--> Abstract: Potassium Mass Balance in Sandstone-Shale Sequences in the Lower Jurassic Statfjord Formation, North Sea: Implications for Permeability Prediction, by M. Ramm; #90937 (1998).
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Abstract: Potassium Mass Balance in Previous HitSandstoneNext Hit-Shale Sequences in the Lower Jurassic Statfjord Formation, North Sea: Implications for Previous HitPermeabilityNext Hit Previous HitPredictionNext Hit.

RAMM, MOGENS, Norsk Hydro E&P, Oslo, Norway

Summary

Formation of diagenetic illite by illitization of kaolins may cause significant Previous HitpermeabilityNext Hit deterioration in the Statfjord Formation during burial Previous HitfromNext Hit 3500 to 4000m. The illite formation needs potassium which is derived Previous HitfromNext Hit dissolution of K-feldspar. If all detrital K-feldspar is diagenetically removed before burial to 3500m, there is an increased possibility for Previous HitpermeabilityNext Hit preservation during burial towards depths greater than 4000m.

Diffusion transport of potassium Previous HitfromNext Hit sandstones to shales during moderate burial (2.5-3.5 km) is probably an important mechanism for removing potassium Previous HitfromNext Hit the sandstones, prior to the onset of kaolin illitization. In the Statfjord Formation, good Previous HitpermeabilityNext Hit has been found in wells Previous HitfromNext Hit the northern North Sea at burial depths exceeding 4000m. In these well, diagenetic kaolins are little affected by illitization and the average potassium content lower than in reservoirs with abundant illite. Similarly, the average potassium contents in the associated mudstones are higher than in shallower wells. The present study thereby suggests that potassium is removed Previous HitfromNext Hit the sandstones during intermediate burial, before illitization commences.

Complete removal of K-feldspar before illitization has, however, not occured in all deeply buried structures, and the average potassium content is high in some deeply buried sandstones with abundant diagenetic illite and low Previous HitpermeabilityNext Hit. Furthermore, the potassium content in the mudstones in one deep well is not significantly different Previous HitfromNext Hit the potassium content in the mudstones in the shallower wells. Why potassium is not transferred Previous HitfromNext Hit the sandstones to the shales is then an important question during Previous HitpermeabilityNext Hit Previous HitpredictionNext Hit in deep structures. One pessimistic, but likely model, is that efficient transport of potassium Previous HitfromNext Hit sandstones to mudstones requires a water saturated pore system, and that hydro-carbon emplacement will stop the process. Hence, if this model is correct it implies that good Previous HitpermeabilityTop in deeply buried Statfjord Formation prospects will be encountered only if the structure is dry, or if hydrocarbon emplacement occured after burial to more than 3500m

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah