Thorium normalization for identification of organic carbon anomalies during the Lower Cretaceous paleolakes in the Southern Hemisphere.

Lagoons are suitable paleoclimatic registers. They carry information about cyclic sedimentary deposits in a paralic and closed environment. The proposed study uses gamma radiation spectrometry to calculate the normalization of thorium in a Brazilian pre-salt well located in the Campos Sedimentary basin during the rift sequence (Cabiunas and Lagoa Feia formations), especially during the deposition of the Buracica and Jiquiá shales. Thorium normalization allows a reduction in risk during exploration, in addition to increasing the potential for success in organic content determination. The radioisotopes of interest in this work are the radioactive nucleotides 40K, 238U, and 232Th, commonly found in underground formations and as constituents of rocks with high total organic TOC (Fertl, 1979). The method proposed by Saunders et al. (1993) was able to correlate airborne gamma radiation anomalies with hydrocarbon occurrences in structural and stratigraphic traps through calibration of 40K and 238U values using 232Th. The model determined the "ideal" 40K and "ideal" 238U as Ki=(Kav/Thav)Ths and Ui=(Uav/Thav)Ths. Where "s" is the measured or sampled value, "i" is the ideal value, and "av" is given as the average value. After this, it is necessary to calculate the measured and idealized values (Saunders et al., 1987) by defining KD%=(Ks-Ki)/Ks and UD%=(Us-Ui)/Us. Where KD decreases, and UD generally increases when hydrocarbons are present. A new DRAD calculation combines the two previous equations with positive DRAD values, indicating the top of hydrocarbon anomalies by calculating DRAD=UD%-KD% and DRAD=((Us/Uav)-(Ks/Kav))/(Ths/Thav). The 40K anomaly typically occurs above rock horizons with high organic content. This work presents the results of the study of thorium normalization for the location and identification of high hydrocarbon content during a continental lagunar paleoenvironment phase in the southern hemisphere.