Inorganic geochemistry as indicator of provenance and sedimentary environment in black shales: the tithonian transgressive deposits of the Neuquén basin (Vaca Muerta Formation), Argentina.

Abstract

This study presents and discusses inorganic geochemical data from the basal Vaca Muerta Formation, deposited during the early Tithonian marine transgression of the Neuquén Basin. This interval is associated with high preservation of organic matter and is considered the most important for oil and gas generation in the basin. The studied succession (< 25 m) consists of finegrained rocks (pelites) ranging in composition from purely silicatic (mudstones) to hybrid silicatic/carbonate (marls). Geochemical analysis of inorganic components, performed on 20 samples from outcrops of the Andean sector of the Neuquén Basin, exhibit pronounced compositional changes that allowed defining several lithological types (mudstones, marly mudstones, muddy marls, marls and calcareous marls). The mudstones are characterized by higher contents of SiO2, Al2O3, K2O, TiO2, Th, Lu, Hf, Sc, Zr, and (especially heavy) RREs. The Sr, considering its affinity with Ca, increases its proportion in the marly sediments. The sampled Vaca Muerta rocks have a composition similar to that of the upper continental crust, but with higher contents of V and U. Provenance diagrams and REE patterns suggest that most of the sediments derived from acidic and intermediate igneous rocks. Clastic input was likely supplied to the basin from different sources, such as the magmatic arc flanking the basin to the west, and from Triassic and Jurassic volcanic terrains located in the passive southeastern margin of the basin; although contribution from the underlying Tordillo Formation cannot be discarded. The high contents of Al2O3, TiO2, Th, Zr and the ratio Th/U in the mudstones, and particularly in deposits of the southern sector of the basin, are considered strong indicators of terrigenous input. On the other hand, redox-sensitive components (Zn, Co, V, Ni, Ni / Co, U, Mo and the Ce negative anomaly), as well as Ca and related elements (Sr, Ba) show fairly high proportions in hybrid rock types, especially those located in the depocentral region (north) of the study area. Together these elements suggest that anoxic conditions were not only common at the water-sedimentary interface, but they also might have increased relatively to the north; in turn, carbonate productivity could have been either less diluted by terrigenous supply or indeed relatively higher to the distal part of the basin. This study shows significant regional changes in inorganic geochemical composition of a relatively thin, highly transgressive interval at the base of the Vaca Muerta Formation, suggesting that accumulation and preservation of those sediments across the entire basin was heavily controlled by intrabasinal factors (rates of supply, productivity, anoxia). These factors should be considered in studies aiming at a better understanding of the unit as source rock and unconventional reservoir across the basin.