Enhancing Lithological and Palaeoenvironmental Resolution in Dryland Successions Using Spectral Gamma Ray Data: Tying Field Observations to the Subsurface

Spectral gamma ray data are important in calibrating outcrop sedimentological observations to subsurface wireline logs. However, the prediction of lithology (VShale) from wireline using the normal gamma ray tool is compromised in some subsurface basins (e.g. the Triassic of the Central North Sea) where reservoir intervals are dominated by highly feldspathic and micaceous sediments.

This study presents a new technique that focuses on the use of thorium and uranium concentration data from spectral gamma ray logs to enhance VShale prediction and characterise depositional environments in dryland successions. High resolution spectral gamma and detailed sedimentological logs collected from outcrop analogues on the Colorado Plateau and East Greenland have allowed the development of this technique which has then been applied to subsurface datasets.

Grain size-VShale (calculated from thorium concentration) correlation coefficients of over 0.3 are an improvement on R2 values obtained from VShale calculated with the industry standard neutron/density cross-plot method that is used to estimate lithology in potassium-rich sedimentary successions. In addition, the generation of VShale cut-offs calibrated to grain size allows for a more accurate determination of lithological heterogeneity both within seals, and within potential sub-net or marginal intervals to deliver enhanced reservoir characterisation in the subsurface.

Field based studies have highlighted the potential for further palaeoenvironmental distinctions to be made. Excess uranium concentration (i.e. that of non-detrital origin) has been utilised as a proxy for water availability in dryland settings. Playas with a low water table show highly negative excess uranium concentrations, whereas gypsiferous playas (with high groundwater), perennial lakes and fluvial environments exhibit positive excess uranium. The combination of VShale-thorium and excess uranium concentration also shows significant potential to predict and characterise palaeosol horizons in dryland floodplain and loess depositional environments, and to differentiate between poorer reservoir quality overbank sandstones and clean, channel sandstones in fluvial settings.