New insights on diagenetic chlorite and its source material in turbiditic sandstones of contrasted reservoir quality in the Lower Cretaceous Agat formation (Duva oil and gas field, northern Norwegian North Sea).

Studies on reservoir quality associated to chlorites in turbidites are quite rare and in recent studies on that topic, chlorite is generally a minor mineral. Moreover chlorite coatings are generally described for their major role in porosity preservation by preventing from quartz cement overgrowths, and only few studies describe their role in reducing permeability when present in excess in the reservoir. The objective of this study is therefore to focus on the material sourcing deep sea turbidites and its impact on the chlorite petrographic/mineralogical characteristics and abundance, and the resulting current reservoir quality. This study focuses on Lower Cretaceous Agat Formation (Måløy Slope, Norwegian North Sea) hosting two reservoirs of contrasted quality. It is based on drill-hole to micrometer scale investigations using QEMSCAN mapping, scanning electron microscopy equipped with an energy dispersive spectrometer, X-ray diffraction, and FEG SEM observation of ultrathin sections. Chlorite of Ib polytype, inherited from berthierine precursor, is observed in both reservoirs. Its homogeneous composition suggests that the geochemical controls of chloritization are similar for the two reservoirs. Grain replacing chlorite, clay replacing chlorite and chlorite coatings are observed in both reservoirs but the grain coatings present different textural configurations. In the upper reservoir, the clay coating is dominated by chlorite platelets whose growth is controlled by geometrical selection. In the lower reservoir, the chlorite coating differs by an additional inner part consisting of a diagenetic-recrystallized clay coat. The preburial material consists of ferric illite (± glauconitic pelloids). The illite morphology is considered to be inherited from an earlier supergene vermiculitization of phyllosilicates in saprolites. As ferrous iron dominates in berthierine/chlorite, a change in redox conditions is needed for Fe illite-berthierine transformation that could be linked to bacteria activity. These results provide a better understanding of the diagenetic process and influence of the material provenance in the evolution of the reservoir properties. • Crystal-chemistry, textural properties and origin of chlorite coating in turbidites formations. • Crystal-chemistry of green "glauconitic" clays. • Dual origin hypothesis for Fe-bearing clays (Fe-illite, glauconite) in the sedimentary material. • Contribution of hinterland weathering profiles of plutonic rocks under tropical climate to the source material of sediments. [ABSTRACT FROM AUTHOR]