Lithofacies distribution within the Vaca Muerta Formation in a sequence stratigraphic context based on subsurface and outcrops data

KIETZMANN, D.A.; GONZALEZ TOMASSINI, F.; SMITH, T.

Congreso; International Convention & Exhibition 2019; 2019

Facies analysis in the Vaca Muerta Formation reveals a wide range of sedimentary and diagenetic processes that are essential for the understanding of this heterogeneous non-conventional shale system. Here we describe 17 of the more representative microfacies in the Vaca Muerta Formation based on textures, grain type, and matrix composition, using standard microscopy textural studies, scanning electron microscopy, and X-ray diffraction techniques. XRD data indicate that most of the mudstones microfacies show quartz-feldspars average values around 40%, carbonate between 20 to 50% and clay lower than 30%, as well as 2 to 10% of total organic carbon. Porosity of crushed rock samples shows common values between 8 to 16%. Porosity observed in SEM can be divided in interparticle, intraparticle and organic-hosted. Coccoliths are commonly found in the Vaca Muerta Formation as calcareous fecal pellets grains that can make up to 35% of the rock volume. Pores within pellets range from 100s of nm to more than 5 µm. Lithofacies lateral and vertical distribution is controlled by the sedimentary environment, their position in the different systems tracts, and by their paleogeographical position within the basin. Transgressive systems tracts (TST) show a relative enrichment in siliceous particles, highest TOC values and abundant organic-hosted porosity. High-stand systems tracts (HST) became carbonate-rich, with lower TOC values, and commonly interparticle and intraparticle porosity. Comparison of the lithological dependent characteristics from the Vaca Muerta Formation with other well-known unconventional plays, shows that the Vaca Muerta Formation shares many characteristics with the Oxfordian ? Tithonian Haynesville/Bossier Shale and the Cenomanian ? Turonian Eagle Ford Shale, such as lithofacies, grains composition, mineralogical composition, diagenesis, %TOC, porosity volume and pore types. All these facies are present in semi-regional cross-sections calibrated with seismic positions in a sequence stratigraphic context, being a great linked to propagate across de basin and correlated with tested landings in different assets and fluids.