The application of archival borehole data to recognizing the primary mineral composition and diagenetic changes in Jurassic sandy rocks of the Polish Basin

Abstract

With emphasis being placed on the re-examining of mature petroleum basins and reduction of the negative impact of the industry on the environment, the use of archival data in geological exploration is essential. This is especially important in regions where the old, Soviet-type tools were used in the past and which are now using modern, western logging equipment. The application of archival geological, geochemical and geophysical data allows recognition of reservoir formations without the use of modern measurements. For the purpose of this research, Jurassic sandy rocks identified in the archival borehole Z-GN4 were analysed. They appear to be a perfect target for further petroleum and geothermal exploration in the Polish Basin; however, variable mineral composition and diagenetic features can cause difficulties while estimating their reservoir properties. The difference between gamma ray and spontaneous potential shale volume parameter (clay difference) was applied together with gamma ray measurement to determine the impact of primary and diagenetic features on each sandy petrofacies. Based on an integration of detailed petrographic analysis of available core samples from Z-GN4 borehole with archival borehole logs, four different sandy petrofacies were distinguished. Moreover, deep resistivity and sonic logs were used to highlight the carbonate-cemented intervals. This study shows how the application of petrofacies analysis in archival datasets can support the interpretation of uncored intervals and upgrade the level of reservoir characterization.