Residual oil as a tool in migration and filling history analysis of petroleum reservoirs, Ghazal Field, Saudi Arabia

Stepwise extraction and molecular analysis of non-recoverable oil from intact reservoir core plugs is used to provide constraints on the accumulation history of petroleum in the Permo-Carboniferous Unayzah reservoirs at the Ghazal Field, Saudi Arabia. When integrated with existing tools such as oil geochemistry, gas geochemistry, stable carbon isotopes, fluid inclusions and thermal modeling, the sequential extraction 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 oil, particularly in the north/northwest of the Ghazal region where paleo-oil accumulations may have been displaced or spilled. The lower portion of the Unayzah reservoir (Unayzah-C) remained water-wet during early filling of the upper reservoir unit (Unayzah-A) and appears to have been filled only by a late oil charge. Anomalous molecular and carbon isotopic behavior of residual oil extracts from the uppermost segment of the Unayzah reservoir, where the free oil (first of the sequential extracts) is less mature (0.95% Rc) and isotopically lighter (δ13C−31.15‰) than the adsorbed oil (last sequential extract, 1.05% Rc; δ13C−29.65‰), suggests a late oil contribution or bitumen contamination from the overlying Khuff Formation.