Journal of Conference Abstracts

Use of Spectral Gamma Ray Signature to Interpret Stratigraphic Surfaces in Carbonate Strata: An Example from the Barents Sea

S. N. Ehrenberg (sne@statoil.com)¹ & T. A. Svånå (tasv@statoil.com)²

¹ Statoil, N-4035 Stavanger, Norway

² Statoil, Pb. 40, N-9401 Harstad, Norway

Spectral gamma ray (GR) profiles have been examined in well 7128/6-1, the stratigraphic "reference section" of the entirely subsurface Upper Carboniferous to Late Permian Finnmark carbonate platform. This example demonstrates how spectral GR patterns can provide evidence for the nature of surfaces having significance for sequence stratigraphic interpretation in carbonate platform strata. Two types of spectral GR peaks are observed in the Moscovian-lower Sakmarian section: K-Th-dominated peaks and U-dominated peaks. Most of the K-Th-dominated peaks consist of a U-rich basal zone (minor to moderate total GR intensity) overlain by a K-Th-rich upper zone (major total GR intensity). Detailed profiling of selected GR peaks shows that the K-Th-rich zones have relatively uniform composition over thicknesses of one to several meters, while the U-rich zones typically consist of multiple thin layers of variably U-rich shale and argillaceous carbonate contained within thicker zones of relatively non-radioactive carbonate. The U-dominated peaks are U-rich throughout, but commonly include a subordinate, moderately K-Th-rich zone.

Abundances of K and Th are mutually correlated in these strata and are interpreted as a direct index of bulk aluminosilicate content (clay, mica, feldspar, and aluminosilicate-associated heavy minerals). U is uncorrelated with K, Th, and all other chemical components measured, but tends to be enriched in thin shale and argillaceous carbonate layers having moderate to high K and Th content. U-enriched shales elsewhere have previously been interpreted as reflecting slow accumulation of organic-rich marine sediments under anoxic conditions. However, this seems unlikely for the present U-rich zones, at least some of which appear to be associated with periods of low accommodation or subaerial exposure rather than deep-water, anoxic conditions. To account for this association, U is suggested to have been concentrated by groundwater oxidation/reduction processes on the exposed platform surface during lowstands of relative sea level. Subsequent or intermittent minor flooding events then caused reworking of U-enriched argillaceous detritus and deposition as thin shaly layers.

The K-Th-dominated GR peaks are suggested to indicate relatively major transgressions during which much of the aluminosilicate detritus was derived from sources more distant than the U-enriched platform surface. The composite low-U/Th over high-U/Th GR structure of many of these GR peaks is suggested to indicate abrupt cycling from minimum to maximum accommodation conditions in the inner-platform setting of well 7128/6-1. The lower, high-U/Th part of the peak reflects multiple exposure and minor flooding events during late base-level fall. The upper, low-U/Th part of the GR peak reflects transgression during the succeeding base-level rise. The U-dominated GR peaks are suggested to indicate relatively minor transgressions during which much of the aluminosilicate detritus was derived from surfaces of extended subaerial exposure, where shallow meteoric groundwater movements had caused extensive U-enrichment.

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