Depositional and Diagenetic History of The Arab C Carbonate Reservoir, Upper Jurassic, Dukhan Field, Qatar

• Main Author: Abdulla, Hezarn Yahya Hezarn
• Published: Royal Holloway, University of London 2008
• Subjects: 553
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.487278

This study investigates the Upper Jurassic Arab C carbonate of the Dukhan field, Qatar. This hydrocarbon reservoir occurs within an anticlinal trap within the Dukhan anticline, western Qatar. The focus of this study is to understand the facies associations, sequence stratigraphy, and diagenesis that together create heterogeneity in this hydrocarbon reservoir. This study also categorizes the Arab C reservoir rocks using a new empirical classification scheme to address the impact of rock heterogeneity within the reservoir. Based on a detailed description and sedimentological and diagenetic analysis of cores from six wells from the Dukhan field the sedimentary development and diagenesis of the Upper Jurassic Arab e carbonate ramp are interpreted. Ten facies are identified and described that fall in four facies associations that represent depositional environments ranging from supratidal, intertidal, lagoon to open marine subtidal environments. The Arab e reservoir rocks are arranged in four, fourth-order deepening-up (more distal) and shallowing-upward (more proximal) cycles of varying thicknesses (2 to 13.5 of metres) that reflect repetition of transgressive and regressive periods during deposition. These four cycles can be traced across the entire study area but are thinner in the crest than the flanks of the Dukhan field anticline. Early sea-floor dissolution and the shallow to deep burial dissolution are the main diagenesis processes that enhance the reservoir quality of rocks deposited during transgressive periods. During highstand phases, evaporation rates were high, especially in the shallower environments, which led to the precipitation of gypsum and anhydrite that destroyed the pore spaces in highstand deposits. The 0180 and ol3e whole rock and crystal phase values suggest that the Arab e calcite cements are similar to original Late Jurassic normalmarine signature and thus not of meteoric water origin. Published reservoir rock classification schemes do not satisfactorily subdivide the Arab e reservoir rocks and an empirical-based classification is proposed that provides a practical method of classifying these rocks to reflect their poroperm values. The reservoir has been divided into ten rock types each with distinct petrographic and petrophysical properties. Excellent reservoir quality occurs predominantly in transgressive systems tracts grainstone shoals that have primary interparticle pores and good secondary mouldic and vug pores. Poor reservoir quality is found mainly in nodular anhydrite, crystalline dolostone, and skeletal mudstone of the highstand systems tracts that have cemented pore spaces. Overall, stratigraphy and depositional setting exercise a pervasive control over heterogeneity in the Arab C carbonate by defining the depositional textures, which in tum control the amount and type of diagenetic modification that occur. The Arab e carbonate of the Dukhan field is comparable with other Jurassic carbonate ramps regionally and globally. However, the Arab C of Dukhan appears to be unique in having the best reservoir quality within transgressive, inner-ramp grainstone shoals.