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Geological Society, London, Special Publications; 2004; v. 237; p. 27-35;
DOI: 10.1144/GSL.SP.2004.237.01.03
© 2004 Geological Society of London

New Approaches, Analytical and Geochemical Developments

Shaken but not always stirred. Impact of petroleum charge mixing on reservoir geochemistry

Arnd Wilhelms1,{dagger} & Steve Larter2,*

1 Norsk Hydro, O&E Exploration, 0240 Oslo, Norway
2 NRG Petroleum Group, School of Civil Engineering and Geosciences, University of Newcastle upon Tyne, UK
* Department of Geology and Geophysics, University of Calgary, Calgary, Canada
{dagger} Norsk Hydro, Technology, 5020 Bergen, Norway

Essentially all petroleums are mixtures with different components charged from source rocks at different temperatures. This heterogeneous charge is the basis for compositional differences in reservoirs that are the basic elements of reservoir geochemical approaches. Because many classical petroleum geochemical tracers of source facies and maturity, such as the cyclic biomarker hydrocarbons, show several orders of magnitude variation in concentration in petroleum systems these compounds do not reliably track facies or maturity signals in mixed oil situations. Light hydrocarbon and aromatic hydrocarbon parameters are more reliable in this sense but, as mixtures are the norm, the concept of the maturity of oils needs revising. We suggest an alternative approach is needed which tracks the maturity/petroleum mass fraction relationships for reservoired oils (mass fraction maturity) and allows the bracketing of source kitchen maturity. We strongly advise against using compound ratios in reservoir geochemical studies without having knowledge of the compounds concentration range variations within the petroleum system being studied.