Geochemical characteristics of light hydrocarbons and their influencing factors in natural gases of the Kuqa Depression, Tarim Basin, NW China

摘要

In order to determine the source of the light hydrocarbons in the Kuqa Depression of the Tarim Basin, Northwest China, their geochemical characteristics are documented, and possible influencing factors are discussed in a broad context on the basis of geochemical analyses of natural gas samples from 19 wells in combination with previously reported natural gas components and carbon isotope values from the same depression. The biomarker analytical results show that (a) methyl cyclohexane (37.5-60.3%) of natural gas predominates the C-7 light hydrocarbon series (methyl cyclohexane, heptanes, and dimethyl cyclopentane), and (b) the heptane content (25.0-54.8%) is also relatively high. The C6-7 light hydrocarbon series (aromatics, normal alkanes, and cycloalkanes) have abnormally high aromatics (benzene and toluene) content (26.2-83.8%). Heptane value (13.31-37.77%) and isoheptane value (2.10-7.64%) indicate that the Kuqa natural gases are of high maturity and typical of coal-derived gas (gas generated by sapropelic source rock). Their light hydrocarbons, however, appear to have some characteristics of mixed-source organic matter. Specifically, the light hydrocarbons in the Dina 2 gas field show characteristics of the coal-derived gas; those in the Dabei gas field show some features of the oil-associated gas (gas generated by humic source rock), whereas those in the Kela 2 and Keshen 2 gas fields and Well Yangtake-101 display characteristics of the mixed source organic matter. The abnormally high content of the aromatics in the Kuqa Depression may have resulted mainly from their high productivity during the thermal maturity of humic type organic matters and the gases accumulation of late stage. The different relative contents of the aromatics in various oil and gas fields may have been accounted by different organic matter types, maturity, and distribution patterns of source rocks.