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High Resolution Non-Destructive Chemostratigraphy of Vaca Muerta Fm: New Evidences of Black Shale Sedimentology Features


This paper is based on soft inorganic geochemistry principles. Soft inorganic geochemistry is defined as the spatial modeling of geochemical data which prioritizes the amount of data, their spatial relationship and their relationship with other data types (geological and geophysical data) over the individual chemical analysis accuracy. The method is based on the statistical concept of processing the entire rock samples population available. It involves thousands of chemical analysis of cutting and core samples, their processing using geostatistical techniques and the integration with well log data, to produce unconventional resources models. Non-destructive analysis was made using rapid handheld X Ray Fluorescence analysis equipment (HHXRF). After, a selection of representative samples were chosen for destructive analysis (XRD and TOC analysis). Finally, geostatistical analysis integration of well log and HHXRF data using 1D Gaussian cosimulation (type II Markov model) to produce geochemical logs was carried out. Two cores belonging to the base of Vaca Muerta Fm. were processed using this non-destructive methodology. A total of 180 readings separated by 10 cm to each other were made, recording 32 chemical elements. Moreover, 13 samples for XRD and TOC destructive analysis were processed. Vaca Muerta Fm. has a pattern of certain chemical elements that are invariably repeated and help to characterize the genesis of euxinic sedimentary environments. Molybdenum and Vanadium metals correlate with TOC content and hence the anoxic level of the sedimentary environment. Additional elements such as chromium, nickel and arsenic are added to the geochemical pattern and indicate that redox front would be well above the water-sediment interface. Other elements such as Zr, Ca and their ratio are important indicators of chemical and clastic alternating environments of eustatic origin in lower Vaca Muerta Fm. The sedimentary cyclicity observed between associations of Mo / Zr / TOC / V and associations of Mn / Ca, allow to infer the existence of probable turbidity currents in a shallow sea, intercalating anoxic levels (Mo / Zr / TOC / V) with dysoxic levels (Mn / Ca) similar to what happened in the Devonian Canadaway Fm. Pennsylvania. The sedimentary cyclicity may be related to atmospheric phenomena that periodically alter the ocean organic productivity, similar to those occurring in the South American Pacific coast known as the phenomena of ‘El Niño’ and ‘La Niña’.