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Manfred K. Hauptmann1, Ralf Bischoff2, Elke Minwegen3
(1) Independent Consulting Geologist, Berlin, Germany
(2) Preussag Energie GmbH, Lingen, Germany
(3) University of Cologne, Cologne, Germany

Abstract: Bioturbation and reservoir properties in nummulitic limestones (Kerkennah-West Permit, offshore Central Tunisia)

A facies analysis of cores and cuttings from Mid-Eocene nummulitic limestones (Reineche Formation) reveals the importance of bioturbation with respect to reservoir properties.

Results are based on samples from five Chergui wells (Kerkennah-West Permit, offshore Central Tunisia) where a small gas discovery has been hit in 1992 immediately adjacent to Cercina Field with current oil production from Reineche nummulitic limestones.

Large post-card sized thin-sections give evidence for a tight network of branching burrows resulting from the activities of various groups of organisms (echinoids, crabs, bivalves). In addition, the material allows to decipher the primary stacking pattern and cyclicity behind the overprint by bioturbation.

Since there is no true evidence for any input of meteoric waters respectively any indication of emersion and erosion, bioturbation helps to explain the irregular patchy association of a broad variety of diagenetic phenomena, especially the complex fractal distribution patterns of dissolution versus cementation but also partial dolomitization and silification of Reineche limestones.

Thus, bioturbation seems another key factor besides fracturing in understanding the distribution of permeabilities and porosities within nummulitic limestones which quite often are characterized by excellent primary intra-particle as well as microporosities in contrast with a lack of connectivities.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana