Abstract

The unpredicted discovery of downdip oil in Permian–Carboniferous Unayzah sandstone reservoirs along the southern flank of the predominantly gas condensate Ghazal field in Saudi Arabia highlighted the need for a thorough assessment of controls on hydrocarbon properties, with the aim to improve prediction and reduce uncertainty. Primary controls on hydrocarbon property variations (condensate-to-gas ratio) here appear to be compartmentalization, multiple charging, presumably from a Lower Silurian Qusaiba kitchen to the south and east within the late-oil to wet-gas generation window, and dysmigration to the north and west. Gas on production represents mostly unmixed thermogenic accumulations of variable maturities, with no sign of biodegradation or thermal cracking. Light oil (1.06–1.21% calculated vitrinite reflectance [Rc]) in the southern part of the field represent late-mature accumulations that apparently escaped flushing by subsequent gas condensate charges generated near the wet-gas generation window (1.41–1.55% Rc) and currently occupying the field to the north. Constant maturity variations for both gasoline range and heavier components in each sample testify to a lack of mixing between the hydrocarbon phases. Petroleum inclusions similarly comprise either light oil or gas condensate that coexist with significantly overlapping homogenization temperatures, suggesting considerable overlap in their trapping history (135–35 Ma). Less mature petroleum has not been identified in the field, possibly because of a lack of trap closure or migration focus prior to the inferred Late Jurassic filling or because of displacement to updip structures, thereby high grading the probability of undiscovered oil west and northwest of Ghazal. If thermal generation instead of retrograde condensation or phase separation controls this petroleum system, then the potential for deeper oil accumulations is limited.

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