Natural gas formation in the western Nile delta (Eastern Mediterranean): Thermogenic versus microbial

摘要

The offshore part of the Nile delta is an emerging gas and condensate province and one of the most promising areas for future petroleum exploration and supply in northern Africa. While the Nile delta’s hydrocarbon potential and its geological and stratigraphic pattern has been the focus of numerous studies, the origin of its gas has been poorly studied. In order to characterize the natural gas and to determine whether microbial or thermogenic processes are the dominant generation pathways of methane, a regional geochemical study was performed on 38 test gases collected from 9 Neogene western Nile delta discoveries. Main constituents of the gases are methane (89.4–99.4%) up to pentane with traces of carbon dioxide and nitrogen. Methane carbon isotopic composition ranges from -66.0‰ to -39.6‰ displaying a mixed microbial and thermogenic character for most of the gases. Microbial gas contributions range from 15% to 95%. The mixed and the more thermogenic gases are frequently accompanied by light oils and condensates (41–59 °API). Calculated maturities of the thermogenic gas proportions suggest source rock maturities between 0.8% and 1.4% Ro, which are actually present in the pre-Miocene section. Evidence for microbial alteration of petroleum accumulations in the Nile delta is largely manifested in anomalously heavy isotopic compositions of both wet-gas components propane and n-butane and preferential removal of higher n-alkanes from condensates. The hydrocarbon accumulations affected by biodegradation are trapped within shallow Pliocene reservoirs (<2400 m) where present day temperatures are below 80 ℃. The coexistence of biodegraded thermogenic gas and condensate with dry microbial gas supports the concept that generation of secondary microbial methane by biodegradation of thermogenic hydrocarbons may occur within the Nile delta.