Seismic geomorphology linked to sequence stratigraphy of an Eocene delta in the Outer Moray Firth, UKCS

The Halibut Delta is a newly described, Lower Eocene delta in the Outer Moray Firth which has been studied in the shallow (<1 s TWT) section of conventional seismic data as an analogue for more deeply buried systems that are potential hydrocarbon reservoirs. The key internal reflectors have been mapped and geomorphological measurements such as clinoform dip, shoreline trajectory, clinothem width and length were taken at multiple sections along strike and dip. These were combined with seismic attribute analysis to characterize the delta and understand the controls on the observed geometries. Grainsize has been inferred using cuttings descriptions from 20 wells that penetrate the interval. The Halibut Delta was divided into three units: a lowermost clay rich unit with a low median clinoform dip of 1.4° and a uniform thickness is overlain by a more sand rich portion with steeply dipping clinoforms (median values 2.0°–4.7°) and a descending, regressive accretionary shoreline trajectory. The upper unit of the delta shows a transgressive accretionary shoreline trajectory with clinoform dips of 3.5°–1.7° and a sandy character. All three units show along strike variation in clinoform dip and lithology with areas of high clinoform dip corresponding to coarser grained lithology that occur at the location of fluvial feeder channels which introduced coarse, immature material to the delta. Towards the edges of the clinothem this material is reworked and redistributed through wave action and longshore transport. The three units of the Halibut Delta represent a succession of transgressive, regressive and transgressive systems tract, the development of which was mainly influenced by eustatic sea level changes. Seen as a reservoir analogue, the clinothems of the Halibut Delta have a short down-dip extent and are divided by mud-draped layers that could baffle fluid flow. In down-dip direction well spacing would have to be below 2 km to ensure production and/or injection takes place within one clinothem and this short down-dip continuity has to be taken into account when correlating clinothems between wells.