PETROPHYSICAL AND SEQUENCE STRATIGRAPHIC ANALYSES OF ‘ORLAWAH’ FIELD, NIGER DELTA

Abstract

Stratigraphic and structural analyses of Orlawah field, Niger Delta were carried out with the aim of evaluating the hydrocarbon potential of the field. A suite of wireline logs and 3D seismic data were quality controlled and processed. Computation of petrophysical properties of reservoir units identified from log data (i.e porosity, permeability and volume of shale) from well-established equations was executed. Lithologic interpretation and hydrocarbon reservoir identification were carried out using gamma ray and resistivity log responses. Log sequence evaluation was done by the analysis of log amplitude and stacking patterns of facies succession. Identification and mapping of network of eleven faults from seismic sections using seismic attributes such as amplitude, reflection geometry and frequency were carried out. The internal reflection configuration of the sequences were studied using physical properties of the seismic events; reflection geometry, continuity, amplitude and frequency to establish the genetic relationship of seismic lithofacies and general stratigraphic architecture influencing reservoir properties in the area of study. Lithostratigraphic and chronostratigraphic correlations were done using the principle of similarities in log responses and rock boundaries. This utilized key stratigraphic surfaces; maximum flooding surface and sequence boundaries in the subdivision of subsurface rock units into depositional sequences. Seismic sequence analysis using reflection termination patterns which are erosional truncation, toplap, onlap and downlap were used for the identification and mapping of the depositional sequences. A reservoir property modelling was carried out to evaluate the spatial distribution of facies.The hydrocarbon bearing reservoirs labelled R1, R2, R3, R4, R5 and R6 occur within a depth interval of 6821 and 10313 ft subsea. R3 is laterally extensive across the field located at a depth interval of 8080 to 10313 ft having an average net to gross sand of 0.8, porosity 38%, hydrocarbon saturation 80%, volume of shale 8% and permeability 1478 md. Three depositional sequences are located at depth interval of 5211 to 11387 ft, accommodating varied number of reservoirs. The depositional environments range from delta plain to shelf and slope. Evidences from isochron maps show that the dominant structure harbouring hydrocarbon in the field is a faulted anticlinal structure. The petrophysical, stratigraphic and structural analyses reveal that the area of study has high hydrocarbon potential and as such, this study has shown that integration of seismic with well log data aided stratigraphic, structural analyses and as well established the environments of sediment deposition in the study area.