Document Type

Article

Publication Date

2-13-2009

Abstract

Stepwise extraction and molecular analysis ol non-recoverable oil from intact reseryoir core plugs is used to provide constraints on the accumulation history of petroleum in the Permo-Carboniferous Unayzah reservoirs at the Chazal Field, Saudi Arabia. When integrated with existing tools such as oll geochemistry, gas geochemistry, stable carbon isotopes, fluid inclusions and-thermal modeling, the sequential extrac- tion approach helped decode a previously unrecognized component of the hydrocarbon accumulation history. In particular, it indicates that the region received early petroleum charges that were less mature than currently produced light oil and gas condensates, .possibly from the prolific Early Silurian Qusaiba 'shale source rock and other sources. This has significant implications for prospecting for additional oll, articularly in the north/northwest of the Ghazal region where paleo-ail accumulations may have been disnlaced or spilled. The lo wer oortion of the Unavzah reservoir ( Unavzah-C) remained water-wet during early filling of the upper reservoir unit (Unayzah-A) and' appears to have been filled nly by a late oil 2 charge, Anomalous molecular and carbon isotopic behavior of residual oil extracts from the uppermost segment of the Unayzah reservoir, where t the free oil (frst the sequential extracts) S less mature (0.95% Rc) and isotopically lighter (8'3C-31,15%c) than the adsorbed oil (last sequential extract, 1.05% te; 29.65%0) suggests allate oil contribution or bitumen contamination from the overlying Khuff Formation.

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