Online Journal for E&P Geoscientists

Colombo, Daniele X.^*1; McNeice, Gary W.³; Keho, Timothy H.²
(1) EXPEC ARC GPT, Saudi Arabian Oil Company, Dhahran, Saudi Arabia. (2) EXPEC ARC GPT, Saudi Arabian Oil Company, Dhahran, Saudi Arabia. (3) EXPEC ARC GPT, Saudi Arabian Oil Company, Dhahran, Saudi Arabia.

Exploratory drilling and coring in the northwest of Saudi Arabia has revealed two distinct igneous rock suites, late Devonian (360 Ma) and late Cretaceous (76-82 Ma). The Devonian sills are significant intrusive events that were caused by extensional and compressional tectonics. The Cretaceous suite is a minor intrusive event related to faulting and a high geothermal gradient along the Al Jawf Graben. The most recent igneous activity, during the Neogene, is seen as an outcropping basaltic sequence. The lithological composition of the Neogene basalt cover is very heterogeneous both vertically and horizontally.

The high velocities of the surface basalts cause strong ray bending in the near surface and result in poor penetration of surface seismic energy. Moreover, the internal heterogeneities in the basalt, as well as the rugosity of the basalt layers, causes severe scattering and image distortion for seismic data. Sub-basalt imaging is one of the most challenging scenarios for seismic exploration worldwide and alternative, or complementary, geophysical solutions are often needed to address this problem. Use of magnetotellurics (MT) and other electromagnetic (EM) methods have proven to be effective in other basalt-covered areas, both as an alternative imaging tool to look deep below the basalt cover and as a method for improving the near surface velocity model.

An MT survey together with ancillary gravity and Time-Domain EM (TDEM) data acquisition was carried out in a region in northwest Saudi Arabia characterized by shallow basaltic layers and poor seismic imaging. The MT inversion results and the interpretation of the multi-geophysics dataset reveal important structural elements, such as deep-seated faults controlling the graben evolution, and allow characterization of the complex basalt layering. Integration of the different geophysical datasets is currently ongoing via simultaneous joint inversion, and these results will help guide the reprocessing of the surface seismic data.