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Using a multi-proxy approach based on core analysis, thin sections and log data from the Jurassic Arab reservoirs in selected wells in the offshore area of Qatar, the reservoir has been divided into a set of distinctive petrophysical units. The Arab Formation consists of cyclic sediments of oolitic grainstone/packstone, foraminifera-bearing packstonewackestone, lagoonal mudstone and dolomite, alternating with anhydrite. The sediments underwent a series of diagenetic processes such as leaching, micritization, cementation, dolomitization and fracturing. The impact of these diagenetic processes on the different depositional fabrics created a complex porosity system. So, in some cases there are preserved depositional porosity such as the intergranular porosity in the oolitic grainstone, but in other cases, diagenetic cementation blocked the same pores and eventually destroyed the pores. In other cases, diagenesis improved the texture of nonporous depositional texture such as mudstone through incipient dolomitization creating inter-crystalline porosity. Dissolution created vugs and void secondary porosity in otherwise non-porous foraminiferal wackestone and packstone. Therefore, creating a matrix of depositional fabrics versus diagenetic processes enabled the identification of different situation in which porosity where either created or destroyed. By correlating such petrographic data with logs, it will become possible to identify certain “faciodiagenetic” signatures on logs which will be very useful in both exploration and production studies. Faculty and PostDoc Science & Engineering A matrix of the main depositional fabrics of the Arab Formation versus the main diagenetic processes that impacted its rocks appears in the table at the top. The main rock types forming the Arab Formation are oolitic-peloidal grainstone-wackestone that formed persistent shoals all over the region, shallow-shelf carbonates formed of limestone and dolomitic limestone containing large benthonic foraminifera with green algae, mudstone with less than 10% of small dwarfed fossils which are believed to form in saline lagoons or restricted shelves, dolomite which represents a replaced texture some of them are difficult to recognize with beds of anhydrite. The main diagenetic processes include dissolution by meteoric water at exposure surfaces or after the final deposition of the formation, bioturbation that include micritization and burrows, cementation which was mainly meteoric blocky cement, dolomitization which may be selective or full dolomitization of the whole rock and fracturing as a result of tectonic activity. Plotting the two components versus each other led to the creation of a matrix that shows the best combination to create or destroy porosity as well as the cases where the original status is preserved. Clear from the table that diagenesis is creating more porosity than destroying it. This best example of this is the impact of dissolution that either enhance the intergranular, intragranular and inter-crystalline porosity or create news vugs and voids by dissolving the less stable components. Dolomitization and fracturing also either improve or create new porosity types. The destruction occurs only by cementation that may block primary or secondary porosity and also some interlocking dolomite that destroy the pores. So in general, diagenesis seems to improve the reservoir characteristics of the formation. Porosity Destruction

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Linking Depositional Environments and Diagenetic Processes to Porosity Evolution and destruction in the Arab Formation reservoirs, Offshore oilfields of Qatar