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Petrophysical Relationships |
School of Geosciences, The Queen’s University, Belfast, BT7 1NN, UK
The distribution of mineral cements in oil fields is critical to the spatial variation of porosity and permeability. The distribution of dolomite cement within fluvial Triassic Chaunoy sandstones in the Paris Basin was studied using core description, petrography, core analysis (porosity and permeability), wireline data interpreted to give mineralogy, porosity and permeability data and geochemical data. Petrographic analysis revealed that dolomite and quartz cements are the main diagenetic minerals. Using sonic transit time, density and neutron porosity log, the overall proportions of quartz, dolomite and shale as well as porosity for each depth interval could be resolved. Petrographic and core analysis data showed that permeability could be calculated from wireline-derived porosity and mineralogy data. There is excellent correlation between core analysis porosity and permeability and their wireline-derived equivalents. There is also excellent correlation between wireline-derived mineralogy data and quantitative petrographic mineralogy data. The wireline-derived mineralogy data show that dolomite is preferentially concentrated at the tops of most sand bodies. Porosity and permeability are consequently lowest at the tops of individual sand bodies due to the localized dolomite cement. There are a number of potential causes for this distribution pattern although geochemical and petrographic data showed that a combination of early pedogenetic dolomite cementation and later recrystallization, possibly due to an influx of organically-derived CO2, are most likely.
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