GEOCHEMICAL DIFFERENTIATION OF SILURIAN AND DEVONIAN OILS FROM ALGERIA

The ability to distinguish crude oils generated from prolific Silurian and Devonian shale source rocks could reduce exploration risk in North Africa. Routine geochemical analyses fail to reliably distinguish these oils. This pilot study shows that non-routine analyses (e.g., compound-specific isotopes of gasoline-range hydrocarbons and gas chromatography-mass spectrometry of diamondoids) distinguish Silurian and Devonian oils.

Hassi Messaoud and Zemlet oils occur in the northern study area where Devonian source rock is absent. These Silurian oils reside in Cambro-Ordovician quartzites below the Hercynian unconformity, where the Silurian overlies the reservoir. Long-distance migration of Devonian oil to the reservoir is unlikely. Oils from Assekaifaf, Zenani, Zarzaitine, and Dome occur in the southeastern study area where both source rocks exist, but migration paths indicate mainly Devonian input.

Diamondoids form in source rocks by clay-catalyzed reactions. They increase relative to other compounds during thermal cracking of oils. Despite higher maturity, the Silurian oils have diamondoid isomer concentrations 2-3 times lower than the Devonian oils due to genetic differences between their source rocks. The diamondoid concentrations in the oils suggest that the Silurian had less clay than the Devonian source rock. Higher dibenzothiophene/2-methylnaphthalene in the Silurian oils supports a source rock with less clay. Iron in clays reacts with sulfides to form pyrite, thus limiting sulfur incorporation into kerogen and related crude oil.

Light hydrocarbons, such as n-hexane, methylcyclopentane, cyclopentane, and methylcyclohexane, are depleted in ¹³C in the Silurian compared to Devonian oils (>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 ¹³C than methylcyclopentane and methylchclohexane in Silurian, but not Devonian oils. The Silurian oils have smaller differences in d¹³C between pristane and nC₁₇ and between phytane and nC₁₈ than the Devonian oils. Different assemblages of organisms likely contributed to the acyclic isoprenoids versus the n-alkanes in the source rocks for these oils.