Trace elements as palaeoenvironmental markers in strongly mature hydrocarbon source rocks: the Cretaceous La Luna Formation of Venezuela

Abstract : The Late Cretaceous La Luna Formation is the main hydrocarbon source-rock formation in Venezuela. In the Lake Maracaibo area, the formation shows a steep maturation gradient affecting sedimentary organic matter (OM) and clay minerals, related to local burial history and thrusting events. The influence of the gradient is examined through geochemical (organic and inorganic) and mineralogical investigations carried out on samples from five cored boreholes in the Maracaibo Basin. The analytical procedure comprises Rock Eval pyrolysis, vitrinite reflectance measurement, palynofacies observation, major- and trace-element content determination, performed on bulk-rock samples or after a sequential leach protocol (DOP calculation) and the examination and analysis of clay-mineral assemblages. The maturation gradient seems to have had no effect upon the distribution of the trace elements referred to as proxy indicators for palaeodepositional conditions of OM-rich sediments (Ba, P, V, Mo, Ni, Co, Cu, Cd, Cr and Mn). Consequently, trace elements appear to be reliable palaeoenvironmental markers. They indicate that, during the La Luna Fm. deposition, the water column must have had variable conditions, with high primary production near the surface and the correlative development of euxinic conditions at the bottom. This provides further arguments for an upwelling system affecting the northern platform of South America during Late Cretaceous time. The relative scarcity of reactive iron in the depositional environment must have taken an active part in a process involving sulphur and acting on the environmental conditions as well as on OM (vulcanisation). This process favours aliphatic OM storage and may be relevant for many other OM-rich facies or hydrocarbon source rocks.
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Thierry Mongenot, Nicolas Tribovillard, Alain Desprairies, Elisabeth Lallier-Vergès, Fatima Laggoun-Défarge. Trace elements as palaeoenvironmental markers in strongly mature hydrocarbon source rocks: the Cretaceous La Luna Formation of Venezuela. Sedimentary Geology, Elsevier, 1996, 103 (1-2), pp.23-37. ⟨10.1016/0037-0738(95)00078-X⟩. ⟨hal-00142193⟩

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