Plagioclase dissolution related to biodegradation of oil in Brent Group sandstones (Middle Jurassic) of Gullfaks Field, northern North Sea

Abstract

Brent Group sandstones from the north side of the Gullfaks Oilfield contain mostly 5–8% albitic plagioclase, whereas plagioclase is almost absent in the same strata in the southern part of the field. 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. Hypotheses for the nature of the inferred leaching event include epigenetic meteoric diagenesis and mesogenetic release of acid components from clay minerals or kerogen, but these explanations are 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 suggest 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. The plagioclase‐free sandstones contain more kaolin than the plagioclase‐bearing sandstones, as would be expected 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, potentially including both epigenetic and mesogenetic dissolution, as well as deposition of detrital kaolin and kaolin precursors.

Creation of pre-oil-charging porosity by migration of source-rock-derived corrosive fluids through carbonate reservoirs: one-dimensional reactive mass transport modelling

“…These modelled processes agree with the observation that carbon dioxide and acetic acid as the main acidic components of oil reservoirs can induce intense plagioclase dissolution and concurrent formation of kaolinite (e.g. in the Gullfaks oil field: Smith & Ehrenberg 1989;Ehrenberg & Jakobsen 2001). Hydrogeochemical modelling by van Berk et al (2009van Berk et al ( , 2013 quantitatively reproduced this proven alteration of mineral assemblages in the Gullfaks field, which was driven by the products of oil degradation (CO 2 , CH 3 COOH and CH 4 ).…”

“…Such organic-inorganic interactions represent the inevitable consequences of chemical thermodynamics of such systems under elevated temperature and pressure conditions; the involved hydrogeochemical reactions tend to reach (metastable) equilibrium conditions (Helgeson et al 1993). The fit of the observations (Ehrenberg & Jakobsen 2001) and corresponding modelling results (van Berk et al 2013), regarding such organic-inorganic interactions in reservoir rocks, indicate that our generic batch modelling of ACF generation provides rough, but plausible, estimates to characterize the composition of ACF expelled from siliciclastic source rocks.…”

“…Consequently, even higher concentrations could be expected in ACF-bearing brines that are generated by kerogen maturation and subsequent oil degradation already in the source rock. In summary, organic-inorganic interactions are driven by the products of kerogen maturation and subsequent oil degradation already in siliciclastic source and reservoir rocks (Smith & Ehrenberg 1989;Ehrenberg & Jakobsen 2001;van Berk et al 2009van Berk et al , 2013. Such organic-inorganic interactions represent the inevitable consequences of chemical thermodynamics of such systems under elevated temperature and pressure conditions; the involved hydrogeochemical reactions tend to reach (metastable) equilibrium conditions (Helgeson et al 1993).…”

Petroleum reservoir quality prediction: overview and contrasting approaches from sandstone and carbonate communities

“…During eo-and meso-diagenesis, gibbsite, goethite, berthieriene and palygorskite tend to react with other (detrital and eogenetic) minerals and aqueous species creating illite and chlorite. Kaolinite can be generated during mesogenetic feldspar alteration (Ehrenberg & Jakobsen 2001), but kaolinite tends to react with detrital K-feldspar to produce illite at elevated temperatures (Bjørkum et al 1993). In the absence of K-feldspar, kaolinite transforms into dickite (a polymorph of kaolinite) at elevated temperature (Beaufort et al 1998).…”

A metagenomic window into carbon metabolism at 3 km depth in Precambrian continental crust

“…Thus far, it has been shown that some of the subsurface microbial communities are sustained by radiolytically derived H 2 and sulfate (Lin et al, 2006b;Chivian et al, 2008). Obvious signatures of biodegradation of the abiogenic hydrocarbon pool, such as elevated isobutane to normal butane ratios (Ehrenberg and Jakobsen, 2001), are not apparent .…”