Abstract The relationship between pore structures was examined using a combination of normalized topographical and topological measurements in two qualitatively different pore systems: organic-hosted porosity, common in unconventional shale reservoirs; and intergranular porosity, common in conventional siliciclastic reservoirs. The organic-hosted pore network was found to be less well connected than the intergranular pore network, with volume-weighted coordination numbers of 1.16 and 8.14 for organic-hosted and intergranular pore systems, respectively. This disparity in coordination number was explained by differences in the pore shapes that are caused by variations in the geological processes associated with the generation of the pore network. Measurements of pore shape showed that the pores in the organic-hosted network were both significantly more spherical and had a more positive curvature distribution than the pores present within the intergranular network. The impact of such changes in pore shape on pore-network connectivity was examined by creating a suite of synthetic pore geometries using both erosion/dilation of the existing network and image-guided object-based methods. Coordination number, Euler characteristic and aggregate porosity analyses performed on these synthetic networks showed that organic-type pore networks become connected at much higher aggregate porosities (35–50%) than intergranular-type pore networks (5–10%).

ABSTRACT This introductory chapter describes and discusses (1) the idea of coopetition that gave birth to this AAPG Memoir, (2) the importance of Geology in exploring and developing unconventional plays, (3) the historical perspective on the studies performed in the Vaca Muerta Formation (Argentina), and (4) the commercial context and the critical conditions that have allowed the Vaca Muerta play to be considered the first economical unconventional self-sourced play outside North America. The experience, research and vision expressed in this chapter provide explorers an analog for future unconventional ventures around the world.

ABSTRACT The Late Jurassic–Early Cretaceous Vaca Muerta Formation deposited in the Neuquén Basin has an exceptional development, in the thickness of its sequences and in the volume of its sedimentary record. These marine deposits, which constituted the source rock of conventional oil and gas for more than a century, continue to be an important source of unconventional hydrocarbons as a producer of oil- and gas-shale and have a large potential future. When the exceptional characteristics of the Vaca Muerta Formation are compared to other basins of Late Jurassic to Early Cretaceous age along the same continental margin, they stand out even more. The basin development and preservation parallel to the continental margin are discontinued by important processes of subduction erosion. The analysis of the thickest and widest part of the basin (the Neuquén Embayment) shows that its large development is related to the combination of two rift systems, a north–northwest system truncated by an almost east–west system. These two extensional systems were developed in the hanging-walls of sutures between different crustal terranes that formed the basement of the Neuquén Basin. The paleogeographic development and the structure of these basement terranes are the control that conferred to the Neuquén Basin and its Embayment their unique characteristics.

ABSTRACT The Vaca Muerta Formation deposited in a retro-arc setting, favored by an under filled depocenter and uplifted peripheral zones. The onset of the deposition corresponds to a rapid flooding of oceanic water penetrated through sudden ruptures along the incipient Andean arc, as a consequence of the Late Jurassic extension during the thermal subsidence stage. The Vaca Muerta Formation forms part of the Lower Mendoza Subgroup, and is defined as the organic-rich calcareous mudstone (av. 5% TOC) deposited above the sharp contact of the continental Tordillo and the Quebrada del Sapo formations; the upper contact with the Quintuco Formation is gradational and diachronous. The Vaca Muerta Formation spans the early Tithonian Virgatosphinctes andesensis to the early Valanginian Lissonia riveroi ammonite Zones. In association with classical stratigraphic characteristics of this unit, this chapter presents a cyclostratigraphic and magnetostratigraphic study displaying an unprecedented accurate time scale for the Vaca Muerta deposition, which took place from Chron M22 to Chron M15 in a timespan of ~10 My (4.86 My within the Tithonian and 5.27 My within the Berriasian).

ABSTRACT This chapter summarizes and integrates some of the most relevant published results related to the geomechanics of the Vaca Muerta Formation and its relationship with structural geology, stratigraphy, and petrophysics. The Vaca Muerta unconventional play is overpressured in most parts of the Neuquén Basin. Direct measurements are not feasible because of the low permeability of these rocks. Indirect measurements used to assess the range of pore pressure derive from diagnostic fracture injection tests (DFIT) taken over several days, and the initial reservoir pressure estimated from production models of horizontal wells. The Neuquén Basin underwent several compressional tectonic episodes. The main direction of the maximum horizontal stress changed from northwest–southeast in the Middle Jurassic, to approximately east–west of present day, associated to the Andean tectonics. Maximum and minimum horizontal stress directions, a critical parameter to design horizontal wells, has been regionally characterized using abundant high-confidence indicators, such as wellbore breakouts, wellbore-induced fractures, and microseismic analysis. The chapter presents and discusses the estimation techniques of overpressure and minimum horizontal stress, the overpressure source mechanism, the overburden variations at the base of Vaca Muerta, a regional characterization of rock strength and elastic properties, and the current understanding of the stress regime and the hydraulic fracture behavior. Significant progress has been made during the past five years in the geomechanical characterization of the Vaca Muerta Formation. As drilling, coring, testing, and production data become available, more comprehensive databases are built and uncertainty is reduced. Whereas a few years ago analogs from other basins were the only available information, today, local databases of triaxial and other rock mechanic laboratory tests, together with well logs, are successfully used to elaborate local correlations.