Geochemistry of aggradational - Progradational sequence sets of the Upper Jurassic – Lower Cretaceous Vaca Muerta shales (Añelo area, Neuquén Basin, Argentina): Relation to changes in accommodation and marine anoxia

Abstract

Unconventional shale plays have received marked attention over the last years because of their economic potential for hydrocarbon generation. The size and geology of the Neuquén Basin of western Argentina lends to large volumes of unconventional shale gas, particularly within the marine shales of the Upper Tithonian — Valanginian Vaca Muerta Formation. This unit is characterized by a well-developed lithological transition from Tithonian monotonous organic-rich marly shales of several hundred metres thickness, to Lower Cretaceous increasingly carbonate-rich marls and marly limestones. Samples from four wells located in the central sector of the Neuquén Basin (Añelo area) were selected for petrographic, X-ray fluorescence and TOC analysis. Based on seismic interpretation of the strata geometry, these data were grouped according to their location in aggradational (AAS) and progradational accommodation sets (PAS). The AAS is composed of basinal black shale facies showing a planar stacking pattern, whereas the PAS is characterized by low-angle sigmoidal strata in which bottomset, foreset and topset deposits (and coeval basinal deposits) are recognized. Petrographic studies allowed to distinguish lithological variations between basinal (organic-rich, laminated unbioturbated shales and marly shales), bottomset (dark gray, diffuse laminated, slightly bioturbated shaly marls and marls), and foreset and topset deposits (gray, massive, bioturbated marls and calcareous marls with a wacky texture). Basinal deposits show a clear tendency towards compositions rich in silica. These distal deposits, together with the distalmost bottomset facies of the clinoforms, are relatively enriched in Al2O3. In turn, the content of CaO, MgO and P2O5 supplied by carbonate biogenic activity are higher in the shallower topset and foreset deposits. Distal (basinal and bottomset) deposits are significantly enriched in redox-sensitive elements (Mo, V, Co, S, Cr) and in organic productivity indicators (Zn, Ba, Cu and TOC). Basinal deposits accumulated under conditions of maximum sediment starvation, restricted circulation and low dissolved oxygen levels. These conditions partially persisted for the more distal progradational (bottomset) deposits, together with a progressive increase in carbonate intrabasinal productivity. Conversely, the foreset and topset deposits of the PAS formed under normal seawater oxygenation. These conditions favoured the increase of carbonate productivity associated with an ostensible decline of organic matter productivity and preservation. This approach to geochemical analysis in the sequence stratigraphic framework of the Vaca Muerta Formation suggests that redox conditions, biogenic carbonate productivity, detrital siliciclastic fluxes and carbon accumulation rates, were closely related to basin isolation, space available for sediment accumulation, and changes in water depth and marine water circulation.