With the aim to improve prediction and reduce uncertainty following a number of unexpected results<br>from drilling, an integrated geological-geochemical-modelling approach was employed to untangle a<br>complex filling history in a predominantly gas-producing Palaeozoic system of Saudi Arabia.<br>The occurrence of light oil of variable condensate-to-gas ratios (CGR) both below and above the fieldwide<br>gas-water contact in the Permo-Carboniferous Unayzah reservoir at the Ghazal field precludes a<br>simple oil-rim setting. Gas-washing, water-washing, biodegradation, oil dropout, and source kitchen<br>variations can all be excluded from exerting major control on CGR, which, instead, appears to be<br>primarily a function of differential charging and discharging, as well as compartmentalization.<br>Whether the basin received petroleum heavier or less mature than that currently being produced (40°<br>- 50° API; calculated vitrinite reflectance, Rc ≥ 1.1%), and the likely fate of that oil remained open to<br>speculation. Fluid inclusions contain only light oil and gas condensate with no evidence for heavier oil.<br>The lack of heavier oil in these inclusions perhaps relate to reservoir temperature (< 90 °C) being<br>insufficient to form a significant amount of inclusions of early oil prior to the Late Jurassic. This may<br>partly explain the paradoxically long lag between the inferred onset of black oil generation (Triassic)<br>and light oil accumulation inferred from co-existing aqueous inclusions to have started in the Late<br>Jurassic. Nonetheless, sequential extraction of traces of residual oil adsorbed onto mineral surfaces or<br>trapped in smaller pores provided temporal resolution of oil charges, including evidence for the<br>“missing” oil, with maturities as low as 0.89% Rc.<br>These results (1) dispute the belief that less mature oil was never expelled from the source rock and<br>(2) suggest the presence of active migration pathways, at least over Ghazal, in Late Jurassic. The<br>presence of a trap at that time is only weakly supported from palinspastic reconstruction, and may<br>need better refined mapping of the overburden. Given the regional geology and maturity trends that<br>suggest charging from south and east, shallower or updip structures located to the west and northwest<br>— where paleo-oil accumulations may have been displaced or spilled — are a good prospect for additional oil.


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