Vaca Muerta Formation: An Example of Shale Heterogeneities Controlling Hydrocarbon Accumulations

Abstract

Vaca Muerta Formation, a thick Upper Jurassic unit covering at least 25,000km 2 , is located in Neuquen Basin and represents the most important source rock in Argentina. Its geological and petrophysical variations are significantly linked to the geodynamic history of the basin strongly related to the transition from extensional reactivation to thermal relaxation which characterized basin s evolution through time. Major cyclic events related to sea oscillations provided the proper marine environment conditions to promote the development of several source rocks within the basin apart from Vaca Muerta Formation, such as Los Molles and Agrio formations. Regional surface data suggests that vertically (time domain) and laterally (space domain) Vaca Muerta Formation comprises a wide variety of lithologies: shales, marls, carbonates, calcareous sandstones and sandstones. Subsurface data, obtained from mud logging, cores and open hole logs also demonstrate it. Both sources provide sufficient sampling density for interpreting the different facies of the unit from the Lower Tithonian to the Lower Valanginian and also considering its areal extent within the basin. These variations are related to the basement fabric, post-rift subsidence interrupted by inverse reactivation, east-west basin asymmetry and climate changes from semiarid seasonally humid to warm and dry. A summary of these characteristics are: a) a prograding NW carbonatic ramp best represents depositional environment; b) formation thickness increases to the west and north; c) carbonates are better developed in proximal areas; d) organic matter content increases at the base of the formation; e) paleotopography clearly controls facies distribution; f) to the north of the basin hydrocarbons are related to organic rich argillaceous limestones, to the center of the basin are mostly pelithic sediments while to NE correspond to a mixture of the above mentioned, to the south the euxinic conditions favored hydrocarbons with increased sulfur concentration. Consequently these vertical and lateral facies variations control key aspects such as: areal distribution, depth, thickness, porosity, permeability, heterogeneity, TOC, mineral composition, reservoir pressure and geomechanics. In this paper a comprehensive analysis using outcrop, well, and laboratory data of the Vaca Muerta Formation are presented summarizing the main hydrocarbon s accumulations with its unconventional reservoir characteristics.