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The Effect of Oil Based Drilling Mud (OBM) on the Assessment of Hydrocarbon Charge Potential: Are We Underestimating Source Rock Potential?


Understanding the level of drilling fluid contamination in rock samples is important when the presence and quality of potential source rocks needs to be evaluated. The use of oil based drilling mud (OBM) is widespread throughout the oil industry, particularly during deep water exploration drilling. Contamination with OBM can significantly affect the geochemical signatures obtained from oil and rock samples, and therefore our understanding of hydrocarbon charge potential in the basin. For this reason, several practices have developed within the oil industry to mitigate its effects on both oil and rock samples. One common practice is the extraction of rock samples, by way of organic solvents, in order to eliminate the effects of drilling fluid contamination. A series of five laboratory experiments, designed to replicate temperature and pressure conditions during deep water drilling, were carried out in order to simulate the rock contamination process during drilling. Contaminated samples were taken through traditional extraction procedures and the resulting values were compared with the original values measured in the undisturbed source rock samples. Results from these experiments show significant differences between values measured in the undisturbed samples versus those that were contaminated and those solvent extracted. In addition to the expected removal of hydrocarbons by the extraction process itself, the measured values surprisingly indicate that oil based drilling fluids are also acting as solvents, altering the source rock generation potential in the contaminated samples. These results raise questions concerning whether or not a “true” assessment of source rock potential and character can be obtained when oil based drilling fluids are used. Interpretations of the basic source rock data must take into consideration both what the fluids are adding and removing, as well as the actions of the sample preparation itself.