H. Troudi¹, A. M'Rabet¹, M.Hedi Acheche¹, H. Ghariani¹, N. Gharieni¹
(1) Etap, 1002 Tunis, Tunisia

Abstract: Diagenetic (lithification and dolomitization) and tectonic (fracturing) controls on the Senonian Abiod carbonate reservoir in Tunisia, North Africa

The Senonian Abiod carbonates are a major oil and gas fractured reservoir in several producing fields (including Sidi El Kilani, Miskar Chorbane East and Zinnia) in Tunisia. They generally consist of two carbonate Members (chalky limestones and dolomitized limestones) separated by a marly unit, having been deposited on a progradational/retrogradational platform. Within the producing fields, fractures with high permeabilities (up to 2-3Darcies) are the main production contributors although the chalky matrix porosity seems to have at least some contribution in the whole hydrocarbon bearing rock volume. In chalky limestones and dolomites , three types of fractures are recognized (open, partially open and occluded fractures) in cores and on wirelogs. The occurrences of these fractures are primarily controlled by diagenesis. Lithification, including early micritic lithification, enhanced the rigidity of the chalky limestones, facilitating the tectonic fracturing of the rock. Based on outcrop and subsurface data, tectonic fracturing is polyphasic and occurred from Late Cretaceous to Early Miocene, prior to the oil/gas migration. On the other hand, early lithification also contributed in the preservation of significant matrix porosity. Shallow burial dolomitization which affected the Abiod chalk near the overlying Late Cretaceous-Early Tertiary unconformities is another diagenetic factor greatly improving the reservoir quality. Dolomites, more dense than limestones, are the most fractured and have better reservoir properties as demonstrated while drilling (severe lost circulation) and on wirelogs.

Prediction of fractured limestone zones and dolomitized zones (based on an integrated approach including petrography, wirelog data and 3D seismic attributes) has crucial implications in terms of field development and Abiod play exploration in Tunisia.

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