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.

Abstract Scanning electron microscopy (SEM) has become a common way to estimate porosity and organic matter (OM) content within shale resource rocks. Since quantitative SEM analysis has emerged as a means for assessing the porosity of shale, a common goal has been to image polished samples at the highest possible resolutions. Because nanopores are visible at pixel resolutions ranging from 5 to 10 nm, it is natural to consider the possibility of a pore regime below 5 nm that could contribute a significant amount to the total porosity of the system. When considering that a molecule of methane gas is on the order of 0.4 nm diameter, pores smaller than 5 nm could contribute significant storage volume and transport pathways in a reservoir. These nanopores may be a significant source of porosity within certain OM bodies, where total detectable pores using SEM (i.e., ~10 nm pore body diameter and up) have been observed to be volumetrically equivalent to the OM body volumes themselves. With the potential to examine the population of pores below ~10 nm in diameter using the helium ion microscope (HIM), it is possible to construct a rock model that is more representative of the varied pore size regimes present. The primary goal of this study was to quantify the amount of organic-associated pores below the resolution of conventional field emission scanning electron microscope (FESEM). In this study, 51 individual imaging locations from 12 organic shale samples were selected for systematic imaging using a HIM. These samples and locations were selected because of the presence of porous OM identified from previously completed SEM imaging. After methodical HIM imaging and digital segmentation, it was concluded that most samples had no significant incremental, resolvable, organic pore fraction below the detection threshold of conventional FESEM imaging. The advanced resolution of the helium ion beam provides sharper definition of pore boundaries, but the total porosity fraction of these <10 nm diameter pores within the OM in most samples was negligible. We also notice that FESEM and HIM can be considered complementary techniques, as each provides beneficial information that cannot be obtained from using only one method.