Stratigraphy and petroleum prospects of Upper Jurassic carbonates in Iraq

Following post-Toarcian rifting in Iraq, basin margin oolitic-peloidal shoals which pass into scattered, low relief sponge-algal buildups were deposited. These sediments form the Najmah Formation. In the basin centre, a condensed section of black shales were laid down (Naokelekan Formation). Later progradation accompanied by slow regression created a bull's-eye evaporitic basin in which the Gotnia evaporitic sediments were precipitated as a lowstand gypsum wedge and basin fill halite. Activation of basement faults during the Late Jurassic may be responsible for the widespread erosion and associated diagenetic features that are exhibited by the Najmah sediments which may suggest post-depositional exposure. The Najmah Formation produced 330 barrels of oil from the Samawa-1 well with oil shows in other wells that make it a potential reservoir. The main reservoir facies are oolitic-peloidal and dolomitic limestones.