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Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain
Challenges Associated with Exploring the Cretaceous Rudist Basin-Margin Buildups of the Arabian Basin
(1) Qatar University, Doha, Qatar.
Rudist buildups were described from many horizons within the Cretaceous strata of the Arabian Basin. The most prominent among these were described from the Shuiaba Formation (Aptian) in the major oilfields of the Arabian Gulf region. They were documented also from the Mauddud Formation (Aptian) in the eastern offshore areas of Saudi Arabia and southeast Iraq and from the Mishrif Formation (Albian-Cenomanian) in the southern Arabian Gulf and southern Iraq (e.g. Majnoon field). More varied buildups were found in the Upper Cretaceous carbonates of the Simsima Formation (Maastrichtian) in Oman and the UAE, the Hartha Formation (Campanian) in central and west Iraq and from Wadi Sir Formation (Turonian) in Jordan.
The buildups were found to occupy either paleotopographic highs resulted from severe erosion as the case with the Simisa Formation in Oman and the UAE, or on horst structures resulted from basement block faulting such as the Aqra Formation in northern Iraq or occupying the crests of growing structures as the case with the Mishrif Formation in Dubai and southern Iraq.
There are several problems associated with the exploration of these bodies resulted from their special nature and geometry. They lack a definite geometry that can be detected in seismic sections or logs. They are relatively narrow and located on the inflection point of the slope and could be easily missed by exploration drilling. They could be extensively altered by diagenesis, removed by erosion or simply overlooked by geologists.
Since rudists do not build “real” wave-resistant reefs, they would be reduced by the action of water and wind into bioclastic grainstone and packstone that may be scattered over areas of tens of kilometers around the original location of the buildups as the case with the Mishrif Formation in Abu Dhabi and southern Iraq. In such cases, this breakdown and distribution of the original components increase the surface area of the potential reservoir rocks. Available data suggest that these rocks have higher porosity and permeability values than the real reefal suites, and hence exploration should be expanded to include all these porous bodies of bioclastic materials. Furthermore, recent investigation of the rudist buildups in the Dujaila Field, southern Iraq suggested that these buildups may act as stratigraphic traps and they have significant oil accumulation beyond the conventional structural traps of the formation.