Subsalt exploration trap styles, Walker Ridge and Keathley Canyon areas, deepwater Gulf of Mexico

Abstract

Abstract Exploration in the deep water Gulf of Mexico has extended well beyond the shelf break, and is now testing hydrocarbon prospects within kilometers of the abyssal plain. We describe the major exploration prospect types associated with the salt nappe, salt canopy, and salt massif systems of Walker Ridge and Keathley Canyon areas in the deepwater Gulf of Mexico (Fig. 1). Although there have been excellent regional synopses of salt deformation in the deepwater Gulf of Mexico (Wang, 1988; Nelson and Fairchild, 1989; Liro, 1992; McGuinness and Hossack, 1993; Shuster, 1995; Rowan, 1995; Rowan et al., 1999), none have been based on area-wide 3D seismic data. 3D prestack depth imaging of seismic volumes over this region allows explicit definition of the salt emplacement and deformation history, and the associated subsalt structural trap styles. In this paper we summarize these subsalt trap styles and comment on their associated petroleum system elements. A limited number of exploratory wells have been drilled in this region, but at the time this paper is being prepared, the results have yet to be released. Salt Emplacement History Significant progradation of unstable siliciclastic shelf margins in the Gulf of Mexico (Winker and Edwards, 1983) began with the Paleocene-Eocene Wilcox trend of the Texas coastal plain. Subsequent loading and progradation of shelf margins are commonly proposed as the primary driving force for salt deformation observed in the deepwater Gulf of Mexico. In response to these sporadic but rapid sedimentary loading throughout the Tertiary and Quaternary, a variety of shallow, laterally extensive salt masses are observed on the outer shelf and slope of the northern Gulf of Mexico. Rowan (1995; 1999) has described the genesis and distribution of these salt features. The southern portion of the Walker Ridge and eastern portion of Keathley Canyon display limited post-emplacement deformation of lateral salt masses, presenting an excellent opportunity to observe the lateral emplacement processes and associated subsalt structures. Salt emplacement in the Walker Ridge-eastern Keatley Canyon region follows a simple history: a series of relatively evenly spaced inferred paleo salt stocks have fed salt canopy, nappe, and massif systems (Fig. 2). Based on ties to abyssal plain stratigraphy, the deepest salt roots were observed at the level of Miocene strata, implying that significant remobilization and lateral extrusion of salt began no later than that time. Stratal geometries in older Tertiary strata in certain portions of the study area suggest inversion structures and withdrawal synclines consistent with cylindrical and domal salt deformation which predate the lateral salt emplacement event. In this area, salt is likely emanating directly from the original Jurassic Louann layer, rather than from intervening salt layers. Peel (2001) has suggested emplacement of a late Mesozoic allochthonous sheet in the region, which would not substantially alter the salt processes and associated structures as discussed in this paper.