Seismic Interpretation And Stuctural Analysis Of Closures In The Greater Ughelli, Central And Coastal Swamp Depobelts Of Niger Delta Basin

ABSTRACT

The Niger Delta is located on a typical passive margin with sediment thicknessesin excess of 12 km at the basin center. The combined occurrence of rift structures, basin subsidence, rapid progradation of deltaic systems, development of growth faults and fold thrust belts in the offshore Niger Delta produced a very unique petroleum habitat with different structural styles and play types. Most of the prospects and producing fields in the region are related to faulted structures. Structural elements such as fault patterns, kinematics, geometry, timing and size of structures therefore control the distribution of hydrocarbons in adjacent fault blocks in most petroleum habitats. The objectives of this study were to: (i) evaluate some failed exploration prospects within a hydrocarbon play; (ii) understand the field and prospect’s kinematics and growth history; (iii) evaluate the timing of fault movements and closure formation relative to hydrocarbon charge; and (iv) understand the fractal nature of faults, trap integrity, faults and structure geometry or size modification.

The methods adopted for this study include both quantitative and qualitative interpretation of well and seismic data.  Well logs, biostratigraphic and biofacies data were integrated to create a correlation panel to aid the understanding of reservoir continuity and gross depositional environment. A synthetic seismogram was generated from density log, sonic log and seismic wavelet to enable well-to-seismic tie for the identification of stratigraphic markers on the 3D seismic data.  A detailed seismic interpretation of faults and horizons was done followed by fault interpretation, quality checking, fault surface mapping, fault pattern, fault growth and kinematics analyses. Sequential restoration of the deformed horizons along the corresponding fault planes using both 2D and 3D approaches was done for the evaluation of episodes of fault motion and closure development through time. Fractal analysis of faults using geostatistical approach aided prediction of sub-seismic faults in three depobelts of the Niger Delta.

The correlation of stratigraphic markers in the three locations shows that they are laterally continuous and easily identifiable on the 3D seismic data with the aid of the synthetic seismogram. The objective intervals are shoreface sandstones between the 33.3 and 31.3 Ma maximum flooding surfaces (MFS) deposited in a middle neritic environment. The seismic interpretation and fault surface mapping yielded a robust sub-surface model which was the main input for the structural analysis. The result of the growth, kinematic analysis and sequential restoration showed that traps formed earlier at all the objective levels at field A, compared to closures B and C, because faulting and closure development started and ceased earlier (at about 18 Ma) around the vicinity of field A. Relative stability of the fault at field A enhanced hydrocarbon retention capacity of the trap at field A compared to closures B and C where multiple episodes of fault movements  modified and breached the paleo-closures (traps), and as a result, trapped hydrocarbons spilled, leaving a paleo-amplitude anomaly at closure B.The fractal analysis shows that the fractal dimension for size frequency distribution increases from Greater Ughelli to the Coastal Swamp Depobelt. The distribution of simulated sub-seismic faults in the fields studied shows a variation from clustering around the major faults where strain is high to anti-cluster distribution at areas with low strain concentration. Sub-seismic faults (also seen in cores) may account for some of the unexpected drop in pressure and production observed in some wells. The results of this study clearly demonstrate the impact of episodic fault movements on closure modification and hydrocarbon retention capacities of hydrocarbon traps in the Niger Delta.