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.

Petrophysical characterization and understanding of pore systems and producibility in unconventional reservoirs remains challenging when evaluating reservoir potential. This study’s main objective is to identify and evaluate the controls on petrophysical rock types in unconventional low porosity, low permeability carbonate reservoirs in Mississippian-aged rocks of the southern Midcontinent. Representative samples selected from cores in the study area are calcareous siltstones and grain-rich packstones to grainstones. Rock fabric, pore types, and pore structure of 23 samples were investigated using multiscale image analysis of optical micrographs and scanning electron microscope (SEM) mosaics. Petrographic observations and quantified pore parameters were correlated with nuclear magnetic resonance (NMR) plug measurements of transverse relaxation times (T 2), pore size distribution, and porosity. Results indicate that pore structure, permeability, and NMR response are closely linked to the dominant pore types, pore sizes, and mineralogy, which are distinctive for specific rocks—allowing for petrophysical rock type (PRT) grouping. NMR signature geometry is distinct in each of these rock type groups. Complex mixed mineralogies in these rocks homogenizes porosity and permeability relationships among rocks of different depositional facies, making it difficult to define clear-cut correlative relationships between pore architecture, rock fabric, and petrophysical response. Petrographic assessment indicates that the primary cause of pore-scale heterogeneity and varying petrophysical response is related to postdepositional diagenesis, such as silicification, cementation, dissolution, and mineralization along pores and pore throats, which produce complicated pore systems and affects matrix permeability. These observations confirm that incorporating geologic information such as mineralogy, diagenesis, and pore types/pore architecture into rock typing workflows in carbonate mudrock reservoirs is critical to understanding petrophysical response. Additionally, the distinct geometries in each petrophysical rock type group establishes the viability of using NMR as a rock typing tool based on the correlative relationships between NMR response, pore types, and facies.

Abstract There are a variety of pore types in unconventional resource mudstones and shales. The currently preferred method by geologists and petrophysicists is to examine and analyze these mudstones and shales by argonion and focused ion-beam milling to produce an ultrasmooth surface, coupled with observation under the field emission scanning electron microscope (FESEM). Potential issues with Ar-ion milled/FESEM preparation and imaging include (1) the small size of sample cubes for upscaling, (2) loss of structural fabric during the milling–imaging process, (3) fewer non-inorganic pore types observed than when observed with an unpolished surface, (4) analog use of pores from one shale to another, although the pore types and composition might differ, and (5) the creation of potential artifacts related to desiccation and rock expansion because of core retrieval and sample preparation. Conventional FESEM images obtained from freshly broken surfaces reveal much more textural detail than those obtained from ion-milled (polished) surfaces. Although conventional FESEM methodology may share some of the same limitations as Ar-ion beam-milled/FESEM technology, FESEM methodology should not be overlooked because it provides a more cost-effective and potentially more accurate analysis for estimating porosity and determining pore types and their distribution in shales. Comparison of FESEM images from ion-milled and fresh, non-ion-milled surfaces reveals that organic matter and internal organoporosity are best viewed on ion-milled surfaces, but shale microfabric and non-organoporosity is best viewed under non-milled surfaces. Complete FESEM imagery for shale characterization should include both types of analyses.