TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractOne of the most challenging issues within Abu Dhabi onshore fields is the definition, characterization and modeling of the main structural heterogeneities. Integration of structural, geometrical and dynamic understanding is the key to defining what is important from a field development and production viewpoint, since it may affect the economics of the fields.A consistent analysis and approach is essential to be able to compare the fields taking into account the three different types of fracture (i.e. faults, fracture corridors and diffuse fracturing), and the potential geological events that postdate the fracturing (fluid circulation, diagenesis).Diffuse fracturing was commonly described across most fields based on core observations and image log interpretation. However, this fracture type alone cannot explain the behavior of many wells.A new look at the data reveals that several fields exhibit fracture corridors and/or subtle faults. Integrating the well behavior suggests this fracture type may have a significant impact in some fields. Unfortunately, this style of deformation is often below the seismic detection threshold.The recent acquisition of 3D seismic across most of the Abu Dhabi onshore fields reveals all fields are faulted at the seismic scale. The fault style is often characterized by small throws. Recent improvements in seismic resolution reveal that what was previously interpreted as one fault is in fact made of numerous fault segments separated by relay areas. This new interpretation has obvious implications for the dynamic behavior of these fault zones. Indeed great discrepancies of the fault zone behavior are observed and discussed using six example fields which encompass a wide variety of carbonate reservoirs with varied depositional environments and reservoir properties. Given such a range of reservoir properties, faults and fractures with similar properties might be expected to have very different expressions in the way they affect fluid flow.

The pre-Khuff principal hydrocarbon reservoir, Unayzah Formation, consists mainly of distal braid plain sandstones characterized by aeolian and sabkha facies with minor fluvial units. It extends between the pre-Khuff and the Hercynian unconformities. In Abu Dhabi, the Unayzah-A is further subdivided into three members, Members 1 and 2 are comprised of sandstone reservoirs and Member 3 consists of siltstone and shale sediments. Facies controls on reservoir quality are weak. The main controls on porosity reduction of the reservoir are mechanical compaction and silica cementation. Quartz cementation tends to be the most severe in the cleanest, coarsest sandstones and near certain fractures. The presence of clay mineral grain coatings, although reducing the permeability, but locally protects the rock from secondary quartz overgrowth and preserve the porosity to great depths of burial. Without the grain coating, porosity will decrease with depth until the reservoir rock is completely tight. Unayzah reservoir seals are provided by the Basal Khuff Clastics, tight Basal Khuff Carbonate and Middle Khuff Anhydrite. The Basal Khuff Carbonate seal does not appear to be regionally extensive but localized and potentially prospect specific. However, there are insufficient data to accurately define the seal for the Unayzah hydrocarbon accumulations. Due to lack of deep penetrations in Abu Dhabi, basin modeling for Silurian hot shale source rock is challenging. Therefore, much of the unknown source and tectonic information were derived from the surrounding countries. This comes from understanding the regional tectonics and depositional trends of the southeastern Arabian plate, which helped to extrapolate the source trends into the Abu Dhabi area. The basin model shows that oil from Silurian source rock was generated early in the basin history and was widespread by the Late Triassic (220 Ma). Significant gas generation occurred during Lower Cretaceous (140 Ma) and dominated the hydrocarbon system by Middle Cretaceous (110 Ma). During the Early Tertiary (50 Ma), the source rock was highly mature for gas generation and at present-day, the charge is still active in the north offshore of Abu Dhabi. The pre-Khuff charge history showed that the southern offshore and onshore structures are underfilled. The filling of these structures ranges between 50% and 80%, but in some onshore structures the filling is less than 50%. The middle and northern offshore structures are expected to be filled to spill point.