JethroThis may explain the heavy oil in Jethro. Poor seal. Not water depth related. From Wikipedia: 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.[citation needed] Heavy oil can be found in shallow, young reservoirs, with rocks from the Pleistocene, Pliocene, and Miocene[18] (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.[citation needed]