Abstract
Gamma-ray logs are widely used as a lithology indicator in wells as part of standard petrophysical interpretations. In cored wells, gamma-ray logs should always be calibrated to the lithology in order to correct the petrophysical model. Gamma radiation is emitted from three elements, K, Th and U (potassium, thorium and uranium) which occur in minerals such as feldspar, mica, glauconite, clay minerals, zircon, titanite and apatite as well as in organic complexes. Organic-rich mudstones usually have high gamma-radiation values and quartz-rich sandstones low values. In many places, upward-coarsening successions are recognisable from the gamma log. The gamma log records the sum of radiation from K, Th and U, and their relative contributions are measured in a spectral gamma-ray log. The present case study focuses on spectral gamma-ray characterisation of the deltaic Atane Formation which shows well-developed, upward-coarsening delta-front deposits in outcrops (Fig. 1C).
Highlights
The present study demonstrates the importance of calibrating petrophysical logs to core data
The cyclicity which characterises the Atane Formation in outcrops and cores (Figs 1, 2), and which would be a means of identifying the Atane Formation in an un-cored well, is obscured in the total gammaray log
The Th log can, to some degree, resolve the lithological variation but the energy is too small to be reflected in the total gamma-ray signal
Summary
The Nuussuaq Basin is a rift basin, which contains the only exposures of Cretaceous and Paleocene sediments along the west coast of Greenland. The siliciclastic sediments are overlain by a thick pile of volcanic rocks (Chalmers et al 1999; Dam et al 2009). During Late Cretaceous, Greenland was characterised by a warm and probably humid climate. The sediments range from alluvial fans overlying deeply weathered Precambrian basement through marginally marine deposits to marine deep-water deposits, all referred to the Nuussuaq Group (Dam et al 2009). The floodplains and delta plains had a rich flora, recorded in well-preserved plant m 60 m
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