Modelling Heterogeneities in the Chalks of the Eldfisk Field, Southern North Sea

Hilde Alexandersen¹, Claus Koch Clausen¹ and Michel Thomas^2
1ConocoPhillips Norway
²Total E&P Norge AS

The Eldfisk Field, located in the Norwegian sector of the southern North Sea, is a large chalk field. The field consists of two northwest-trending, asymmetric anticlines, Alpha and Bravo, with the northern structure slightly offset to the east. The Eldfisk Field also includes the East Eldfisk, a domal structure located northeast of the Alpha structure. 

The main reservoir intervals consist of fractured chalks of the Maastrichtian age Tor Formation and the Ekofisk Formation of Danian age. Sedimentation of the chalk was governed by a number of factors, including tectonically controlled local reworking, formation of hardgrounds and other erosional features due to currents or winnowing, local thinning and pinch-out related to changes in depositional centres and sedimentary bypass of the field area.

All of these are challenges in reservoir modelling. The challenge is further enhanced by approx. 50% of the field area not being seismically resolved (‘obscured area’). Correlation between wells generally is not a problem over larger distances, as packages of sedimentation can be recognized, but local correlation between wells a few hundred meters apart can be very difficult – i.e. the system is extremely dynamic on local scale but systematic on a larger scale. This creates large problems in a traditional model building approach and thus new approaches have been developed. 

The overall goal is not to distribute chalk facies in the geological model, but to recognize rock types that relate to flow units both vertically and laterally. Thus focus on faults and sedimentary barriers is natural and important – the lateral extent of these must be a combination of data and concept driven. Concepts can be determined from outcrops – the chalk cliffs at Etretat, France are a very good analogue to the rapid variations that occur in current dominated chalk deposition. This therefore also raises the question of depositional water depths for the chalk – most people agree that chalk is a deep-water deposit, but how deep was actually deep? Evidence from the neighbouring field Valhall suggests that chalk deposition there took place in the shallow marine environment. This might also be the case for the Eldfisk Field. However, changes in depositional thickness show that the answer is not as simple as that – while the Alpha structure was source of locally reworked chalk during Maastrichtian that was deposited at Bravo, the Danian section is fairly uniform over both structures. 

The effect of the depositional variations are further enhanced by diagenetic alterations and then further complicated by extensive faulting. The present goal for the model is to predict initial water movements related to the water injection program started year 2000, and thus the model must reflect water movements, in contrast to previous focus on drainage volumes during initial depletion phase from 1977 to 2000. 

All in all, the modelling of the Eldfisk reservoir is challenging and exciting. Understanding of depositional systems leads not to a facies but to a rock type model, that ties directly into the engineering understanding of flow behaviour in three dimensions, and thus can only be achieved by a truly integrated team.