Estimation of clay minerals from an empirical model for Cation Exchange Capacity: An example in Namorado oilfield, Campos Basin, Brazil

Clay minerals are important components of shales and occur predominantly in clastic sedimentary basins. Although the classification of such minerals arouses the interest of oil exploration practitioners, this is a hard task to accomplish. In order to typify the main clay minerals of turbidite sandstones in Campos Basin, this study comprises an empirical model to estimate Cation Exchange Capacity. The basic parameter for applying the empirical model is shale volume, whose values are calculated from natural gamma ray, neutron and bulk density measurements. Two-dimensional maps of Cation Exchange Capacity in specific depths are also provided in order to extend the analysis to the whole Namorado oilfield, Campos Basin, in southeast Brazil. The application of this methodology in two wells drilled in Namorado oilfield reveals, despite significant differences in shale content estimation, the predominance of kaolinite in the selected sedimentary interval, while layers of illite and chlorite are noticed at the top of the sediments. There is a total absence of smectite in all studied area. Two-dimensional maps for specific depths of Namorado reservoir enhance discrepancies associated with limitations of both methods to obtain shale content.