No abstract

The Zubair Formation in Raudhatain Field is a prolific producer of oil for last 5 decades. Currently, it produces the maximum amount of dry oil in North Kuwait. The Formation was deposited in paralic to offshore environments giving rise to reservoirs of variable quality and heterogeneity. An integrated study on structural and stratigraphic interactions led to discovery of bypassed oil in an extended area. The discovered area was developed with four producers which resulted in sustaining production from Zubair. The concept could be useful in discovering more thin pay sands for future development. Identification of nature of the faults, pattern and compartmentalization are the key contributors to the process of locating bypassed oil areas. Faults were mapped from wide ranging seismic signatures varying from refection discontinuity to attribute variations supplemented by information from geology, drilling, well logs and reservoir performance. Fault calibrations from well cuts were useful to define the throws and distinct faults were identified to have varying throws with depth. Strike slip faults were mapped from more seismic attributes. The fault seal analyses were carried out from performances of multiple reservoirs which are in mature stages including the ones lying 4000 ft above the target zones, variation in fluid contacts with time and space, juxtaposition, structural back stripping and fill-spill history of the traps. An east-west trending fault Zone with several secondary faults was identified from multiple criteria. It is the most dominant fault trend and separate the southern part of Raudhatain field from main field and has significant control over fluid distribution during later stages of field life. Lack of communication across the fault was observed from pressure response in overlying Mauddud reservoir. Distinctly low reservoir pressure in the main field during early stages of Mauddud injection got inverted during later stages of water injection provided the direct proof of fault sealing. Water encroachment mapping in Lower Burgan was supplemented to the study showed island of dry oil in Layered sands while the area in the main field showed water. Three wells drilled close the fault zone indicated drilling complications from brecciated rocks directly associated with faulting. Finding thick oil columns in attic zone gave credence to the concept of fault sealing in this area. Another well lying in the graben area shows a thick immovable oil zone in Burgan massive sand which is not seen in anywhere else in the field. This immovable strip is the fault zone which enabled significant up-structure migration of oil. Sand discontinuity due to shorter channels and rapid variation in water depths in adjacent areas contributed to the existence of by-passed oil. The oil bearing layers are stratigraphically controlled with lower net/gross. More arenaceous intervals show communication across the faults. One of the eight identified fault compartments was drilled with an appraisal well and it showed 79 ft of bypassed oil in Zubair. Heavy oil/Tar is normally observed close to faults: NMR logs and pressure response indicated presence of light oil. The reserves added by the finding would last for 30 years with existing producers in the area. The effort helped in extending the platue period of the reservoir. Being closer to oil-water-contact and with significant volume of residual oil in flushed zones, the area is the target for future exploitation with the appropriate facilities and completion technology.

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.