SOURCE-RESERVOIR-SEAL RELATIONSHIPS, IN ZUBAIR FIELD’S OIL ENTRAPMENT, SOUTHERN IRAQ

Abstract

Today’s approach of integrated petroleum exploration is practiced to insure: first, a genesis of petroleum and its timing and direction of its primary migration; second, the formation of a trap at a time prior to the time of primary migration with its sizeable reservoir and type of barrier , be it hydrodynamic and / or a rock seal ; third , early or late entrapment which necessitates a diagenetic assessment of the reservoir rock ; and finally, the type of trap being stratigraphic, structural with or without hydrodynamic. This study comes to emphasize this approach. Zubair oil field, southern Iraq is in its advanced stage of development and has a wealth of raw data available, including cores and well logs. It is selected for this study to shed light into what one should do with fields in their early stage of development or those yet undiscovered. Data regarding the quantity and quality of the source rock organic matter of early Cretaceous age (Sulaiy , Yamama , Ratawi , and shales of Zubair Formations ) are available in the literature . And since Sulaiy Formation is found of high thermal maturity and known, from the literature, by its suitable organic matter, quantitatively and qualitatively, it is considered the major source rock for the vertically migrated oil in the direction of Zubair's two pay members. The timing of oil migration is determined as early Eocene; thus it is a late migration indicating a vertical route. As for Mishrif reservoir oil, the Albian source rocks situated to the east of Iraqi Iranian border is considered its oil supplier and indicating a lateral migration, in addition to what was expelled through vertical supply. The first structural closure was formed in Zubair Formation during Aptian time and became structurally stable during late Cenomanian. This was followed by the formation of a structural closure in Mishrif reservoir during late Cenomanian - early Turonian which became stable during early Campanian. The Ghar - Fatha reservoir appeared with a structural closure during middle Miocene and became stable late Miocene. Lithologically, Zubair's sandy pay members is characterized by high porosity and permeability and, to a lesser extent, with respect to Mishrif Formation. Both reservoir had undergone several diagenetic processes which led to a porosity reduction in some of their parts. These diagenetic processes have no clear imprints on Ghar - Fatha Formations because of their shallow depth. Zu- bair's upper sand member had a higher porosity as a result of early pre - burial diagenesis causing a hinderance to compaction. Based on stratigraphic distribution of source rocks and the volumetric estimation of migrated oil from Sulaiy Formation and entrapped oil in overlying reservoirs, the vertically migrated oil, in spite of its immense quantity, is not enough to supply oil to all southern Iraqi traps. Therefore, an assumed lateral oil supply from thermally matured source rocks situated east ward is probable.