Extreme Thin-Beds Formation Evaluation

Chanh Cao Minh¹ and Olivier Billon^2
1Schlumberger, Katy, TX
²Total, Luanda, Angola

Extreme thin-beds are encountered when shale volume fraction exceeds 80%. In these cases effective porosity in the sand layers rarely exceeds 5 p.u. The combination of high shale content and low porosity pushes formation evaluation to its limits. Not only new technology tools are needed, but a good interpretation procedure is important as small parameters change can alter the petrophysical results dramatically. Finally, control of the petrophysical results is also critical as the thin-bed zones are seldom tested due to their low storage volume and producibility.

We present a thin-bed case in West Africa where the shale volume averages 90% and the sand effective porosity averages 3 pu. Other complications include the presence of numerous tight cemented streaks and bad holes. We use a triaxial induction and NMR logs in the formation evaluation of extreme thin-beds. The workflow explains how to:
- recognize the extreme thin beds from the above logs,
- choose the shale point in a formation that averages 90% shale volume,
- select the shale anisotropy parameters and the shale porosity,
- discriminate anisotropy caused by tight streaks,
- overcome bad holes,
- estimate the fluid type and hydrocarbon content in the thin sands.
The primary goal is to establish the vertical continuity of the hydrocarbon column for the field rather than the reserves in the thin beds.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery