Heterogeneity of oil composition within a reservoir as a reflection of accumulation history

The unaltered petroleums of a small oil field in the F.R. Germany reveal pronounced spatial heterogeneities in bulk and molecular composition. Over most of the field area, the oil in high-porosity/ high-permeability sand intervals bears a higher proportion of saturated hydrocarbons and has a higher maturity chemical signature as compared to oils occupying lower-porosity/lower-permeability zones. For all sands with less than about 13% porosity a regular relationship was observed between oil composition and porosity/permeability: with a decrease of the latter petrophysical properties, increasingly lower chemical maturities and increasingly different bulk chemical compositions (lower proportions of saturated hydrocarbons and higher proportions of NSO-compounds) were recorded. Maximum contrasts measured for chemical maturities are, e.g., 0.63 versus 0.82% vitrinite reflectance equivalents ( R c -values calculated on the basis of the methylphenanthrene index according to Radke and Welte, 1983), 0.31 versus 0.84 for the ratio of triaromatic to mono- plus triaromatic steroid hydrocarbons, and 0.38 versus 0.47 for the 20S/ 20S+20R- ratio of C 29 5α(H),14α(H),17α(H)-steranes. These spatial heterogeneities in the composition of the oils are interpreted to reflect 1. i) successive stages of trap filling due to episodes of increasing burial and maturation of the relevant source rocks, in combination with 2. ii) low rates of in-reservoir mixing of these individual oil charges by convection or diffusion. Recognition of compositional heterogeneities within the oil column of individual fields may not only provide a basis for the reconstruction of reservoir filling history, but may also have important implications for improving petroleum production.