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Application of Gamma Spectrometry to Hydrocarbon Anomalies in Shale Cores at Pomerania Region, Poland

Abstract

Shales in Eastern Pomerania region (Poland) occurs at depth from 1000m to 4500m in Lower Paleozoic age, with documented gas potential levels. Identifying hydrocarbon saturated intervals in the boreholes requires wireline measurements and petrophysical interpretation. More accurate measurements of the core material are usually carried out under laboratory conditions. The purpose of this work is to apply a mathematical method (named DRAD) for quick evaluation of hydrocarbon occurrence in borehole, with the use of gamma spectrometry data (40K, 238U and 232Th) from core measurements. Data from two boreholes (B1 and B2) were used for the proposed goal. B1 contained a set of geophysical measurements and involves petrophysical interpretation with hydrocarbon saturation and lithology, which were used to correlate the results and hydrocarbon occurrence. Data from B2 involved only spectral gamma measurements on the drilling core that had been measured by the gamma logger. The device is adopted to one meter core samples, it was built with a BGO detector (Bi4Ge3O12) due to the high atomic number of bismuth and its high density, to achieve high efficiency. To further evaluate the hydrocarbon presence in analysed boreholes, calculation model was introduced, based on the assumption that in hydrocarbons 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 are not to be affected by hydrocarbon occurrence. Results for B1 showed interesting correlation between DRAD and hydrocarbon saturation. There is a clear relation between the increase of hydrocarbon saturation (HS) and increase of DRAD values. Data from B2, allowed to create 62m length profile of K, U, Th (with additional laboratory test) and DRAD curve. Obtained results showed three intervals (about 10m thick) with hydrocarbon occurrence and positive DRAD values. In conclusion, hydrocarbon anomalies calculated from spectrometric measurements on the core, can be an easy way to indicate saturated intervals in the borehole profile, before petrophysical interpretation and sampling for further laboratory measurements. This technique can aid early stages of petroleum exploration, showing levels of hydrocarbons with increasing probabilities of occurrence.