Dry Shale Density and Depth Variation

Most petrophysical interpretations use density porosity. For the analysis of shaly sands and seismic petrophysical integration a petrophysical interpretation must also include an accurate determination of shale porosity. To interpret shale the density porosity equation requires the dry shale density as an input parameter. Shales do not naturally occur in a dry state. The dry shale density is an abstract value which cannot be measured in conventional core analysis. Furthermore, shale composition is highly variable and the density of shale is only broadly constrained by mineralogical knowledge. Dry shale density is an essential interpretation parameter but it is very difficult to quantify. Values of dry shale density are best calculated from wireline NMR and density measurements. The correct value is established when, through a shale interval, the density porosity matches the NMR total porosity. In this study, dry shale density values have been calculated using log data from Norway's national data repository, DISKOS. 332 wells with wireline NMR and density data were used. The Norwegian Continental Shelf comprises three geological areas: the North Sea, the Norwegian Sea & the Barents Sea. The well data is split fairly evenly between these. Notably, the North Sea and Norwegian Sea basins have very similar geological histories and are dominated by subsidence. On the other hand, the Barents Sea is a more complex geological area containing multiple basins and platforms. In the Barents Sea between 1000 m and 2500 m of geological uplift has occurred since the Late Cenozoic. Dry shale density values from these three areas are considered separately and comparisons are made. Consideration is also given to the changes in shale mineralogy that occur with burial. The results calculated are of high quality. The large number of wells used gives statistical validity. The derived dry shale density values match well to the values expected from mineralogy and are observed to increase with depth. This accords with the chemical transformations that are known to occur in shales with increasing burial. Variations in montmorillonite content mean that dry shale densities at shallow depths are extremely variable. Thus, at shallow depths the density-depth relationship is poorly defined. At greater burial depths montmorillonite undergoes diagenetic transformation to illite. Identical density-depth relationships are derived for the North Sea basin and the Norwegian Sea basin. In the Barents Sea the density-depth relationship shows the same underlying trend but with the overprint of the Late Cenozoic uplift that has occurred in this area. This study is a demonstration of how a high-quality regional database can be used to investigate and define a fundamental petrophysical parameter. The density-depth relationshipis well definedbelowapproximately2000m TVDBML and is sufficiently robust for quantitative interpretational use. At shallower depths, for accurate evaluation of shale porosity, a logging program should include NMR and density logs. The consistency observed in the values of dry shale density and similarities seen in the different Norwegian sedimentary basins suggest that the density-depth relationship derived may be a general relationship that is applicable to all basins dominated by long-term subsidence.