Abstract
The reconstruction of a lithological profile based on geophysical logs of chemical composition provided by geochemical gamma-gamma well logging probes has been increasingly used for geophysical interpretation. A chemical profile, analogous to the measurements mentioned above, can be determined based on measurements made with a portable X-ray fluorescence spectrometer (pXRF). This paper presents a methodology for determining the mineral composition of drilled, clastic, as well as clay-rich rocks on the basis of both inexpensive and timesaving pXRF measurements as well as models combining the results of chemical composition analysis with results of mineral composition analysis (XRD). The results of chemical composition analysis obtained with a portable XRF spectrometer were calibrated based on a detailed analysis produced with ICP-OES and ICP-MS methods. A significant advantage of the proposed method is the possibility to apply it with regard to drill cuttings as well as archival cores. However, considerable discrepancies in the results obtained were identified while comparing the results of chemical composition analysed directly on the core and milled material. The analysed material comprised Carboniferous rocks derived from three boreholes located in Poland: Kobylin-1 as well as Biesiekierz-1 and -2. It was possible to directly compare the lithological profile obtained based on measurements taken on drill cuttings with the results of the lithological interpretation of a geochemical probe log.
Highlights
Geophysical logging has been a main tool used for hydrocarbon exploration by oil and gas companies
The analysis was conducted with regard to the impact of rock material comminution degree on results obtained with the use of a portable X-ray fluorescence spectrometer (pXRF) owing to the fact that, during hydrocarbon exploration, mineralogical investigation is carried out both on solid samples coming from full size and sidewall cores as well as on drill cuttings collected during borehole drilling when cores are not collected
Graphs presenting calculation results for contents of particular mineral components based on the pXRF measurements (Figures 14 and 15), apart from earlier described cumulative columns containing lithological composition, illustrate contents of particular minerals determined on the basis of the XRD method
Summary
Geophysical logging has been a main tool used for hydrocarbon exploration by oil and gas companies. The interpretation of geophysical logs is based on an assumed lithological profile The more this profile responds to the real lithology of rocks drilled in a borehole, the more precise the interpretation of the remaining deposit parameters will be. The importance of the knowledge of mineralogy of a drilled geological formation led to the situation whereby the reconstruction of a lithological profile along with an interpretation of geophysical logs has become increasingly often conducted based on geophysical measurements of chemical composition. It is usually taken with geochemical probes of a LithoScaner (Schlumberger) or GEM (Halliburton) type
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