S. N. Ehrenberg1, K. G. Jakobsen2
(1) Statoil, N-4035 Stavanger, Norway
(2) Statoil, Bergen, Norway
ABSTRACT: Plagioclase Dissolution Caused by Biodegradation of Oil in Brent Group Sandstones (Middle Jurassic) of Gullfaks Field, Northern North Sea
Brent Group sandstones from the north side of the Gullfaks Oilfield contain mostly 5-8% albitic plagioclase feldspar, while the same strata from the southern part of the field have mostly zero plagioclase content. Absence of plagioclase throughout the entire vertical extent of the Brent Group in the southern wells seems to rule out provenance as the principal explanation for differing plagioclase content, which is therefore interpreted as the result of diagenesis. Various hypotheses for the nature of the inferred leaching event have been considered, including epigenetic meteoric diagenesis and mesogenetic release of acid components from clay minerals or kerogen, but these models seem unable to account for the observed spatial distribution of the plagioclase-bearing and plagioclase-free sandstone intervals. However, overall correspondence between the area lacking plagioclase and oil compositions having both anomalously high CO2 and organic geochemical indications of advanced biodegradation suggests a link between plagioclase dissolution and biodegradation of the present oil column. It is therefore proposed that acid components from biodegradation selectively reacted with albitic plagioclase to form kaolin, releasing sodium bicarbonate into the residual water phase.
The plagioclase-free sandstones contain more kaolin overall than the plagioclase-bearing sandstones, as would be expected due to aluminum conservation. However, the wide and overlapping ranges of kaolin content in both groups suggest that most of the kaolin originated from processes other than biodegradation-driven plagioclase alteration, potentially including both epigenetic and mesogenetic dissolution, as well as deposition of detrital kaolin and kaolin precursors. Petrographic observations indicate that a large proportion of the kaolin present is not derived from feldspar dissolution, but formed from mica and lithic grains, either before or after sand deposition.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado