Seismic Based Fluid and Lithology Discrimination in Turbidite Systems: Case Studies from West Africa, Angola

By

Greg J. Schurter¹, Steve R. Smith¹

(1) BP, Sunbury, United Kingdom

 Discoveries in the deep-water blocks of offshore Angola are dominated by Miocene and Oligocene mid-slope turbidite reservoirs. The seismic data are ideal for identifying AVO anomalies and fluid contacts as the rock and fluid properties combine to produce some of the highest quality seismic data in the world. As the cost of deepwater appraisal wells is huge, there is an increasing dependence on seismic data to predict critical reservoir parameters such as net-to-gross and hydrocarbon presence, often far from well control. In response, BP now routinely generates fluid and lithology specific seismic attribute volumes in Angola in order to capture information historically generated through appraisal well drilling.

Seismic reservoir characterization in deep-water environments has been aided recently by extended elastic impedance theory, which can be used to determine seismic projections that enhance or diminish fluid and lithology responses. Seismic projection angles are determined by analyzing fluid and lithology relationships on seismic or log cross-plots, and those angles are used to generate seismic attribute volumes. In offshore Angola, the resulting lithology and fluid attribute volumes have consistently yielded additional information about the reservoir, often information not readily available from conventional stacks. Deriving the best projection angles is not yet an exact science and angles derived from seismic and logs are not always the same. This paper will present, through a series of case studies, a methodology for deriving projection angles from log and seismic data, and some of the striking seismic image enhancements that can be achieved through utilizing those projections.