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Impact of Diagenesis on Reservoir Quality Evolution of Fractured Cretaceous Carbonate Reservoirs from Abu Dhabi, United Arab Emirates

Al Harethi, Amena D.*1; Morad, Sadoon 1
(1) Geoscience, Petroleum institute, Abu Dhabi, United Arab Emirates.

The impact of diagenesis on reservoir quality evolution of the Upper Cretaceous Formation in one of the Abu Dhabi onshore fields, United Arab Emirates, is controlled depositional facies (subtidal to supratidal), sequence stratigraphy, and fracture development. Thirty three thin sections were examined from core plugs. Diagenetic processes include micritization, dolomitization, cementation by calcite, recrystallization, calcitization of dolomite, dissolution of grains and cement, chemical compaction, and thermal reduction of sulfate cements. Eogenetic processes in the subtidal wackestones and packstones, particularly in the transgressive systems tracts (TST), are dominated by micritization of bioclasts, dolomitization, and cementation by calcite (fibrous, bladed, syntaxial overgrowths), and framboidal pyrite. Fall in the relative sea and concomitant meteoric water percolation resulted in grain and cement dissolution, calcitization of dolomite, cementation by equant calcite, and recrystallization of micrite, particularly below the sequence boundaries (SB) and in the highstand systems tracts (HST). Recrystallization may also have continued during burial diagenesis. Mesogenetic processes include the development of dissolutions seams and replacement of nodular and bladed gypsum/anhydrite by calcite or coarse-crystalline pyrite, which are suggested to be due to thermal sulfate reduction. The nodular gypsum/anhydrite is inferred to be supratidal in origin. Therefore, recognition of the latter diagenetic processes can have important implications for paleo-environmental interpretations and modeling dolomitization across the platform. Sabkha and seepage reflux are the most probable models for this early dolomite. Nevertheless, dolomitization had relatively limited and local impact on reservoir quality improvement, particularly when later calcite cement has filled the created intercrystalline pores. Fractures, which were developed due to the Zagros orogeny, are either empty or filled partly by equant calcite, have greater impact on reservoir quality than diagenesis.

 

AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain