Organic-rich Stratigraphic Units in the Vaca Muerta Formation, and their Distribution and Characterization in the Neuquen Basin (Argentina)

摘要

The Vaca Muerta Formation consists of marine, dark, fine-grained mudstones, marls and limestones, and it is the main proven source rock for conventional reservoirs in the Neuquen Basin. In recent years, it has also been intensely studied and tested as an unconventional shale reservoir. The main objective of this contribution is to define and evaluate the distribution of the organic-rich intervals (TOC >2%), as well as their geological controls, within a regional sequence stratigraphic framework for the Vaca Muerta Formation in an area of 28,000 km2 of the Neuquen Embayment. The workflow includes a first stage of interpretation of 2D and 3D seismic data, followed by the correlation of 172 exploration wells evenly distributed across the Neuquen Embayment. The result is a regional sequence stratigraphic model that consists of three 'low-frequency sequences' (LFS). Regional seismic lines show that the fine-grained facies of the Vaca Muerta Formation display a predictable pattern of thickness changes, from 100 to 800 m in a SE-NW direction, in the gas window region, which corresponds to bottomset and foreset segments of prograding clinoforms. However, the organic-rich intervals (unconventional target), which range from 80 to 450 m in thickness, do not follow the same pattern. In order to asses this variability, these 'high TOC' intervals were integrated within the stratigraphic model, and were assigned to five distinct organic-rich stratigraphic units (OVM1 to OVM5). The properties of each unit (effective porosity, water saturation, TOC, and carbonate, quartz and clay content) were modeled from well logs and laboratory sample measurements for wells in the area of study. The statistical analysis of the results provided the basis for understanding the vertical and lateral variations of properties of the Vaca Muerta Formation 's unconventional target at exploration scale. The preparation of thickness maps for each unit enabled the analysis of the spatial distribution of each organic-rich stratigraphic unit and the possible geological controls, which range from stratigraphic (i.e., evolution of the prograding clinoforms) to tectonic (i.e., tectonic setting, differential subsidence in depocenters, etc.). The data integration regarding the areal distribution of each OVM, thermal maturity, fluid type windows and the activity of O&G companies with focus on the Vaca Muerta Formation provides a very useful analysis tool with strong impact on future strategies for the development of this unconventional play.