Where is the Vaca Muerta Sweet Spot? The Importance of Regional Facies trends, Thickness Variations, and Thermal Maturity in Generating Play Concepts

Abstract

The Vaca Muerta is composed of dark, fine-grained, organic-rich mudstones, marlstones and limestones. This marine succession (Tithonian-Valanginian) is located in the Neuquén basin of western Argentina and was deposited in a restricted, low energy, back-arc setting (proto-Andes) with a predominant prograding direction to the northwest (paleo-pacific ocean). Andean Cenozoic compression uplifted the western and distal parts of the basin, whereas the eastern portion remains in subsurface as one of the country´s best O&G productive basins. Regional seismic lines (100-180 Km) from proximal to distal facies (south-east to north-west), show a highly regressive stacking pattern for the Vaca Muerta interval. When analyzed in detail, the succession can be divided into at least 5 major cycles or depositional sequences with distinctive seismic stacking patterns. The base is marked by a regional flooding event followed by prograding clinoforms with well-defined topsets, foresets and bottomsets in proximal and central areas of the basin (mostly subsurface). However, as we move north-west into the distal facies (subsurface and outcrops), the Vaca Muerta shows a horizontal "railroad track" stacking pattern. This is due to the distal stacking of multiple clinoform bottomsets, which results in a much thicker organic-rich section (~350m) than in proximal areas (~150m). Based on the analysis of cores, well data, biostratigraphy, petrophysics, geochemistry, and 2D/3D seismic data, we were able to correlate (chronostratigraphically) distal facies of Vaca Muerta to proximal basin-margin settings. As a result we obtained a conceptual and predictive sequence stratigraphic model to explain lateral and vertical changes in facies, TOC, and mineralogical composition. The combination of seismic stratigraphic and geomorphic techniques allowed obtaining regional interpretations of depositional systems, slope break delineation, direction of progradation and mapeable facies belts, all of which provide a basis for reservoir prediction, lithology distribution, and sweet spot delineation for unconventional exploration. Additionally, the thermal maturity along the depositional dip ranges from "Peak oil" window in the east to "dry gas" in the west, having cross-cutting relationships with facies belts. Having a clear understanding of these relationships is critical in defining reservoir property maps and regional basin play concepts.