Plagioclase dissolution related to 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, whereas plagioclase is almost absent in t he same strata in the southern part of the field. Absence of plagioclase th roughout the entire vertical extent of the Brent Group in the southern well s seems to rule out provenance as the principal explanation for differing p lagioclase content, which is therefore interpreted as the result of diagene sis. Hypotheses for the nature of the inferred leaching event include epige netic meteoric diagenesis and mesogenetic release of acid components from c lay minerals or kerogen, but these explanations are unable to account for t he 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 suggest a li nk between plagioclase dissolution and biodegradation of the present oil co lumn. It is, therefore, proposed that acid components from biodegradation s electively reacted with albitic plagioclase to form kaolin, releasing sodiu m bicarbonate into the residual water. The plagioclase-free sandstones cont ain more kaolin than the plagioclase-bearing sandstones, as would be expect ed due to aluminium 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, po tentially including both epigenetic and mesogenetic dissolution, as well as deposition of detrital kaolin and kaolin precursors.