Sequence architecture and heterogeneities of a field – Scale Vaca Muerta analog (Neuquén Basin, Argentina) – From outcrop to synthetic seismic

Abstract

The Quintuco – Vaca Muerta System in the Neuquén Basin (Argentina) has received much attention due to its potential as a major unconventional play. This study combines field geology with satellite imagery interpretation to document the sequence architecture of the prograding mixed carbonate-siliciclastic system over several kilometers from the shelf to the basin. Detailed sections allow the allocation of bed-scale lithofacies changes within this sequence stratigraphic framework. Synthetic seismograms are constructed from the observed facies distribution and petrophysical properties in order to compare this architecture with subsurface seismic data.The field area, the Sierra de la Vaca Muerta, shows a prograding system along a 10 km long and 1 km thick cross section. The studied interval can be subdivided into eight depositional sequences based on the facies distribution, representing a very similar scale as the seven sequences documented in the global sea level chart in this time interval. This suggests a eustatic control on the system.The depositional sequences are dominated by calcareous shale and siliciclastics but have variable amounts of carbonates admixed both temporally and laterally, leading to different motifs of cyclicity and geometry. Shales and calcareous silt- and sandstones are the dominant facies. Pure carbonate or pure siliciclastic portions are very rare and restricted to sequence tops. Carbonate-rich portions are characterized by abrupt lateral facies transitions and breaks within their steep depositional profile, whereas siliciclastic-rich intervals show gently inclined profiles with no breaks and more gradual facies changes.In the temporal scale, heterogeneities occur both as cycles and events. Meter scale alternations of pure shale with calcareous shale are the typical cyclic expressions, which occur down-slope of carbonate-dominated systems. Mixed turbidites commonly follow just after carbonate sequences, triggered by the steeper depositional gradients. This distribution indicates that carbonate-rich intervals offer both more brittle volumes for fracturing (calc. shales and silty wackestones) and potential relatively permeable layers (turbidites) within the surrounding tight shale. These sweet spots follow the sequence stratigraphic framework and typically occur in close relation with tops of carbonate dominated cycles.These carbonate cycles prove to be easy to detect in the synthetic seismograms due to their strong acoustic impedance and geometrical expressions. These findings could offer a way to identify sweet spots for unconventional exploration in the basinal transition, since they are laterally related to carbonate sequence tops, which have a distinct seismic signature.