SEQUENCE STRATIGRAPHY ANALYSIS OF “UNIK” FIELD, ONSHORE NIGER DELTA, NIGERIA

Abstract

The sequence stratigraphic analysis of “Unik” Field located in the Coastal Swamp area of Niger Delta Basin has been carried out. Geophysical wire-line logs from nine wells and a 3-dimensional seismic data in SEGY format were utilized. The objectives of the research are to identify lithologies and detect the potential reservoirs, predict the reservoir characteristics and detect the paleodepositional environment of the sediments, interpret the structural framework of the study, and identify hydrocarbon leads. The analysis of the wire-line logs and the seismic data was done using PETRELTM (2009) Software. Lithology was identified on the wire-line logs using the gamma ray and self potential logs of each well, followed by the lithostratigraphic correlation of the wire-line logs. Depositional environments, major transgressive/regressive cycle, sequences and systems tracts were identified on well logs patterns. On the 3-Dimensional seismic data, structural interpretation was carried out by mapping faults. Stratigraphic interpretation involving mapping sequences and systems tracts were also carried out on the 3-Dimensinal seismic data by identifying discontinuities from reflection terminations. Well-to-seismic tie was carried out using check-shot data to tie well log depth information to seismic time information. Seismic facies in terms of reflection configuration, continuity, amplitude and reflection geometry were analyzed on the 3-Dimensional seismic data. Ten electrofacies characterizing the main facies of the depositional environments were interpreted within correlative well log depth range of the “Unik” field. These electrofacies include: floodplain, transgressive shoreline, point bar, channel fill, multistory channel, tidal flat, barrier bar, barrier foot, stream mouth bar and prograding wedge. Interpretation of the seismic data shows intense faulting with faults trending east-west and the sequence stratigraphic elements of the “Unik” field support that of the silisiclastic environment. Six depositional cycles (sequences) comprising the lowstand systems tracts (LST), transgressive systems tracts (TST) and highstand systems tract (HST) were identified on the geophysical wire-line logs. These sequences were tied to seismic data to give a good tie. The LST (prograding wedge) composed of interbedded sandstones and shales, TST composed of shale interpreted to be as a result of the retrogradation of sediments and the prograding and aggrading HST. Seismic facies analysis of each sequence reveals that the reflection configuration patterns are generally of parallel to sub-parallel, divergent with sheet or wedge shaped external form typical of a shelf environment characterized by a high-energy sedimentation environment with high to medium reflection amplitudes. The interpretation of “Unik” Field shows that the resource areas are the pinch-outs and channel sand fills within the identified sequences of the field. However, this study creates a better understanding of the subsurface geology, and identification of some hydrocarbon leads that may be useful for further future exploration and development plans within the area.