Facies Anatomy and Heterogeneity of a Shallow-Marine Sandstone Reservoir: The Mid-Jurassic Garn Formation in the Kristin Field, Offshore Norway

The Garn Formation of Halten Terrace, Mid-Norway Continental Shelf, has long been recognized as a succession of shallow-marine arenitic sandstones recording early syntectonic sedimentation of the Jurassic rifting phase, but its exact origin remained uncertain and rather controversial, which also led to simplistic reservoir heterogeneity models. The present study from the Kristin field indicates that the sand was deposited in an actively subsiding, incipient tectonic graben within the narrow Jurassic seaway linking the Boreal and Tethys open seas. The deposition involved repetitive cycles of tide- to wave-dominated sedimentation, attributed to subsidence-driven relative sea-level changes. The transgressive-regressive cycles formed an overall transgressive parasequence set ~100 m thick, culminating in a major marine flooding due to the regional climax of the rifting phase with the shelf structural collapse. Sedimentological analysis reveals three main facies assemblages and indicates that the sedimentation involved formation of tidal sand ridges, with a concurrent accumulation of sand in the inter-ridge swales and episodic storm influence. Fairweather wave action with erosion and sand bypass prevailed once the accommodation became exhausted by seafloor aggradation, until a new increase in accommodation would occur due to tectonic subsidence. The recognized facies anatomy of the formation bears important implications for the reservoir heterogeneity model, with the tidal sandstone ridges as a main architectural element. These sandstone bodies consist of dune cross-strata sets that are highly anisotropic permeability mini-compartments and the main source of reservoir heterogeneity. A statistical approach is used to estimate the frequency distribution of cross-set volumes from the measured cross-set thickness distribution, which allows the ridge sandstone bodies in reservoir model to be populated with realistic percentages of cross-set volumes. Such quantitative volumetric estimates are highly valuable for the characterization and modelling of petroleum reservoirs, where only bed thicknesses can be measured from the well cores.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California