Heavy oil is generally characterized by high specific gravity, and high contents of heavy components. It can also possess increased levels of heavy metals, sulfur and nitrogen. It is generally assumed that high-viscosity heavy oils are the result of alteration processes of crude oils by deep biosphere microbial activity. The biogenic alteration strongly impacts the economic potential of an oil accumulation because these properties affect both the recovery process and its value. We investigated heavy oils from different basins worldwide using selected biomarker parameters obtained from gas chromatography-mass spectrometry (GC-MS) analysis. Additionally, data from stable carbon isotopes, SARA analysis, elemental analysis, GC-FID and freezing-point depression were evaluated to characterize these oil samples. The aim of this study was to investigate new correlations of fluid properties and geochemical parameters with oil viscosity and density for heavy oils from different basins. On a regional scale, oil viscosity (i.e., viscosity for oils from the same petroleum system) is a function of maturity and the magnitude of alteration; thus, reasonable correlations between selected biomarkers and oil viscosity within a petroleum system can be made in which the magnitude of alteration is the primary factor controlling variation in fluid properties. The present study shows, however, that the degradation–viscosity correlation cannot be globally applied to compare heavy oils from different basins; heavy oil viscosity and density are predominantly determined by original thermal oil maturity and organofacies characteristics when comparing respective oils from different basins. Further, correlations between the extent of biodegradation and oil viscosity are commonly observed in regional studies but cannot be applied to a worldwide dataset because of the effects of other post-generation processes, source-rock heterogeneity, or oil mixing. Although biomarker characteristics of heavy oils from different reservoirs do not show a direct correlation to oil viscosity, they can be used to help limit potential oil viscosity ranges based on maturity and organofacies characteristics. In addition, biomarker studies show promise as a method to predict the ranges of viscosity and density for individual reservoirs. For example, biomarkers can be utilized to explain why some oil fields will never reach the viscosities and densities of others. Furthermore, the geochemistry of oil samples can explain outliers and why, in many cases, trends in the physical data have been difficult to establish (i.e., mixing of oils). The results of this study show that predictive models of oil viscosity using oil compositional characteristics require taking geochemical parameters on original maturity and organofacies into account.