K. L. Henry1 and B. J. Radovich2
1Texaco Exploration, Bellaire, TX
2Texaco Upstream Technology, Bellaire, TX
ABSTRACT: Integration Of Sequence Stratigraphy And Seismic Attributes To Characterize Deepwater Fans, Central Offshore Nigeria
Interpretation of key seismic attributes within a sequence stratigraphic framework can facilitate deepwater fan identification. The regional sequence framework guides the selection and use of the most appropriate attributes. As increasingly larger surveys are acquired, the interpreter is faced not only with the challenge of accurately detecting and characterizing the most extensive fan types, often in areas far from well control, but also with making this interpretation on large data volumes in a timely manner. Fortunately the exponential increase in the volume of 3D data acquired over the last three years has been coupled with the development of highly sophisticated visualization and interpretation tools. We can now custom-create multiple attribute volumes that highlight the expected response of our fan targets. With these modern tools we are more readily beginning to see the detailed 3D geomorphology of ancient deepwater fan systems on seismic data.
Seismic sequence interpretation of the Central offshore Nigeria region reveals a predictable spatial distribution of fan types that stack to form an overall progradational profile as the facies belts have shifted basinward through time. On the seismic data we see a clear vertical change from the older sequences that are dominated by basin floor fans, to sequences that are dominated by distal slope fans grading to sequences containing proximal slope fans. The very youngest sequences show the cut and fill signature of an active slope environment. Wells that are today being drilled in deeper water environments and to greater depths are testing very different fan types to the shallower, more inboard wells.
The concepts of sequence stratigraphy are still very important because they provide clear guidelines for interpreting the seismic data and for understanding what the observed seismic architectures represent. The deepwater section in Central Offshore Nigeria is characterized by alternating clusters of high and low amplitude reflections. This amplitude cyclicity is apparent on a regional scale, largely independent of structuring, and is interpreted to reflect energy variations associated with changes in relative sea level. The sequence boundaries in the deepwater are picked at the base of each high amplitude cluster. The transition from high to low amplitude above each sequence boundary is interpreted to reflect a change in lithofacies associated with a change from high to low depositional energy from early lowstand to highstand times. In areas far from well control, reasonable chronostratigraphic predictions can be made ahead of the drillbit by counting down the cycles or sequences from younger, more readily correlatable control points.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado