Controls on Chlorite Coatings in Shallow Marine Sandstone Reservoirs: An Illustration From the Mid-Jurassic of Norway

Lafont, Francois¹; Matthews, James C.²; Capron, Alexis³; Schulbaum, Laurent^4
1Clastic Sedimentology, TOTAL SA, Pau, France.
²Inorganic Geochemistry, TOTAL SA, Pau, France.
³Geosciences Division, TOTAL Gabon, Port Gentil, Gabon.
⁴Geosciences Division, TOTAL Indonesie, Balikpapan, Indonesia.

Grain-coatings can preserve good reservoir properties in deeply buried sandstones. The coatings inhibit quartz diagenesis by precluding nucleation of overgrowths on quartz grains. A common case is clay coating by chlorite.

This study is focussed on Victoria, a large gas accumulation in the Jurassic of the Halten Terrace, in the Norwegian Sea. Core data have allowed a detailed study of chlorite coating distribution. Coating formation is a very early process that was initiated during sediment deposition: Precipitation occurred where salt and fresh waters mixed, in deltaic to estuarine areas.

The development of the coatings follows a sequence stratigraphic forcing. They are better developed in the early transgressive units than in the regressive ones. During regression, with a prevailing deltaic setting, the fresh water tended to be flushed seaward. Thus, the mixing should have occurred in plumes at the sea surface away from the sand deposition area. During early transgressive periods, the salt water invaded further up the delta plain, mixing occurred higher on the profile and precipitation occurred directly within the channels and bars.

In the best preserved reservoirs, the Victoria data show that for a given porosity, the channel sandstones are more permeable than the bar ones. It is explained by the coarser grain size of the channels fills, thus with larger pore throats leading to better permeability than in the bar sands. At this scale, the basic control on reservoir quality is the depositional environment.

The most permeable facies of the Victoria cores are found in a coarse cross-bedded unit which cut into fine-grained lagoon / bay series. It is interpreted as a lowstand channel cut during a 4th order sea level fall. As it was linked to an allocyclic process, it stood stable for a longer duration than the autocyclic channels and then underwent more extensive coating.

In conclusion, the efficiency of porosity preservation by chlorite coating in these shallow marine clastic reservoirs is controlled by both autocyclic and allocyclic processes. At first order, sequence stratigraphy explains the stratigraphic distribution of the preserved reservoirs. At a more detailed scale, differences in depositional sub-environments control the variations in reservoir quality.

This work is published with the kind permission of TOTAL Norge, operator of PL211, Statoil and ENI Norge.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012