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Abstract: Seismic Modeling for Gas Field Development in Offshore Niger Delta: A Case History

Ayorinde Olusegun Idowu

In practice, seismic data has been used to map the geometry of events in the subsurface, mainly from reflection continuity and the character of reflection packages. Seismic modeling techniques are attempts to mathematically and geometrically represent the geology of the subsurface and to depict the interaction of that geology with a propagating seismic wave front. In the Niger delta, recent development in stratigraphic exploration has induced the examination of more subtle features of reflection, mainly polarity, amplitude, and waveform, to define the limits of seismic resolution and hence predict the geometry of the subsurface fluid and solid interfaces.

The field being appraised for development in this study is located approximately 35 km offshore southeast of the Niger delta in a water depth ranging from 20 to 25 m, and the project involves a modeling evaluation of five sand units encountered at different levels in the subsurface. The sands being modeled occur within the Agbada Formation (Miocene age), an alternation sequence of transitional sands and shales characterized by high-amplitude, anomalous seismic events over the Nkop field. Structurally, the field is characterized by seaward facing, contemporaneous growth faults consisting mainly of a rollover anticline located in a regional fault couple, bounded to the north and south by a growth fault.

The main objective is to model the reservoir geometry of the gas field's pay zones by obtaining empirically a good match between the synthetic model and the seismic data, and thereby establish the dominant frequency content, amplitude, and phase of the seismic data. An evaluation of seismic events emanating from substitution of combinations of oil and gas models was also derived in terms of their geologic significance.

Three geologic models were proposed to enhance the field development by choosing an appropriate source wavelet (5-35 Hz minimum phase), and picking reflection times from an acoustic marker corresponding to lithologic interfaces of intercalated sands and shales. For a given geologic structure, a zero offset two-dimensional record section is generated according to Kirchoff's solution of Hugyen's principle. The geologic structure is approximated with plane segments, whereby each segment has a reflection coefficient and an interval velocity above the boundary (or segment) and an interval velocity below the boundary associated with it. An appropriate reference shot pulse is chosen, and the wavelet is convolved with the geologic model to generate the equivalent synthetic seismogram.

The reservoir geometry (including that of the pay zero) was consequently well approximated and the respective pay thicknesses were accurately determined (confirmed by later drilling). This procedure effectively improved subsurface geologic interpretation and estimation of the P + P reserves to aid optimum development of the Nkop gas field. The method further demonstrated that structural and stratigraphic modeling are effective tools for testing the mapability of a geologic concept and are able to evaluate the significance of reflectivity changes or anomalies on uncalibrated seismic data.

AAPG Search and Discovery Article #90982©1994 AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, August 21-24, 1994