Statistical descriptive analyses were performed taking into account sulphur, vanadium and nickel concentrations in several nonmature crude oil samples collected in the last few years from various Venezuelan and Spanish basins. Techniques used included flame atomic absorption spectrometry and energy-dispersive X-ray fluorescence spectroscopy. The results allow us to obtain a generic diagram in which four zones are well defined by their limit V/Ni ratios or Ni contents. Although previous works established intervals with reference to V/Ni ratios, this is nonetheless one of the first times that data are statistically considered with the aim of obtaining and evaluating the validity of a diagram plot to make it possible to determine the type of environment in which organic matter was deposited prior to any crude oil. Such a diagram is useful in establishing the palaeoenvironmental redox conditions that prevailed in the corresponding source rocks during their deposition. It is important to emphasize that this diagram can be applied as a geochemical exploration tool in medium-crude oils from other petroliferous basins in order to significantly reduce the costs and time of oil prospecting. © 2008 Elsevier B.V. All rights reserved.