Characterization of Small-Scale Cycles of the Vaca Muerta Formation in Short-Cores and Outcrop, Neuquén Basin, Argentina

Abstract

Abstract

High-frequency cycles of the Vaca Muerta in the Sierra de la Vaca Muerta area display variations of facies, carbonate content, and total organic carbon (TOC) that are linked to the transgressive and regressive portions of each cycle. In the studied section the mudstone cycles are around 1 to 4 m thick, coarsening and thickening upward. Mudstones are fissile and laminated. The regressive portion of the cycles is thinner and grades from faintly laminated calcareous mudstone to silty-calcareous mudstone up to wackestone with fragmented skeletal pieces like ammonites and bivalves. The top of the cycles is often capped by a more resistant calcareous bed and occasionally by concretions. The capping beds of yellow-brown color are dolomitic mudstones filled with fine, 5 mm thick, calcite veins.

11 short-cores (∼1m long) from the Vaca Muerta Fm were drilled in the south of the basin. They were scanned every 0.5 cm using an XRF scanner retrieving ultra-high resolution variability of elemental composition. The cores are rich in Ca, Fe, Si, S, and K. Calcium provides a first index for the calcium carbonate content and helps to define cm-scale carbonate rich cyclicity. Fe and S high peaks reflected bioturbated areas with pyrite, and Si enrichment is attributed mainly to replacement of quartz.

The cores also display in great detail some of the facies transitions in the basin. In the south, the basal core of the Vaca Muerta is a black mudstone-wackestone with ammonites and oysters that directly overlies the eolianite of the Tordillo Formation, documenting a rapid flooding of the basin. The peak of this initial flooding is 37m above the flooding surface. It displays the highest TOC value of in this part of the basin. Cores from this high TOC interval reveal a fissile, very finely laminated mudstone. Here the cycles are capped by concretions in which the sedimentary structures and the fossil content are well preserved. Up-section at 95-127m a carbonate-rich interval, the Los Catutos Member, marks a dramatic facies change. In short cores, these carbonates are alternations of light brown faintly laminated and highly bioturbated mudstones with some fossils replaced by pyrite. 5 short-cores were drilled from the center of the basin where the underlying Tordillo Formation is a lacustrine brown-weathered claystone and the base of the Vaca Muerta is a light gray cryptalgal laminated bindstone that transitions into calcareous mudstone with skeletal fragments.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016