Occurrence and distribution of vanadyl porphyrins in Saudi Arabian crude oils

Ten crude oil samples, covering wide range of maturity (API = 18.5-36.1), were collected from the major producing fields in the area of the Arabian Gulf. The oil fields were Safaniya, Abqiq, Ain-Dar, Wafra, Marjan, and Zulf. Vanadyl porphyrins were separated from the residual fraction (200 degrees C +) by means of adsorption chromatography on silica gel and alumina using solvents of increasing polarity. The chromatographic separation process was monitored by ultraviolet-visible spectrophotometry. The isolated vanadyl porphyrins were identified and characterized by ultraviolet-visible spectrophotometry and mass spectrometry. The porphyrins were found to be a mixture of etioporphyrins (Etio) and deoxophylloeorythroetioporphyrins (DPEP) homologues in the carbon number range of C-23-C-43. Sigma DPEP/Sigma Etio, derived on the basis of the ion molecular peaks abundance, was found to be close to unity. Except for Wafra/Iucene crude oils, the abundance of vanadyl porphyrins exhibited good relationships with the content of sulfur and asphaltene, The effects of reservoir depth and maturation level of the crude oils on the abundance of vanadyl porphyrins were also considered. It was suggested that the content of vanadyl porphyrins was governed by a thermal stress of depth. (C) 1998 Elsevier Science B.V. All rights reserved.