Implementation of Interval Velocity Data in the Analysis of Vertical Fluid Migration Routes and Charging Offshore Qatar

Oil production from the Al Shaheen Field in Block 5 and Block 5 Extension, offshore Qatar, relies on horizontal drilling and careful stimulation of the very thin and low-permeable Cretaceous carbonate reservoirs. Although the field has been in production since 1994 some uncertainties remain, particularly concerning fluid migration routes and charging of the field. The field appears to be effected by vertical anomalies which have the potential to act as vertical migration paths for fluid migration possibly impacting fluid distribution and characteristics at present day reservoir levels. This study uses 3D seismic data combined with a calculated interval velocity volume to analyse the vertical anomalies in more detail. The anomalies are vertically extensive and typically characterized by lower velocities than the surrounding strata. They have been mapped according to their vertical extent, average velocity, any associated lateral effects, initiation and termination levels and their relation to other features in the area. The results of the mapping suggest that the majority of the velocity anomalies are related to fluid migration with gas causing the anomalous low velocities. The present day gas cap in the Kharaib reservoir may have formed due to vertical migration of gas along the velocity anomalies, which in the area of the gas cap are associated with very low velocities and all terminate at the level of the reservoir. In the areas north of the gas cap, the velocity anomalies generally terminate shallower suggesting that fluid venting probably continued to the seafloor thereby preventing gas accumulation in the reservoirs. The study successfully implements velocity data in the analysis of fluid migration routes and supplements the current understanding of some of the uncertainties related to the Al Shaheen Field. The results represent new inputs to the characterisation of fluid migration within the reservoirs and the existing production and may furthermore provide new input in the assessment of the future development of the field.