TRACE-ELEMENTS AS PALEOENVIRONMENTAL MARKERS IN STRONGLY MATURE HYDROCARBON SOURCE ROCKS - THE CRETACEOUS LA LUNA FORMATION OF VENEZUELA

The Late Cretaceous La Luna Formation is the main hydrocarbon source-r ock 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 analy tical procedure comprises Rock Eval pyrolysis, vitrinite reflectance m easurement, palynofacies observation, major- and trace-element content determination, performed on bulk-rock samples or after a sequential l each protocol (DOP calculation) and the examination and analysis of cl ay-mineral assemblages. The maturation gradient seems to have had no e ffect 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 a ppear to be reliable palaeoenvironmental markers. They indicate that, during the La Luna Fm. deposition, the water column must have had vari able conditions, with high primary production near the surface and the correlative development of euxinic conditions at the bottom. This pro vides further arguments for an upwelling system affecting the northern platform of South America during Late Cretaceous time. The relative s carcity of reactive iron in the depositional environment must have tak en an active part in a process involving sulphur and acting on the env ironmental conditions as well as on OM (vulcanisation). This process f avours aliphatic OM storage and may be relevant for many other OM-rich facies or hydrocarbon source rocks.