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New Insight into the Genesis and Early Diagenesis of Microporous Carbonates

Volery, Chadia 1; Davaud, Eric 1; Foubert, Anneleen 2; Caline, Bruno 3
1 Earth and Environmental Sciences, University of Geneva, Geneva, Switzerland.
2 Earth and Environmental Sciences, University of Leuven, Heverlee, Belgium.
3 CSTJF, Total, Pau, France.

Many Cretaceous carbonate reservoirs in the Middle East consist of shallow marine microporous limestones. However, despite an important economic interest, their genesis remains poorly understood. A detailed database of shallow marine microporous carbonate reservoirs (MCR) in the Middle East has been reconstructed in order to clarify their stratigraphic context and their relationship with respect to the Mg/Ca ratio of seawater (calcite sea vs. aragonite sea) and with eustatic changes. This study shows that MCR from the Middle East were formed during calcite sea periods. During the Cretaceous, a typical calcite sea period, 11% of the carbonate reservoirs are shallow marine and microporous, while such reservoirs do not exist during late Carboniferous to Triassic times, a period characterized by an aragonite sea. Moreover, during the Cretaceous, MCR developed mainly during major transgressions or sea level highstands.

It can be supposed that the following factors are necessary to develop microporous limestones: 1) A precursor mud mostly composed of low-Mg calcite crystals. Such a low-Mg calcitic mud is relatively stable and may partly retain its primary microporous fabric during early diagenetic processes. 2) An early and limited overprint of meteoric diagenesis. Freshwater fluids may stabilize the sediment by dissolution of the unstable crystals and reprecipitation by overgrowth on the more stable crystals (Ostwald ripening). This dissolution-reprecipitation process contributes to harden the sediment while preserving the original fabric and therefore reduce physical compaction during burial. However, exposure to meteoric fluids must be limited in time to prevent intense diagenesis. These favourable conditions best occur during transgressive episodes, where meteoric water lenses are easily replaced by interstitial seawater.

Recent shallow marine carbonate muds are mainly composed of aragonite and high-Mg calcite. Therefore, they cannot be used as valuable analogues to study the formation of Cretaceous shallow marine microporous limestones. However, recent and ancient low-Mg calcitic lacustrine sediments may represent good analogues. The study of Upper Miocene lacustrine microporous limestones from the Madrid Basin reveals strong petrographical and petrophysical similarities with Cretaceous MCR of the Middle East and allows better understanding of their genesis and diagenesis.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009