DIAGENETIC-GEOCHEMICAL PATTERNS AND FLUID EVOLUTION HISTORY OF A LOWER JURASSIC PETROLEUM SOURCE ROCK, MIDDLE ATLAS, MOROCCO
M. Rachidi1, F. Neuweiler1* and D. Kirkwood2
1Dépt de Géologie et Génie géologique, 1065 Av. de la Médecine, Université Laval, Québec, Canada, G1V 0A6.
2Commission géologique de Canada, 490 Rue de la Couronne, Ville de Québec, Canada, G1K 9A9.
Pliensbachian to earliest Toarcian marls and argillaceous limestones exposed at the surface near Aït Moussa (Boulemane Province, Middle Atlas) include the only examples of effective petroleum source rocks so far known in the Moroccan Atlas rift basins. The outcrop interval includes hemipelagic, peri-Tethyan low latitude source rocks with Type II kerogen (total thickness of 2.5 m with mean TOC of around 3.2%). Early diagenetic, anoxic remineralisation of sedimentary organic matter resulted in hydraulic fractures, calcite cementation, a negative shift of carbon and oxygen stable isotopes relative to marine values (Δδ13C = -1.1; Δδ18O = -2.0), framboidal pyrite, and relative enrichment of the middle weight rare earth elements (REE). In combination, these attributes of early diagenesis may assist in the identification of other source rock intervals of similar age and setting.
Progressive burial produced three generations of Fe-calcite cemented veins, followed by three generations of replacive dolomite and concluded by ankerite replacing dolomite. Compaction fluids initially caused a slight positive shift of δ13C values (Δδ13C = +0.4), a flattening of the REE distribution pattern and an increase in REE content, together related to the dewatering of clay. Dolomitizing fluids (dol-2 and dol-3) record a positive shift of δ13C values (Δδ13C = +0.9) suggesting the effects of methanogenesis or an uptake of heavy d13C from underlying rock formations during fluid migration. Dol-3 is an Fe-bearing saddle dolomite that carries a positive Eu-anomaly (Eu/Eu* = 8.1) best explained by ascending hydrothermal fluids which are presumably of Middle Eocene age.
A first migration of bitumen is recorded as fluorescent inclusions in dol-2 (Late Jurassic -- Cretaceous), but bituminous fluids remained normally pressured until the establishment of inclined stylolites during Late Eocene tectonic compression. Comparative organic-geochemical analyses (GC, NMR of inclusions, non-expelled and expelled bitumen) indicate that thermal maturation advanced significantly after the onset of migration. Differences in terms of Pr/n-C17, Ph/n-C18 ratios and aromaticity corroborate the effects of differential expulsion. An exploration strategy should consider both secondary migration via opened tectonic stylolites in association with late-diagenetic fractures and a persisting tightness that then could have created an unconventional oil reservoir.
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