Litologi og diagenese i Froyhogdaomrádet, Norskehavet, basert på petrografi og bronnlogdata.

Abstract

Thin sections and petrophysical log data from a drill core west of the Frøya High, Norwegian Sea, have been studied to identify lithostratigraphy and diagenesis. The rock types and diagenesis has been interpreted based on optical microscopy of thin sections from sidewall cores, supplemented by XRD data and SEM analysis. Petrophysical well log data were compared with the mineralogical data and intergrated into the lithostratigraphic interpretation.The interpreted succession can be divided into 1) basement gneisses and igneous rocks, 2)mixed carbonate-siliciclastic sandstones, 3) arkosic sandstones, 4) limestone and 5) claystonesat the top. The igneous rocks include rhyolites and quartz monzodiorites. The basement rocksshow evidence of weathering and retrogression such as chloritization, sericitization andfracturing. The microfractures are partly filled by late phase calcite cement. The retrogradealterations have probably occurred during uplift and faulting.

Comparing with the lithostratigraphy of the area, the Triassic and Early-Middle Jurassic rocks are absent. These sediments are most likely eroded away by uplift (late Early to MiddleJurassic) and rifting (Late Middle to Upper Jurassic) of the area. The calcilithic hybridarenites (mixed carbonate-siliciclastics) overlie the basement rock unconformably. They have been interpreted as Upper Jurassic successions, possibly equivalent to the Spekk Formation.The arkosic sandstones occur within the calcilithic hybrid arenites. The calcilithic hybridarenites may have been deposited in shallow marine (shelf) environments and may possibly have been reworked. The arkosic sandstones of the Rogn Formation are considered as possible reservoir rocks. Cementation, dissolution and clay mineral authigenesis are the mostsignificant diagenetic processes affecting the reservoir quality. The arkosic sandstones are overlain by lime rich carbonate rock, interpreted as Early Cretaceous Lyr Formation. The grayish green noncalcareous claystones, Lange Formation, overlie the carbonate rocks of the Lyr Formation. The general depositional sequence has been influenced by eustasy and synsedimentary tectonics.