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ABSTRACT: Potassium-Leached Zones at the Contacts of the Garn Formation, Mid-Norwegian Continental Shelf: Evidence for the Role of Shale in Sandstone Diagenesis

S. N. Ehrenberg

The Middle Jurassic Garn Formation consists mainly of subarkosic arenite in which K-feldspar is more abundant than plagioclase. In cores from eight wells, the top of the formation (upper 6-12 m) appears to have undergone relatively intense alteration of K-feldspar to kaolinite, resulting in a loss of bulk potassium content. this cannot be explained as a result of weathering because in two of these wells the same pattern of alteration is observed at the base of the formation. In all cases the sandstone displays

gradual increase in kaolinite content and decrease in the ratio of bulk-rock potassium-aluminum over a distance of several meters approaching the formation boundary. These trends are interpreted to result from introduction of acid released by the maturation of organic matter in adjacent shales.

The cores studied record different stages in the following depth-related sequence of diagenetic events: (1) early kaolinization throughout the formation by meteoric water flushing; (2) formation of additional kaolinite near formation contacts by influx of acid from surrounding shales; and (3) extensive alteration of kaolinite to illite at 3.7 km depth. Illitization has been impeded in the leached zones near formation contacts due to local deficiencies in reactive potassium.

These observations have the following implications. (1) A substantial proportion of the acid generated from kerogen is capable of escaping into adjacent sandstone layers without being neutralized by reaction with the shale matrix. (2) Aluminosilicate dissolution caused by the influx of such acid results in little or no net increase in sandstone porosity and permeability. (3) Extensive illitization of sandstone can occur by short-range diffusion in a chemically closed system.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990