~84% of those for coal (0.078/0.093) - higher than CO2 emissions of conventional oil. Environmental Research Web has reported that "because of the energy needed for extraction and processing, petroleum from Canadian oil sands has higher life cycle emission" compared with conventional fossil fuels; "up to 25% more." Geological origin Most geologists agree that crude becomes ‘heavy’ as a result of biodegradation, in which lighter oils are preferentially consumed by bacterial activity in the reservoir, leaving heavier hydrocarbons behind. This hypothesis leans heavily on the techniques of petroleum geochemistry. Poor geologic reservoir sealing exposes the hydrocarbons to surface contaminants, including organic life (such as bacteria) and contributes to this process. Heavy oil can be found in shallow, young reservoirs, with rocks from the Pleistocene, Pliocene, and Miocene (younger than 25 million years). In some cases, it can also be found in older Cretaceous, Mississippian, and Devonian reservoirs. These reservoirs tend to be poorly sealed, resulting in heavy oil and oil-sands. Chemical properties Heavy oil is asphaltic and contains asphaltenes and resins. It is "heavy" (dense and viscous) due to the high ratio of aromatics and naphthenes to linear alkanes and high levels of NSOs (nitrogen, sulfur, oxygen and heavy metals). Heavy oil has a higher percentage of compounds with over 60 carbon atoms and hence a high boiling point and molecular weight. For example, the viscosity of Venezuela's Orinoco extra-heavy crude oil lies in the range 1000–5000 cP (1–5 Pa·s), while Canadian extra-heavy crude has a viscosity in the range 5000–10,000 cP (5–10 Pa·s), about the same as molasses, and higher (up to 100,000 cP or 100 Pa·s for the most viscous commercially exploitable deposits). A definition from the Chevron Phillips Chemical company is as follows: The "heaviness" of heavy oil is primarily the result of a relatively high proportion of a mixed bag of complex, high molecular weight, non-paraffinic compounds and a low proportion of volatile, low molecular weight compounds. Heavy oils typically contain very little paraffin and may or may not contain high levels of asphaltenes. Heavy crude oil is generally categorized in two ways: Those that have over 1% sulfur (high sulfur crude oils), with aromatics and asphaltenes. These are mostly found in North America (Canada (Alberta, Saskatchewan), United States (California), Mexico), South America (Venezuela, Colombia and Ecuador) and the Middle East (Kuwait, Saudi Arabia). Those that have less than 1% sulfur (low sulfur crude oils), with