Understanding of HC migration in sedimentary basins (present state of knowledge)

Abstract

Great advances were made during the decade of 1975–1985 in the understanding of HC migration by establishing that the main mechanisms were polyphasic flow, whatever its stage, and that migration in solution in water and/or by molecular diffusion was insignificant, except for methane, ethane and benzene. Research is now focusing on points which still have not been satisfactorily answered such as:(1)Behaviour of source-rocks during HC expulsion: the main driving force for fluid expulsion has been recognized to be excess fluid pressure within the source-rock with respect to its surrounding environment of permeable beds. The pressure build-up is due mainly to accommodation by the fluids of part of the mechanical stresses which are exerted on the rock (mainly sedimentary load but also stresses developed by tectonics), to thermal expansion of water, and to HC formation from kerogen in response to increasing temperatures. However, it is not clear whether or not petroleum expulsion proceeds preferentially along oil-wet networks (kerogen or oil-wet mineral surfaces). Nor it is not clear whether or not microfracturing, provoked by an internal fluid pressure higher than allowed by the mechanical strength of source-rocks, is necessary for HC expulsion.(2)Modifications of the compositions of HC phases (e.g. “oil” versus “gas”, or the distribution of biomarkers during migration,) and losses of hydrocarbons along the migration pathways. Although careful observations of natural sample series, and also experimentation, have been done, no clear picture has emerged for the moment. However, compositional effects seem to be due more to changes in thermodynamic conditions along migration pathways, which provoke inter alia hydrocarbon phase separation (retrograde condensation), than to differential retention by mineral surfaces.