Caracterización geoquímica y mineralógica de la Formación Agrio y su potencial como roca generadora de hidrocarburos en el centro-oeste de la Cuenca Neuquina.

Abstract

This doctoral thesis constitutes a multi-proxy study that includes sedimentological, stratigraphic, mineralogical, and geochemical information from the deposits of the Pilmatué and Agua de la Mula members of the Agrio Formation (late Valanginian to early Barremian) spanning from the Arroyo Cieneguitas (in the south of Mendoza province) to the Pichaihue locality (in the north of Neuquén province). Four localities and seven boreholes were selected along two correlation transects oriented NW-SW and NW-SE to carry out the research. These transects include distal (i.e, Arroyo Cieneguitas), intermediate (i.e., YPF.Nq.FM.x-40) and proximal sections (i.e. Pichaihue) of a low-gradient mixed siliciclastic-carbonate depositional system. Detailed sedimentological profiles were elaborated and systematic sampling was conducted for mineralogical, petrographic, and geochemical analyses in all sections. Based on the analysis of the sedimentary records of outcrop and subsurface, total of eleven lithofacies were defined and grouped into silicoclastic, mixed, carbonate and pyroclastic facies, which allowed determining nine facies associations that characterize four main depositional systems: deltaic systems, offshore systems, mixed (siliciclastic and carbonate) and carbonate-dominated basin systems. The compositional characterization required of energy dispersive X-Ray Fluorescence (EDXRF), X-Ray Diffraction (XRD) and petrographic studies. All the analyses revealed that the succession is characterized by fine-grained material and compositionally homogeneous with the same association of mineral phases, though in differents proportions depending on the studied. The most abundant components identified were detrital quartz and calcite with subordinately clays, feldspars, plagioclases, algal and terrigenous organic matter and bioclastic fragments. Overall, the succession is characterized by a vertical and lateral decrease in carbonate content and a vertical and lateral increase in terrigenous components. For the geochemical characterization, the results obtained from Rock Eval pyrolysis analysis, organic petrography and C isotope and the estimations made with the Passey’s method and Gamma-Ray measurements were integrated. Correlation panels were designed to establish internal variations where the fluctuations of the different geochemical parameters were identified. A statistical analysis was also conducted to estimate the average values of total organic carbon (COT) and pyrolysis per member, sedimentary facies, and organic levels in each section. Finally, the organic matter type and quality, the hydrocarbon generation potential and thermal maturity were defined using distribution of samples interpreted from geochemical diagrams (S2 versus COT, modified Van Krevelen, IH versus Tmax). Based on all the data it was established that, in general, the Pilmatué Member presents higher organic content compared to the Agua de la Mula Member, and that together they present a total of eight organic levels (NO1-NO8) that disappears towards the southern sector. This decrease in organic content was linked to higher dilution rates due to the input of detrital material and/or degradation of organic matter in an environment with higher energy and oxygenation in more proximal areas. The NO1-NO2 organic levels in the Pilmatué Member and NO5-NO6 in the Agua de la Mula Member exhibit the highest organic content and lateral continuity, with NO1 and NO5 being prevalent along the entire transect. Regarding the facies, all those with generating potential were analyzed regardless of their relative abundance in the diferents sedimentary profiles. This allowed the identification of laminated mudstone-wackestone facies as the most organic-rich, followed by laminated siliciclastic shales (Psl) and massive mudstone-wackestone facies. The geochemical diagrams evidenced a mixture of kerogens ranging from Type I to Type IV, with a predominance of Types II/II-III. Based on the organic petrography, macerals did not show significant changes in their composition, therefore the cyclic variations in pyrolysis data reflect variations in organic matter preservation and/or the effects of thermal maturity. The thermal maturity range for the Agrio Formation was established between 401°C and 532°C of Tmax (%Ro measured and equivalent: ≤0.6-1.84 %Ro). The Pilmatué Member ranged between early maturity and the peak of oil generation in the north and even overmature in the south, associated with greater burial in more internal positions of the basin. The Agua de la Mula Member generally reached the stage of early maturity. Finally, the hidrocarbon generating potential was classified as fair to very good and in specific areas even excellent, to continue generating mainly oil in the north and gas in the south. As for its reservoir potential, two new intervals were identified in the Pilmatué Member associated with the NO2 and NO3 organic levels and one in the Agua de la Mula Member linked to the NO5 organic level. These levels, particularly NO2, coincide with the most enriched deposits in carbonate facies (MWm) and present better quality organic matter (Type I/II-II/III), which makes it possible to define them as those with the best potential and possible targets in the evaluation as unconventional reservoirs.