CENTRAL SAUDI ARABIAN CRUDE OILS: A GEOCHEMICAL INVESTIGATION

Abstract

ABSTRACT A suite of Paleozoic oils from Central Arabia was studied in order to evaluate conventional hopane and sterane biomarkers, as well as non-conventional markers such as phenanthrene and its methylated isomers. Because it was uncertain whether conventional biomarkers would be found in these light oils, additional analyses were performed on the oils for correlation purposes. These include light-hydrocarbon and mid-range micro-scale correlation techniques. Paleozoic oils and condensates from several fields from Central Arabia as well as Paleozoic condensate and two Jurassic oils from the Eastern Province (for comparison) were studied. Bulk properties, such as API gravity, percent sulfur, and nitrogen content were determined. The oils and condensates were then fractionated into their major compound classes: saturates, aromatics, polars, and asphaltenes. The full range gas chromatographic analysis of the saturate and aromatic fractions did not permit meaningful distinctions for bimodal distribution of normal alkanes or identification of aromatic compounds. Micro-scale correlation technique was used to correlate mid-range hydrocarbons and the mid-range star diagrams (MRSD) are drawn. The MRSD showed that all Paleozoic fluids (Central Arabian oils) follow patterns that are different from those followed by the Eastern Province. Bulk properties, pristane/phytane ratios, methyl penanthrene ratio (MPR), and calculated reflectance (% R c) values showed that Paleozoic oils from Central Arabia differ significantly from Jurassic oils of the Eastern Province. Central Arabian Paleozoic oils and condensates are high-API gravity, exceedingly low sulfur, low nitrogen and low metals fluids. They have pristane/phytane ratios that are greater than one indicating dysoxic conditions at the time of deposition of their source rocks. Their MPR and % R c values are indicative of differences in timing of generation from the source rock as well as in maturity and, perhaps most significantly, indicative of post-generative alteration such as water washing. Eastern Province Jurassic oils are: medium-gravity, high sulfur, high nitrogen crude oils. They have pristane/phytane ratios that are less than one indicating highly reducing conditions at the time of source rock deposition. Their MPR and % R c values are indicative of generation from a source rock that was at peak oil maturity (VRe ≈ 0.80–1.0%).