Morphology of the basement and Hormuz salt distribution in offshore Abu Dhabi from constrained 3-D inversion of gravity and magnetic data

Abstract

Many seismic surveys and exploration wells have been conducted in offshore Abu Dhabi, United Arab Emirates. However, the Neoproterozoic basement and the Infracambrian Hormuz salts were not imaged or penetrated by seismic and well data. To delineate the morphology of the basement and distribution of salt bodies, we applied a 3-D constrained inversion method to aerogravity and aeromagnetic data covering offshore Abu Dhabi. Well and seismic data were used to constrain and validate the inversion results. The magnetic inversion model resulted in a robust basement model that varies in depth from 8 to 10 km with two highs at 8 and 8.5 km highlighting possibly magmatic bodies. In contrast, the gravity inversion model does not delineate very well the basement morphology, possibly due to the lower density salt bodies masking the gravity response of the higher density basement. By calculating and removing the gravity response of the magnetic basement model from the observed gravity data, we recovered the residual gravity response of the Hormuz salts. The 3-D gravity inversion of the residual shows that the Hormuz salts are more wide-spread and thicker than previously thought with a depth to top of salt varying from 5.5 to 8.5 km. This salt model suggests that the giant oilfields in offshore Abu Dhabi, such as Umm Shaif, Nasr, Abu Al-Bukhoosh, Zakum, Sarb and Hail, occur exactly above crests of salt pillows. In addition, a series of smaller oilfields are found located at the margin of the northern basement high, and three structural lineament trends (NE-SW, NW-SE, and N-S) were identified from the inverted basement and salt models. Reactivation of these fault trends and associated diapiric salt uplifts, as observed in the seismic sections and curvature map, played a major role in shaping the basement morphology and mobilization of the Hormuz salts.