Identifying Reservoirs in Bazhenian Suite Using a Combination of Spectral Gamma Ray and Other Logs

Formations of Bazhenian suite are characterized by increased radioactivity caused by the decay of uranium series, increased abundance of hydrogen, reduced density of the matrix, and high electrical resistivity. The analysis of mineral composition shows that Bazhenian formations consist of siliceous material, solid organic matter and small quantity of clayey minerals. Proposed is a technique for distinguishing reservoirs of Bazhenian suite using an extended system of nuclear logs: spectral gamma, neutron log, and formation density logs. From the core analysis of Bazhenian suite of Surgut arch the following results were produced. Abnormally high concentration of uranium is related to a solid organic matter enclosed by rock matrix. Concentration of potassium and thorium mostly depends on the quantity of clayey minerals. Hydrogen content is controlled by abundances of solid organic matter, clayey minerals, and oil. Matrix density decreases with the growth of organic matter. These results have brought us to a mineral componential model of rocks of Bazhenian suite for one of the fields of the Surgut arch and find petrophysical correlations allowing to interpret well logs and identify depth intervals characterized by an increased porosity. Presented are comparisons of reservoir intervals identified from the technique proposed and well test results. <br><br>