The Garn Formation (Bajocian‐Bathonian) in the Kristin Field, Halten Terrace

Abstract

The Garn Formation of Halten Terrace, Norwegian Continental Shelf, has long been recognised as a gas-bearing and condensate-bearing succession of shallow-marine arenitic sandstones deposited by early syntectonic sedimentation during Jurassic rifting. However, the origin and spatial architecture of this formation remained unclear and rather controversial, which also led to simplistic models of reservoir heterogeneity. The present study from the Kristin Field indicates that the sand was deposited in an actively subsiding, incipient graben within the narrow Jurassic seaway that linked the Boreal and Tethys open seas. Deposition involved repetitive cycles of tide-dominated to wave-dominated sedimentation, attributed to subsidence-driven relative sea-level changes. Sedimentological analysis indicates that sedimentation involved the development of tidal sand ridges, with episodic storm influence and a concurrent accumulation of sand in inter-ridge swales. Fairweather wave action with ridge erosion and sand bypass prevailed once accommodation became filled by sea floor aggradation, until a new increase in accommodation occurred due to tectonic subsidence. The transgressive-regressive cycles formed an overall transgressive parasequence set ˜ 100 m thick, culminating in major marine flooding due to the regional climax of the rifting phase with associated structural collapse of the shelf. The suggested facies anatomy of the formation has important implications for reservoir heterogeneity, with the tidal sandstone ridges as a main architectural element. These semi-isolated sandstone bodies are more permeable than the surrounding sandstone facies and consist of dune cross-strata sets that form highly anisotropic reservoir mini-compartments. The two types of anisotropy are considered to be the main source of reservoir heterogeneity and a probable cause of the rapid pressure decline and low gas recovery in the Kristin Field. Considerable amounts of gas condensate may potentially be entrapped in the tidal sandstone ridges as the relative water saturation rises and its capillary effect rapidly increases. As an improvement for reservoir model, the dimensions of the sandstone ridges are estimated and a statistical approach is used to estimate the frequency distribution of cross-set volumes from the measured cross-set thicknesses. The estimates allow these sandstone bodies in the reservoir model to be populated with realistic percentages of cross-set volumes. The new heterogeneity model may facilitate more reliable simulation of condensate flow in the reservoir and a better assessment of gas recovery efficiency.