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Newcastle Research Group in Fossil Fuels & Environmental Geochemistry, Drummond Building, The University, Newcastle upon Tyne NE1 7RU, UK
Although large-scale compositional heterogeneities in petroleum accumulations have been known for some time (Sage & Lacey 1938; Schulte 1980; Hirschberg 1984), it is only recently that petroleum column heterogeneities have started to be interpreted from the perspective of petroleum geochemistry (England et al. 1987; Larter et al. 1990). Here, compositional variations in petroleum accumulations may be interpreted as being due to source facies and maturity variations in the petroleum charge feeding the accumulation. Over time, the varying composition of the charge to the field is integrated and preserved as compositional variations in the petroleum column due to incomplete mixing of the petroleum in the reservoir (Horstad et al. 1990).
England & Mackenzie (1989) have suggested that, in permeable reservoirs, diffusive or density-driven mixing may eliminate inherited compositional variations on a reservoir thickness scale (around 100 m) in a short time (c. 1 Ma), but that diffusion cannot equilibrate lateral compositional gradients on a kilometre scale in large petroleum reservoirs. These resulting compositional variations may then be interpreted to indicate field filling directions and to place limits on the probable location and type of source rocks filling an accumulation. While generally confirmed by other workers (Leythaeuser & Rückheim 1989; Karlsen & Larter 1991) it is evident that other processes, including interaction of polar petroleum compounds with reservoir surfaces, do perturb this general rule and may prevent homogenization even on small scales (Larter et al. 1990).
This area of geochemical study may prove to be one of the most powerful applications of petroleum geochemistry to basin evaluation problems.
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This 250-word extract was created in the absence of an abstract.
