Geochemical differentiation of Silurian from Devonian crude oils in eastern Algeria

Abstract

The ability to distinguish crude oils generated from prolific Silurian and Devonian source rocks provides a means to map their petroleum systems and thereby reduce exploration risk in North Africa. Routine geochemical analyses commonly fail to reliably separate these oils. This study demonstrates that non-routine analyses, such as compound-specific isotopes of light hydrocarbons and gas chromatography-mass spectrometry of diamondoids, can distinguish Silurian from Devonian oils in Algeria. Oil samples from Zemlet field and the giant Hassi Messaoud field were obtained from the northern part of the study area in Algeria where the Devonian source rock is absent. These oils originated from thermally mature Silurian source rock and migrated updip to fractured Cambro-Ordovician quartzite reservoirs below the Hercynian unconformity. Oil samples from the Assekaifaf, Oued Zenani, Zarzaitine, and Dome fields occur in the southern part of the study area, where both source rocks exist, but migration paths indicate input mainly from Devonian source rock. Despite higher maturity, the Silurian oil samples have diamondoid isomer concentrations 2–3 times lower than the Devonian samples. Because diamondoids form in source rocks by clay-catalyzed reactions and increase relative to other compounds during thermal cracking of oils, the diamondoid concentrations in the oil samples suggest that the Silurian source rock had less clay than the Devonian source rock. Higher dibenzothiophene/2-methylnaphthalene and generally higher sulfur in the Silurian oil samples support a source rock with less clay. For the Devonian oil samples, slightly more iron in clays reacted with sulfides to form pyrite, thus limiting sulfur incorporation into the kerogen and the generated crude oil. Light hydrocarbons, such as n -hexane, methylcyclopentane, cyclopentane, and methylcyclohexane, are depleted in 13 C in the Silurian compared to Devonian oil samples (> 1.1,0.6, 1.0, and 0.2 respectively). Patterns of isotopic ratios among these compounds also differ ( e.g. cyclohexane is more depleted in 13 C than methylcyclopentane and methylcyclohexane in Silurian, but not Devonian oil samples). The Silurian oil samples have smaller differences in δ 13 C between pristane and the C 17 n -alkane and between phytane and the C 18 n -alkane than the Devonian samples. Different assemblages of organisms likely contributed to the acyclic isoprenoids versus n -alkanes in the source rocks for these two oil families.