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Chemostratigraphic Characterization of the Vaca Muerta Formation in the Neuquén Basin, Argentina: Implications for Depositional Environment and Stratigraphic Distribution of Hydrocarbon Play Elements


The Vaca Muerta Formation is an emerging unconventional resource play of late Jurassic (Tithonian) to early Cretaceous (Valanginian) age. Depositional sequences of mudstones with variable contributions from carbonates and clastics/volcaniclastics are found admixed throughout the succession. Vertical and lateral distribution of mudstone lithofacies indicate westward progradation of the system, resulting in temporal and geographic controls on hydrocarbon play elements quality and distribution. This study utilizes x-ray fluorescence (XRF) on drill cuttings and rotary sidewall cores (RSWC) for 3 vertical wells to enable high-resolution chemostratigraphic differentiation of mudstone packages within the Vaca Muerta. In addition, drill cuttings from a 1200 meter horizontal well were analyzed to assess lateral compositional variability. XRF measurements were calibrated to inductively coupled plasma mass spectrometry (ICP-MS), with significant agreement between the different instrument data (r2=0.973). On the basis of high-resolution chemostratigraphy, 5 disparate parasequence-sets were identified with characteristic chemical attributes and stratigraphic trends. Within the Lower Vaca Muerta, 2 parasequence-sets are recognized and show systematic thinning to the west reflecting decreased sediment flux away from carbonate source areas to the east. This is corroborated by a gradual decrease in Ca and an increase in Si and TOC westward (decreased dilution). Similar elemental trends are observed in the Upper Vaca Muerta. However, stratal architecture of 3 parasequence-sets shows a systematic thickening westward, suggesting erosion and/or sediment bypass with subsequent progradation to the distal areas of the basin. This is further evidenced by seismic toplap terminations (sequence boundary) at the Lower-Upper Vaca Muerta contact and progradational to aggradational parasequence stacking patterns. This study aims to impact pre-drill decisions through delineating vertical and lateral variations in mudstone properties that are critical for reservoir characterization. Moreover, elemental ratios can be used to identify pay intervals, as well as to understand lateral continuity and potential geomechanical/frac barriers (i.e., Ca/Al, Si/Al, Ti/Al, Zr/Al). The results illustrate the importance of XRF chemostratigraphy in geosteering as well as facilitating subsurface characterization, resulting in more precise economics (P10/P50/P90) while reducing costs and uncertainty.