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The K/Rb Ratio and Its Geological and Petrophysical Implications: A Test Case of the San Jorge Gulf Basin, Argentina


This article presents the conclusions of inorganic geochemical studies from cutting and cores belonging to wells located on the southern flank of San Jorge Gulf basin, Argentina. Chemical analysis was performed by a nondestructive X Ray fluorescence analysis with rapid handheld equipment. Ten wells were studied and chemical analysis was performed on 2500 cutting samples and 27 m. of core with readings separated 10 cm from each other. San Jorge Gulf basin is an extensional basin developed on continental rocks of Paleozoic age. Basin fill began during rifting stages from late Jurassic to early Cretaceous and successive sag stages during middle to upper Cretaceous. Basin fill is predominantly of lacustrine (D129 formation, the source rock) and fluvial origin (Mina del Carmen, Cañadon Seco and Meseta espinosa formations, the reservoirs) where the main petroleum system is developed. Geochemical logs were built with major and trace elements and they were compared with core lithology, petrophysic and oil saturation logs. The most important observation was the correlation between de K/Rb ratio and oil saturation. Moreover, the source rock interval of D129 Formation matched with an increasing of K/Rb ratio. D 129 Formation was developed as stacked lacustrine systems with an organic matter content responsible of I/II to II/III kerogen type, while TOC values ranges from 0.5–3%. This would indicate a lacustrine environment with contribution of plant material belonging to vascular plants. Paleobotany indicates that paleoenvironment corresponds to a tropical-subtropical forest ecosystem. Since Potassium (K) is an essential macro nutrient for higher plants, its presence in large quantities suggests the transport via fluvial system to the lakes. As observed by several authors, organic raw material such as oil has a high K/Rb ratio with values greater than 600 units, while for clastic rocks this relationship is about 100 units. Here, values over 200 units were observed matching with source rock intervals and reservoir beds with oil shows and production. As a result we show the importance of the K/Rb ratio for the evaluation of organic matter thickness in continental source rocks like D129 formation. Moreover the K/Rb high values in reservoirs with oil saturation allow to discriminate between oil and water bearing sands.