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Diagenesis Impact on Permeability of a Large Carbonate Reservoir

Abstract

Preliminary results of a diagenetic study of an Aptian carbonate reservoir in the Middle East indicate that both porosity and permeability started to be modified soon after deposition. However, present day permeability is mainly controlled by late, randomly distributed calcite cements. Diagenesis started with the development of micritic envelops that occurred in restricted marine conditions, commonly associated with precipitation of small cubic pyrite crystals. Soon after, these still loose sediments became influenced by meteoric waters, as a result of a sea level drop, producing chalkification of mollusk fragments. Neomorphism from aragonite to calcite of rudist and corals and development of millimetrer-thick irregular fractures probably occurred at this time. Subsequent dissolution generated centimeter-size vugs, mouldic porosity and enlargement of previous fractures, which were partially filled with sediment and totally occluded by calcite cement. Pedogenic features, such as microcodium-like calcite, also developed. Later, calcite cement rims around bioclasts precipitated under phreatic conditions. A second dissolution episode generated most of the present day mouldic porosity. The last calcite cementation event was characterized by isolated large crystals in interparticle and in the previously formed mouldic porosity. At least two events of pyrite precipitation have been recognized: pyrite replacing bioclasts and pyrite filling vugs. Some of the observed key points are: 1) Intraparticle porosity in orbitolinas has remained uncemented since deposition; 2) Vuggy porosity shows a different distribution pattern than mouldic porosity within the studied rocks pointing to different dissolution events; and, 3) The late isolated large calcite crystals occluded porosity very locally but their spatial distribution within the pore throats are a key control in permeability modification. This work indicates that the fundamental understanding of the diagenetic modifications of the pore system is essential to reservoir modelling and prediction at well, interwell and reservoir scale.