The Ordovician Glaciation in Saudi Arabia — Exploration Challenges Part 2: Geophysics (Seismic, Potential Field Data and Seismic Interpretation)

Ashraf Khalil¹, Geoffrey Pike¹, Pieter Van Mastrigt¹, and John Smale^2
1Evaluation Department, South Rub Al-Khali Co. Ltd., Al-Khobar, Saudi Arabia
²Exploration, Shell International Exploration & Production B.V., Rijswijk, Netherlands

Extensive post-well geophysical studies have been utilized in support of ongoing seismic interpretation to address recent well results and the overall prospectivity of Ordovician glacial sediments in the South Rub Al Khali Basin. The interface at the base of the Silurian Qusaiba Shale and the top of the Ordovician reservoirs reflects the combination of structuration and paleo-topography caused by glacial erosion during Late Ordovician time and hence cannot be fully described by picking timelines on the seismic data. The presence of seismic multiples and a limited impedance contrast between the Sarah (peri-glacial) and Qasim Formation (pre-glacial) sediments hinders the recognition of the base glacial unconformity on seismic.

Integrated geophysical technologies deployed in the exploration effort include airborne and land-based gravity, high-resolution magnetic data, passive low-frequency seismic, magneto-telluric, multi-azimuth VSP and acoustic impedance data.

Seismic interpretation suggests that the glacial sedimentary succession drapes much of the study area and points to overfill depositional models. Internal sedimentary architecture may consist of nested incised valleys within a broader incised valley. Field analogs suggest that glacial incisions are common, often resulting in a complex architecture of buried hills and adjacent glacial valley systems. Such topographic features and valleys appear to be influenced by the regional basement structural features.

Constrained depth inversion of potential field data has been used to determine if the glacial deposits, rich in diamictites and possibly having a unique density and/or magnetic susceptibility contrast, can be identified and mapped regionally. The combined results of this study helped explain exploration well results and aided the development of more accurate play maps to focus future exploration campaign.

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