Natural fracture characterization of the Los Catutos Member (Vaca Muerta Formation) in the southern sector of the Sierra de la Vaca Muerta, Neuquén Basin, Argentina

Abstract

The Vaca Muerta Formation is the main source rock of the Neuquén Basin in Argentina. The interest in this unit has grown over the years as it is considered an excellent unconventional exploration target. It is characterized by a naturally fractured succession of dark organic-rich shales deposited due to a marine transgression during the Late Jurassic to Early Cretaceous. In the Sierra de la Vaca Muerta area, the southernmost sector of the Agrio fold and thrust belt, a carbonate-dominated interval in the Vaca Muerta Formation is outcropping characterized by tabular micritic limestones intercalated with marls known as the Los Catutos Member. This member usually exposed large pavements that allow good observations of the fracture pattern and lengths. We performed a natural fracture network analysis over the mid-upper Tithonian Los Catutos Member outcrops. 585 fractures were measured in the field, and 665 fracture traces were manually mapped using an aerial image acquired by an unmanned aerial vehicle (UAV). Four main sub-perpendicular to the bedding fracture sets were identified, trending WNW-ESE, N–S, NW-SE, and NE-SW. The fracture length distribution was analyzed using the maximum likelihood estimator technique, revealing a lognormal distribution as the best fit for the sets recognized in the UAV image (WNW-ESE, N–S, and NE-SW). Also, the fracture system was classified as stratabound given the spatial and length distribution. The relative chronology between the four fracture sets was established based on the observed abutting relationships; furthermore, based on a fold test, a pre-folding origin was deduced. Finally, a regional interpretation for fracture formation is proposed considering the structure and tectonic history of the basin. The WNW-ESE oriented set formed by the longest fractures together with the orthogonal N–S oriented cross-joints, give rise to a ladder pattern; this arrangement likely responds to an along-foredeep stretching scenario that characterized this area during the Paleocene. Then, the NW-SE and NE-SW oriented fracture sets were emplaced, probably associated with the layer parallel shortening phase installed before the onset of major thrusting and folding from the late Miocene onwards.