Abstract

As exploration migrates into deeper water, crude-oil quality becomes increasingly important. Variations in oil quality, which are reflected in such properties as API gravity, viscosity, sulfur content, and acid number impact both value and producibility. In fact, issues of oil quality in deep water may, in some cases, be more critical than issues of hydrocarbon volume. Problems associated with deep water are commonly thought to be amplified largely as a result of the expansion of the biodegradation window. The expanded window is a result of lower temperatures at the mud line. A review of data from the Gulf of Mexico and the Gulf of Guinea reveals that other factors may have a greater influence on oil quality. For example, in the Gulf of Mexico, strong evidence exists that the nature of the source rock is a major factor in establishing sulfur content and API gravity. Oils derived from an Upper Jurassic Oxfordian calcareous source rock generate oils with higher sulfur contents than those derived from Cretaceous argillaceous source rocks. In the Gulf of Guinea, although many of the newly discovered pools are shallowly buried and evidence for biodegradation exists, crude-oil quality is mitigated by multiple charging events. In both regions, evidence also exists for phase segregation, which introduces light oils and condensates into the shallow part of the sedimentary sequence. Both phase segregation and multiple charging events appear to be largely a result of an individual trap's structural evolution. The available data, therefore, suggest that some of the risks associated with oil quality may be reduced through a more detailed assessment of a prospect's filling history and structural evolution and an understanding of the nature of the source facies. However, there clearly are such situations as offshore Brazil, where the risks associated with oil quality appear more difficult to mitigate.

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