--> Lithology and TOC at the Base of the Vaca Muerta Formation, Neuquén Basin, Argentina

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Lithology and TOC at the Base of the Vaca Muerta Formation, Neuquén Basin, Argentina

Abstract

Detailed outcrop and geochemical analyses provides a new understanding to fluctuations in lithology and organic matter at the base of the Vaca Muerta Formation in the Neuquén Basin. Total organic carbon content (TOC) varies (<0.1% − ~16%) throughout the entire Vaca Muerta with the highest values in the initial 70 m and a general decrease upsection. Spectral Gamma Ray (SGR) data collected shows a positive correlation between uranium concentrations and TOC that could be beneficial in unconventional exploration. In the Sierra de la Vaca Muerta (SdlVM) area, five sections were measured to observe the lithologic and TOC changes from a proximal to a distal setting. The strata rich in total organic carbon are confined to the fore and bottom sets of a prograding mixed carbonate siliciclastic shelf on top of the mostly eolian strata of the Tordillo Formation. The onset of marine sedimentation in the Vaca Muerta Formation consists of 20 m thick succession of intercalated sands and siltstones after the initial marine mudstone, and another, thinner (10–15 m) package occurs 40 m above the contact, separated by marine mudstones high in organic content. The TOC varies in concert with sequence stratigraphy; the highest observed TOC value is found just below the maximum flooding surface that is associated with the initial flooding of the basin. In the first sequence, average TOC values are slightly higher in the distal section than in the proximal section (4.6% and 3.7% respectively). Up-section the TOC decreases significantly. The distribution, however, follows a sequence stratigraphic control as packages of richer concentrations coincide with the maximum flooding intervals of subsequent sequences. Correlating SGR and source quality has been used with varying success in other systems. This study's findings in the Sierra de la Vaca Muerta show that uranium concentrations show peaks at stratigraphic intervals that coincide with higher TOC values. Based on these observations, trends in organic material follow predictable patterns (controlled by lithology and sequences) with the highest concentrations associated with the initial flooding of the basin.