Geochemical Investigation of Crude Oils from Different Oil Fields of the Potwar Basin

Abstract

Crude oils (12) from the Potwar Basin, have been analyzed to obtain geochemical information on source and depositional environment of organic matter, maturation, biodegradation and geochemical correlation. Bulk and molecular parameters have recognized three groups of petroleum within the study area, Group I of heavy oils, Group II of medium to light oils and Group III, comprising of a single oil Adhi, of light oil. Based on samples location in ternary diagram, Group I oils were classified as paraffinic naphthenic (PN), while Groups II and III were classified as paraffinic (P) oils. Gas chromatographic (GC) fingerprints of Group II and III oils showed full suite of n-alkanes, low isoprenoid to n-alkane (Pr/nCl7 and Ph/nC18) ratios, absence of UCM (unresolved complex mixture) and higher saturates/aromatics ratios. These features suggested non-biodegraded nature of • biod radation on account of these parameters. Carbon Preference Index (CPI) and Odd Even redominance (OEP) 1.0 for Group Il and Adhi and <1.0 for Group I oils revealed slightly lower thermal maturity of the latter. Terrestrial to aquatic ratios (TAR) suggested that all the samples contained mixed organic matter (OM) with predominant contribution of marine geolipids for Groups 1 & II and terrestrial OM for Group III. Pristane to phytane (Pr/Ph) ratios < 1 along with high sulfur content suggested typical marine carbonate dysoxic sediments generating Group I oils, while Pr/Ph > 1 and very low sulfur content recommended oxic depositional settings for Group I oils and Adhi_ The differences in the location of Joyamair and Minwal on geochemical plots were interpreted to difference in thermal maturity compared to other oils of this group.