CRETACEOUS – NEOGENE STRUCTURAL EVOLUTION OF SE ABU DHABI, UNITED ARAB EMIRATES

M. Y. Ali^{1 *} and A. Farid¹

¹ The Petroleum Institute, P O Box 2533, Abu Dhabi, UAE.

^* Corresponding author, email: mali@pi.ac.ae

Regional 2D seismic profiles and 2D cross-sections extracted from high-resolution 3D seismic data were interpreted together with well data to reconstruct the sequence stratigraphy of the Late Cretaceous - Miocene sedimentary succession in SE Abu Dhabi. This region is a major hydrocarbon province with numerous giant oilfields producing from large-scale anticlines. The paper focusses on the structural development of the Sahil, Asab and Shah fields.

Six sequence boundaries within the Aptian to Miocene succession were identified on the basis of erosional truncations and onlap patterns. The deepest-lying erosional surface is the top-Albian sequence boundary. This was identified by angular truncation of the Nahr Umr Formation in the SE of the study area near an uplifted domal structure which hosts the Mender and nearby Lekhwair fields. The unconformity is interpreted to result from reactivation of the basement together with a eustatic fall in sea-level which resulted in regional uplift and erosion. A second phase of uplift occurred during the mid-Turonian and resulted in significant erosion of the Shilaif Formation along structural highs. The top- Shilaif Formation unconformity forms a sequence boundary and is observed at the Shah, Zararrah and Sahil anticlines. The unconformity is coeval with the onset of obduction of the Semail Ophiolite onto the Arabian margin, and the associated flexural bulge probably resulted in structural uplift and reactivation of basement structures. A global sea-level fall is also reported during middle Turonian time. The third sequence boundary occurs in the lower Campanian as an angular unconformity surface between the Halul and Fiqa Formations. This event is most obvious along the Shah and Zararrah anticlines and is interpreted to have occurred as a result of flexural uplift related to a further episode of obduction of the Semail Ophiolite. The fourth sequence boundary occurs in the Campanian - Maastrichtian and was identified by angular truncations of the Fiqa Formation against the Simsima Formation, an event which caused widespread erosion of a significant portion of the Fiqa Formation. This tectonic activity was most likely controlled by the final emplacement of the Semail Ophiolite, and probable reactivation of basement faults. The Simsima Formation is bounded above by the fifth sequence boundary, which coincides with the Cretaceous/Tertiary boundary and which is associated with the final stages of Late Cretaceous tectonic activity. The final sequence boundary is the supra-Dammam unconformity, which resulted in a peneplain surface associated with widespread erosion and an extended hiatus.

Major trap formation in the study area occurred from the Cenomanian to the Maastrichtian, and coincided with obduction-related emplacement of allochthonous thrust sheets onto the plate margin in Oman. However, the structural growth history of the Shah, Asab and Sahil anticlines suggest multiple episodes of uplift. Growth of the Shah anticline was initiated during deposition of the Shilaif Formation in Cenomanian – early Turonian times, but neither the Asab nor Sahil anticlines show significant growth or faulting older than Campanian. In all the anticlines, the most pronounced tectonic activity observed was in the Campanian during deposition of the Fiqa Formation. Significant growth of structures was completed by Paleocene time, suggesting that the Zagros orogeny had little structural influence in SE Abu Dhabi.

Keywords: Cretaceous, Abu Dhabi, sequence boundary, Shilaif Formation, Fiqa Formation, Simsima Formation, structural growth, anticlines, Semail Ophiolite, obduction.

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