Online Journal for E&P Geoscientists

Watson, Jonathan ^*1; Jackson, David ¹; Tefera, Girma ²
(1) interpretation, ARKeX, Cambridge, United Kingdom. (2) SWE, Addia Ababa, Ethiopia.

Summary

A high resolution gravity gradiometry and magnetic survey was acquired early in 2011, over an area of 30,000 sq.km.

The main objectives were to define the basin architecture (depicting basin depth, sub-basins and half-grabens) and to highlight focus areas for the acquisition of new seismic.

Introduction

The Ogaden Basin extends across one-third of Ethiopia, and is the major Ethiopian structure east of the East African Rift.The hydrocarbon potential of the Ogaden Basin has been proven, mainly in the southwest and central regions.

Gravity gradiometry

Airborne gravity gradiometry is a former military technology using full tensor gradiometers built by Lockheed Martin. The airborne nature of the survey means that a large region can be studied, while preserving the high resolution signal of the ftg data.

Qualitative Interpretation

Using all available data (surface geology, topography, magnetic, gravity, gravity gradiometry and maps produced from limited seismic interpretation) a preliminary structural elements map is produced.

Extracting more information from the recently acquired and old datasets becomes more and more important as we progress from basin and sub-basin definition, to promoting a ranked portfolio of leads.

Quantitative interpretation

2D gravity and magnetic modelling of a number of seismic lines. The modelling workflow involves assigning rheological properties to a series of bodies derived from an initial structural interpretation. This work is combined with iterative depth to basement estimates to derive a depth-converted 2D model from the time domain, which can be used as our starting model for gravity, gravity gradiometry, and magnetic 2.5D modelling. The 2.5D modelling on a series of lines provides a fence distribution of 2D geological cross-sections, which can then be used to semi-constrain 3D modelling, such as gravity inversion modelling, to produce a 3D basement surface.

Conclusions

The recently acquired gravity gradiometry and magnetic surveys have enabled a refined understanding of the development of this part of the Ogaden Basin.

The 2011 seismic acquisition program has taken this preliminary work into account, and where possible, changes in seismic acquisition geometries have been made to maximize imaging quality, and to target the highest ranked areas.

In addition, the new ftg dataset has enabled solid interpretation of previously difficult 2D seismic data and a refined model of basin development.