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.