Regional study of controls on reservoir quality in the Triassic Skagerrak Formation of the Central North Sea

Abstract

An improved understanding of the controls on reservoir quality is key to ongoing and future exploration of the Central North Sea Triassic play. This paper presents a regional integrated study of 50 000 ft of wireline log data, 10 000 ft of core, 4431 routine core analyses measurements and 377 thin sections from 86 cored wells.Triassic Skagerrak Formation sandstones represent thin-bedded heterogeneous reservoirs deposited in a dryland fluvial–lacustrine setting. Fluvial-channel facies are typically fine–medium grained and characterized by a low clay content, whilst lake-margin terminal splay facies are finer grained, more argillaceous and micaceous. Lacustrine intervals are mud-dominated. Primary depositional textures retain a primary control on porosity evolution through burial. Optimal reservoir quality occurs in aerially and stratigraphically restricted fluvial-channel tracts on the Drake, Greater Marnock, Puffin and Gannet terraces, and the J-Ridge area. These primary textural and compositional controls are overprinted by mechanical compaction, the development of early overpressure and diagenesis. Anomalously high porosities are retained at depth in fluvial sandstones that have a low degree of compaction and cementation, including chlorite. Forward modelling of reservoir quality using Touchstone™ software has been validated using well UK 30/8-3 where reservoir depths are >16 000 ft TVDSS (true vertical depth subsea).

The influence of stratigraphy and facies distribution on reservoir quality and production performance in the Triassic Skagerrak Formation of the UK and Norwegian Central North Sea

“…The reservoir quality control that facies and facies associations exert in the Triassic of the Central North Sea is well documented (e.g. Akpokodje et al, 2017), together with the impact of diagenetic processes and burial history (Nguyen et al, 2013;Grant et al, 2014;Stricker et al, 2018).…”

“…Due to its burial depth the Skagerrak Formation remains one of the last major exploration targets and forms an important secondary reservoir in a number of fields, such that an increased understanding of controls on reservoir quality is important. Skagerrak Formation reservoirs in the Central North Sea represent deposits of distributive fluvial, terminal fluvial, axial fluvial and lacustrine systems (McKie & Audretsch, 2005;McKie et al, 2010;Kape et al, 2010;McKie, 2011;Akpokodje et al, 2017). These heterogeneous reservoirs have been interpreted to exhibit variable architectures and show varying reservoir quality and production profiles (McKie & Audretsch, 2005;McKie et al, 2010;Kape et al, 2010;McKie, 2011;Akpokodje et al, 2017).…”

“…The reservoir quality of Triassic reservoirs in the Central North Sea has been investigated with respect to diagenesis and compaction (Nguyen et al, 2013;Grant et al, 2014;Stricker & Jones, 2016;Stricker et al, 2018) and linked to depositional facies (McKie, 2011;Akpokodje et al, 2017). In addition to reservoir quality, reservoir architecture and production style have been detailed, specifically in UK Quads 22 and 30 of the Central Graben (Fig.…”

“…1; McKie & Audretsch, 2005;Kape et al, 2010;McKie et al, 2010). However, as recognised by Akpokodje et al (2017), a disconnect exists between localised field-scale studies of reservoir quality and production, and more regional scale basin-wide depositional models (e.g. McKie, 2014).…”

“…The Triassic depositional model comprises fluvial systems derived from both the east (Norwegian landmass) and west (Scottish landmass), these systems are likely large, fan shaped point sourced systems which coalesce and form a southerly directed axial system. Opinions vary between various authors as to the size of the different fluvial systems through time, the relative importance of easterly versus westerly input and the lateral extent of the axial system (Mange-Rajetsky, 1995;McKie, 2014;Mouritzen et al, 2017;Akpokodje et al, 2017). These differences illustrate that at the basin-scale our understanding of facies distributions is inadequate.…”