Technical Challenges in Appraising Deep, Sub-Salt Miocene-Age Reservoirs, Deep-Water Gulf of Mexico: Knotty Head Discovery

Kent Wilkinson, Mike de Vries, John Wagner, Eric Kubera, David Pope, and Adam Bishop
Nexen Petroleum U.S.A., Dallas, TX

Over the past decade as supra-salt opportunities have become limited, long-held ideas of drilling sub-salt for potentially larger structures were made possible by advancing drilling and seismic technologies. These structures were generally defined as exploration prospects on Narrow Azimuth Seismic data with poorly imaged sub-salt resolution. However, recent discoveries in the Gulf of Mexico (GOM) have led to an increase in Wide Azimuth Seismic acquisition for evaluating uncertainties in both structural and reservoir architecture. Knotty Head is a discovery in the central GOM where delineation of the reservoir at great depths remains a key technical challenge. Knotty Head is located in Green Canyon Block 512 at a water depth of 1085m. Oil was discovered in 2005 in multiple mid-Miocene age reservoirs. The BP#2 well holds a world record for achieving a total depth of 10420m in a hydrocarbon bearing well.

The reservoir intervals lie below 4500m of allochthonous salt. The evacuation of the underlying autochthonous salt formed a large four-way turtle structure. The salt body geometry and depth of the reservoirs create significant structural uncertainty due to challenges in seismic imaging, frequency content and velocity modeling. Over 180m of core indicate the reservoirs are associated with an organized deep-water fan system dominated by gravity-flow deposits. Facies range from low-relief channelized lobes (sheets) to debris flows. Porosity-permeability development and preservation at these depths are an integral part of the overall evaluation. Future appraisal well locations are being planned to minimize structural and reservoir uncertainty. Due to significant drilling risks such as internal salt suture zones, mobile tars and reservoir pressure regressions, careful well planning, execution and world class equipment is required.

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