Low Temperature oil Polymerisation and Hydrocarbon Expulsion from Continental Shelf and Continental Slope Sediments

摘要

The low temperature (<20 -150° C), medium pressure, polymerisation of methane and carbon oxides, from biogenic and thermogenic sources, via the reaction xCH4 + yCO2 + zH2 = oil (alkanes + alkenes) + water, is considered to be a natural response to increasing pore pressure and temperature below the BSR. A general kinetic model for "non-catalytic" polymerised oil formation through carbon oxide methanatio'n and hydrogenation as a function of time, temperature and pressure is outlined. A number of oils from continental shelf and continental slope sediments which are currently interpreted as thermogenic, are reinterpreted as low temperature (<20 - <90° C) catalytic polymerisation products and possible polymerisation routes identified. The hydrocarbon concentration and expulsion mechanism from the source rock through seismic chimneys to overlying strata is considered. Analysis of test data indicates that the development of macropores provides a major primary high volume mechanism for pressure relief in unconsolidated sediments during the initial phases of porosity expansion. The macropores may be subsequently superseded by the partial or complete fluidisation of the migration conduits. A quantitative model for the expulsion of hydrocarbons from a pressure mound during a poro-elastic cycle is outlined.