Estructuras sedimentarias inducidas por la actividad microbiana (ESIAM) en sistemas petroleros no convencionales (Los Molles-Lajas), cuenca Neuquina: implicancias para el análisis paleoambental.

Abstract

This doctoral thesis was carried out in deposits of the Los Molles Formation (Toarcian - Lower Callovian) located in the Chacay Melehue, Arroyo Lapa and Picún Leufú Anticline localities within the Neuquén Basin. The Los Molles Formation is one of the oldest source rocks in the Neuquén Basin and constitutes an unconventional shale gas reservoir of great economic importance. In recent years, the presence of levels with microbial mats has been recognized in this unit, although their characteristics and implications have not yet been evaluated. For this reason, the main objective of this study was to analyze, characterize and document the presence of microbial mats in the Los Molles Formation, as well as to discuss their sedimentological and ichnological implications. Finally, the initial results obtained from the comparison between the microbial levels of the Los Molles Formation and those recognized in the Vaca Muerta Formation, are presented. Field work consisted of detailed sedimentary sections and lithological profiling at the three localities, as well as level sampling with MISS. In the laboratory, the samples were analyzed by stereo microscope, petrographic microscope and scanning electron microscope. Five facies associations were recognized and described for the Los Molles Formation: offshore, platform, deltaic front, prodelta, turbidite. Most of the levels assigned to microbial mats were found on deposits belonging to the offshore and platform facies associations and, to a lesser extent, on the turbidite facies associations. The microbially induced sedimentary structures found in the Los Molles Formation comprised Kinneyia-type wrinkle structures, biolaminations, micro mat fabrics, possible dome constructions, reticulate patterns, small pinnacles and elephant skin. In the thin sections, textures and structures characteristics of microbial mats were recognized, such as anastomosing laminations, oriented grains, filaments and heavy minerals bound to the mat layers. Kinneyia-like wrinkled structures, biolaminations, mat microfabrics and pinnacles were also petrographically characterized. Scanning electron microscope images revealed the presence of structures resembling filaments and filament sheaths, spheroidal structures similar to coccoids, extracellular polymeric substances (EPS) and framboidal pyrite in isolation, in addition to EPS linked to filament-like structures and pyrite. Trace fossils assigned to Trichichnus isp., Chondrites intricatus, Chondrites patulus, and Phycosiphon incertum were recognized at the Chacay Melehue and Arroyo Lapa localities. These results reveal that the levels found and analyzed in the Los Molles Formation may have had a biological origin. The structures and textures identified may have originated from microbial mats located on the seafloor of the Neuquén Basin during the Toarcian – Lower Callovian. These communities were established in offshore and platform environments at depths between 200 and 400 m, in paleoecological and paleoenvironmental conditions that allowed their development, such as low oxygenation, low sedimentation rate, and low light. From the integration of ichnological and geobiological analyses, it was inferred that both Trichichnus isp. and the microbial mats could have a common origin from the biological activity of some genera of the Beggiaotoaceaea family. On the other hand, the microbial mats could have been a food source for benthic organisms, which would be reflected in trace fossils of undermat miners found in the organic levels. Comparison with microbial mats from the Vaca Muerta Formation indicates that the similarities in the thin section and the scanning electron microscope are unquestionable despite having formed in different environments. A very interesting aspect, which will be evaluated in the future, is to analyze the possibility that the biolaminated levels have been an important source of organic matter to the system, and thus may have contributed to the generation of hydrocarbons.