Diagenesis of part of the Brent Sand Formation (Middle Jurassic) of the northern North Sea Basin

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Abstract
Sandstone cores from the Brent Sand Formation were studied by petrographic and reflected light microscopy, cathodoluminescence, scanning electron microscopy and X-ray diffraction. Mineral abundances were obtained by point counting, and porosities and permeabilities by petrophysical methods. The diagenetic changes found by these techniques include (1) early cementation by poikilotopic calcite or, more commonly, pressure solution of quartz with deformation of micas and the development of quartz euhedral overgrowths; (2) partial to complete alteration of feldspars and micas resulting in authigenic growth of euhedral kaolinite and recrystallization of fibrous illite: these clay minerals form bridges across gaps and obstruct pore throats; (3) the occasional growth of small rhombohedra of calcite, dolomite or siderite or more extensive poikilotopic calcite cement with replacement of earlier-formed minerals; and (4) the development of authigenic pyrite and siderite when organic matter is present. These diagenetic changes, which result in loss of porosity and drastic reduction of permeability, are related to original lithology (Grain size and shape, mineralogy and primary porosity) and to subsequent burial history, in which the observed changes are brought about by rise of temperature and increase in pH of circulating pore solutions.

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