The Logan Discovery, Southern Walker Ridge, US Gulf of Mexico: An Oil Accumulation in a Complex Setting

Abstract

The sub-salt Logan discovery well was drilled during 2011 in Block 969 of the Walker Ridge Protraction area, in the US Gulf of Mexico. At the time of drilling, the well was the most southerly drilled in the US Gulf of Mexico. The well encountered potentially commercial oil volumes overlying normal salinity formation water in sandstones of the Paleogene Wilcox Fm. A subsequent appraisal well, drilled in 2013 to gain more understanding of the reservoir and reduce the uncertainty in the oil water contact, encountered poorer reservoir properties and, surprisingly, fresh formation water with a very much reduced salinity compared to the discovery well. 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. 1000ʹ), 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. The Wilcox Fm. reservoir is both stratigraphically and structurally compartmentalized, although a detailed description of the reservoir is hampered locally by poor seismic imaging. The reservoir underlies an overburden that includes an allochthonous salt canopy, rafted Paleogene and Mesozoic carapace sediments, and shallow secondary mini basin sediments deposited on top of the allochthonous salt canopy. The complexity of the overburden presents a challenge to even the most advanced seismic imaging techniques. As a result of the poor seismic imaging, the location of potential dynamic flow barriers segmenting the accumulation must be largely inferred and critical spill areas of the structural closure are poorly constrained. Following the disappointing results from the appraisal well, the Logan partners decided not to proceed with further evaluation of the Logan discovery and the remaining acreage was relinquished by the partnership in 2015.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015