--> Abstract: Structurally Dependent Reduction of Reservoir Quality by Authigenic Kaolin Mineral Growth, Late Miocene, Pannonian Basin, by A. Juhasz and A. Matter; #90937 (1998).

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Abstract: Structurally Dependent Reduction of Reservoir Quality by Authigenic Kaolin Mineral Growth, Late Miocene, Pannonian Basin

JUHASZ, ATTILA and ALBERT MATTER, Institute of Geology, University of Berne, Switzerland

The Late Miocene to Pliocene coastal sandy conglomerates of the Bekes Formation and the turbiditic sandstones of the Szolnok Formation are important hydrocarbon reservoirs of the Pannonian Basin in southeastern Hungary. Kaolin minerals reduce reservoir quality down to 2 km by filling primary and secondary pores, replacing feldspars, and by growing between mica plates.

X-ray diffractometry reveals depth related clay mineralogy in both formations: kaolin dominated facies are located close to the flanks of basement highs, whereas basin centers are characterized mainly by illite, illite/smectite with minor chlorite and kaolin minerals. No dickite polytype was found down to 1.5 km, whereas kaolinite free samples were observed below 3.0 km. The intermediate interval is characterised by rapidly increasing dickite to kaolinite ratios with increasing burial depth.

Petrographic observations show that kaolinite (d18O=+11.9 to +16.0^pmil, dD=-65 to -94 ^pmil) is an early (< 70°C) diagenetic mineral and is intimately associated with dissolution of detrital aluminosilicates. Mass-balance calculations indicate that alteration of plagioclase, biotite and muscovite, caused by meteoric influx recharged from the surrounding topographic highs, provides the aluminium for kaolinite precipitation.

Dickite (d18O=+14.0 to +16.8^pmil, dD=-46 to -72^pmil) from the centre of the basin is a late (> 110°C) authigenic phase and precipitated from an evolved pore-water enriched in heavy oxygen isotopes.

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