ABSTRACT: Preservation of Anomalously High Porosity in Deeply Buried Sandstones by Grain-coating Chlorite: Examples from the Norwegian Continental Shelf

EHRENBERG, S. N., Statoil, Stavanger, Norway

Five different Early to Middle Jurassic sandstone formations from Statoil-operated licenses provide examples of porosity preservation by Fe-rich chlorite.

1. Tilje Formation, Haltenbanken. Zones of anomalous porosity are observed in two widely separated areas: block 6506/12 (Smorbukk and Smorbukk Sor fields) and approximately 100 km northeast in block 6507/3. Extreme vertical heterogeneity in sand:mud ratio reflects local progradation of sand-dominated delta lobes into a tidally influenced, shallow marine environment. Wide porosity variations in clean sandstones correlate with abundance of grain-coating chlorite and consequent inhibition of quartz cementation.
2. Tofte Formation, Haltenbanken. Coarse, poorly sorted fan-delta sandstones were rapidly deposited into deep water from a nearby tectonic uplift. Cores from three wells show porosity preservation nearer the fluvial source and extensive quartz cementation only 3-6 km basinward.
3. Garn Formation, Haltenbanken. Grain-coating chlorite occurs in only 1 of 16 wells studied. Quartz cementation appears inhibited, such that porosity was lost mainly by compaction.
4. Statfjord Formation, Veslefrikk Field. A 220 m thick alluvial plain sequence is overlain by 3-6 m of transgressive marine sandstone containing chlorite-coated anomalous porosity.
5. Intra-Dunlin Sand, Veslefrikk Field. Three shale-rich upward-coarsening cycles culminate in an uppermost zone of high-porosity, chlorite-rich sandstone.

Concentrically layered chlorite ooids are a characteristic minor component in all five examples. The ooids formed by mechanical accretion as the grains were rolled by water currents, indicating that concentration of Fe-rich clay occurred syndepositionally. This clay, probably similar to minerals of the modern verdine facies and the ancient oolitic ironstone facies, precipitated in rapidly accumulating, high-energy sands where Fe-rich fresh water discharged into various near-shore marine settings.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)