The Use of Outcrop-calibrated Gamma Logs from East Greenland to Aid Interpretation of Triassic Mudstone Depositional Environments in the Central North Sea

The Julius and Jonathan Mudstone Members of the Skagerrak Formation in the Central North Sea (UKCS) form seal horizons to the two principal reservoir units in the Triassic succession. A detailed understanding of vertical and lateral mudstone facies variability is currently lacking, but is critical in the assessment of seal efficiency at exploration scales. A greater understanding of mudstone facies and their genesis can also aid in the contextualisation of adjacent reservoirs to improve sandstone characterisation.

Due to a lack of core material, interpretation of mudstones in the offshore is based mainly on wireline signatures supplemented by lithology data from cuttings. This study aims to investigate whether a more detailed record of mudstone facies and their depositional environments can be extracted and correlated from wireline data. To achieve this, the use of outcrop analogues allow the calibration of gamma log signatures to facies identified from detailed sedimentary logs, which are then compared to subsurface wireline data.

High resolution sedimentary and hand-held, spectral gamma logs have been constructed from three well exposed sections through the Upper Triassic of East Greenland. The sections examined encompass the Kap Seaforth Member of the Gipsdalen Formation, together with the Edderfugledal, Malmros Klint and Ørsted Dal members of the overlying Fleming Fjord Formation. This succession provides a continuous record of climatically controlled lacustrine deposition, and is considered to be analogous to the Triassic of the Central North Sea basin.

Detailed sedimentological analysis and comparison with associated spectral gamma data have allowed the characterisation of lacustrine facies, as well as transitions between various lacustrine settings and an upward transition into the overlying fluvial-dominated Ørsted Dal Member. The facies and spectral gamma log signatures can also be correlated at both field and regional scale. Comparison is made with spectral gamma data from the Triassic succession of the Central North Sea to gain a better understanding of sedimentology and palaeoenvironmental significance of the Julius and Jonathan Mudstone members.