Grain Association, Petrography, and Lithofacies

Abstract

This chapter analyzes the different lithofacies of the Vaca Muerta Formation using macroscopic observations, standard microscopy textural studies, scanning electron microscopy (SEM), and x-ray diffraction (XRD) techniques and presents them in a paleo-environmental and a sequence stratigraphic context. Microfacies-scale analyses in the Vaca Muerta Formation reveal a wide range of sedimentary and diagenetic processes that are essential for the understanding of this heterogeneous self-sourced unconventional reservoir. Seventeen of the more representative lithofacies in the Vaca Muerta Formation are described based on textures, grain type, and matrix composition. XRD data indicate that most of the mudstones microfacies show quartz–feldspar values above 40%, carbonate between 20% and 40%, clay lower than 35%, and total organic carbon (TOC) between 2% and 8%. Porosity of crushed rock samples shows common values between 10% and 16%. Porosity observed in SEM can be divided into interparticle, intraparticle, and organic-hosted type. Coccoliths are commonly found as calcareous fecal pellets that can make up to 35% of the rock volume. Pores within pellets range from hundreds of nanometers to more than 5 µm. Lateral and vertical distribution of lithofacies is controlled by the sedimentary environments and the systems tracts. Transgressive systems tracts (TST) show an enrichment in siliceous particles, the highest TOC values, and abundant organic-hosted porosity. Highstand systems tracts (HST) are carbonate rich, with lower TOC values and 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, percentage of TOC, porosity volume, and pore types