The Value of Integration of Gravity Gradiometry with Seismic and Well Data – an example from a frontal thrust zone of the UAE-Oman Fold Belt

The east of the Emirate of Dubai is dominated by the geologically complex western thrust front of the northern Omani Mountains. This deformation front is the boundary between the western foredeep basin and the eastern Omani fold-and-thrust belt. Complex geology makes conventional exploration challenging. The reservoir (Thamama Group) structures are thrusted anticlines with the overlying Tertiary units showing large-scale thrusting as well. The Lower Cretaceous Thamama Group limestone is one of the main hydrocarbon reservoirs in the Middle East. It forms a major hydrocarbon-producing reservoir in the U.A.E., Iraq, Bahrain and Oman and has a high hydrocarbon potential in southeast Iraq, offshore Oman and offshore northeast Saudi Arabia. Due to the significant density contrast between the reservoir and the overlying sediments, Margham Dubai Establishment commissioned an airborne gravity gradiometry (GG) survey to improve the confidence in top reservoir location and to aid in ongoing exploration activity. GG, magnetic and LiDAR data were acquired and used in an integrated interpretation with existing seismic and well data. The integration of these data allowed a better understanding of the thrust linkages at different levels, and a better insight into the interaction of thrusts, backthrusts, detachment levels, imbricated zones, and lateral ramps. The survey is designed around the airborne GG technology known as the Full Tensor Gravity Gradiometer (FTG). GG measures the rate of change of the Earth’s gravitational field<br>while conventional gravity (CG) measures the vertical acceleration. Acquired from an aircraft, GG has a strategic advantage over CG due to the resolution limitation imposed by the Differential Global Positioning System (DGPS). The DGPS limits airborne gravity<br>resolution to > ~4000m wavelengths while GG can resolve wavelengths of > ~300m. The shorter wavelengths are crucial to accurately model the geology above the reservoir. In complex geology, multiple lithological units contribute to the GG signal. To map the<br>reservoir, it is vital to isolate its response from the overlying geology. The high resolution GG data facilitated an accurate investigation of the 3D Shallow Earth Model (SEM). Modelling of seismic sections constrained by GG and magnetic data exploits their complementary nature. Seismic data respond well to horizontal discontinuities while potential field data respond to vertical discontinuities. This produces a geologically realistic SEM. Forward calculation of the GG signal from the 3D SEM was performed and then subtracted from the observed signal. The SEM corrected data were then used to interpret the Thamama reservoir structure.