Paleozoic Petroleum System of the Northern Arabian Platform

A. S. Alsharhan
Middle East Geological Enterprises, Al Ain, United Arab Emirates

Oil and gas fields were discovered in central Saudi Arabia in the sandstone of the Permian Unayzah and Devonian Jauf formations, and from basal transgressive marine sandstones and cyclic dolomitic shelf-carbonates of the late Permian Khuff Formation. The reservoir is dominated by alluvial and fluvial sequences of sandstone and siltstone or silty sandstones whose reservoir characteristics range from good to excellent with average porosity usually greater than 20% and permeability locally exceeding 4D. It sealed by overlying tight carbonate-evaporite strata, and by subconformity pinchouts of pre-Unayzah reservoir units against impermeable facies. The reservoir contain light (48-52° API gravity), sulfur free (0.02-0.07%) crude oil (Arabian super light) and also sweet gas and condensate (65° API gravity). The source rock for the oil in these reservoirs is the early Silurian Qusaiba hot shale with a maximum TOC content of 6.15%. It was matured during the late Jurassic with expulsion beginning during early Cretaceous, with onset of oil generation began in about 160 Ma.

In western Iraq high gravity oil (42° API) was discovered in Silurian sandstone and sweet gas in Ordovician sandstone reservoirs and sealed by lower Silurian shales. The principal source rock was the highly mature marine shales of Ordovician and Silurian age, with onset of oil generation began about 250 Ma.

Structures that trap hydrocarbons are mostly large gentle anticlines formed from reactivated basement fault blocks and/or salt domes that resulted from halokinesis. Other structures occurs in the horst/graben-related traps that formed prior to, during and after Hercynian deformation.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005