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AAPG GEO 2010 Middle East
Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain

Petroleum Charge and Discharge in Central Arabian Basin

Khaled Arouri1; Pierre J. Van Laer2

(1) EXPEC ARC, Saudi Aramco, Dhahran, Saudi Arabia.

(2) Exploration Resource Assessment, Saudi Aramco, Dhahran, Saudi Arabia.

With the aim to improve prediction and reduce uncertainty following a number of unexpected results from drilling, an integrated geological-geochemical-modelling approach was employed to untangle a complex filling history in a predominantly gas-producing Palaeozoic system of Saudi Arabia.

The occurrence of light oil of variable condensate-to-gas ratios (CGR) both below and above the field-wide gas-water contact in the Permo-Carboniferous Unayzah reservoir at the Ghazal field precludes a simple oil-rim setting. Gas-washing, water-washing, biodegradation, oil dropout, and source kitchen variations can all be excluded from exerting major control on CGR, which, instead, appears to be primarily a function of differential charging and discharging, as well as compartmentalization.

Whether the basin received petroleum heavier or less mature than that currently being produced (40° - 50° API; calculated vitrinite reflectance, Rc ≥ 1.1%), and the likely fate of that oil remained open to speculation. Fluid inclusions contain only light oil and gas condensate with no evidence for heavier oil. The lack of heavier oil in these inclusions perhaps relate to reservoir temperature (< 90 °C) being insufficient to form a significant amount of inclusions of early oil prior to the Late Jurassic. This may partly explain the paradoxically long lag between the inferred onset of black oil generation (Triassic) and light oil accumulation inferred from co-existing aqueous inclusions to have started in the Late Jurassic. Nonetheless, sequential extraction of traces of residual oil adsorbed onto mineral surfaces or trapped in smaller pores provided temporal resolution of oil charges, including evidence for the “missing” oil, with maturities as low as 0.89% Rc.

These results (1) dispute the belief that less mature oil was never expelled from the source rock and (2) suggest the presence of active migration pathways, at least over Ghazal, in Late Jurassic. The presence of a trap at that time is only weakly supported from palinspastic reconstruction, and may need better refined mapping of the overburden. Given the regional geology and maturity trends that suggest charging from south and east, shallower or updip structures located to the west and northwest — where paleo-oil accumulations may have been displaced or spilled — are a good prospect for additional oil.