Abstract
The existence of tilted fluid contact and extension of hydrocarbon accumulation beyond the structure spill point in eastern Abu Dhabi has been suggested by available drilling activities. This is an evidence of the impact of the Miocene Zagros tectonic movement over basin flow, hydrocarbon re-migration and trapping in the primary Cretaceous reservoirs. Therefore, the characterization of the structure and the reservoir evolution related with the Zagros movement reveals potential for the presence of hybrid structural-diagenetic traps in this region.
The fault development, cementation and hydrocarbon distribution of reservoirs were calculated and mapped for different structures in a regional model, in order to quantify the Zagros tilting effect over the hydrocarbon trapping and preservation. This was performed based on the reconstruction of regional paleo-structure maps of the Cretaceous reservoirs at different geological ages. Also, key geological parameters were considered, such as distance to the matured Diyab and Shilaif source rock kitchens, distance to the Zagros Mountain, structure relief, orientation and scale, lateral shift distance, change in closure height and gross volume through geological time, reservoir thickness and continuity.
As a consequence of the Zagros compression to the northeast of Abu Dhabi, westward shift of existing structures triggered the westward re-migration of structurally trapped hydrocarbons resulted from the Maastrichtian-Early Tertiary Masirah collision in the Cretaceous Thamama and Mishrif reservoirs. The depression in the east due to the Zagros compression created a westward basin flow and hydrodynamic potential. However, the reservoir cementation below the paleo hydrocarbon column formed bottom and lateral seal during the structural tilting. Besides, it created obstruction to the lateral migration across the paleo hydrocarbon-water contact. The sealing capacity of the cement overcame the hydrodynamic pressure of the westward basin-flow and caused the contact tilting towards east following the continuous depression. The Zagros event also partially destroyed the closure volume of the paleo-structures. Nevertheless, due to the cement sealing along the tilted contact, hydrocarbon that had exceeded current closure volume can be trapped at the northeast flank that used to be the crest of the structures. As such, they have formed hybrid structural-diagenetic traps that extend the upside potential of the Cretaceous reservoirs in current producing fields.
Consequently, the hybrid structural-diagenetic trap in Eastern Abu Dhabi is an integrated product of the paleo structural trap, diagenesis and hydrocarbon re-migration associated with the Zagros movement. On one hand, the closer to the source rock kitchen and the tilt deflection the more continuous the reservoir. On the other, the gentler the structure geometry, the larger volume variation in structure closure during the Zagros movement and the higher possibility to develop a hybrid structural-diagenetic trap with tilted hydrocarbon-water contact.
