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A Simple Geotracer Correlation Analysis Reveals Charge and Filling Patterns

Abstract

We propose a simple, but powerful geotracer correlation analysis approach to reveal charge and filling patterns among different fields. The geotracers we used are carbazoles, a family of neutral pyrrolic nitrogen compounds existing naturally in crude oils. The concentration of carbazoles in crude oil decreases during migration due to their removal from oil by adsorption to solid surfaces and partitioning into water, which in turn results in concentration variations among different fields. Our approach is based on the concept that fields more closely related with respect to their filling should have a stronger correlation of carbazole compositions, than those that are less related. That is correlation coefficients of carbazole compositions can quantify the charge and filling relationship among different fields. We tested the approach in an exploration area in Saudi Arabia. Our results show that the correlation coefficients of C0 and C1 carbazoles and C0 benzocarbazoles vary significantly among the six fields studied. According to their correlation coefficients these fields can be divided into two groups. Within each group, the correlation coefficients between any two fields are very high (0.87 – 0.94). In contrast, correlation coefficients between any two fields from opposite groups are much lower (0.28 – 0.72). Supported by other evidence, we infer that these six fields are located along two different migration pathways, with each apparently being charged from a different part of a common source kitchen. Although in line with our current understanding of general migration trend and overall migration picture inferred from geotracers data (Arouri et al., 2011), our new approach offers further resolution into migration pathways. The correlation coefficients of C3 – C6 carbazoles and C1 – C3 benzocarbazoles are similarly high among all the fields studied. This suggests that these compounds are less sensitive to migration, but more reflective of a common source rock type. In conclusion, carbazoles correlation provides a high-resolution tool to define charge and filling patterns among different fields, and perhaps also helps infer source similarities of the oils in question. Arouri K.R., S.K. Panda, S.A. Satti, Y. Yang, 2011. Migration Tracers reveal long-range migration in the Summan exploration area. The 25th IMOG, Interlaken, Switzerland, Sept. 2011.