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ABSTRACT: Identifying and Evaluating Producing Horizons in Fractured Basement

Lloyd, Peter M.1, Peter M. Tandom2, Nguyen H. Ngoc2, Dr. H. D. Tjia3
(1) Schlumberger, Kuala Lumpur, Malaysia 
(2) Carigali Vietnam, Ho Chi Mihn City, Vietnam
(3) Petronas Research & Scientific Services, Kuala-Lumpur, Malaysia

This paper describes the results of development drilling in the basement reservoir of the Ruby Field in the Mekong Basin, Offshore Vietnam.

Studies on the Ruby discovery wells and onshore basement exposures were used to develop an interpretation model for the fractured and weathered granite reservoir. This model has proven robust, and has been further enhanced by applying more quantitative formation evaluation techniques to determine fracture aperture and porosity. Various acquisition techniques and computation algorithms have been tested to derive an optimized (single trip in the hole) logging program; giving consistent results compared with core analysis and production data.

With a geological model for the field, and carefully matched well data with reprocessed 3D seismic, it is possible to identify faults and fracture zones. Fracture zones can be recognized by their lower acoustic impedance, and by the generation of characteristic diffraction patterns. These effects are often quite subtle, but with the help of Coherence Cube techniques, and after detailed comparison with the fracture properties where we have well control, not only faults, but also zones of fracturing can be identified with some confidence.

Directional wells drilled towards the NE and SW (perpendicular to the direction of maximum horizontal geostress), and penetrating sections where there is evidence of both faulting and fracturing on the seismic, have indeed proven to be the best producers. It is possible to predict with some certainty the most prospective hydrocarbon bearing intervals within each well.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia