Integrating rock physics and sequence stratigraphy for characterization of deep-offshore turbidite sand system

Abstract

Definition and understanding deep water depositional environment is essential, especially in exploration and development phase. This study is aimed at improving characterization and environment of deposition assessment of deepwater turbidite sands systems as a way of evaluating reservoir geometry and quality in “NOJA” field, deep-offshore Niger Delta. The data used for the analysis comprise 3D full angle stack seismic, biofacies and logs from five wells. Seismic sequence analysis using reflection termination patterns were used for the mapping of depositional sequences. The sequences have internal reflection geometries ranging from parallel, subparallel and chaotic. Seismic structural maps gave insight into rock deformation and hydrocarbon potential of the field. The probable structure responsible for the trapping of oil and gas was a faulted anticlinal structure. Amplitude maps of the two stratigraphic surfaces gave insight into sand and shale distributions and possible environment of sediment deposition. Rock physics concepts were utilised to evaluate reservoir continuity, diagenetic and environment of deposition effects influencing reservoir properties. Two reservoirs (R1 and R2) of interest occurred at a depth interval of 2398–2461 m. Reservoir (R1) has average volume of shale of 22%, effective porosity 20%, hydrocarbon saturation 53%, and permeability 556 mD. Reservoir (R2) has average volume of shale of 10%, effective porosity 26%, hydrocarbon saturation 79%, and permeability 1044 mD. Environment of sediment deposition of the reservoirs was weakly confined with distributary channel complexes. Reservoir (R1) had low degree of connectivity (loosely amalgamated turbidite sand system) and reservoir (R2) possessed high degree of connectivity (highly amalgamated turbidite sand system). Rock physics templates of constant, contact and friable showed that the reservoir sands were poorly cemented to unconsolidated. These models indicated that the reservoir sands were both influenced by depositional and depth related diagenetic effects.