Depositional and Diagenetic History of the Permian Unayzah A Reservoir, South Haradh, Saudi Arabia with Implications for Deep Gas Exploration and Development
Althani, Lulwah Faisal
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The Early Permian Unayzah A member in the Ghawar area of east-central Saudi Arabia is a prolific gas producer but is characterized by significant reservoir heterogeneity related to complex interbedding, on the scale of 5 meters or less, of eolian dune, sand sheet, interdune and ephemeral (playa) lake facies. Diagenetic products in Unayzah A developed during a continuum of eogenetic and mesogenetic reactions. Early or eogenetic cements are dominated by clay rims that formed at temperatures below 70oC. Oil migration along stylolites probably from Silurian source rocks occurred during the early mesogenetic stage followed by barite, quartz and carbonate cementation. Oil degradation at temperatures between 150o and 225oC produced acidic pore fluids that caused dissolution of earlier formed carbonates to generate secondary intragranular, moldic and micro pores. Thin, early clay and hydrocarbon rims as well as possible early microquartz cements inhibited cementation of primary intergranular pores that are only partially filled with quartz outgrowth crystals. Total porosity, including primary open pores, secondary pores, and bitumen-coated and filled pores ranges up to 27 percent. Reservoir quality and heterogeneity are a function of depositional environment and diagenesis in which eolian dune and sand sheet facies contain the highest total porosities and hence the best reservoir properties. Some previously recognized Stoke's surfaces are characterized by higher concentrations of quartz cement further compromising reservoir interconnectivity.
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