Occurrence of heavy crude oil in the Persian Gulf

Some of the most prolific petroleum reservoirs with high gravities of oil in the world occured in the Upper Jurassic and Cretaceous carbonate formations in the Persian Gulf area .Most of the reservoirs are composed of pelletal,oolitic ,or bioclastic grainstones and reefal limestone that have high primary porosity and permeability .These reservoirs are sealed either by tight limestone ,massive anhydrite, or by impermeable rocks. The seals are effective throughout most of the Persian Gulf and surrounding areas, Accumulations of heavy oil and natural asphalts on the Iranian side of the Persian Gulf extend NE to SW along the Qatar arch. some of the geological structures which are located along this trend contain heavy oil and natural asphalts within post- Jurassic formations. The major geologic factor which caused accumulations of heavy oil within some geological structures is the Qatar arch. As a result ,some differences appear in lithologic units in two sub-basins northwest and southeast of the Qatar Arch. In general , the occurrence of heavy oil in this trend can be explained as follows; • Fracturing and joints within formation ; • Reduction of thickness of post- Jurassic sediments; • Facies change of Arab reservoirs, cap rock (Hith anhydrite), in some structures from anhydrite to dolomite and also pinchout; The absence of proper environment for the generation of high gravity oil may also be of importance .It should be mentioned that "F" structure (Ferdows), with huge amount of oil in Ratawi and Sulaiy carbonate reservoirs(Lower Cretaceous), is one of the largest heavy oil fields along this trend .The Farsi "B" structure, ( Farzad), with a thick Jahrum formation (Eocene) which has excellent reservoir properties, is filled with natural asphalts.