Effects of biodegradation on the carbon isotopic composition of natural gas - A case study in the bamianhe oil field of the Jiyang Depression, Eastern China

Abstract

To study the characteristics and the effects of biodegradation on the carbon isotopic composition of gaseous hydrocarbons of the natural gases in the Bamianhe oil field of the Jiyang Depression, five gas samples and three associated oil samples from the Bamianhe oil field of the Jiyang Depression, Eastern China, were collected and analyzed. The results present here an example of gas biodegradation. The main component of natural gas is methane, in the range of 94.8 to 99.2%. There are only small amounts of ethane and heavier wet hydrocarbon components in the gas, with methane/(ethane + propane) ratios (C1/(C2 + C3)) ranging from 78 to 551. The carbon isotopic compositions of methane and ethane rang from -45.0 to -50.0‰ and from -34.2 to -39.8‰, respectively. The carbon isotopic compositions of propane, iso-butane and normal butane display significant variability, ranging from -2.6 to -31.1‰, -14.6 to -29.5‰, and -3.7 to -29.2‰, respectively. The δ31C of propane and normal butane are generally more enriched in 13C than that of iso-butane, and there exists a direct correlation between δ13C values of propane and normal butane of biodegraded gases. The carbon isotopic compositions of CO2 tend to become depleted in 13C with the increasing δ13C values of propane and n-butane. This study reveals that δ13C values of methane and ethane basically have not been changed by the biodegradation; biodegradation of natural gas can lead to drier gas compositions; the effects of biodegradation on the δ13C of normal butane are greater than that on the δ13C of iso-butane; and that propane and normal butane are attacked by microorganisms in a synchronous process. Normal alkanes in the oil samples from the Bamianhe oil field are lost partially or totally with the increasing degree of biodegradation, resulting in the rising of the ratios of (Acyclic Isoprenoid Alkanes/(Normal Alkanes and the ratios of (pristane + phytane)/(nC17 + nC13). With the increasing of the above ratios, the carbon isotopic compositions of propane, iso-butane and normal butane become enriched in 13C and the C1/(C3 + C3) ratios increase. It confirms that the abnormal gas components and carbon isotopic compositions of natural gases of the Bamianhe oil field are the result of biodegradation. Copyright © 2005 by The Geochemical Society of Japan.