Airborne Radiometric Survey data processing and interpretation to find Hydrocarbon anomalies in the Carboniferous units of the South Portuguese Zone

Abstract

This works aims to assess the exploration potential of shale gas concentrations, based on the processing and interpretation of gamma radiation data (thorium, uranium and potassium) recorded by airborne radiometric survey, in Mértola, Mira and Brejeira Formations. These formations, mostly composed by shaly and sandy turbidites, outcrop in the South Portuguese Zone (SPZ). The SPZ is located in the southernmost sector of the pre-Mesozoic Iberian Variscan Chain, which is composed by highly deformed upper Paleozoic rocks, of low grade metamorphism. However, in some areas, it is possible to observe the same rocks practically undeformed.

The radiometric data used in this work were obtained by RTZ in 1991 and generously donated by the National Laboratory of Energy and Geology (LNEG) from Portugal. The data were provided with the file format “grid” and the unit of measurement is counts per second (cps). To convert the cps values to ppm (U and Th) and % (K) the standard values suggested by Geosoft® were used.

In order to identify and explain the radiometric anomalies related with hydrocarbon emanations, several models have been developed by different authors. One of the most successful models is the one where the relationship is based on the fact that, in oil fields context, the potassium (40K) decreases and uranium (238U) tends to increase. Thorium (232Th) is considered to be highly retained in local rocks and soils and not to be affected by hydrocarbon ascension or seepages. Based on these assumptions, the ideal potassium and the ideal uranium have been defined in literature as: Ki = (Kav/Thav) X Ths and Ui = (Uav/Thav) X Ths.

Where “s” refers to the measured or sampled value; “i” is the ideal value; and “av” is the average value for the studied area, normally, at least five times bigger than the predicted anomaly. The difference between the average and the ideal values is then calculated as follows: KD = (Ks-Ki)/Ks and UD = (Us-Ui)/Us. When hydrocarbons are present, KD decreases and UD increases. In order to capitalize these two relationships a new variable, called DRAD, has been defined as: DRAD = UD-KD or DRAD = ((Us/Uav)-(Ks/Kav))/(Ths/Thav). Therefore, hydrocarbon anomalies are characterized by positive values of DRAD.

The results point to a higher probability of having present-day hydrocarbon microseeps in the Mértola and Mira Formations, when compared with the Brejeira Fm. area partially covered by the airborne radiometric survey. The results will also be used to assist the surface geochemical prospecting in order to identify the best areas to evaluate the hydrocarbons occurrences in soil and the consequent unconventional petroleum system characterization.

AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015