SEQUENCE STRATIGRAPHIC CORRELATION OF OVERPRESSURE AND ENVIRONMENT OF SEDIMENT DEPOSITION IN ‘DEB’ FIELD, NIGER DELTA

Abstract

Several researches carried out in the Niger Delta field for overpressure prediction, have been based on the use of wireline logs, pressure data analysis and seismic. However, sequence stratigraphy has been sparingly used in overpressure prediction. This research, therefore employed sequence stratigraphy as a tool for overpressure prediction to aid better prediction by delineating environment of deposition associated with it. The sequence stratigraphic formations were delineated using electrosequence log motif. The tops and bases of overpressured zones were delineated using the sonic log, which were later transferred to the seismic and mapped across the field using well tops picked from the delineated wells. The same overpressured zones were also mapped by extracting interval velocity from velocity model generated from seismic and plotted with depth. The environment of deposition were delineated from well information and seismic using reflection termination and stacking patterns of the sediments. These environment of sediment deposition mapped were correlated with the overpressured zones in order to map out the environment of sediment deposition associated with overpressure. Three seismic facies were mapped on the seismic section at 2600 ms, 3400 ms and 3700 ms using the reflection termination patterns. Four sequence boundaries were mapped at 2182 ms, 2340 ms, 2905 ms and 3209 ms, three of which correlated to the ones mapped on the log at 2340 ms, 2905 ms and 3209 ms. Three maximum flooding surfaces were also mapped at 2502 ms, 2695 ms and 3127 ms. Also, two transgressive surfaces were mapped at 2174 ms and 2839 ms and both correlated with the one delineated on the log. Three sequence stratigraphic formations were identified on the log and correlated in the field of study with the facies delineated on seismic. The second formation was found to be overpressured and the overpressure zones ranging from 3600 m to 4500 m were delineated and mapped. Environments of deposition determined in the field are prograding wedge, slope fan, basin floor (Lowstand System Tracts), fluvio-deltaic sand (Highstand System Tracts) and alluvial, coastal plain (Transgressive System Tracts). This study has been able to establish a relationship between overpressure zones and environment of deposition; fluvial sands, which is in the Highstand system tracts respectively is the environment of deposition associated with overpressure.