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High-Resolution Geochemical Analysis of Cycles of the Vaca Muerta Formation, Neuquén Basin

Abstract

The early Tithonian-Valanginian Vaca Muerta (VM) Formation in Neuquén Basin, Argentina, contains cycles of fine-grained calcareous and siliciclastic sediments with variable amounts of total organic carbon (TOC). The cycle motifs change from the proximal to distal portion of the basin and from the base of the VM Fm. XRD and XRF measurements help assess geochemical and lithological associated changes. Facies associations and their stacking patterns are used to define cycles in outcrops in the Sierra de la Vaca Muerta (SdlVM) and the Puerta Curaco (PC) regions of the basin. The observed lithofacies are clastic mudstones, partially calcareous to calcareous mudstones, wackestones, packstones, floatstones, sandstones, and altered volcaniclastic deposits. Basic cycles (5th order) are ~1-3m thick; stacked basic cycles form cycle bundles (4th order) and are ~ 3-25m thick. Mixed cycles are dominant in the basin; the regressive hemicycle is calcareous dominated while the transgressive portion has very low-to-low carbonate content (<30%). However, almost pure carbonate cycles are also found, (60m-thick in VM-member in SdlVM, and <60m-thick in PC at ~ 350, ~450, ~ 500 m up in section) representing an increase in periplatform sediments. Cycles in the more proximal position (SdlVM) are thicker and more carbonate-rich than the cycles in the basin center at PC. Concentrations of U, Th, and K were measured every m with a handheld gamma-ray spectrometer and a TOC sample was taken at each location. Nearly 100 short cores of approximately 1-m in length were drilled within the sections for detailed analysis of TOC, mineralogy, and elements within the cycles. The cores were scanned every 0.5 cm using an XRF scanner retrieving ultra-high resolution variability of elemental composition. 22 major and trace elements were found in all cores. Preliminary results show that enrichment of Ti, Zr, and Si elements correlates with intervals where clastic sediments and quartz are dominant; Ca enrichment and depletion coincide with carbonate content. S and Fe peaks are directly linked to volcaniclastic deposits, helping to differentiate from other clastic deposits (enriched in Ti, and Zr). Fe and S peaks also coincide with pyrite. In addition, high correlations were found between V, Mo, Ba, Ni, and TOC. Mo is associated with anoxic to low-oxygen conditions. This high-resolution elemental and mineralogical study attempt to refine the characterization of the cycles in the Vaca Muerta formation.