West Africa Deep-Water Developments: Subsurface Learning's
Authors: Calavan Chuck W. , Garfield Timothy R. 
Published: October 2008

Abstract
The focus of this talk is ExxonMobil's subsurface learnings and application of those learnings to West Africa deep-water developments. The learnings fall broadly into three areas: evolution of deep-water depositional models and concepts; deep-water depletion and well planning; and integration of subsurface technologies into development planning.
Our knowledge of deep-water depositional systems has rapidly evolved over the last decade due to heightened industry focus and advancements in data quality and quantity. Deep-water fan depositional models of the 1970's and early 1980's have been improved and enhanced by these industry activities. We now recognize a spectrum of channel complexes and related architectural elements in deep-water reservoirs, whose complicated facies and flow unit architecture require innovative development strategies.
As the cost of deep-water drilling and development is very high, it is critical to develop robust depletion plans that maximize recovery, minimize well count and maintain flexibility to react to dynamic change. Cross-functional subsurface teams (geoscience, reservoir engineering, drilling and development planning), utilizing rapidly advancing 3D visualization technology, have proven effective in developing such plans. These teams must identify critical uncertainties and plausible scenarios that drive resource ranges. Robust development concepts must then be identified that minimize risk across the range while achieving acceptable returns in the best estimate case.

Background
During the 1990's, an explosion of oil exploration activity occurred in deep-water basins, ignited by advances in deepwater drilling and subsea production technologies. Coincident with these breakthroughs, deep and ultra-deep-water concessions in offshore West Africa were offered to industry. Drilling results in the mid-to-late 1990's in this region were outstanding, thrusting it to the forefront of upstream focus moving into the new millennium. Key discoveries during this period included Girassol, Dalia and Kizomba (Hungo, Chocalho, Kissanje and Dikanza) in Angola, Zafiro in Equatorial Guinea, and Bonga and Erha in Nigeria (Figure 1).
ExxonMobil established a substantial acreage position in this region and has equity in over 40 significant discoveries todate. Stemming from these discoveries, ExxonMobil is participating in over 15 major development projects that span the maturity spectrum from emerging (post-discovery) to planning/designing (concept selection to funding) to implementation (funding to first oil) and those in early production phase. The subsurface learnings that follow are based on experiences in these projects and include technical learnings, work process learnings and development planning learnings.

Evolution of Deep-water Depositional Models
Recent advances in our understanding of deep-water deposition are in direct response to heightened industry focus on deep-water reservoirs. With significant advancements in seismic data quality (including high resolution 3-D), a growing log and core database, tools to interpret and visualize data in 3-dimensions and the advent of side-scan sonar and other water-bottom imaging tools, our knowledge of deepwater depositional processes and architecture has evolved greatly in the past 10 to 15 years. This allows construction of higher quality geologic and simulation models earlier in the project lifecycle. As more projects enter development drilling and production phases, time-lapse (4D) seismic is acquired and history matches achieved, it is our belief that the details of these models will likely further advance and become more robust at the flow-unit scale.