Abstract This chapter describes three-dimensional (3-D) stochastic modeling of the Jotun field, which was initially undertaken in 1998 and updated after the first four wells came on production. The Jotun field contains both differential compaction traps (Elli and Elli South four-way dip closures) and a stratigraphic pinch-out trap (Tau West). It produces from the distal parts of the Paleocene Heimdal Formation sand-rich submarine-fan system. A predrill (1997) deterministic oil-in-place geological model was history matched so that simulated pressure drop resulting from Heimdal field gas production matched the observed pressure drop in the Jotun appraisal wells, with aquifer size and conductivity as the main history-matching parameters. With a development plan strategy of four producers on Elli, two on Elli South, and five producers on Tau West (all highly deviated or horizontal), the predicted aquifer support was such that predrilling water injectors for pressure support was unnecessary. This saved the considerable capital expenditure of three water injectors. Predicted hydrocarbon recovery was influenced by vertical sweep efficiency, dependent on vertical permeability (kv)/horizontal permeability (kh), controlled, in turn, by the architecture of the interlayer shales. Reservoir heterogeneity was introduced in the 1998 model as architectural facies bodies in a 3-D object-based (Roxar STORM software) stochastic geological model. This captured and integrated the core-scale features in a seismic-scale stratigraphic and structural framework, using rules from outcrop analogs to fill in the missing scale. Subsurface realizations reflected different geological possibilities while preserving their influence in the upscaled dynamic simulation models. One of the major uncertainties in the geological modeling was considered to be the extent of faulting, sandstone injection, and slumping as features, which disrupt shale continuity at core scale and which might greatly increase vertical communication and, hence, recovery. If such features are not common, significant volumes of oil could be trapped beneath laterally continuous shale barriers. Different scenarios of geometry, properties, and distribution were used to investigate the significance of such features on the expected ultimate reserves of the field. Both the large aquifer support and a high level of connectivity between the separate structures were confirmed by the first four producers brought on stream.

Abstract Accumulated knowledge from the early pioneering wells and subsequent exploration history led to the discovery of several fields in the Utsira High area during the 1990s. One of these, the Jotun Field, was discovered in 1994. The Jotun Field consists of three structures and is located on the western flank of the Utsira High, close to the eastern pinchout of the Tertiary submarine fan system. The reservoir at Jotun comprises Paleocene Heimdal Formation sands shed from the East Shetland Platform and transported across the Viking Graben area onto the Utsira High by high density gravity flow processes dominated by sandy turbidites. These distal gravity flow deposits display both thin-bedded sands alternating with shales and thicker, more massive sandstones (tens of metres thick). Minor sand injections occur throughout the field but are volumetrically insignificant. The production wells in one of the structures are completed in a slump and injection complex above thick massive reservoir sands. The Jotun Field development strategy was designed to optimize oil capture and minimize risk based on the interpreted reservoir geology. The initial development comprised 14 development wells: 12 horizontal producers, 1 water injector and 1 water disposal well. Production from the Jotun Field started in October 1999 and reached peak production in June 2000, with 140 000 BOPD. After exceeding initial expectations, declining production and rising water cut prompted an infill well programme, time-lapse seismic data acquisition and production logging in 2002. The first two wells were disappointing due to facies transition to interbedded sands and shales on the flanks of the structure and underprediction of oil-water contact movement. The newly available time-lapse seismic data were then integrated with production logging and updated depositional facies studies to evaluate additional drilling opportunities. The discovery, appraisal and development history of the Jotun Field serves as a good example of the key challenges in the Tertiary Utsira High play and the strength of applying multidisciplinary team efforts, partner co-operation and involvement to optimize asset value through tailored drilling and data acquisition programmes.