Petrographic, mineralogical and geochemical characterization of organic-rich, fine-grained deposits in the Pilmatué Member of the Neuquén Basin: implications for the explorations and development of unconventional reservoirs

MANUEL ISLA; MARIANO REMIREZ; LUIS SPALLETTI

Buenos Aires

Congreso; International Conference and Exhibition 2019; 2019

American Association of Petroleum Geologists

Sedimentological attributes of mudstones have important implications in unconventional reservoirs. Understanding the variations of the fine-grained deposits in a depositional system constitute a key point for the exploration and development of such reservoirs. The aim of this contribution is to evaluate the influence of terrigenous input, carbonate productivity and redox conditions of a mixed (clastic-carbonate) marine ramp, through high-resolution sedimentology and inorganic geochemistry in the offshore deposits of the Pilmatué Member, Agrio Formation (Late Valanginian ? Early Hauterivian, Neuquén Basin, Argentina).Six stratigraphic sections (350-650 m thick) were logged in a north-south transect of 50 km in the northern Neuquén province. A total of 400 samples were processed using handheld X-ray fluorescence equipment. The elements used for this characterization were: SiO2, K2O, TiO2, Al2O3, Th and Zr, as detrital supply proxies; CaO and Sr as proxies of carbonate productivity; and V, Co, Cu, Mo, S and Ni as redox proxies. Additionally, textural and mineralogical features were defined through X-ray diffraction for mineralogical studies and thin sections for textural definition.Results allowed to define two different accommodation sets: aggradational (AAS) and progradational (PAS). The AAS comprise the lower half of the Member and is mostly composed of carbonate-rich and marly fine- to medium-mudstones rich in organic matter, whereas the PAS is dominated by fine- to coarse-siliciclastic-rich mudstones. These variations are also recorded through geochemical studies showing that the AAS is more enriched in CaO, Sr and redox proxies whereas the PAS is enriched in SiO2, K2O, TiO2, Al2O3, Th and Zr and depleted both in carbonate productivity and redox proxies. Moreover, lateral variations within the sets are recorded, exhibiting a subtle increase in the grain size and clastic components proxies towards the south, with the consequent decrease in carbonate components and redox proxies.Understanding the relationship between sediment supply and carbonate productivity and the redox conditions in a mixed system, will allow to predict the variability of related properties in unconventional reservoirs. In the studied case, higher proportions of carbonate-rich lithologies are linked to the best conditions for production and preservation of organic matter.