First Break - Volume 31, Issue 12, 2013

Sissel Grude, Jack Dvorkin, Anthony Clark, Tiziana Vanorio and Martin Landrø present laboratory work on geochemical reactions to show how saturation, pressure and mineral precipitation effects could be resolved from seismic data.

Josephin Mühlbach, Steffen Klumbach, Astrid Hirsch, Marco Kromer and the COBRA Team explore the findings of the COBRA (CO2 Borehole Research Apparatus) project to see if it is possible to achieve safe geological CO2 storage in depleted hydrocarbon reservoirs and deep saline aquifers.

Will fears that oil companies are holding back their spending and that the boom in capital spending is over be justified?

Electromagnetic geophysical applications exploit a petrophysical relationship governing the electrical properties of rocks. Given the extensive nature of many airborne electromagnetic surveys it seems natural to classify, and hence simplify, the spatially aggregated conductivity information on the basis of rock lithology. This provides an important link between lithological rock parameters and the petrophysical parameters controlling bulk conductivity. This classification procedure has been applied to more than 8 million conductivity estimates obtained from airborne surveys conducted in the UK over a period of ten years. The analysis has provided new information on the conductivity characteristics of many UK geological formations and allowed a new UK national map of bedrock conductivity to be developed. Here, we highlight the complex conductivity variations observed in mudstone and shale rocks across a Carboniferous basin. The potential interplay of increasing conductivity due to clay content and decreasing conductivity due to the presence of pore-scale hydrocarbons is noted. The geological-geophysical analysis described is a new procedure that provides a framework for further detailed petrophysical understanding of rock properties at depths accessible to EM techniques.

Brian W. Horn explores the application of ideas and technology with a common philosophy: basins are best understood from the bottom up.

Craig Koch, Iain Brown, Ron Borsato, Severino Cardoso and Lumen Sebastiao examine how the development of modern 2D seismic techniques has led to expectations of new prospectivity in a West African basin that was abandoned more than a decade ago.

Neil Hodgson and Anongporn Intawong show how the drilling of four wells in deep-water offshore Namibia have reduced the risk for forthcoming drilling campaigns in what were thought to very unpromising plays.

We describe an example of a common geological scenario which results in magnetic bodies of limited depth extent. We then present examples of the errors in depth estimates, as high as 40% or more, incumbent in the use of many classic magnetic depth techniques when considering bodies of limited depth extent. Then using interpretation by characteristics applied to generalized dyke models we develop correction nomographs for a wide variety of classic magnetic depth techniques. We show the improvements in depth estimation that are obtained using our nomographs when applied to the ideal dyke model as well as a conceptualized passive margin cross-section model. We use the principle of isostatic equilibrium to develop one simple methodology for estimating magnetic crustal thickness in the absence of a priori information. We then demonstrate the effectiveness of these techniques by application to the 3D Bishop magnetic model. We show that by using reasonable geological assumptions one can develop acceptable first-pass estimates of the maximum thickness of magnetic bodies, which are key to accurate depth estimates. Using estimates of crustal thickness and applying appropriate thickness corrections based on the developed nomographs, errors of up to 40% in depth estimation can be reduced to a few per cent under ideal assumptions of isolated bodies and absence of noise.

In December 2010 BP commenced acquisition of a 5000 km2 3D seismic survey over the Risha Concession in Eastern Jordan. The survey utilized, BP’s Distance Separated Simultaneous Sweeping Technique to acquire over 2,4 million vibrator points in 6 months, with peak productivity of 23,851 vibrator points in a single 24 hour day on flat gravel plain, equating to 59 km2 per day. After ramp-up, 18 operational vibrators acquired up to 1500 km2 of high fold (1100 at 7 km offset) seismic data per month. The wide azimuth, high fold data provided a step change in BP’s subsurface understanding of Risha’s Ordovician prospects. The high fold, long offsets, and fully sampled source wavefield provided flexibility and enhancement of noise attenuation techniques enabling removal of the extreme scattered noise which has contaminated previously acquired data. For the first time, coherent reflectivity can be seen and seismic attribute products have provided insights into the lithology and fracture distribution in the area.