very high, with benzene as the main peak in the gas chromatogram. The relative content of aromatic compounds generally amounts to over 30% of the C-6-C-7 light hydrocarbons, especially in the Kela 2 gas field where light aromatics comprise more than 70% of the C-6-C-7 light hydrocarbons. Compared with 255 coal derived gas samples in many other sedimentary basins in China, an abnormal distribution of hydrocarbon composition is observed in the study area. In order to investigate this abnormal distribution, light hydrocarbon yields generated from low maturity coals at different pyrolysis temperatures were simulated quantitatively. The results showed that the yield of light aromatic compounds (benzene and toluene) gradually increases with increasing pyrolysis temperature. Furthermore, the relative content of aromatic hydrocarbons generated from coal samples at high temperatures, similar to an instantaneous accumulation mode under geologic conditions, can amount to 90% of C-6-C-7 light hydrocarbons, while that of aromatic hydrocarbons generated cumulatively at high temperatures, similar to a cumulative accumulation mode in natural geologic bodies, and is no more than 46.6%. These phenomena have not yet been reported in the literature. By combining these data with carbon isotopic data for natural gas components, it is proposed that an abnormal distribution of aromatic hydrocarbons in gases from the Kuqa depression may imply that the Kela 2 gas field was mainly formed by instantaneous gas accumulation, while the Dabei and Dina 2 gas fields were largely formed by cumulative gas accumulations. (C) 2014 Elsevier Ltd. All rights reserved.