S. N. Ehrenberg1, T. A. Svånå2
(1) Statoil, N-4035 Stavanger, Norway
(2) Statoil, Harstad, Norway
ABSTRACT: Use of Spectral Gamma Ray Signature to Interpret Stratigraphic Surfaces in Carbonate Strata: an Example from the Finnmark Carbonate Platform (Carboniferous-Permian), Barents Sea
Spectral gamma ray profiles were examined in well 7128/6-1, the stratigraphic "reference section" of the entirely subsurface Finnmark carbonate platform. Detailed bulk-chemical profiling of selected GR peaks shows that potassium and thorium are mutually correlated and are a direct index of siliciclastic (aluminosilicate) content, while uranium is uncorrelated with K, Th, and all other chemical components measured. U is enriched in thin shale and argillaceous carbonate layers within otherwise carbonate-dominated intervals. U is thus associated with aluminosilicate minerals (although not linearly correlated with their abundance), and shows no apparent relationship to dolomitization.
Two types of GR peaks are observed. K-Th-dominated peaks are suggested to indicate relatively major transgressions during which aluminosilicate detritus was derived from sources interior to the Fennoscandian shield. U-dominated peaks correspond with more minor (higher-order) transgressions within intervals of cyclic shallow-water carbonate deposits. U-enrichment is suggested to be the product of extended subaerial exposure of the platform surface, during which U was concentrated by meteoric groundwater movements. Subsequent flooding then reworked the U-enriched argillaceous detritus to form thin shale and shaly carbonate layers. These results can be useful as a basis for applying spectral GR signature as a tool for stratigraphic interpretation in uncored or "incipiently understood" carbonate sections.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado