The Birgitta Field, Block 22/19a, UK North Sea

Abstract

Abstract Birgitta Field was discovered by well 22/19-1 which encountered a 230 ft gas–condensate column in Triassic Skagerrak Formation and tested at a combined rate of 38.3 MMscfgd and 3750 bcpd. The tilted-fault block trap forms the crest of a Triassic ‘pod’ or mini-basin formed by salt withdrawal during Triassic extension, further rotated and eroded during Jurassic extension. Field extent is supported by apparent seismic hydrocarbon indications. An early oil charge was likely converted to condensate by Plio-Pleistocene gas influx and rapid burial, whilst an underlying palaeoresidual gas column reflects some trap leakage. Birgitta typifies certain Triassic reservoir characteristics in this part of the Central North Sea. The thick, relatively high net:gross reservoir comprises moderate to poorly sorted, sub-lithic to sub-arkosic sandstones deposited in a dryland braided fluvial system. Pore-lining chlorite overgrowths dominate the pore fabric, reducing pore throat sizes and contributing to appreciable levels of non-effective micro-porosity and hence elevated water saturation. Key petrophysical challenges are the accurate determination of effective porosity and water saturation. Birgitta approaches high pressure–high temperature conditions and illustrates some of the challenges of progressing small, unappraised field tie-backs. These include resource uncertainty, compartmentalization risk, infrastructure access and marginal economics. Evaluated for development several times, Birgitta presently remains undeveloped.