Compaction microstructures in quartz grains and quartz cement in deeply buried reservoir sandstones using combined petrography and EBSD analysis

Abstract

Sandstone diagenesis involves porosity loss by mechanical and chemical compaction. In this study we have examined relations of diagenesis and microstructures in quartz grains and quartz cement in sandstones from relatively deep burial depths offshore mid Norway. Electron backscatter diffraction analysis (EBSD) has been combined with optical and cathodoluminescence petrography study of samples with different degrees of compaction and quartz cementation. Quartz cement is shown to be syntaxial to the nearest host domain (grain or subgrain), both for monogranular, polygranular, undeformed and deformed quartz. The quartz cement growth may have been initiated at different subgrain surfaces, and preserving pre-depositional deformation structures. EBSD inverse pole figure imaging shows dauphiné twins to be common in all samples, both in quartz grains and quartz cement. The dauphiné twins appear in grain-grain contacts and in cement-crystal boundaries, and commonly crossing grain–cement boundaries. On basis of twin distributions we suggest that both inherited twins from the source area and twins formed by compaction-induced grain boundary deformation are present. A general problem in sedimentary rocks is to distinguish inherited from sedimentary compaction-induced intra-grain deformation structures. This study shows that the role of ductile deformation mechanisms (e.g. twinning) in sediment compaction can be clarified by more comprehensive following up studies using EBSD analysis in combination with other techniques.