Reservoir challenges of heterolithic tidal sandstone reservoirs in the Halten Terrace, mid-Norway

Production from the Halten Terrace hydrocarbon province (Mid-Norwegian shelf) is mainly from heterolithic siliciclastic successions as well as diagenetically altered sandstones. Eight hydrocarbon fields are currently in production, which produce c. 840 000 BBL oil equivalent per day, with several new fields expected to come on stream in the next decade. This paper is an introduction to a thematic set on the characterization and modelling of heterolithic reservoirs and focuses on the three main types of heterogeneity: (1) heterolithic facies, (2) faulting and (3) diagenesis. Challenges vary according to field setting: shallow (1-3 km burial depth), deep (3-5 km) or very deep (currently up to 5.6 km). Water depths vary from 200 m to 500 m. Heterolithic sedimentary packages are composed of shale or siltstone layers intercalated with clean, but often thin, sandstone layers of varying lateral extent. These were deposited in Lower Jurassic tide-influenced or tide-dominated deltaic and estuarine environments along the margin of a shallow seaway. Hydrocarbon traps are formed by faulted and rotated fault blocks created during rifting. Faulting of these heterolithic facies is a critical parameter for fluid flow, with fault transmissibility and fault position often difficult to determine. Complex patterns of diagenetic cementation are an additional aspect of heterogeneity in the deeply buried reservoirs, such as the Smorbukk and Kristin fields. However, grain coatings of chlorite, illite/chlorite and illite have prevented or hindered the development of quartz overgrowths and allowed the preservation of anomalously high porosity and permeability. Modelling and assessing the impact of these reservoir uncertainties has included development of novel tools and methods, leading to a much-improved level of understanding, better prediction of recoverable reserves and significantly increased recovery factors.