ABSTRACT: Geophysical Modeling of Reservoirs in Offshore Niger Delta

IDOWU, AYORINDE OLESEGUN, Nigerian National Petroleum Corporation, Lagos, Nigeria

The prospect under 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 within the paralic sequence in a growth faulting/rollover structural configuration. The sands being modeled occur within the Agbada Formation (Miocene age), an alternation of transitional sands and shales characterized by high amplitude anomalous seismic events over the Nko field.

The main objective is to model the reservoir geometry of the 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.

A total of three geologic models were proposed for the field by choosing an appropriate source wavelet (5-25 Hertz Minimum Phase), and picking reflection times from an acoustic marker corresponding to lithologic interfaces of intercalated sands and shales. Using a computer program, the geologic structure is approximated with segments of planes, each of which is assigned a reflection coefficient, and an interval velocity above and below the segment to generate equivalent geophysical models.

The reservoir geometry (including that of the pay zones) was consequently well approximated and the respective pay thicknesses

were accurately determined (confirmed by later drilling). This procedure greatly improved subsurface geologic interpretation and estimation of the P+P reserves, enabling optimum development of the Nko field.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)