Abstract: Expulsion and Trapping Potential of Oil and Gas in the Paleozoic Hydrocarbon System, Central Saudi Arabia

Abu-Ali, M. A. - Saudi Aramco; J. L. Ruddkiewicz - IFP; J. G. McGillivray - Saudi Aramco; and J. M. Gaulier - IFP

The Paleozoic oil and gas system of Central Saudi Arabia is discussed with respect to hydrocarbon generation and migration history through geologic time. 2D and 3D basin simulators have been used to understand gas and oil expulsion, migration and entrapment to assess future exploration potential.

The Paleozoic system in Central Saudi Arabia consists of the Early Silurian Qusaiba hot Shale as the major source rock and the Permian Unayzah sandstone as the main regional reservoir. The thermal regime and migration directions were reconstructed through time using a 3-D data set comprising the major formations from Paleozoic to Cenozoic over the 480 x 550 km study area. Present day bottom hole temperatures and past maturity indicators were used for calibration. The 3-D data set was then backstopped to reconstruct paleoslopes for all layers, with special emphasis on the Unayzah and Qusaiba formations. The generation of hydrocarbons was modeled with integrated primary and secondary compositional cracking. Several 2-D cross sections, from kitchen to structure, were used to account for gas/oil ratios.

The expulsion history of the Qusaiba source rock shows a peak expulsion younger than 52 Ma, with a significant amount of gas being created from secondary cracking of oil retained in the source rock. Paleostructure of the Unayzah reservoir and associated capture areas were computed at different times, especially after peak expulsion. The expelled amounts of hydrocarbons from the Qusaiba source rock were then added up to rank the different structures according to their charge volume and the nature of trapped components.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil