Análisis petrográfico y procedencia de las areniscas líticas glauconíticas de la Formación Magallanes inferior en el sector central de la cuenca austral, sur de Santa Cruz

Abstract

The Lower Magallanes Formation (Upper Cretaceous-Lower Eocene) hosts the productive reservoirs of the most important hydrocarbon deposits in the Austral basin. The reservoir informally called M1 is located in this Formation, which was discovered as a productive gas and condensate reservoir in the Campo Indio field in 2001 with well CI.a-23. The petrophysical heterogeneities of the reservoir to the north and east of the reservoir caused these extensions to be relegated and in the background, focusing the initial commercial production in the western sector between 2002 and 2008, where the best petrophysical properties predominate. Much of the information obtained in the development and extension wells allowed to start a new stage of development in 2016 in the eastern sector, which managed to quintuple the production of the Campo Indio deposit until the beginning of 2018. The main objective of this work is to understand the composition and origin of the elements that make up the clastic fraction of the rocks of the M1 reservoir in the Lower Magallanes Formation and how the lithological, compositional and biogenic factors affect the petrophysical characteristics of the rock. For this, a detailed petrographic and mineralogical study of samples belonging to crown cores from 4 wells of the Campo Indio deposit was carried out, integrating quantification studies and chemical microanalysis using SEM-EDS. In this way, it was possible to identify that the rocks correspond mainly to fine feldspar-quartz-lithic and quartz-feldspar-lithic sandstones, with a high degree of bioturbation evidenced by biogenic structures and glauconitized fecal pellets. From the modal counts performed, it was observed that the samples present poor to regular grain size selection and clasts with angled to sub-rounded edges. Based on the joint analysis of these characteristics, it was possible to infer a reduced degree of transport and a pattern of origin of transitional magmatic arcs, with imprints of dissected and non-dissected arcs. The distinctive and indisputable feature of the clastic fraction are the autochthonous glauconites, recognizable in the form of glauconitized fecal pellets and identified as sedimentary lithics, which present characteristic green colors and particular morphologies. The diversity in the colors of the glauconites shows different evolutionary stages, as a consequence of the variation of the iron and potassium content. The quantity of the pellets and the evolutionary degree of the glauconites coincide with the moments in which the rhythms of the sedimentary rates present variations, which would be related to the time of permanence of the same in the water-sediment interface or at greater depths, due to the infaunal habit of many organisms that produce the pellet.. Glauconitized pellets in an advanced evolutionary stage are conditioned with periods of relatively low sedimentation rates, where they remain on the seabed for long enough times reflecting periods of no sedimentary deposition. What is observed is that when there are pauses in sedimentation, the evolutionary degree of the pellets and their abundance is greater, for that moment and place in the basin. In addition to the aforementioned, the study and quantification of glauconitized fecal pellets showed that they do not cause deterioration in the total porosity values of the crowns, so they would not mean a problem in the petrophysical properties of the reservoir rock. Linked to this, the presence of disseminated and framboidal pyrite also increases as the pellets are inverted, which confirms its biological origin under microreductive conditions. On the other hand, it is necessary to take into account that both the pyrite and the Fe content in the glauconitized pellets have a severe impact on the electrical response of the reservoir. The scope of this work tries to reveal the composition of the clastic fraction of the rocks that make up the M1 reservoir of the Lower Magallanes Formation, and also to achieve a petrographic approach to the products of the bioturbation of the substrates, which in this case are glauconitized fecal pellets. Thanks to the degree of detail achieved, it will be evaluated how to improve them in the composition and petrophysical properties of the sandstones, causing variations in these parameters. These aspects should undoubtedly be taken into account for a better characterization of the sandstones and as a possible element to identify key correlation surfaces.