Improved Seismic Imaging of the Sub-Salt Logan Reservoir: Ultra Deep Water Setting in the U.S. Gulf of Mexico

Abstract

Abstract

The sub-salt Logan discovery well was drilled by Statoil and partners during 2011 in Block 969 of the Walker Ridge Protraction area. At the time of drilling the well was the most southerly drilled in the US Gulf of Mexico, located in an ultra-deep water setting and beneath a geologically complex overburden that challenges the most advanced seismic acquisition and processing methods. The discovery well encountered potentially commercial oil volumes in the upper part of the heterolithic Paleogene Wilcox Fm. Information acquired in the well indicates that the reservoir is compartmentalized by faulting and by stratigraphy. A subsequent appraisal well, drilled in 2013 to gain more understanding of the reservoir and reduce the uncertainty in the oil water contact, confirmed the segmentation of the complex reservoir. The Logan structure is typical of the outer fold belt underlying the southern edge of the Sigsbee salt canopy in the US Gulf of Mexico, with the Paleogene reservoir deformed into a moderate relief (ca. 1000'), 4-way dip closed structure, overlying a pillow of autochthonous evaporites of the Callovian Louann Fm. Top seal to the accumulation is provided by Eocene and younger claystones stratigraphically overlying the reservoir interval. Hydrocarbon charge is assumed to be from Tithonian organic rich shales vertically underlying the accumulation. A detailed description of the Wilcox Fm. reservoir is hampered locally by the low resolution seismic image, primarily caused by the complex velocity structure of the overburden. This complexity includes: an allochthonous salt canopy; rafted high velocity Paleogene and Mesozoic carapace sediments; low velocity secondary mini basin sediments deposited on top of the allochthonous salt canopy. At best, the seismic image gives a low resolution image incapable of resolving the reservoir's stratigraphic complexity: at its worst the location of potential dynamic flow barriers segmenting the structurally complex crest of the accumulation must be largely inferred and critical spill areas of the structural closure are poorly constrained. In an effort to improve the seismic image and understand the reservoir complexity, Statoil and partners undertook an extensive seismic re-processing effort of the commercially available narrow and wide angle seismic datasets, concentrating on improving the description of the complex velocity structure and incorporating available well information.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016