AAPG Annual Convention and Exhibition

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Application of Oil Gravity and Sulfur Content Relationships to Oil Typing and Source Rock Kinetics


API gravity and sulfur (S) content of crude oil are a function of the organic composition and thermal maturity of the source rock as well as oil alteration effects such as biodegradation, water washing, and thermal cracking. Orr (1986, 2001) has shown that the API gravity versus S content relationship is a function of the S content of the source kerogen, and he defined linear boundaries that differentiate oils derived from high sulfur kerogen (Type II-S), medium sulfur kerogen, or low sulfur kerogen. This study builds on Orr's work to develop a method to utilize gravity versus S relationships in chemometric oil typing studies and in basin modeling studies where source-rock organic S content and oil generation kinetics are unknown. Assuming an x axis intercept of zero % S at 40 degrees API, a slope value for an oil can be calculated: wt% S/(API gravity – 40). Oils with the same slope are derived from source kerogen with the same S content. For example, an oil with a slope of -0.125 (or greater) is derived from Type II-S kerogen, similar to Orr's line that separates the Type II-S and Type II zones. Oil families with known kerogen-sulfur content can be used as calibration for oils of unknown source to determine the appropriate kinetic parameters to use in basin modeling. Examples will be presented illustrating the application of this method to oil typing chemometrics and basin modeling and the limitations of this approach due to oil alteration effects.