On a worldwide basis, approximately 70 to 80% of the 9 to 10 trillion bbl original oil in place of viscous oil reserves (in-situ 100 cp) occur in unconsolidated sandstones, with high porosity and permeability. The remaining viscous oil reservoirs are hosted within fractured carbonates. Both types of viscous oil reservoirs are characterized by highly variable in-situ reservoir conditions. Different technologies are best for different lithostratigraphic and geometric conditions. New and emerging production and drilling technologies (i.e., horizontal wells, multilaterals, logging while drilling, and others), along with production-technology sequencing, allow tailoring of the drilling and production schemes to each specific reservoir considering the inherent geologic variability of these unconventional reservoirs. Geotailoring for viscous oil production generates a need for geoscience and engineering screening criteria.