HENDRY, JAMES P. and NIGEL H. TREWIN, Department of Geology and Petroleum Geology, University of Aberdeen, Aberdeen, UK

ABSTRACT: Calcite-Cemented Layers in Deep-marine Sandstones: Lower Cretaceous Scapa Field, Witch Ground Graben, North Sea

Calcite-cemented layers within shallow marine sandstones have been subject to much recent research. Cementation is generally ascribed to local re-distribution of biogenic carbonate, producing simple cement sequences involving only one or two generations of calcite. This study examines analogous cemented layers in deep marine reservoir sandstones of the Valhall Formation in the Scapa Field. The sands were deposited as richly bioclastic but clay-poor turbidites, sourced from a shallow marine shelf located on an earby horst. The sandstones contain abundant, relatively thin (approx. >2 cm-1 m) calcite-cemented layers and nodules. Preliminary results reveal a highly complex and variable cementation history. Up to five distinct calcite cement generations occur within individual layers or nodules, and cementation patterns differ from layer to layer.

Earliest calcite cements are syntaxial overgrowths on bioclasts, isolated patches of poikilotopic spar associated with lignite fragments, and more pervasive non-ferroan, fine-grained spar. Coarse or poikilotopic ferroan spar and subsequent strongly ferroan calcite both postdate episodes of fracturing and corrosion of the earlier cements. Thus two or more calcite phases are frequently intergrown on a sub-millimeter scale. Further complexity arises from an abundance of sponge spicules, which have variously been replaced by different calcite generations. Additional cements include feldspar overgrowths, at least two generations of silica, and late-stage sulfides, ankerite and kaolinite.

The cemented layers clearly formed through a variety of processes and diagenetic environments. The fine-scale mixtures of cements necessitate extremely cautious geochemical sampling strategies, and highlight the importance of detailed petrographic study as a pre-requisite for all investigations of calcite-cemented layers and concretions in sandstones.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.