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Geochemical criteria for reservoir characterization

Abstract This paper shows how the basinal analysis of formation waters and petroleums may be used to deduce the general geological setting of the source-reservoir system. There are two extreme cases, principally depending on the relative volumes of the source and reservoir regions. If the source rock region dominates, a ‘closed’ system results. Otherwise, where the reservoir volume is large the system is ‘open’. The ‘reservoir’ may be thought of as including the ‘carrier bed’ if appropriate. In the ‘closed’ system, for example the carbonate reefs of the Western Canada Basin, water-soluble products of petroleum generation (e.g. benzene and H2S) are preserved in the reservoir. There is insufficient water to remove these components by aqueous solution. The partial pressure of methane is expected to be the same in formation waters and petroleums, since they can equilibrate in a ‘closed’ system. In an ‘open’ system the volume of water in the ‘reservoir’ is large compared to the source. Water and petroleum expelled from the mature source rock kitchen cannot reach equilibrium with the vast volume of formation fluids. Regions which connect with local source kitchens are expected to experience alterations in salinity and dissolved organics. Concentrations of water-soluble components will therefore be reduced in petroleums. Data from ‘open’ and ‘closed’ systems are presented from the Devonian of the Volga-Ural province. A reduction in water salnity below 3 km, near the source kitchen is attributed to expulsion of a low salinity water during oil generation. Related changes in aqueous benzene concentrations are also seen.