Natural radioactivity in rocks is usually the result of the presence of natural isotopes: potassium - 40K, uranium - 238U, 234U, 235U, and thorium - 232Th and their decay products. Initially contained in acid igneous rocks, these isotopes are released through weathering processes and subsequently transported to sedimentary sites, where they primarily accumulate by adsorption onto clay mineral particles. Direct measurements of potassium, uranium and thorium activity in sedimentary rocks provide crucial insights into their geochemistry and mineral content.
Methods proposed the use of gamma ray logs as lithological indicators in siliciclastic sedimentary rocks suggest that high gamma ray values indicate the presence of fine-grained deposits or those rich in clay minerals, while low values suggest coarse-grained rocks or carbonates. However, they are inconsistent in identifying deposits containing high concentrations of potassium-rich minerals. The ability to distinguish between the gamma emissions of uranium, thorium and potassium means that the gamma ray spectral profile can be used to identify various clay minerals.
The use of this profile to identify minerals and clay minerals in a siliciclastic environment is well established for an assemblage composed of the main minerals found in these sedimentary environments (e.g., chlorite, kaolinite, smectite, illite-mica and its mixed layers, glauconite and feldspars). However, for environments rich in Mg-clay minerals, this study has not yet been developed.
The concentrations of Si+4, Mg+2 and Ca+2 cation provided by the lithogeochemical logs and their ratios can efficiently indicate intervals rich in Mg-clay and the mineral species present after adjustments to their mineral model.
The aim of this work is to establish a methodology for identifying the clay minerals present in the rocks of BVE using gamma ray spectral and lithogeochemical logs, on the base of samples where the Mg-clay minerals have already been identified using x-ray diffraction (XRD) data.
The data analysis shows that samples rich in stevensite (>95%), a Mg-smectite, are associated with higher thorium and uranium values and intermediate potassium values. The kerolite-stevensite interstratified samples occur at points with intermediate thorium values and lower potassium values, but without a diagnostic behavior for uranium. Samples rich in kerolite (>95%) are associated with lower potassium and uranium values, and intermediate thorium values. Saponite-rich samples (>95%), which is a Mg-smectite rich in Al+3, are associated with lower potassium values and intermediate to low thorium values, showing dispersed behavior in relation to uranium. Samples with intercalation between saponite and kerolite behave more like kerolite, with low potassium and uranium values and intermediate thorium values.
In addition to XRD data for identifying Mg-clay minerals, total organic carbon (TOC) data can be incorporated to understand the variation in gamma ray values, especially the behavior of uranium at points with high TOC contents. The correlation of these values can provide light on the BVE lacustrine sedimentology.