Microfracture Characterization in the Lower Vaca Muerta Formation, Neuquén Basin, Argentina

Abstract

The Vaca Muerta Formation in the Neuquén Basin, Argentina, is an emerging resource play containing shale oil and gas in Late Jurassic to Early Cretaceous foreland basin strata. Like many black shales, it is characterized by the presence of abundant bedding-parallel microfractures within the organic-rich matrix. Sub-vertical fractures are also observed across the studied section, adding complexity to characterization of the mudrocks. The lower Vaca Muerta is composed of alternating mudstones-wackestones, bioclastic siltstones, carbonate mudstones, and bentonite facies. The mudstone-wackestone facies is characterized by an impermeable, wavy, clay-rich matrix that encompasses the kerogen grains. The concept of bedding-parallel microfracturing has been related to thermal maturation of kerogen in organic-rich black shales. Within the oil maturation window, overpressure at the lower Vaca Muerta interval is correlated with significant total organic content values (up to 10 wt%). These conditions eventually dominate the mechanical behavior of the formation. Observed microfractures are filled or partially filled with calcite cement. Thus carbon and oxygen stable isotope chemostratigraphy together with thin-section petrography aid in establishing the paragenetic sequence, and ultimately identify the origin of the aqueous and hydrocarbon fluids. In addition, microfractures may have the potential to contribute to overall effective permeability of the matrix depending on their length, aperture orientation, and connectivity to larger fractures. Thus, microfracture characterization and their relation to facies is key for identifying the sweet spots for future production.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015