--> ABSTRACT: Deep Burial Diagenesis in Rotliegende Reservoirs of the NW German Basin, by R. Gaupp, A. Matter, K. Ramseyer; #91003 (1990).

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ABSTRACT: Deep Burial Diagenesis in Rotliegende Reservoirs of the NW German Basin

R. Gaupp, A. Matter, K. Ramseyer

A deeply buried Permian continental sequence forms the major gas reservoir in northwest Germany. Deposits of fluvial, eolian, and playa lake shoreline facies show the most favorable reservoir properties. Burial diagenesis is greatly influenced by primary depositional textures and eogenetic processes. However, growth of authigenic clay minerals (illite, kaolinite/ dickite, chlorite) relates to changes in the chemistry and flow rate of formation waters.

Three different mesogenetic fluid types can be recognized: (1) Alkaline fluids from basin center red beds: The presence of pore-lining chlorite in porous subarkoses of the playa shoreline facies probably is related to a compaction-driven influx of alkaline waters from the shaly, red bed sequences of the basin center. (2) Acidic fluids from coal-bearing Late Carboniferous sediments: An aureole of dickite/kaolinite, several hundred meters wide, is developed in Rotliegende arkosic sands where they are juxtaposed against Carboniferous horsts. In this zone, almost all feldspars were destroyed and the formation of dickite/kaolinite was followed by illite growth and bitumen impregnation. In an outer aureole with less extensive feldspar destruction, kandite minerals are not present, but a den e meshwork of well-crystallized, platy illite fills the pores. The intensity of illitization diminishes away from the Carboniferous sediments (i.e., with increasing distance of fluid migration). K-Ar ages reveal that this illite precipitated within a period when organic maturation products were expelled from the coal measures into Rotliegende sediments and when Paleozoic faults were reactivated. (3) Brines from overlying Zechstein evaporites: During late mesogenetic uplift, local influxes of these brines resulted in the formation of pore-plugging carbonate and sulfate cements.

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