Structural and Stratigraphic Control on the Trapping Machanism in Stacked Reservoirs of Kuwait

Abstract Two giant faulted domal structures are separated by a narrow saddle on the northern extension of Burgan Arch in Kuwait. The structures hold oil at multiple levels in Zubair, Burgan, Mauddud and Tuba Formations with varying entrapment mechanisms. The Zubair Formation is a major oil reservoir in Eastern structure but water wet in the structurally higher Western structure while the Tuba Formation is oil bearing only in the West and wet in East. Multiple phases of oil migration, filling, up-fault migration and spilling of the traps due to tilting and faulting have been postulated from an integrated study of geochemistry, trapping mechanism and structural history. The early charge of immature and bio degraded oils have been replaced by more mature lighter oil. Part of the spilling and redistribution of oil is due to the structural tilting initiated during the Zagros orogeny. Deeper relict oil, geometry of a heavy-oil wedge, presence of higher porosity in carbonates, residual oil rims below the original oil water contacts indicate structural tilt towards the north and west. Another interesting feature is the presence of high asphaltene oil in the form of a massive "mushroom" plug in the central part surrounded by lighter oil in and Burgan formation of western structure. The immovable oil and nearby lighter oil at same depths have similar biomarkers indicating a common source but different spill/charge histories. The lighter hydrocarbons have moved up through faults in the crestal area leaving behind the asphaltene rich heavy oil. In Eastern structure, the major part of Zubair is devoid of movable hydrocarbons in structural traps due to up-fault migration, lesser charge and inefficient seal of cavernous Shuaiba limestone. In contrast, giant structural oil traps in Western structure are to due to the smaller oil leakage along the less intense faults and more charging from the southwest. Additional support to the migration theory is that all the overlying traps are filled to spill point other than Zubair. Hydrocarbons are trapped in Burgan and Mauddud due to seals provided by carbonate cemented sands of Middle Burgan and marine shales of Wara. Understanding the redistribution of oil in structural traps and the basinal oil charging have been pursued in discovery and delineation of the stratigraphic traps of Zubair and Ratawi Formations of the Eastern structure. These traps are much smaller compared to the giant structural traps.