Weathering profiles and clay mineralogical developments, Bornholm, Denmark

Saprock-saprolite associations were studied by field and laboratory methods (optical microscopy, X-ray powder diffraction, scanning electron microscopy, electron microprobe) in order to describe regolith development in the crystalline rocks of the Nygård kaolin pit (Bornholm, Denmark). The clay sequences and stages of porosity development are similar to those observed for reservoir rocks from the Utsira High (Riber et al., 2016). The weathering of the parent granite began before the end of the Mesozoic. Two stages of syn-/pre-burial alteration, followed by diagenesis during burial, and then post-uplift weathering have been recognized. In stage Ι, plagioclase and some biotite (biotite-vermiculite-kaolinite) reacted to form elongate booklets of highly-ordered kaolinite or smaller, blocky pseudohexagonal crystals. Stage II represented more extreme weathering developed along local fracture systems. The higher potential for fluid flow in the fractures caused highly-ordered kaolinite to alter to halloysitic, poorly-ordered kaolinite. Plagioclase, biotite, and K-feldspar continued to interact with formation water and formed additional quantities of secondary clay minerals. As a consequence of inter-, intra-, and trans-granular cracks, the original rock fabric deteriorated and porosity increased. During later burial, diagenetic siderite crystals filled voids and chlorite formed at the expense of kaolinite. After re-exposure, calcite precipitated in localized zones. Smectite formed on a broader scale wherever high ionic strength groundwaters were present. A mineralogical key to predicting where high porosity and permeability zones occur is the recognition of halloysitic, poorly-ordered kaolinite. Highly-ordered kaolinite is abundant, but it is ubiquitous. Siderite, chlorite and smectite adversely affect permeability. Riber, L., Dypvik, H., Sørlie, R. and Ferrell, R. (2016) Clay minerals in deeply buried paleoregolith profiles, Norwegian North Sea. Clays Clay Miner. DOI: 10.1346/CCMN.2016.064036. •Two major stages of clay mineral alteration and porosity development are present in the weathered Nygård kaolin pit.•Horizontal changes in the fabric of samples less than 3 m apart illustrate the high variability of closely spaced samples.•The weathering profiles in the Nygård kaolin pit can be correlated with reservoir rock fabrics from fields on Utsira High.•The mineralogical key to predicting a potential reservoir is the presence of halloysitic, poorly-ordered kaolinite