Generation, Migration, Accumulation, and Dissipation of Oil in Northern Iraq: Middle East

"Most of the known oil accumulations of Northern Iraq probably originated by upward migration from earlier, deeper accumulations which were initially housed in stratigraphic or long-established structural traps, and which are now largely depleted. The earlier concentrations had their source in basinal sediments, into which the porous, primary-reservoir limestones pass at modest distances east of the present fields. Development of the region favored lateral migration from different basinal areas of Upper Jurassic and Lower-Middle Cretaceous time into different areas of primary accumulation. Important factors affecting primary accumulation included--1. early emergence and porosity improvement of the reservoir limestones, followed by burial under seal-capable sediments; 2. the timely imposition of heavy and increasing depositional loads on the source sediments, and the progressive marginward advance of such loads; 3. progressive steepening of gradients trending upward from source to accumulation area; 4. limitation of the reservoir formations on the up-dip margin by truncation or by porosity trap conditions. In late Tertiary time, large-scale folding caused adjustments within the primary reservoirs and associated fracturing permitted eventual escape to higher limestone reservoirs, or to dissipation at surface."