Diagenesis of Lower Jurassic sandstones, Block 211/13 (Penguin Area), UK northern North Sea

Abstract

The Penguin area is a cluster of three hydrocarbon accumulations located in Block 211/13 in the UK sector of the North Sea. The reservoirs are principally Middle and Upper Jurassic sandstones, but include the Lower Jurassic Statfjord and Nansen formations. The Statfjord Formation was deposited in a fluvial environment and is overlain by sandstones deposited during a marine transgression (Nansen Formation). Fluvial channel sandstones of the Statfjord Formation are extensively cemented by carbonates that precipitated soon after deposition, whereas sandstones of the Nansen Formation were cemented by grain-rimming chlorite and dolomite. Subsequent diagenesis involved the extensive precipitation of kaolin and the formation of minor secondary porosity at the expense of detrital feldspars and carbonate cements. Petrographic, isotopic and structural considerations suggest that the latter diagenesis was caused by meteoric water ingress during the formation of the late Cimmerian unconformity. Meteoric water was not responsible for extensive porosity modification, and the reservoir quality of the Lower Jurassic sandstones is poor and has not been signficantly improved by the formation of secondary porosity.