Use of Seismic Forward Modeling and Rock Property Analysis to Evaluate and Classify Resources, Deepwater Offshore Nigeria
Sunkanmi Iyiola, Alan D. Fuqua, and Jeffrey S. Ogilvie
ChevronTexaco, Bellaire, TX
ChevronTexaco's exploration success in the Niger Delta is due in large part to the use of seismic attributes for appraisal and classification of hydrocarbon-bearing reservoirs. Located in OPL 213 and 249, the Aparo discovery is currently being evaluated for potential field development. Exploration wells drilled to date encountered numerous oil-bearing reservoirs within the Upper and Middle Miocene. Pertinent data consisted of good-quality 3D time-migrated seismic data (PSTM) and excellent-quality wireline log suites.
Extensive well log analysis, rock property analysis, and seismic forward modeling were carried out for multiple zones to aid with resource classification. Direct Hydrocarbon Indicators (DHI) included NMO gather anomalies, fluid vector indicators from gradient-stack analysis, and various other reconnaissance amplitude-vs-angle attributes.
The study indicated for all hydrocarbon-bearing reservoirs present in sufficient sand thickness and quality, DHIs should be expected. In addition, modeled sensitivities to the seismic response included: porosity; clay volume; reservoir thickness; net-to-gross; and top seal heterogeneities (porosity, intrinsic anisotropy, clay volume). Thinner sands were shown to exhibit subdued seismic responses due to tuning effects. Variations in top seal heterogeneities were also found to significantly affect seismic amplitude character and magnitude. Seismic response ambiguity was associated with the greatest shale variability. These forward modeled seismic responses (AVA, etc.) and uncertainties related to shale variability were quantified, catalogued, and tabulated for each reservoir unit...ultimately providing seismic support for discovered reserve/resource classification.